First medium energy neutral atom (MENA) Images of Earth's magnetosphere during substorm and storm-time

Abstract

Initial ENA images obtained with the MENA imager on the IMAGE observatory show that ENAs emanating from Earth's magnetosphere at least crudely track both Dst and Kp. Images obtained during the storm of August 12, 2000, clearly show strong ring current asymmetry during storm main phase and early recovery phase, and a high degree of symmetry during the late recovery phase. Thus, these images establish the existence of both partial and complete ring currents during the same storm. Further, they suggest that ring current loss through the day side magnetopause dominates other loss processes during storm main phase and early recovery phase.

Full text

GEOPHYSICAL RESEARCH LETTERS, VOL. 28, NO. 6, PAGES 1147-1150, MARCH 15, 2001
First medium energy neutral atom (MENA) images
of Earth’s magnetosphere during substorm
and storm-time
C.J. Pollock,1K. Asamura,2M.M. Balkey,3J.L. Burch,1H.O. Funsten,4M.
Grande, 5M. Gruntman,6M. Henderson,4J.-M. Jahn,1M. Lampton,7M.W.
Liemohn,8D.J. McComas,1T. Mukai,2S. Ritzau, 4M.L. Schattenburg,9E.
Scime,3R. Skoug,4P. Val ek1,10 and M. W¨uest1
Abstract. Initial ENA images obtained with the MENA
imager on the IMAGE observatory show that ENAs ema-
nating from Earth’s magnetosphere at least crudely track
both Dst and Kp. Images obtained during the storm of
August 12, 2000, clearly show strong ring current asymme-
try during storm main phase and early recovery phase, and
a high degree of symmetry during the late recovery phase.
Thus, these images establish the existence of both partial
and complete ring currents during the same storm. Further,
they suggest that ring current loss through the day side mag-
netopause dominates other loss processes during storm main
phase and early recovery phase.
1. Introduction
Energetic ions in Earth’s magnetosphere charge exchange
with the extended neutral atmosphere to produce energetic
neutral atoms (ENA) that are imaged by ENA ‘cameras’
on the Imager for Magnetopause-to-Aurora Global Explo-
ration (IMAGE) observatory [Burch et al., 2000.] The Low
(LENA; [Moore et al., 2000), medium (MENA; [Pollock et
al., 2000]) and high (HENA; [Mitchel l et al., 2000]) energy
neutral atom imagers on IMAGE observe ENAs from 15 eV
up to 500 keV per nucleon, allowing visualization of the mag-
netosphere. This enables exploration of global structures
and processes, and their response to solar wind driving.
The ring current, plasma sheet, cusp, and their low alti-
tude extensions produce ENA fluxes in the MENA energy
range. Increases in ENA flux are induced by plasma in-
jections associated with geomagnetic storms and substorms
[Roelof , 1987; Henderson et al., 1997], as well as by en-
hanced magnetospheric convection [Liemohn et al., 1999;
M. Thomsen, private communication, 2000]. At IMAGE,
the dominant source of ENAs above a few keV is the ring
current. ENA fluxes are therefore expected to correlate with
1Southwest Research Institute, San Antonio, TX
2Institute of Space and Astronautical Sciences, Japan
3West Virginia University, Morgantown, WV
4Los Alamos National Laboratory, Los Alamos, NM
5Rutherford Appleton Laboratory, Oxfordshire, England
6University of Southern California, Los Angeles, CA
7University of California at Berkeley, Berkeley, CA
8University of Michigan, Ann Arbor, MI
9MIT Center for Space Research, Cambridge, MA
10Auburn University, Auburn, AL
Copyright 2001 by the American Geophysical Union.
Paper number 2000GL012641.
0094-8276/01/2000GL012641$05.00
ring current enhancements as indexed by Dst [Tinsley, 1979;
Williams et al., 1992; Fo k e t a l . , 1996; Jorgensen et al.,
1997]. Ring current ENAs have been discussed from the
point of view of mid-latitude aurora and plasma heating by
Pr¨olss [1973] and, with photometric observations, by Tinsley
[1979]. Tinsley cited Dessler and Parker [1959] and Sckopke
[1966] in noting the proportionality of the time derivative
d(Dst)/dt and ring current loss processes, including ENA
emissions. Roelof et al. [1985] discussed ring current loss
by ENA emission and the implication of the Dessler-Parker-
Sckopke relation in detail. Using the CEPPAD instrument
on Polar, Jorgensen et al. [1997] found that storm time 30–
50 keV ENA flux was proportional to Dst, particularly dur-
ing storm recovery.
Observations consistent with an incomplete or asym-
metric ring current have been presented and discussed e.g.
by Frank et al. [1970], Kawasaki and Akasofu [1971], and
Greenspan and Hamilton [2000]. Recently, models [Liemohn
et al. 1999] have shown a compact asymmetric ring current
during storm main phase, and its evolution into a larger,
more symmetric configuration during late recovery. This is
due to enhanced convection during the main phase, which
places the bulk of the ring current on open drift paths. De-
creasing convection during recovery allows the ring current
to grow radially and become more symmetric as drift paths
circumscribe the Earth.
We present initial MENA observations and compare them
with both Dst and Kp, thereby demonstrating the sensitiv-
ity of observed ENA rates at IMAGE to magnetospheric
activity. From the morphology of the ENA fluxes observed
during the storm of August 12, 2000, we clearly observe evo-
lution from a compact, asymmetric ring current during main
phase to an expanded and more symmetric one during late
recovery.
2. Observations
MENA images respond sensitively to geomagnetic activ-
ity and graphically portray magnetospheric dynamics. We
present observations from two days. One (July 26, 2000)
displayed mildly disturbed (Kp =3−4) conditions. The
other (August 12, 2000) allows study of storm dynamics
and global ring current evolution.
July 26, 2000 Figure 1 shows a full day of obser-
vations from July 26, 2000. This day was mildly active,
with two instances of enhanced activity (Figure 1f). Most
of the day was characterized by negative IMF Bzcompo-
nent (mean value at ACE between 0400 and 2300 UT was
−6.1 nT). Solar wind speed and density displayed typical
values near 350 km s−1and 10 cm−3.
1147

1148 POLLOCK ET AL.: FIRST MEDIUM ENERGY NEUTRAL ATOM (MENA) IMAGES
Figure 1. MENA observations and magnetospheric activity indices on July 26, 2000. Bottom panels show quantities plotted versus
UT and orbital parameters. Dst (left) and Kp (right) appear in the bottom panel. The second panel from the bottom shows MENA
coincidence rates, with IMAGE spin phase plotted on the ordinate and time on the abscissa. Two white lines indicate Earth’s limb.
Geophysical ENA emissions are ordered with respect to Earth. Detector voltages are reduced in the radiation belts, leaving gaps
early on this day, and also between 1200–1600 UT. They are also reduced each spin for sunward viewing. Vertical bands of counts
are due to charged particles energetic enough to overcome the electrostatic collimator deflection. The four panels across the top show
4-minute MENA images. Each is annotated with geomagnetic dipole field lines at MLT = 6, 12, 18, and 24 hours and at L=4 and
L= 8. Noon and midnight field lines are labeled “S” (sunward) and “A” (anti-sunward). The circle at the center of each image
indicates Earth. The four images are of ENAs from 5.2–12 keV, assuming the species is hydrogen. Separate color bars to the right
provide logarithmic scaling for the coincidence rates and the images.
Figure 1e shows few ENA counts through the first quar-
ter of the day. Beginning near 0615 UT, counts are observed
from near Earth. These are due to ENAs from the ion in-
jection that gives rise to increases in Kp and Dst.After
1600 UT, ENA fluxes subside and broaden due to reduced
activity and the lower IMAGE altitude. Then, near 1830
UT, another injection yields enhanced count rates of ENAs
and a small increase in Kp.
ENA images from before and after the first ion injection
are shown in a 180◦fisheye projection in Figures 1a (0600–
0604 UT) and 1b (1030–1034 UT). Few ENAs are observed
prior to the onset of activity (1a, 1e). Subsequently (1b),
substantial emissions from the anti-sunward region are seen,
as expected for a night side plasma injection (counts near
the top of the image are due to contamination from the
sunward direction and should be ignored). Evident in these
and other ENA images for specific observation geometries
is the dominance of low altitude emissions. These originate
from the MLT region opposite the spacecraft location and
arise from the large density of charge exchange targets at
low altitudes [Roelof, 1997]. As the magnetospheric activity
subsides, so do the ENA fluxes, though not quite back to
the low levels seen prior to 0615 UT.
Figures 1c (1715–1719 UT) and 1d (1910–1914 UT) show
the inner magnetosphere before and after a second ion in-
jection on July 26. Reduced fluxes are seen at 1715 UT,
from an expanded volume as compared with those in 1b,
still emanating primarily from the night side. Then, an-
other injection is observed near 1830 UT (1d), on the night
side.
August 12, 2000 Figure 2 displays observations of a
geomagnetic storm on August 12, 2000, in a format similar
to Figure 1. Images from three energy channels and at three
times are arranged in columns (times) and rows (energy).
We note here a saturation nechanism in MENA that is not
yet fully understood. When the flux is large, we observe
elongation in the imaging direction that is not geophysical.
This is most evident in Figure 2g and 2h. It does not affect
the main results of this paper, but will modify quantitative
results in Table 1.
Dst peaked on this day near 1000 UT, at a value near
−230 nT. ENA observations are obscured during the first
seven hours of the day. There is a large difference between
the MENA images obtained during the storm main phase
and those obtained during late recovery. Emissions are more
intense and localized at the earlier times (2a, 2d, and 2g)

POLLOCK ET AL.: FIRST MEDIUM ENERGY NEUTRAL ATOM (MENA) IMAGES 1149
Figure 2. MENA observations and magnetospheric activity indices from August 12, 2000 (DOY 225). Format is similar
to that of figure 1, except that 4-minute images in three energy ranges, assuming the species is hydrogen, are shown.
than they are later (2c, 2f, and 2i). The viewing geometry
is similar and optimal (from over the pole) at the two times
so that any effect due to viewing angle is minimal.
We can quantify ring current symmetry properties at
these two times by measuring the ratio of counts in the
dawn, noon, and dusk quadrants to those near midnight.
Results for the energy range 5.2–12 keV are shown in Ta-
ble 2. This demonstrates the asymmetry in the main
phase ring current, as compared to the late recovery phase.
The center column (2b, 2g, 2h) shows the ring current
during early recovery phase. The viewpoint here is from
lower latitude. Viewing effects thus make it more difficult to
determine the ring current distribution in MLT, nevertheless
the emissions are clearly asymmetric.
These results are consistent across a range of MENA en-
ergies, though at the highest energy there are too few counts
to confirm a symmetric ring current at 2240 UT. Most no-
tably, the count rate decreases monotonically with energy.

1150 POLLOCK ET AL.: FIRST MEDIUM ENERGY NEUTRAL ATOM (MENA) IMAGES
Table 1. ENA counts from the dawn (0200–0900), noon (0900–
1500), dusk (1500–2100), and midnight (“MN”; 2100–0300) MLT
quadrants during storm main phase (0930 UT) and late recovery
phase (2200 UT) are compared.
Cdawn
CMN Cnoon
CMN Cdusk
CMN
0930 UT 0.63 0.45 0.77
2200 UT 0.99 0.98 1.2
This is driven both by the reduction of source ion flux and
the decreasing charge exchange cross section with increasing
ion energy.
3. Discussion
The near-Earth magnetosphere emits more ENAs during
periods of geomagnetic activity than during quieter times
[Tinsley, 1979]. MENA observations during mildly dis-
turbed and storm times confirm this dependence. Further,
they show a remarkable variety of emission morphology, ow-
ing to both geophysical and geometric viewing effects.
MENA imagery from the August 12, 2000, storm clearly
shows evolution from asymmetric to symmetric ring current.
The ratio of ENA flux from the noon quadrant to that from
the midnight quadrant is ∼0.45 during main phase and
early recovery, and ∼0.98 during the late recovery phase.
These observations confirm the existence of both partial
and symmetric ring currents, a subject of current interest
[Grafe, 1999; Greenspan and Hamilton, 2000]. Further, they
show that the partial ring current evolves to a symmetric
ring current at these energies over the life of the storm.
Our observations suggest that the ring current lies mostly
on open drift paths during storm main and early recovery
phases and on closed drift paths during late recovery phase
[Liemohn et al., 1999].
Acknowledgments. We are grateful to many unnamed
individuals at SwRI and collaborating institutions whose hard
work has enabled these observations. We thank P. Gonzales and
G. Waters for assistance in manuscript preparation. This work
was supported at SwRI under NASA contract NAS5-96020.
References
Burch, J. L., Image Mission Overview Sp. Sci. Rev., 91, 1, 2000.
essler A. J., and E. N. Parker, Hydromagnetic theory of geomag-
netic storms, J. Geophys. Res., 64, 2239, 1959.
Fok, M.-C., et al., Ring current development during storm main
phase, J. Geophys. Res., 101, 15311, 1996.
Frank, L. A., Direct detection of asymmetric increases of extrater-
restrial ‘ring current’ proton intensities in the outer radiation
zone, J. Geophys. Res., 75, 1263, 1970.
Grafe, A., Are our ideas about Dst correct?, Ann. Geophys., 17,
1, 1999.
Greenspan, M. E., and D. C. Hamilton, A test of the Dessler-
Parker-Sckopke relation during magnetic storms, J. Geophys.
Res., 105, 5419, 2000.
Henderson, M. et al., First energetic neutral atom images from
Polar , Geophys. Res. Lett., 24, 1167, 1997.
Jorgensen, A., et al., Global energetic neutral atom (ENA) mea-
surements and their association with the Dst index, Geophys.
Res. Lett., 24, 24, 1997.
Kawasaki, K., and S.-I. Akasofu, Low-latitude DS component of
geomagnetic storm field, J. Geophys. Res., 76, 2396, 1971.
Liemohn, M., et al., Analysis of early phase ring current recovery
mechanisms during geomagnetic storms, Geophys. Res. Lett.,
26, 2845, 1999.
Mitchell, D., et al., High Energy Neutral Atom (HENA) imager
for the IMAGE mission, Space Sci. Rev., 91, 67-112, 2000.
Moore, T. E., et al., The Low Energy Neutral Atom imager
for IMAGE, Space Sci. Rev., 91, 155-195, 2000.
Pollock, C. J., et al., Medium Energy Neutral Atom (MENA)
imager for the IMAGE mission, Space Sci. Rev., 91, 113, 2000.
Pr¨olss, G. W., Decay of the magnetic storm ring current by the
charge-exchange mechanism, Planet. Space Sci., 21, 983, 1973
Roelof, E. C., et al., Energetic neutral atoms from the ring cur-
rent: IMP7/8 and ISEE1, J. Geophys. Res., 90, 10991, 1985.
Roelof, E. C., Energetic neutral atom image of a storm-time ring
current, Geophys. Res. Lett., 14, 652, 1987.
Roelof, E. C., ENA emissions from nearly mirroring magneto-
spheric ions interacting with the exobase. Adv. Space Res., 20,
361, 1997.
Sckopke, N., A general relation between the energy of trapped
particles and the disturbance field near the earth, J. Geophys.
Res., 71, 3125, 1966.
Tinsley, B. A., Energetic neutral atom precipitation during mag-
netic storms—optical emission, ionization, and energy deposi-
tion at low and middle latitudes, Geophys. Res. Lett., 6, 291,
1979.
Williams, D. J., et al., Global magnetospheric imaging, Rev. Geo-
phys., 30, 3, 183, 1992.
J. L. Burch, J.-M. Jahn, D. J. McComas C. J. Pollock
and M. W¨uest, SwRI, 6220 Culebra Road, San Antonio,
TX 78238, USA. (e-mail: jburch@swri.edu; jjahn@swri.edu;
dmccomas@swri.edu; cpollock@swri.edu; mwuest@swri.edu)
K. Asamura and T. Mukai, ISAS, 311 Yoshinodai Sagamihara,
Kanagawa 229-8510, Japan
M. M. Balkey and E. Scime, WVU, Physics Department, Box
6315, Morgantown, WV 26506, USA.
H. O. Funsten, M. Henderson, S. Ritzau and R. Skoug, LANL,
CSSE/NIS-1, Los Alamos, NM 87545, USA.
M. Grande, RAL, Oxfordshire OX11 0QX, England
M. Gruntman, USC, Department of Aerospace Engineering,
Los Angeles, CA 90089-1191, USA.
M. Lampton, UCB, Space Sciences Laboratory, Centennial
Drive at Grizzly Peak, Berkeley, CA 94720, USA.
M. W. Liemohn, UM, SPRL, 2455 Hayward St., Ann Arbor,
MI 48109, USA.
M. L. Schattenburg, MIT CSR, 77 Mass. Ave., Cambridge, MA
02139, USA.
P. Valek, Auburn University, Department of Physics, 206 Alli-
son Laboratory, Auburn, AL 36849, USA.
(Received November 13, 2000; revised January 15, 2001;
accepted January 18, 2001.)