Energy-dispersed ions in the plasma sheet boundary layer and associated phenomena: Ion heating, electron acceleration, Alfvén waves, broadband waves, perpendicular electric field spikes, and auroral emissions

Abstract

Recent Cluster studies reported properties of multiple energy-dispersed ion structures in the plasma sheet boundary layer (PSBL) that showed substructure with several well separated ion beamlets, covering energies from 3 keV up to 100 keV (Keiling et al., 2004a, b). Here we report observations from two PSBL crossings, which show a number of identified one-to-one correlations between this beamlet substructure and several plasma-field characteristics: (a) bimodal ion conics (<1 keV), (b) field-aligned electron flow (<1 keV), (c) perpendicular electric field spikes (~20 mV/m), (d) broadband electrostatic ELF wave packets (<12.5 Hz), and (e) enhanced broadband electromagnetic waves (<4 kHz). The one-to-one correlations strongly suggest that these phenomena were energetically driven by the ion beamlets, also noting that the energy flux of the ion beamlets was 1–2 orders of magnitude larger than, for example, the energy flux of the ion outflow. In addition, several more loosely associated correspondences were observed within the extended region containing the beamlets: (f) electrostatic waves (BEN) (up to 4 kHz), (g) traveling and standing ULF Alfvén waves, (h) field-aligned currents (FAC), and (i) auroral emissions on conjugate magnetic field lines. Possible generation scenarios for these phenomena are discussed. In conclusion, it is argued that the free energy of magnetotail ion beamlets drove a variety of phenomena and that the spatial fine structure of the beamlets dictated the locations of where some of these phenomena occurred. This emphasizes the notion that PSBL ion beams are important for magnetosphere-ionosphere coupling. However, it is also shown that the dissipation of electromagnetic energy flux (at altitudes below Cluster) of the simultaneously occurring Alfvén waves and FAC was larger (FAC being the largest) than the dissipation of beam kinetic energy flux, and thus these two energy carriers contributed more to the energy transport on PSBL field lines from the distant magnetotail to the ionosphere than the ion beams.

Full text

Ann. Geophys., 24, 2685–2707, 2006
www.ann-geophys.net/24/2685/2006/
© European Geosciences Union 2006
Annales
Geophysicae
Energy-dispersed ions in the plasma sheet boundary layer and
associated phenomena: Ion heating, electron acceleration, Alfv´
en
waves, broadband waves, perpendicular electric field spikes, and
auroral emissions
A. Keiling1, G. K. Parks1, H. R`
eme2, I. Dandouras2, M. Wilber1, L. Kistler3, C. Owen4, A. N. Fazakerley4, E. Lucek5,
M. Maksimovic6, and N. Cornilleau-Wehrlin7
1Space Sciences Laboratory, 7 Gauss Way, UC Berkeley, USA
2Centre d’Etude Spatiale des Rayonnements, 9 ave de Colonel Roche, Toulouse, France
3University of New Hampshire, 39 College Road, Durham, USA
4Mullard Space Science Lab, Holmbury St. Mary, Dorking, UK
5Space and Atmospheric Physics, Imperial College, London, SW7 2B2, UK
6LESIA, Observatoire de Paris, Meudon, France
7CETP, 10/12 Ave de l’Europe, Velizy, France
Received: 30 November 2005 – Revised: 8 June 2006 – Accepted: 29 August 2006 – Published: 20 October 2006
Abstract. Recent Cluster studies reported properties of mul-
tiple energy-dispersed ion structures in the plasma sheet
boundary layer (PSBL) that showed substructure with sev-
eral well separated ion beamlets, covering energies from
3 keV up to 100keV (Keiling et al., 2004a, b). Here we
report observations from two PSBL crossings, which show
a number of identified one-to-one correlations between this
beamlet substructure and several plasma-field characteris-
tics: (a) bimodal ion conics (<1 keV), (b) field-aligned elec-
tron flow (<1keV), (c) perpendicular electric field spikes
(∼20 mV/m), (d) broadband electrostatic ELF wave pack-
ets (<12.5 Hz), and (e) enhanced broadband electromagnetic
waves (<4kHz). The one-to-one correlations strongly sug-
gest that these phenomena were energetically driven by the
ion beamlets, also noting that the energy flux of the ion beam-
lets was 1–2 orders of magnitude larger than, for example,
the energy flux of the ion outflow. In addition, several more
loosely associated correspondences were observed within the
extended region containing the beamlets: (f) electrostatic
waves (BEN) (up to 4kHz), (g) traveling and standing ULF
Alfv´
en waves, (h) field-aligned currents (FAC), and (i) au-
roral emissions on conjugate magnetic field lines. Possi-
ble generation scenarios for these phenomena are discussed.
In conclusion, it is argued that the free energy of magne-
totail ion beamlets drove a variety of phenomena and that
the spatial fine structure of the beamlets dictated the loca-
Correspondence to: A. Keiling
(keiling@ssl.berkeley.edu)
tions of where some of these phenomena occurred. This
emphasizes the notion that PSBL ion beams are important
for magnetosphere-ionosphere coupling. However, it is also
shown that the dissipation of electromagnetic energy flux
(at altitudes below Cluster) of the simultaneously occurring
Alfv´
en waves and FAC was larger (FAC being the largest)
than the dissipation of beam kinetic energy flux, and thus
these two energy carriers contributed more to the energy
transport on PSBL field lines from the distant magnetotail
to the ionosphere than the ion beams.
Keywords. Ionosphere (Wave-particle interactions) – Mag-
netospheric physics (Auroral phenomena; Magnetosphere-
ionosphere interactions)
1 Introduction
Ion beams are a characteristic feature of the PSBL during
all levels of geomagnetic activity (Lui et al., 1978). Many
studies have characterized their properties from low altitudes
out to the distant magnetotail (e.g., Forbes et al., 1981; East-
man et al., 1984; Parks et al., 1984, 1998; Takahashi and
Hones, 1988; Zelenyi et al., 1990; Bosqued et al., 1993; Hi-
rahara et al., 1997; Sauvaud et al., 1999; Sergeev et al., 2000;
Lennartsson et al., 2001; Grigorenko et al., 2002; Kazama
and Mukai, 2003). To explain their existence, various pro-
cesses have been proposed for the energization of ions in the
Published by Copernicus GmbH on behalf of the European Geosciences Union.

2686 A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer
SC 1: Outbound crossing
SC 1: Inbound crossing
(b)
(a)
PSBL
N
S
Figure 1
AB
beamlets
beamlets
?
Fig. 1. Cluster crossings of the PSBL in both hemispheres on 14
February 2001, showing ion energy-time spectrograms for (a) the
outbound crossing and (b) the inbound crossing (adapted from Keil-
ing et al., 2004b).
magnetotail (e.g., Lyons and Speiser, 1982; Hasegawa, 1987;
Schindler and Birn, 1987; Wygant et al., 2005). It is possible
and even likely that different types of ion beams are gener-
ated by different mechanisms under different conditions. For
example, it was found that the active and quiet magnetotail
can lead to ion beams with different properties giving justifi-
cation for the existence of several energization mechanisms
(Sauvaud and Kovrazhkin, 2004).
In spite of ample observations, little has been conclu-
sively confirmed about the impact of PSBL ion beams on
auroral and magnetotail dynamics. The free energy carried
in ion beams is clearly a potential source for driving other
phenomena in the PSBL. Two general beliefs are that ion
beams contribute to creating the aurora (e.g., Kan and Aka-
sofu, 1976; Lyons and Evans, 1984) and to the formation
of the central plasma sheet (CPS) (e.g., Lyons and Speiser,
1982). Early studies also found that ion flows in the PSBL
simultaneously occur with various other field and plasma
signatures (e.g., Parks et al., 1984; Eastman et al., 1984).
In more recent studies, correlations between ion flows and
other phenomena were further established. For example,
energy-dispersed ions in the magnetotail have been directly
associated with ionospheric and plasma sheet (PS) activities
(Sauvaud et al., 1999; Sergeev et al., 2000; Kazama and
Mukai, 2003). Some observational evidence has been re-
ported that ion beams could lead to broadband electrostatic
noise (BEN) (e.g., Gurnet et al., 1976; Grabbe and East-
man, 1984) via beam-related instabilities. Magnetotail ion
beams have been invoked for the acceleration and heating
of ionospheric ion outflow to form ion beams and conics,
respectively (Alfv´
en and F¨
althammar, 1963; Schriver et al.,
2003; Lennartsson, 2003; Janhunen et al., 2003). Further-
more, Elphinstone et al. (1995) associated velocity-dispersed
ion structures (VDIS) with the double oval, and Janhunen et
al. (2003) suggested that the free energy associated with ion
shell distributions in the PSBL could lead in a sequence of
events to the acceleration of auroral electrons causing auro-
ral arcs.
In this paper, we describe observations of phenomena
which are associated with energy-dispersed ion structures in
the PSBL for two events using a comprehensive set of instru-
ments onboard Cluster. Both events have previously been
investigated with regard to properties and generation mecha-
nism of the dispersed ion structures (Keiling et al., 2004a, b).
This new investigation will assess the impact of dispersed
ion structures on their plasma environment and, in general,
on magnetosphere-ionosphere (M-I) coupling. One of the
chosen events is one of the most complex ones found in the
three-year Cluster data base (2001 to 2003). On 14 February
2001 multiple energy-dispersed ion structures were recorded
in the PSBL by the Cluster spacecraft while on an inbound
crossing (Fig. 1b). At about 00:45 UT (00:10 MLT, 4.5Re),
Cluster 1 crossed the lobe-PSBL interface first followed by
Cluster 3 (shown later). The ion data of Cluster 1 reveal four
smaller-scale structures (called beamlets) which are grouped
such as to resemble an extended dispersed ion structure (A);
the second ion structure (B) contains two distinct beamlets.
At ∼00:57 UT (arrow with question mark) a fainter ion struc-
ture is apparent but it is unclear whether it is a beamlet struc-
ture. Although this event is complex, its fine structure is very
clear and shows well separated ion beamlets, allowing us in
this study to less ambiguously identify relationships between
ion beamlets and other phenomena, because we not only have
one dispersed ion beam but several ion beamlets during one
crossing, each of which can be compared to other local phe-
nomena. These favorable circumstances warrant this com-
prehensive case study.
About two hours after Cluster crossed the PSBL in the
Southern Hemisphere, it crossed the opposite (northern)
boundary of the PS at about the same radial distance and
local time but this time during an outbound motion of the
spacecraft (Fig. 1a). Although the energetic ion signatures in
the E-t spectrogram appear at first glance to be very different
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A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer 2687
(best seen by comparing the dispersion slopes of the ions)
from the inbound event (Southern Hemisphere), we proposed
that in fact the same injection scenario operated during both
inbound and outbound crossings creating the dispersion sig-
natures (Keiling et al., 2004b). Consequently, one might ex-
pect that the impact of the dispersed ions on their plasma
environment should be the same for both events, in spite of
their differences in appearance in the E-t spectrograms, and
we will show evidence for this.
The most significant result reported here will be to estab-
lish convincing one-to-one correlations among particle and
field signatures which will lead us to propose a scenario in
which the magnetotail ion beamlets are the driver of ion con-
ics, perpendicular electric field spikes, field-aligned electron
energization, broadband electrostatic waves, and electromag-
netic waves. Furthermore, it will be proposed that the ion
beams can be the driver of Alfv´
en waves and indirectly au-
roral emissions (via these Alfv´
en waves). In this study we
will not, however, discuss the properties of the ion beam-
lets themselves and their possible generation mechanism in
the far-tail but refer the reader to Keiling et al. (2004a, b)
where properties such as energy range, pitch angle, compo-
sition, dispersion slopes and possible injection scenarios are
discussed in detail. We also refer the reader to the simulation
study by Ashour-Abdalla et al. (2005), who investigated one
of the two events to identify the source region of these ion
beamlets but came to an alternative interpretation.
2 Data sources
The observations presented here are from the Cluster space-
craft which are placed in a 57-h orbit with perigee and apogee
of 4 and 19.6 REgeocentric distance, respectively (Escou-
bet et al., 1997). Data used in this study come from the
ion instrument (CIS), the electron instrument (PEACE), the
electric field instrument (EFW), the plasma wave instrument
(STAFF), and the Fluxgate magnetometer (FGM). In addi-
tion to the Cluster data, we utilized ground magnetic field
data from the International Monitor for Auroral Geomagnetic
Effects (IMAGE).
3 First event: inbound crossing of the PSBL
During the inbound crossing of the PSBL on 14 February
2001 at about 00:45 UT the substorm recovery phase pre-
vailed according to ground magnetometer data from the IM-
AGE network (not shown). In this section we will show
the following particle and field signatures occurring during
this inbound crossing: field-aligned currents (FAC), heated
ion outflows, accelerated electrons, broadband electrostatic
and electromagnetic waves, electric field spikes, DC electric
field, Alfv´
en waves, and energy flux calculations of the var-
ious magnetotail energy carriers. We will mostly describe
observations from Cluster 1 but will point out differences
between Cluster 1 and Cluster 3. First, we present a brief
overview of this crossing including the tail lobe and the CPS
to emphasize features that are unique to the PSBL.
3.1 Plasma regions
The Cluster spacecraft crossed several plasma regions during
this event which can be identified in the energetic ion data
(>1keV) (Figs. 2a and g), the low-energy ion data (<3 keV)
(Figs. 2b and h), the electron data (Figs. 2c and i), the mag-
netic field (Figs. 2d and j) and electric field (Figs. 2e and k)
data, and plasma wave data (Figs. 2f and l).
Figure 2a shows the Cluster 1 encounter (first dashed line
from left) with the magnetotail ion beamlets as was already
shown in Fig. 1b. The beamlet-carrying region lasted un-
til 00:56 UT (second dashed line) and is denoted the PSBL.
Cluster 3 encountered the first ion beamlet about 1.5 min af-
ter Cluster 1 (separated by ∼500 km in the direction of mo-
tion and by ∼100 km in the azimuthal direction) but recorded
a very different beamlet structure in the high energy ion data
(Fig. 2g). Three larger scale structures (A, B, and C) were
recorded of which A and B also show substructure in the
form of beamlets but their separation is not as clear as it was
for Cluster 1 (cf. Keiling et al., 2004a). This has direct conse-
quences for establishing one-to-one correlations between ion
beamlets and other phenomena. The region to the left of the
PSBL is devoid of high energy ions and is the tail lobe. Fol-
lowing the PSBL to the right is a more structureless, thermal-
ized ion population, which corresponds to the CPS (except of
the faint structure at ∼00:57 UT indicated in Fig. 1b).
The three regions, identified on the basis of high energy
ion data (>1 keV), can also clearly be seen in the low-energy
ion data (Figs. 2b and h). From the left, Cluster 1 first en-
countered outflowing ions with energies <100eV in the tail
lobe. Later we will show that this is O+. No O+ was recorded
by Cluster 3. This region is followed by a different type
of ion outflow covering the energy range up to about 1keV
(starting at the first dashed line from the left). At the second
dashed line a sharp transition occurs to a third kind of ion
outflow, namely inverted ion V’s with energies of 0.3–3keV.
The electron data also show characteristic signatures in
each region (Figs. 2c and i). First, to the left, polar rain is
present which is the strongest evidence for the tail lobe. From
00:42 UT (Cluster 1) and 00:44 UT (Cluster 3) onward there
is a gradual increase in electron energy up to several keV. The
region of keV electrons (between both dashed lines) also co-
incides with the presence of lower energy electrons (<1 keV)
which are intense and very structured. This structured signa-
ture is similar to the low energy ions (see Sect. 3.4 for more
detail on this similarity). At the second dashed line, coincid-
ing with the onset of the CPS as determined from the ions,
the electron peak energy (>10keV) is raised on Cluster 1
but not on Cluster 3, and the low energy electrons (<1 keV)
show a drop-out.
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2688 A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer
PSBL CPS
Tail lobe
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Fig. 2. Particle and field data during the lobe-plasma sheet crossing on 14 February 2001. (a–f) are for Cluster 1. (a and b) Energy-
time spectrogram of all ions with >1keV and with <3 keV. (c) Energy-time spectrogram of electrons. (d). The azimuthal component of
the magnetic field in field-aligned coordinates (4-s spin modulation removed). (e) Electric field component in x direction (field-aligned
coordinates). (f) Sum of the two electric power spectral densities. (g–l) The same as (a–f) but for Cluster 3.
The plasma sheet (PSBL and CPS) was accompanied
by field-aligned currents (FACs) which are inferred from
the azimuthal magnetic field component (model subtracted)
(Figs. 2d and j). On entering the ion PSBL, Cluster 1
recorded a downward FAC, followed by an upward FAC at
approximately the time of entering the CPS. This is in con-
trast to Cluster 3, which recorded both downward and up-
ward currents inside the PSBL. The significance of this dif-
ference is topic of Sect. 3.2. Note that the smaller-scale fluc-
tuations superposed on the large-scale FAC in the beamlet-
carrying region for both spacecraft are due to Alfv´
en waves
which will be further discussed in Sect. 3.5.
The division in different regions is also apparent in the
electric field. On entering the PSBL, the fluctuations in
the electric field increased (Figs. 2e and k), and on leav-
ing the PSBL the fluctuation reduced significantly. Finally,
we note that broadband electrostatic waves were enhanced
and reached higher frequencies (up to 4 kHz) in the PSBL
(Figs. 2f and l).
3.2 Field-aligned currents
In Sect. 3.1 it was pointed out that upward and downward
FACs were crossed by Cluster 1 and 3. The main differ-
ence between both spacecraft was the current flow direc-
tion change recorded by Cluster 3 inside the PSBL (Fig. 2j
at ∼00:50 UT). This shows that the PSBL is not uniquely
defined by the current flow direction. Consequently, all ion
beamlets are found in the downward current region for Clus-
ter 1 (cf. Figs. 2a and d), whereas for Cluster 3 (cf. Figs. 2g
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A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer 2689
and j) they are found in both downward and upward currents.
This difference can be explained by separating injected ion
beamlets from bouncing echoes of these injections. It was re-
cently argued by Keiling et al. (2004a, 2005a) that the beam-
lets of structure A (Figs. 2a and g) are separate, individual
injections occurring in the distant tail, whereas the beamlets
of structures B and C are not new injections but bouncing
echoes of A. This distinction organizes the injected beam-
lets according to the current flow direction, namely all of the
injected beamlets (structures A) are found in downward cur-
rents for both Cluster 1 and 3. This suggests that the injection
region of the ions in the distant tail is a region of downward
current.
On the other hand, the echo beamlets (structures B and C)
can be found in both downward (Cluster 1) and upward cur-
rents (Cluster 3), suggesting that the echoes are independent
of the current direction. Note that for Cluster 1, one echo
beamlet of structure B overlaps with an injected ion beam-
let of structure A at ∼00:52 UT. If the downward current
is indeed of importance for the acceleration and injection
of magnetotail ion beamlets, it is not inconsistent that the
echo beamlets occur in upward FAC regions as well and this
could be taken as additional evidence that these ion structures
are echoes rather than new injections. The FAC flow direc-
tions with respect to the other plasma and field signatures are
also of importance for establishing cause-effect relationships
among various phenomena as will be described later.
The large-scale currents in the PSBL are associated with
large-scale convective electric fields (see Sect. 3.5) and carry
Poynting flux toward the ionosphere. This Poynting flux is
shown in Sect. 3.7 and compared to the energy flux of other
energy carriers in the PSBL such as ion flow and Alfv´
en
waves.
3.3 Ion outflow
In Sect. 3.1 it was shown that the ion outflows could be
divided into three regions on the basis of the variations of
the ions’ energy range. We now show for Cluster 1 ad-
ditional ion composition and pitch angle data for each re-
gion which will reinforce the fact that the outflow in each
region is distinct. Figures 3a and b are for reference show-
ing the ion beamlets and the ion outflow (not mass-resolved
from HIA). Figures 3c and d show mass-resolved E-t spec-
trograms of H+ and O+ (from CODIF) for energies <3 keV.
Figures 3e–g show, respectively, pitch-angle spectrograms
for non-mass-resolved ions, H+, and O+ for the same en-
ergy range (<3keV) as used in the corresponding E-t spec-
trograms. The horizontal black lines in Fig. 3e are visual aids
to emphasize the different pitch angle ranges in each region.
The ion outflow in the tail lobe is identified as purely
O+ (Figs. 3b–d) with peak energy and pitch angle range of
<100 eV and <50◦, respectively. The ion outflow in the
PSBL contains both H+ and O+. Energies and pitch angle
ranges for H+ are 10–800 eV and <90◦, respectively. O+, on
the other hand, has the same pitch angle range but is less en-
ergetic than H+. In contrast, the ion outflows of H+ and O+ in
the CPS are more energetic and the pitch angle range of <30◦
is significantly narrower compared to the PSBL ions. Two
instrumental considerations are noted here. First, the ion en-
ergy measurements are with respect to the floating spacecraft
potential. Since this potential is between 10 to 20 eV in the
PSBL (as determined from the electric field instrument EFW,
not shown), the energy values given above need be corrected
upward. Second, to verify that O+ is not an artifact (i.e., spill-
over from H+) in the CODIF measurements, we additionally
verified the presence of O+ by analyzing time-of-flight his-
tograms (not shown here). A word of caution is that these
histograms required time intervals of several minutes to ac-
cumulate enough data points to be certain that the noise level
was exceeded. Thus, individual O+ structures could not al-
ways be independently verified. The analysis that follows
will thus focus on H+.
The CPS ion (H+) outflow shows inverted V structures
(see arrows in Fig. 3b) which have typical beam distribu-
tions (Figs. 4e and f). The distribution functions of the H+
outflows in the PSBL, on the other hand, are conical with
varying cone angles, showing both parallel and perpendicu-
lar acceleration (Figs. 4a–d). The distribution in Fig. 4c is
bowl-shaped (white dashed line) and is very similar to those
discussed in Kumplar et al. (1984), where it is argued that
the ions undergo at least two acceleration mechanisms, one
field-aligned and the other perpendicular to the background
magnetic field. The variation in cone angles indicates that
the transverse energization occurred at various altitudes from
near the spacecraft (Fig. 4a) to ∼6000 km below the space-
craft (cone angle of 90◦– Fig. 4b). The latter estimate is
based on the assumption of constant energy and constant
magnetic moment during travel. Since some conics were
energized well below the spacecraft, energy dispersion over
the energy range of the conics might be expected. However,
no dispersion was present within the limit of the detector’s
time resolution. One possible explanation is that the heat-
ing region was moving with the large-scale E×Bdrift (see
Sect. 5.1 for further discussion).
Because of their different properties (note the abrupt
change at ∼00:56 UT), it is to be expected that the energiza-
tion of the ion outflows in the PSBL and those in the CPS
are the result of different processes. It was noted above that
there is a clear distinction of down- and upward current in
the PSBL and CPS, respectively, for the crossing recorded
by Cluster 1. This, however, was not the case for the cross-
ing recorded by Cluster 3 where both down- and upward cur-
rents existed in the PSBL. Since ion outflow existed through-
out the PSBL crossed by both spacecraft, we rule out that
the FACs were the driver of the ion outflows in the PSBL.
On the other hand, the large-scale FAC in the CPS are up-
ward for both Cluster 1 and 3. Field-aligned potential drops
created in upward FAC are probably the cause for the in-
verted V ion beams in the CPS (Carlson et al., 1998). The
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2690 A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer
Tail lobe PSBL CPS
Figure 3
(a)
(b)
(c)
(d)
(e)
(f)
(g)
mag.tail beamlets
conics
inv. V’s
Fig. 3. Composition and pitch angle data for Cluster 1 on 14 February 2001. Energy-time spectrograms of (a) all ions with >3keV, (b)
all ions with <3 keV, (c) H+ with <3 keV, and (d) O+ with <3 keV. (e–g) Pitch angle versus time spectrograms for all ions, H+ and O+,
respectively.
most obvious difference between the ion conic region and the
neighboring regions is the simultaneous presence of magne-
totail ion beamlets in the ion conic region, which strongly
suggests their importance for the energization of the ion out-
flows. Additional support for the scenario in which the mag-
netotail beamlets are responsible for the conic generation is a
close spatial/temporal relationship between ion beamlets and
individual ion outflow structures for Cluster 1 (dashed lines
with arrows in Figs. 5a and b). The ion outflow structures
show a similar periodicity as the magnetotail ion beamlets
suggesting that the latter causes the former. Moreover, it is
important to note that the ion outflow structures occurred at
or in close vicinity to the “edges” of the magnetotail beam-
lets. Figure 5d shows higher time resolution (12 s versus 16s
in Fig. 5b) non-mass resolved ions (i.e., including both H+
and O+ which accounts for the differences to Fig. 5b) con-
firming this association with the “edges”. Figure 5c shows
particle flux rather than energy flux as in Fig. 5b. A compar-
ison of these two figures shows that there is also ion outflow
away from the beamlet edges but the most energetic outflow
occurred close to the beamlet edges. If the beamlets (in par-
ticular their edges) are responsible for the ion heating, as will
be argued in this report, it might be argued that the conics that
were energized far below the spacecraft should be spatially
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A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer 2691
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400 00:53:07 - 00:53:23
B
y
H+
H+
H+
00:59:31 - 00:59:47
-27
log
10
(df/s
3
cm
-6
)
-28 -22
0
1
0
1
01:03:31 - 01:03:47
-26 -25 -24
log
10
(df/s
3
cm
-6
)
-28 -22
0
1
0
1
Figure 4
PSBL
CPS
-400 -200 0 200 400
V||
-400 -200 0 200 400
V||
-400 -200 0 200 400
H+H+
V⊥V⊥V⊥
V⊥V⊥
(a) (b) (c) (d)
(e) (f)
H+
-400 -200 0 200 400
-400
-200
0
200
400 00:55:31 - 00:55:47
V⊥0
xx
0001-005-00050001
0001-
005-
0
005
0001
V⊥0
z
0001-005-00050001
.
:
z
x
0001-
005-
0
005
0001
log
10
(df/s
3
cm
-6
)log
10
(df/s
3
cm
-6
)
Fig. 4. Velocity space distribution functions of Cluster 1 for specific times during the plasma sheet crossing on 14 February 2001. (a–d)
show ion conics in the PSBL, and (e) and (f) show ion beams in the CPS.
displaced from the ion beamlets because of the presence of
large-scale convection during the time period the conics trav-
eled from their source region to the spacecraft. This lack of
significant displacement can be explained with the simulta-
neous E×Bdrift of the ion beamlets and the ion conics (see
Sect. 5.1 for further discussion).
In Sect. 3.5, we will identify Alfv´
en waves collocated with
the beamlet-carrying region which also needs to be consid-
ered as a driver of ion heating. However, we will argue
against such a scenario (see Sect. 5.1).
For SC 3, ion outflows also occurred in the same region as
the magnetotail beamlets. The energy range of this outflow
is the same as observed by Cluster 1. However, no convinc-
ing one-to-one correlation was apparent (see Figs. 2g and h).
This could be explained with the lack of beamlet separations
with clear edges during the Cluster 3 crossing. A second
event will be presented later (Sect. 4) which will reinforce
the importance of beamlet edges for the energization of ion
conics. It is also noted that two outflow structures (between
00:47–00:50 UT in Fig. 2h) showed energy dispersion in the
Cluster 3 data; all other structures showed no dispersion. For
the moment, we hypothesize that these structures were cre-
ated by a separate mechanism which will be further investi-
gated elsewhere.
3.4 Accelerated electrons
The electrons (<1 keV) show a structured signature in the
beamlet-carrying region (PSBL) (Figs. 5e–g) which abruptly
stops at 00:56 UT coinciding with the exit of the beamlet-
carrying region, which is also equivalent to leaving the ion
conic outflow region. At about 00:57 UT electron flow was
again recorded which coincides with more intense PS ions
(Fig. 5a, arrow #6). Although we did not classify this ion
structure as a PSBL beamlet in the previous sections, it could
be argued that it is a thermalizing remnant of a bouncing
PSBL beamlet as discussed in Keiling et al. (2005a). No
outflowing ion conic was associated with this ion structure.
The most interesting observation regarding the electrons is
that at times of ion conic outflows (arrows labeled 1 through
5), the electron flow was more intense and reached higher
peak energy values (>100 eV) compared to the adjacent elec-
trons. The energies were comparable to the ion outflow en-
ergies. The one-to-one correlation between ion conics and
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2692 A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer
PA 0deg
10
-6
10
-5
10
-4
ergs/cm**2-s-str-eV
dEF
PA 90deg
PA 180deg
10
2
10
3
eV
10
2
10
3
eV
Spacecraft potential
10
2
10
3
eV
10
1
10
2
10
3
eV
00:42 00:45 00:48 00:51 00:54 00:57 01:00
12 3 4 5
Figure 5
6
(a)
(b)
(c)
12 3 4 5 6
(d)
(e)
(f)
(g)
Fig. 5. Comparison of ion and electron signatures during the PSBL crossing on 14 February 2001 for Cluster 1. The dashed lines with arrows
in panel (a) show the correlation with ion beamlets and ion conics (a–d). Note that the color scale in (b) and (c) is energy flux and flux,
respectively. Panel (d) shows non-mass resolved ion data which has a higher time resolution (4 s) than the H+ data (12s) in (b). The electron
data are separated in (e) field-aligned, (f) perpendicular, and (g) anti-field aligned energy-time spectrograms.
Ann. Geophys., 24, 2685–2707, 2006 www.ann-geophys.net/24/2685/2006/

A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer 2693
electron “bursts” is best seen for the structures labeled 2, 3,
and 4 which also coincides with times that show the clearest
beamlet separation. Furthermore, the electron bursts 2 and 3
do not show low energies (<100eV, white arrows in Figs. 5e
and g) as in the neighboring regions suggesting that the elec-
trons were simply raised in energy from their background
values. These observations suggest that the mechanism that
caused the heating of the ion conics was also directly or in-
directly causing the electron acceleration. However, in con-
trast to the unidirectional ion outflow in this region, the elec-
tron flow direction is more complex; electrons are observed
in all three angular sectors (panels e–g) in combinations of
field-aligned, anti-aligned and perpendicular motion that de-
pend upon time and/or location. For example, the electron
bursts labeled 2 and 3 are seen at all pitch angles with the
downward direction (panel g: PA 180 degree) being the most
intense. Downgoing electrons suggest that electron accelera-
tion processes also occurred above Cluster’s altitude. Where
down- and upgoing electrons occurred together, it is proba-
ble that the upgoing electrons were the reflected downgoing
electrons; note that in these cases both upgoing and down-
going electrons had the same energies (e.g., burst #3). Per-
pendicular flowing electrons are probably locally mirroring
ones. Some electron structures (e.g., burst #4) show mostly
upward flow, suggesting that the electrons were accelerated
below the spacecraft. Consequently, Cluster was immersed
in a region which experienced field-aligned electron acceler-
ation above and below the spacecraft.
In Sect. 3.3, we established a one-to-one correlation be-
tween magnetotail ion beamlets and ion conics, and thus we
argued that the former provides the energy for the latter. The
same can be suggested for the electrons, that is, the ion beam-
lets are causally related to the energization of the electrons.
However, it is not necessary that both ion conic and elec-
tron energization occurred at the same location simultane-
ously, since we showed electrons that were energized above
the spacecraft but were recorded together on the same field
lines with ion conics that came from below the spacecraft
(see, for example, the ion and electron structures labeled 2).
3.5 Electric and magnetic field variations
On crossing the PSBL, enhanced electric field activity per-
sisted (Figs. 2e and k). Two expanded views of this cross-
ing are shown in Figs. 6 and 7 for Cluster 1. The elec-
tric field shows several features which are superposed such
as small-scale electric field spikes, higher frequency elec-
trostatic fluctuations, the perturbation field of low-frequency
Alfv´
en waves, and a DC electric field shift. Below we in-
vestigate each of these features. The magnetic field, on the
other hand, shows low-frequency variations superposed on
the large-scale FAC. These low-frequency variations are as-
sociated with the Alfv´
en waves. Higher frequency electro-
magnetic waves are also present. E and B are shown in field-
aligned coordinates, with z along B, y westward, and x mak-
ing a right-handed coordinate system (i.e., nearly radially in-
ward).
3.5.1 Electric field spikes
Obvious features in the electric field inside the PSBL are
isolated, larger amplitude spikes (arrows in Fig. 6b). These
spikes have a periodicity of 1–3min which is comparable to
the periodicity of magnetotail ion beamlets. Moreover, the
spikes are often found at the edges of individual beamlets
(best seen for the largest spikes). This correlation is demon-
strated in a different format in Fig. 6c, where both the den-
sity of the ion beamlets with energies >3keV and a filtered
(>0.3 Hz) component of the electric field are overlaid. (Note
that the density plot is 12-s time resolution whereas the E-t
spectrogram is 4-s time resolution.) Individual larger spikes
with amplitudes up to 20 mV/m are located at density gradi-
ents. This one-to-one correlation suggests that the spikes are
causally related to the ion beamlets, in particular their edges
or density gradients.
3.5.2 Broadband waves
In addition to large electric field spikes, extremely low fre-
quency (ELF) electric field turbulence (Fig. 6c) as deter-
mined from EFW (Nyquist frequency of 12.5Hz) as well
as broadband electrostatic noise (BEN) with frequency up
to 4 kHz as determined from the plasma wave instrument
STAFF (Fig. 6d) occurred throughout the beamlet-carrying
region. There are no corresponding magnetic field fluctu-
ations (Fig. 6e), except at specific times (see below) when
broadband electromagnetic waves were recorded by the wave
instrument.
According to the EFW data, three broad regions of en-
hanced ELF electrostatic waves, indicated by horizontal bars
in Fig. 6c, can tentatively be identified. The two regions
to the left coincide with the magnetotail beamlets (bars in
Fig. 6a), but the region to the right is located inside the CPS.
Collocated with the ELF region in the CPS was a somewhat
increased intensity in ion energy flux. This weak ion struc-
ture has already been pointed out in Sect. 3.4 where it was
shown that it also coincided with enhanced electron flow.
The regions of ELF waves show signatures of wave pack-
ets. Thus an obvious question is whether these wave pack-
ets are associated with other particle signatures. In Fig. 6g
magnetotail ion beamlets and wave packets are shown for a
sub-interval with vertical dashed lines drawn above individ-
ual wave packets. Many of these lines line up with the start
and end of individual beamlets or with smaller ion features.
For example, the third wave packet (#3) lines up with a faint
beamlet feature that is located between two larger beamlets
of structure A (cf. Fig. 2a). Between wave packets #5 and #6
and between #9 and #10, wave turbulence prevails without
clear wave packets. These two periods correspond to beam-
lets that lasted somewhat longer (1–2min). Wave packets,
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2694 A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer
Ex
mV/m
10 mV/m
Ex
(a)
(b)
(c)
(d)
(e)
Figure 6
UT: 0042 0044 0046 0048 0050 0052 0054 0056 0058 0100
SUM OF THE THREE
MAGNETIC POWER-
SPECTRAL DENSITIES
SUM OF THE TWO
ELECTRIC POWER-
SPECTRAL DENSITIES
0042 0044 0046 0048 0050 0052 0054 0056 0058 01000042 0044 0046 0048 0050 0052 0054 0056 0058 01000042 0044 0046 0048 0050 0052 0054 0056 0058 01000042 0044 0046 0048 0050 0052 0054 0056 0058 01000042 0044 0046 0048 0050 0052 0054 0056 0058 01000042 0044 0046 0048 0050 0052 0054 0056 0058 01000042 0044 0046 0048 0050 0052 0054 0056 0058 01000042 0044 0046 0048 0050 0052 0054 0056 0058 01000042 0044 0046 0048 0050 0052 0054 0056 0058 01000042 0044 0046 0048 0050 0052 0054 0056 0058 01000042 0044 0046 0048 0050 0052 0054 0056 0058 01000042 0044 0046 0048 0050 0052 0054 0056 0058 01000042 0044 0046 0048 0050 0052 0054 0056 0058 01000042 0044 0046 0048 0050 0052 0054 0056 0058 01000042 0044 0046 0048 0050 0052 0054 0056 0058 0100
1
0042 0044 0046 0048 0050 0052 0054 0056 0058 0100
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
B (nT
2
/Hz)
10
100
1000
f (Hz)
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
E (mV
2
/m
2
/Hz)
10
100
1000
f (Hz)
(f)
(g)
12345 67891110
Fig. 6. Comparison of ion beamlets with electric field and wave data for the 14 February 2001 event. (a) E-t spectrogram of ion beamlets.
(b) Excomponent of the electric field (field-aligned coordinate system). (c) High-pass filtered electric field overlaid on the density of the
ion beamlets (>3 keV). (d, e) Electric and magnetic field power spectral densities (up to 4kHz). (f) E-t spectrogram of ion conics for a
comparison with the electric and magnetic field data. (g) Expanded view of the filtered electric field from panel (c) together with the ion
beamlets from panel (a).
Ann. Geophys., 24, 2685–2707, 2006 www.ann-geophys.net/24/2685/2006/

A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer 2695
spikes and beamlet gradients are possibly related which will
be further discussed in Sect. 5.3.
On entering the PSBL, BEN with frequencies >100 Hz
were recorded which continued deep into the CPS. How-
ever, coinciding with the beamlet-carrying region (PSBL) the
frequency reached values up to the limit of the instrument
(4 kHz). BEN was striated throughout the PSBL so that a
simple and convincing one-to-one correlation with the wave
packets of the ELF waves was not possible to establish. Sim-
ilarly, it is more ambiguous as to whether there was a clear
one-to-one correlation with ion conics (Fig. 6f). However, at
specific times (arrows above panel e), broadband magnetic
fields were recorded with corresponding broadband electric
fields (panel d), thus showing the presence of electromag-
netic waves. Interestingly, these waves were correlated with
the edges of ion beamlets (Fig. 6a) (or, equivalently, outflow-
ing ion conics), again suggesting a causal relationship.
3.5.3 Alfv´
en waves
The nature of the low-frequency fluctuations can be seen in
the band-pass filtered (40–160 s) electric and magnetic field
data (Figs. 7c–f). This filter range shows the lowest fre-
quency Alfv´
en waves during this crossing but higher fre-
quencies waves were also present. Figures 7a and b show
E (unfiltered) and B (filtered: >4 s). To compare the phase
relationship between E and B, Fig. 7e shows the electric field
overlaid with the Hilbert-transformed magnetic field. The
Hilbert transform shifts all frequency components by 90◦
without changing their magnitudes. This technique has been
demonstrated by Dubinin et al. (1990). It can be seen that
both fields show similar waveforms with reduced phase shift
during the time period indicated by the horizontal bar. Be-
cause B was phase-shifted by the Hilbert transform, this re-
sult indicates that the E and B fluctuations were partially
standing Alfv´
en waves. We also plotted in the last panel
E and B (without Hilbert transform) to show that these two
wave forms do not match as well. It is noted that the E-to-B
ratios are smaller than the local Alfv´
en speed; however, a de-
viation is to be expected for a mixture of traveling and stand-
ing Alfv´
en waves (Mallinckrodt and Carlson, 1978). We em-
phasize that the identification of traveling or standing waves
in the PSBL depends on the chosen frequency range (Keil-
ing et al., 2005b). In other frequencies ranges more traveling
wave power was observed for this crossing.
After the region of Alfv´
en waves, a strong current was
encountered (∼00:56 UT), and there E and H(B) are not in
phase; instead E and B are in phase. Furthermore, the E-to-B
ratio in this case is significantly smaller than the local Alfv´
en
speed, indicating that this is a static current structure.
3.5.4 DC electric field
Finally, a DC electric field of 5–10mV/m was present in
the beamlet-carrying region (Fig. 6b). This DC field to-
Figure 7
SC 1 14/Feb/2001
Minutes after 00 UT
down FAC up FAC
Alfven waves
FAC
Ex
mV/m
Bx
nT
By
filtered
Ex
filtered
-Ex
1.5 H(By)
-Ex
1.5 By
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 7. Electric and magnetic field data during the PSBL crossing on
14 February 2001 for the analysis of the low-frequency variations.
(a) Unfiltered and (b–f) filtered electric and magnetic fields. The
field data are presented in field-aligned coordinates. Shown are the
Alfv´
en wave components in the range from 40–160s. The last two
panels (e, f) show comparisons of E, B, and the Hilbert-transformed
B. 90◦phase-shifts or lack thereof are indicators for standing and
traveling Alfv´
en waves.
gether with the total magnetic field yield a large-scale con-
vective plasma flow ((E×B)/B2) of the order of 10 km/s in
the azimuthal direction (determined from EFW and FGM;
not shown). This convective plasma flow is possibly associ-
ated with shear flow in the distant tail which mediates mag-
netic stress via FAC and DC electric field to the Cluster loca-
tion.
Furthermore, the DC electric field and the magnetic field
of the field-aligned current carry Poynting flux towards the
ionosphere which is calculated and compared to other energy
carriers in Sect. 3.7.
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2696 A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer
-0.002
0.000
0.002
0.004
Ions
Energy (eV)
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
Q
||
E > 3keV
ergs/cm
2
-s
S
||
0.006
0.008
ergs/cm
2
-s
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
PSBL CPStail lobe
Q
||
E < 3keV
ergs/cm
2
-s
Q
||
E < 3keV
ergs/cm
2
-s
Figure 8
(a)
(b)
(c)
(d)
(e)
(f)
S
||
ergs/cm
2
-s
B1 B2
A1 A2 A3 A4
Ion outflow
Ion outflow
Magnetotail
ion beamlets
Alfven waves
FAC
Fig. 8. Energy flux comparison of (b, c) ion outflow, (d) magne-
totail ion beamlets, (e) Alfv´
en waves, and (f) FAC. (a) shows the
E-t spectrogram of ions as a reference. Note that (b) and (c) show
the same ion data (<3keV) but with different y scale. The vertical
dashed lines bracket the PSBL.
3.6 Auroral emissions
At the time of the PSBL crossing on 14 February 2001, ultra-
violet images from the IMAGE satellite show a double oval
(as reported by Keiling et al., 2004a) which is a typical re-
covery phase signature (Elphinstone et al., 1995). Allowing
for the mapping uncertainty, it is possible that the ion beam-
lets were conjugate to the poleward arc of the double oval,
and that the ion beamlets were indirectly responsible for the
auroral emissions. In particular, the question arises whether
the sharp beamlet gradients that showed large perpendicular
electric field spikes were conjugate to individual arcs. An
alternative source for the poleward arcs of the double oval
are the Alfv´
en waves that were simultaneously present. Both
possibilities are discussed in Sect. 5.7.
3.7 Energy flux
In order to determine the energy source of the phenomena
that have been presented in the previous sections, such as
the ion heating and electron acceleration, broadband waves,
and auroral luminosity, it is necessary to investigate the en-
ergy flux flowing towards Earth and crossing the region that
was traversed by Cluster. Contributors to this energy flux are
the magnetotail ion beamlets, FAC and the Alfv´
en waves.
Figure 8 shows in the first panel the E-t spectrogram of the
ions with individual beamlets being labeled. The following
panels show the field-aligned energy flux associated with the
ion outflow (<3keV) with two different scales (panel b has
the same scale as panel d for better comparison), the mag-
netotail ion beamlets (>3 keV), the Alfv´
en waves (filtered:
6s, 180 s), and the large-scale FAC (E and B were detrended
with a 5-min running average to obtain the DC component).
Once again there is a clear distinction in the three regions (tail
lobes, PSBL, CPS) with respect to the energy flux. There is
near zero energy flux in the tail lobe for all quantities. The
energy flux of the ionospheric ion conics in the PSBL is about
one order of magnitude smaller compared to the inverted V’s
in the CPS. For all magnetotail drivers (panels d, e, and f), the
energy flux is largest in the PSBL. The energy flux of each
driver is 1–2 orders of magnitude larger than the ion conics
energy flux, and, therefore, the energy flux of each driver is
in principle sufficient to energize the ion conics.
It is noted that the energy fluxes of the magnetotail ion
beamlets and the Alfv´
en waves show a tailward directed con-
tribution. For Alfv´
en waves, this is due to reflected Alfv´
en
wave components as shown in Sect. 3.5. The tailward en-
ergy flux for the ions is caused by temporal/spatial effects
which can be seen in velocity space distributions (Fig. 9).
Some distributions have already been reported in Keiling et
al. (2004a). For example, Figs. 9b, e, and h show full shell
distributions whereas Figs. 9a, c, and i show partially filled
distributions. These variations reflect temporal/spatial ef-
fects. For example, the first three distributions (Figs. 9a–
c) show the changes associated with beamlet A1. The first
Ann. Geophys., 24, 2685–2707, 2006 www.ann-geophys.net/24/2685/2006/

A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer 2697
A1 A2
A3
A4 and B1
Figure 9
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
Fig. 9. Selected velocity space distribution functions (Cluster 1 – HIA) for various ion beamlets during the PSBL crossing on 14 February
2001. Negative (positive) velocities indicate downward (upward) flow directions.
encounter with A1 shows mostly downgoing ions (panel a)
followed by downgoing and mirroring ions further into the
PSBL (panel b). On leaving A1, mostly mirroring ions were
recorded (panel c). Moreover, a closer inspection reveals
that the peak upward velocity in panel (b) is slightly larger
than the peak downward velocity. These variations are con-
sistent with the well known velocity profile of ion beams
inside the PSBL (Takahashi and Hones, 1988). The spa-
tial/temporal pattern can thus locally lead to strong net Earth-
ward and/or tailward energy flux. There is no one-to-one
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2698 A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer
X
Z
Figure 10
Cluster
ion outflow
mag.tail ion beamlets
(a)
(b)
Fig. 10. Outbound crossing of the PSBL on 14 February 2001. (a)
E-t spectrogram of ions (non-mass resolved) with >1 keV. (b) E-t
spectrogram of ions with <1keV.
correlation between enhanced downgoing kinetic energy flux
of ions and upflowing kinetic energy flux of ion conics. How-
ever, since the energy flux of the magnetotail ions is over-
whelmingly larger (1–2 orders of magnitude) than that of the
upflowing ion conics, only small energy differences between
downward and upward traveling ion beamlets are required to
energize the ion conics. These small energy differences to-
gether with the temporal/spatial variations are probably im-
possible to identify in this energy flux comparison. Similarly,
no one-to-one correlation between upflowing ion conics and
either FAC or Alfv´
en waves exists. Thus, we conclude that
this energy flux comparison does not provide any confirma-
tion as to which of the three energy carriers was the actual
driver but it shows that each of them carries enough energy
flux to be the driver of ion conics. However, as argued above
and also summarized in the discussion section, other reasons
suggest that the ion beamlets were the driver.
The energy flux required to power weak auroral arcs, on
the other hand, requires much more energy flux than was car-
ried by the here reported ion outflow, and we will argue in
Sect. 5.7 that the Alfv´
en waves were likely the driver.
4 Second event: outbound crossing of the PSBL
The importance of edges of magnetotail ion beamlets is fur-
ther demonstrated with this second event. About 2.5h af-
ter Cluster crossed the PSBL in the Southern Hemisphere, it
crossed the opposite (northern) boundary of the PS at about
the same radial distance and local time but this time dur-
ing an outbound motion of the spacecraft (Fig. 10). Sub-
storm recovery was also prevailing during this crossing as it
was for the inbound event. At about 03:04 UT Cluster en-
countered energy-dispersed ions which lasted until 03:14 UT
in the spacecraft frame while crossing the PSBL (Fig. 10a).
These energetic ions were accompanied by intense ion out-
flow (Fig. 10b). In the following analysis, we will focus on
the time period from about 03:08 to 03:14 UT (Fig. 11).
Figure 11 shows several magnetotail ion beamlets and as-
sociated phenomena. It is noted that most ion beamlets are
not separated from one another which is different compared
to the beamlet substructure reported for the inbound event
(Sect. 3). One common signature between beamlets of both
inbound and outbound events are the abrupt energy steps (or
edges) (see arrows in Fig. 11a) of individual beamlets occur-
ring from one sample interval (4 s) to the next. Only the most
apparent energy steps are marked with dashed lines and num-
bers but additional steps can be found in this interval. These
steps separate individual beamlets from one another. In Keil-
ing et al. (2004b), it was determined using multiple space-
craft measurements that the steps were caused by crossing
spatial boundaries of flux tubes carrying the magnetotail ion
beamlets (as opposed to a sudden appearance of a beamlet).
The sharp boundaries have spatial scales of <60km since the
ion fluxes changed abruptly from one sample to the next (4-s
time resolution of the ion instrument) and the relative mo-
tion of spacecraft and ambient plasma was about 15km/s. In
comparison, the gyroradius of a 10-keV ion is about 30 km
at this location.
The most important observation in the context of this study
is that these boundaries occurred simultaneously with other
particle and field signatures. The boundaries numbered 1
through 5 are associated with simultaneously occurring en-
hanced ion outflows in comparison to their immediate sur-
roundings, suggesting a causal relationship as was argued for
the inbound event (Sect. 3). It is however also noted that ion
outflow existed between the boundaries but with lesser in-
tensities. The outflow shows pitch angles up to 90◦, which
is similar to the inbound event (note that the magnetic field
direction is reversed compared to the inbound event, i.e., a
pitch angle of 180◦corresponds to upward motion).
Electron acceleration coincided with the ion outflow re-
gion from 03:10 to 03:12 UT (Figs. 11d–f). Intense down-
flowing electrons were present as well as upflowing ones;
both showing fine structure. A clear one-to-one relation-
ship with all boundaries (labeled 1 through 5) was not appar-
ent, although the strongest downgoing electron flow (white
arrow in panel d) coincides with #2. This lack of a clear
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A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer 2699
12345
Figure 11
PA 0 deg
10
-6
10
-5
10
-4
ergs/cm**2-s-str-eV
dEF
PA 90 deg
PA 180 deg
10
2
10
3
eV
10
2
10
3
eV
Spacecraft potential
10
2
10
3
eV
1
10
2
10
3
eV
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(i)
(h)
SUM OF THE TWO
ELECTRIC POWER-
SPECTRAL DENSITIES
1
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
E (mV
2
/m
2
/Hz)
10
100
1000
f (Hz)
SUM OF THE THREE
MAGNETIC POWER-
SPECTRAL DENSITIES
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
B (nT
2
/Hz)
-4
10
100
1000
f (Hz)
Fig. 11. Comparison of various quantities for the outbound crossing of the PSBL on 14 February 2001. (a) Ions with >3keV, (b) ions with
<1 keV, (c) pitch angle plot for ions with <1keV, (d–f) electrons for different pitch angle ranges, (g) Excomponent of the electric field in
field-aligned coordinates, (h, i) electric and magnetic field power spectral densities up to 4kHz.
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2700 A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer
x
y
z
Ions injected from isolated
islands forming beamlets
Alfven waves generated
by ion beamlets
Accelerated electrons
Excited plasma
waves
Upflowing heated ions
(ion conics)
Double oval
Ion beamlets
and Alfven waves
j
Field-aligned currents
Azimuthal convection
Radial convection
Figure 12
j
Perpendicular electric
fields
Fig. 12. Simplified cartoon showing the various phenomena that are possibly causally associated with the energy-dispersed ion beamlets
observed during the PSBL crossings.
one-to-one relationship could be because the ion outflow
was present during the entire time interval of electron flow
(03:10–03:13:20 UT) without intermittent gaps (i.e., no ion
outflow).
The perpendicular electric field, Ex, is shown in Fig. 11g.
The arrows above this panel are placed at the same times
as those above the first panel. In particular, the largest two
spikes (arrows #1 and #2 and to a lesser degree #3) line up
well with the beamlet boundaries (Fig. 11a), giving further
support to the scenario that these boundaries of the ion beam-
lets created perpendicular electric fields which in turn heated
the upflowing ionospheric ions.
BEN and Alfv´
en waves were also present inside the beam-
let carrying region, but simple and convincing one-to-one
correlations as for the ion conics are not apparent; instead
the waves cover the entire beamlet-carrying region as was
the case for the inbound event. It is also noted that BEN was
present deep inside the CPS with similar intensity as in the
PSBL. The wave magnetic field data (Fig. 11i) showed iso-
lated enhancements which approximately coincided with the
beamlet boundaries. Thus, as for the inbound event, electro-
magnetic waves appear to be associated with these bound-
aries.
In conclusion, this outbound event supports the view that
was developed for the inbound event, namely that the magne-
totail ion beamlets, in particular their sharp edges (or bound-
aries) are driving other phenomena.
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A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer 2701
5 Discussion
An important topic in M-I coupling is the impact of PSBL ion
beams (or ion beamlets) on auroral dynamics (cf. Sect. 1). To
investigate this topic, we presented two case studies showing
various particle and field signatures in association with multi-
ple ion beamlets that occurred during PSBL crossings at geo-
centric distance of about 4.5 Re. We compared the beamlet-
carrying region with neighboring regions to show that the
signatures in the beamlet region were distinct from those of
the other regions. On the basis of the particle and field sig-
natures reported here, the following impacts (Fig. 12) of the
ion beamlets are proposed and further discussed below:
–generation of ion conics
–generation of field-aligned electrons
–generation of broadband waves
–generation of electric field spikes
–generation of Alfv´
en waves
–association with FAC
–association with poleward arcs of double oval
We emphasize that our conclusions are based on the one-to-
one correlations of ion beamlets with the various particle and
field signatures observed by Cluster 1 during the inbound
PSBL crossing on 14 February 2001. Although the Cluster 3
observations during the same crossing also showed distinct
particle and field signatures in various quantities inside the
beamlet-carrying region, they did not show the clear one-to-
one correlations as observed in Cluster 1 data. However, we
find it plausible that it is the exceptional separation of indi-
vidual ion beamlets recorded by Cluster 1 that made it possi-
ble to see clear and convincing one-to-one correlations. Our
findings were also supported by observations from a second
event occurring during an outbound PSBL crossing which
did not show well separated beamlets but showed well devel-
oped boundaries between beamlets similar to those recorded
by Cluster 1 on the inbound PSBL crossing.
5.1 Generation of ion conics
Here we reported ion outflow in the form of bimodal ion con-
ics at 4.5 Re in a region which also carried the energetic ion
beamlets from the magnetotail. A similar association of en-
hanced large pitch angle ion outflow and velocity-dispersed
ions was recently reported in a Polar-spacecraft-based study
at distances of 6Re and higher (Lennartsson, 2003). In ad-
dition, here we found that the enhanced conical outflow co-
incided with the edges (boundaries) of energetic magnetotail
ion beamlets. These edges had perpendicular spatial scales of
the order of gyroradii. Such boundaries were also reported in
Lennartsson et al. (2001). The energy range of the conics was
between 10 and 1000 eV, which is typical for ion conics (An-
dre and Yau, 1997). It was inferred that the observed conics
were heated at altitudes from 16 000 to 22 000 km which is
within the range of previously observed ion conics (Kintner
and Gorney, 1984; Peterson et al., 1992).
The free energy carried in ion beams is a potential source
for the energization of ionospheric ion outflow. It has re-
cently been suggested based on observations (using the Polar
satellite) and simulations that ion shell distributions associ-
ated with PSBL ion flow excite Bernstein waves which in
turn heat ion outflows to create the observed ion conics (Jan-
hunen et al., 2003; Olsson et al., 2004). The simulated waves
covered a frequency range of 50–500Hz. In an alternative
scenario, also based on Polar observations and theoretical
considerations (Lennartsson, 2003), perpendicular electric
fields, generated by charge imbalances due to ion gyroradii
differences between energetic magnetotail ions and electrons
at density gradients of filamentary ion structures, are pro-
posed to transversely accelerate outflowing ionospheric ions,
thus creating ion conics. In turn, the acceleration and dis-
placement of the ions are proposed to generate electric field
turbulence. Thus, in this scenario, the ion conics cause the
electric field turbulence whereas in Janhunen and Olsson’s
scenario, the electric field turbulence causes the heating of
the ion conics.
Our observations can be compared with both scenarios.
First, the ion beamlets reported here showed shell distribu-
tions in velocity space together with BEN (up to 4kHz) and
ion conics which is similar to the Janhunen and Olsson sce-
nario (except the different frequency range). However, we
found that the most intense and energetic ion conic outflows
were found at or in the vicinity of beamlet edges (bound-
aries). Their scenario does not provide an explanation for
the preferential edge location of the ion conics as was shown
here. Furthermore, the edges often showed larger electric
field spikes and ELF (<12Hz) wave activity in the form of
wave packets. These observations are consistent with the
Lennartsson model (see also Sect. 5.4).
Either way, the clear and convincing one-to-one correla-
tion between ion beamlets (or more precisely their edges)
and ion conics strongly suggest that the beamlets provided
the energy – which was shown to be sufficient – for the ion
heating. We also rule out that Alfv´
en waves heated the out-
flowing ionospheric ions in a way described by Chaston et
al. (2004) for ion heating below 1 Re altitude because at Clus-
ter’s location there was no indication of small-scale Alfv´
en
waves with frequencies of 1Hz or higher which are required
for resonant heating of outflowing ions.
We showed that the ion conics were accelerated in the par-
allel field direction to produce bimodal ion conics as first
reported by Klumpar et al. (1984). The current model by
Lennartsson (2003) does not account for parallel electric
fields. A plasma sheet mechanism first proposed by Alfv´
en
and F¨
althammar (1963) and later simulated by, for exam-
ple, Schriver (1999) can generate upward directed electric
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2702 A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer
fields due to the differences in mirror points of energetic PS
ions and electrons. However, the simulation by Schriver et
al. (1999) shows that most of the electric field is created at
altitudes below the region from where our observed conics
came. Nevertheless, a combination of the mechanisms pro-
posed by Alfv´
en and F¨
althammar (1963) and by Lennarts-
son (2003) might yield additional parallel acceleration of
the ion conics. This possibility needs to be further investi-
gated in future studies. On the other hand, the simulation by
Janhunen et al. (2003), already mentioned above, produced
field-aligned electric fields in addition to transverse electric
fields in association with Bernstein modes. In their study it
was argued that these fields accelerate electrons but it was
not addressed whether they can contribute to parallel ion ac-
celeration.
A constraining feature for the ion conic generation was
that the conics showed no energy dispersion. It was in-
ferred that the source regions of these conics was at times
6000 km below the spacecraft. Thus, the ions with the lowest
(∼30 eV) and highest (1 keV) energies require at least 85s
and 14 s, respectively, to reach the spacecraft. This would
argue that the outflows lasted for at least 71s (=85 s–14 s).
If the source region of the ion conics convected with the
E×Bdrift during this time, no energy dispersion would oc-
cur as was observed. This convection would also cause an
ion conic to convect in its entirety to a different L shell be-
fore it reached the spacecraft. Since we argued above that the
edges of ion beamlets were associated with the heating of the
ion conics, one might expect that the ion conics would be dis-
placed from the edges due to this E×Bdrift which however
was not observed. In Keiling et al. (2004b) it was concluded
that the edges of ion beamlets on 14 February 2001 were
caused by a distant tail source which itself convected with the
E×Bdrift, thus creating a broad energy range on the same
field line (see also Lennartsson et al., 2001). Therefore, the
E×Bdrift of the source regions of both magnetotail beam-
lets and ion conics – both located on the same flux tubes –
could explain why no separation of beamlets and ion conics
occurred.
In conclusion, in our opinion, the clear correlation of ion
conics (and the electric field spikes, see Sect. 5.4) and the
edges of magnetotail beamlets favors the Lennartsson (2003)
model for the ion conic generation reported here.
5.2 Generation of field-aligned electrons
A distinct electron signature occurred in the beamlet-
carrying region. Electron flow with energies below 100eV
was present throughout the region; but most importantly, the
energy was raised to levels up to 1keV at times when en-
hanced ion conic outflows (or equivalently, at times when
magnetotail ion beamlets) occurred. The electrons were
field-aligned and anti-field-aligned with varying dominance
in a particular direction; 90◦mirroring electrons were also
observed at times. The downward and upward electron flow
suggests that the electron acceleration process occurred both
below and above the spacecraft. Moreover, the one-to-one
correlation with the magnetotail ion beamlets again suggests
that a process driven by the ion beamlets was causing the
electron energization.
Electron acceleration in the auroral acceleration region
by field-aligned potential drops is well known (e.g., Evans,
1974; Mozer et al., 1980). But it has also been shown that
electrons are accelerated at much higher altitudes. Kinetic
Alfv´
en waves have been associated with the acceleration of
electrons in the field-aligned direction in both the auroral
region and the PSBL (Chaston et al., 2000; Wygant et al.,
2002). Further, Janhunen et al. (2003) showed in simulations
that electrons can be accelerated in the field-aligned direc-
tion by Bernstein modes. Field-aligned and heated electrons
in the PSBL have been broadly associated with the presence
of ion conics and magnetotail ion beams although not nec-
essarily in a causal relationship (e.g., Klumpar and Heikkila,
1985; Schriver et al., 1990).
Our observations favor a scenario in which the ion beam-
lets energetically drive the acceleration of the electrons be-
cause of the above-mentioned one-to-one correlation. This
acceleration might be mediated via the generation of broad-
band waves. In particular, several studies showed correla-
tions between density gradients and the presence of broad-
band waves (Marklund et al., 2001; Vaivads et al., 2003;
Wahlund et al., 2003). Backrud et al. (2004) showed that
these waves can have a component parallel to the ambient
magnetic field. The scenario by Janhunen et al. (2003) (see
previous paragraph) is consistent with these reports and thus
a possible candidate for the electron acceleration observed
here, although the authors did not associate the wave genera-
tion and the subsequent electron acceleration with the edges
of ion beamlets. Kinetic Alfv´
en wave acceleration of elec-
trons is not likely because the observed Alfv´
en waves did
not show the small scales (frequencies of 1Hz and higher)
required to provide significant parallel electric fields at the
location of 4–5 Re (Wygant et al., 2002).
Finally, it was peculiar that the electron energies were
comparable to the ion conics’ energies which will need to
be explained in future studies.
5.3 Generation of broadband waves
Several broadband wave modes were recorded inside the
beamlet-carrying region. First, broadband electrostatic noise
(BEN) activity (up to 4kHz and possibly higher) was en-
hanced during the crossing of the beamlet-carrying region,
although significant activity was also recorded deep into the
CPS which was at times as strong as those in the PSBL.
Second, electrostatic ELF waves (<12.5Hz) were observed,
sometimes in the form of wave packets. These wave packets
appeared to be uncorrelated to the higher frequency BEN,
but instead were often collocated with the boundaries of ion
beamlets. ELF was also found inside beamlets that lasted
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A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer 2703
for 1–2 min. Third, broadband electromagnetic waves were
collocated with the boundaries of beamlets.
It has long been known that BEN is enhanced across the
PSBL (Gurnett et al., 1976). However, it has proven to be
very difficult to identify the wave modes and their driving
source associated with BEN (e.g., Lakhina et al., 2000, and
references therein). Many simulation studies exist showing
different sources of BEN, but clear and unambiguous obser-
vational evidence does not exist because too many parame-
ters are simultaneously observed. FAC (e.g., Ashour-Abdalla
and Thorne, 1977), ion beams (e.g., Grabbe and Eastman,
1984), electron beams (e.g., Schriver and Ashour-Abdalla,
1989), and Alfv´
en waves (Backrud et al., 2004) have all been
put forward as the driver of BEN. All these signatures were
present during our events, making an identification more am-
biguous.
BEN was enhanced during both downward and upward
FAC inside the beamlet-carrying region, suggesting that the
FAC were not the generator of BEN inside the beamlet-
carrying region. Janhunen et al. (2003) proposed that ion
shell distributions similar to those reported here could excite
several Bernstein modes in the frequency range (50–500Hz).
It was suggested by these authors that these waves could
yield the signature of BEN. The lack of clear one-to-one cor-
relations of the striated BEN and the beamlets, as reported
here, might be due to the fact that BEN propagates obliquely
to the ambient magnetic field.
Intense BEN was also present further into the CPS. If the
ion beamlets did provide the energy for BEN, then it is also
clear that other sources can drive BEN since in the CPS no
ion beamlets were observed. In the CPS, upward FAC and
inverted ion V’s were present which are possible sources for
those BEN.
In contrast, the here reported ELF wave packets (10Hz)
were often found at the boundaries of magnetotail ion beam-
lets. Such ELF turbulence can be explained with the electric
field generation process as proposed by Lennartsson (2003)
(see Sect. 5.1). Furthermore, the correlation of broadband
electromagnetic waves and boundaries of ion beamlets sug-
gests that the waves were energetically driven by the beam-
lets. The mechanism is as of yet unknown.
5.4 Generation of electric field spikes
Inside the broad region of electric field turbulence, larger
electric field spikes were reported which often coincided with
the edges of the magnetotail ion beamlets. No magnetic
counterparts of the electric field spikes were present. Thus,
we rule out that the spikes were of Alfv´
enic nature.
Instead, the generation of electric fields at beamlet edges is
readily explained with the model by Lennartsson (2003) (see
also Sect. 5.1). Lennartsson did not actually show a one-to-
one correlation of electric field spikes and individual beam-
let edges, but it was shown that the turbulent electric field
region coincided with the region of filamentary magnetotail
ion flow at 6Re and above. The filamentary ion structures
showed gradients of the order of gyroradii. It was suggested
that the generation of electric fields at the gradients and the
subsequent acceleration of cold ions would lead to further
electric field turbulence.
5.5 Generation of Alfv´
en waves
The region of beamlets was threaded by low-frequency
Alfv´
en waves which showed both traveling and standing sig-
natures. The Alfv´
en wave electric field amplitudes were
of the order of mV/m which is 1–2 orders of magnitude
smaller compared to Alfv´
en waves in the PSBL during times
of substorm expansion phase (Wygant et al., 2000; Keiling
et al., 2000). The event here occurred during the recovery
phase. The signature of standing Alfv´
en waves is readily ex-
plained by the reflection of Alfv´
en waves off the ionosphere
(Mallinckrodt and Carlson, 1978).
An association of Alfv´
en waves and ion flow in the lobe-
PSBL region was recently proposed by Zelenyi et al. (2004).
Dispersionless ion structures at 25 Re were found on fluctu-
ating magnetic field lines which were identified as Alfv´
enic.
It was suggested that the ion structures injected in the dis-
tant magnetotail excite Alfv´
en waves on the same flux tube
via the tail firehose instability. Although our ion beamlets
show clear energy dispersion signatures, and thus are differ-
ent from those reported in Zelenyi et al. (2004), it is also pos-
sible that our beamlets triggered a firehose instability which
produced Alfv´
en waves. The condition for the firehose in-
stability has to be met near the source region in the distant
tail and cannot directly be verified for our event, but as ar-
gued in Zelenyi et al. (2004), this condition can be met in the
far tail. Takada et al. (2005) also showed the simultaneous
occurrence of Alfv´
en waves and ion flows in the magneto-
tail (>15 Re), and proposed that an ion cyclotron anisotropy
instability could generate the Alfv´
en waves. In accordance
with these two studies, it is thus possible that the Alfv´
en
waves observed together with the beamlets by Cluster were
generated by the beamlets in the far-tail region.
5.6 Association with FAC
The ion beamlet region carried FAC which carried significant
Poynting flux towards the ionosphere. During the inbound
event, both spacecraft first encountered downward FAC on
entering the PBSL. The multiple ion beamlets were however
not associated exclusively with one current direction. The
ion beamlets of the first large-scale ion structure A (Figs. 2a
and g) were located in the downward current region for both
spacecraft and only secondary and tertiary structures (B and
C) were located in the upward FAC region. Note that the
secondary and tertiary ion structures were likely echoes of
the first structure (Keiling et al., 2005a). Thus, it is possible
that the downward currents are related to the generation of
ion beamlets; additional investigations are required.
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2704 A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer
The currents were associated with large-scale azimuthal
convection at Cluster’s location. This convection is possi-
bly mediated via the FAC from the source region where az-
imuthal shear flow created this FAC. Such shear flow in the
tail has been associated with bursty bulk flow (Angelopoulos
et al., 2002). At this time, it remains open whether this shear
flow is related to the beamlet injection mechanism.
5.7 Association with poleward arcs of double oval
As previously reported, both events occurred during a double
auroral oval and the footprints of the beamlets mapped into
the vicinity of the poleward arcs (Keiling et al., 2004b). The
conjugacy of ion beamlets and auroral activity begs the ques-
tion whether the ions were responsible for the auroral emis-
sions. An association of VDIS with the double oval is consis-
tent with the observation by Elphinstone et al. (1995). Other
authors have also suggested that ion beams could drive au-
roral arcs (e.g., Marghitu et al., 2001; Janhunen et al., 2003;
Olsson et al., 2004). For example, Olsson et al. (2004) sug-
gested that the free energy contained in ion shell distributions
observed in the PSBL plays a role in the energy transfer lead-
ing to stable auroral arcs.
The energy flux required to cause visible aurora
(>1 erg/cm2s at ionospheric altitude) is significantly larger
(1–2 orders of magnitude) than the energy flux required to
drive, for example, the ion conic heating. Although the
ion beamlets carried such energy flux towards Earth (up to
4 erg/cm2s when mapped to ionospheric altitudes using a
mapping factor of 100), most of their energy flux however
mirrored back, thus creating the ion shell distributions. Con-
sistent flattening of these distributions was also not observed
(Janhunen et al., 2003). Therefore, only relatively small
amounts of beamlet energy were actually dissipated below
the spacecraft. Instead, Alfv´
en waves were largely dissipated
below the spacecraft showing a net Poynting flux toward
the ionosphere of >1 erg/cm2s (mapped to ionospheric alti-
tudes). This is above the threshold of visible aurora. Alfv´
en
waves traveling in the PSBL have been associated with the
driving of conjugate auroras (Wygant et al., 2000; Keiling et
al., 2002). Keiling et al. (2002) showed a positive correlation
of the Poynting flux of Alfv´
en waves and auroral luminosity.
The Poynting flux reported here corresponds to the weaker
auroras reported in Keiling et al. It is also noted that the FAC
was downward on entering the PSBL, thus ruling the current
out as a driver; at least in a way that is associated with up-
ward FAC and auroral acceleration. We thus propose that
the Alfv´
en waves generated by ion beams in the distant tail
could drive the poleward arcs of the double oval. In this sce-
nario, the ion beamlets are thus indirectly responsible for the
auroral emissions.
6 Conclusions
It is well known that a rich phenomenology exists on PSBL
field lines, but the causal relationship among these phenom-
ena is not well established. For example, temporal or spa-
tial correlations among phenomena are observed and theo-
retical work is provided to support a cause-effect relation-
ship; however, several phenomena are often simultaneously
present and the correlations are often only approximate, i.e.,
in the “close” temporal or spatial proximity, which makes
an unambiguous identification of the cause-effect relation-
ship difficult. Consequently, alternative scenarios are often
proposed which can explain the same observations. The dis-
crimination of competing scenarios is one of the challenges
in magnetospheric physics.
Cluster observations presented here show that energy-
dispersed ions in the PSBL energetically drive the heating of
ion outflows forming ion conics, the field-aligned accelera-
tion of electrons, and the generation of perpendicular electric
fields and ELF turbulence. These conclusions were foremost
based on convincing one-to-one correlations of the phenom-
ena with the beamlet substructure of the dispersed ions. The
beamlet substructure facilitated the identification of cause-
effect relationships because it was possible to explain the fine
structures of these associated phenomena with the beamlet
substructure. An important signature was that the various
correlations occurred at the boundaries (with narrow spatial
scales of the order of a gyroradius) of individual ion beam-
lets. These boundaries “focused” the correlations to narrow
regions which left less room for ambiguities. The association
with beamlets was further supported by the abrupt changes
that occurred at the boundaries to the regions adjacent to the
beamlet-carrying region.
Additional support for our conclusions is that they are
consistent with the observations and theoretical considera-
tions of Lennartsson (2003) but also extend their observa-
tions. Lennartsson proposed a model where ion beam gra-
dients with small perpendicular scale of the order of gyro-
radii create perpendicular electric fields which locally heat
outflowing ions. This model accounts well for the observa-
tions of ion conics and electric field generation occurring at
the boundaries of ion beamlets as reported here. Lennarts-
son, however, did not comment on plasma wave activity and
parallel acceleration of both ions and electrons as reported
here which would have allowed further comparisons of his
observations to our event.
Furthermore, it was shown that the dispersed ions reported
here could provide the free energy for BEN and electromag-
netic broadband waves. The ions showed shell distributions
in velocity space which have been proposed by Janhunen et
al. (2003) and Olsson et al. (2004) to create BEN. Although
convincing one-to-one correlations as shown for the ion con-
ics, field-aligned electrons, and electric field spikes were not
observed for BEN, it is very probable that the BEN was ener-
getically driven by the ion beamlets because other scenarios
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A. Keiling et al.: Energy-dispersed ions in the plasma sheet boundary layer 2705
were found to be less likely, and the perpendicular propaga-
tion of BEN might have obscured this one-to-one correlation.
In contrast, electromagnetic broadband waves showed a one-
to-one correlation with the ion beamlets.
In addition, we showed new observations regarding the
source region of the magnetotail ion beamlets. In Keiling
et al. (2004b), an injection scenario was proposed for the
highly structured multiple dispersed ion beams seen in the
events reported here. Two new observations add to their sce-
nario, namely (1) that the source region of the magnetotail
ion beams generated a downward FAC which connected az-
imuthal shear flow in the far tail with the ionosphere, and (2)
that both traveling and standing Alfv´
en waves accompanied
the ion beamlets. We proposed that the Alfv´
en waves were
possibly generated by the beamlets themselves in the distant
tail via ion beam-related instabilities (Zelenyi et al., 2004;
Takada et al., 2005). Additional event studies are required to
determine whether these observations are common.
This work can also be viewed in the framework of how
the energy coming from the magnetotail is distributed among
various energy carriers. Both theoretical and observational
works show that FAC, ion beams, and Alfv´
en waves con-
tribute to this energy transfer (e.g., Schriver et al., 2003).
Here we showed that the FAC carried the largest amount of
energy flux (almost one order of magnitude larger than for
the other two energy carriers) on 14 February 2001. This en-
ergy flux however was not dissipated to create the phenom-
ena discussed in this report but must have been dissipated at
much lower altitude possibly the ionosphere (for example,
via Joule heating). Alfv´
en waves and ion beamlets carried
comparable amounts of energy flux. Much of the Alfv´
enic
energy flux was dissipated below the spacecraft; perhaps into
auroral emissions as argued here. In contrast, most of the
ion beamlet energy flux was mirrored at lower altitude and
returned to the magnetotail (which was manifested in the
shell distribution). Such mirroring ion beamlets can lead to
bouncing ion clusters inside the PSBL (Ashour-Abdalla et
al., 1993; Keiling et al., 2005a) and thus can again drive the
same processes – as described here – on their return. It was
shown that only small amounts of energy flux are required to
drive the locally observed ion outflow, and thus most of the
beamlet energy is eventually thermalized in the CPS. Thus,
we conclude that Alfv´
en waves and FAC contributed more
energy flux towards M-I coupling for this particular event –
which might be typical for the recovery phase - compared to
magnetotail ion beamlets. Nevertheless, many phenomena
were driven by these ion beamlets in regions above the au-
roral acceleration region. Furthermore, if the Alfv´
en waves
were indeed generated by ion beamlets in the distant mag-
netotail, then this shows that ion beamlets have an impact
on magnetotail dynamics not only at Cluster’s location and
below but already starting at the source region.
Acknowledgements. This work was supported by NASA grant
NNG04GF23G. We thank F. Mozer for providing Cluster electric
field (EFW) data, and A. Viljanen from the Finnish Meteorological
Institute for the IMAGE ground magnetometer data. We also thank
the reviewers for their critical comments.
Topical Editor I. A. Daglis thanks W. K. Peterson and M. Hira-
hara for their help in evaluating this paper.
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