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Rotationally driven quasi-periodic radio emissions in the Jovian
magnetosphere
A. Morioka,
1
H. Nozawa,
2
H. Misawa,
1
F. Tsuchiya,
1
Y. Miyoshi,
3
T. Kimura,
1
and W. Kurth
4
Received 12 December 2005; revised 17 January 2006; accepted 26 January 2006; published 29 April 2006.
[1]The occurrence characteristics of Jovian multiple quasi-periodic (QP) bursts are
investigated based on Galileo spacecraft observations. Multiple QP bursts recurrently
appeared in a group with a planetary spin period of 10 hours. Their appearance was
synchronized with the planetary rotation, i.e., they were preferably excited when
Jupiter had a particular spin phase angle with respect to the Sun (at a subsolar longitude of
System III around 260–320). We also found that the burst groups were activated during
magnetospheric disturbances probably caused by solar wind pressure variations. The
rotationally driven polar disturbances accompanying quasi-periodic particle accelerations
and radio wave bursts are discussed as being one of the daily unloading processes of
planetary rotational energy.
Citation: Morioka, A., H. Nozawa, H. Misawa, F. Tsuchiya, Y. Miyoshi, T. Kimura, and W. Kurth (2006), Rotationally driven quasi-
periodic radio emissions in the Jovian magnetosphere, J. Geophys. Res.,111, A04223, doi:10.1029/2005JA011563.
1. Introduction
[2] The Jovian magnetosphere is essentially rotationally
driven by means of the fast planetary spin forming it into a
disk-like magnetosphere, in contrast to Earth’s magneto-
sphere which is driven by the solar wind forming it into a
streamer-like magnetosphere. The Jovian magnetosphere
also has significant interactions with the solar wind due to
its huge cross section with respect to the direction of the
solar wind flow. The magnetospheric disturbances at Jupiter
would therefore have both components caused by the
unloading of planetary rotational energy and solar wind
energy. It is also considered that the coupling of both solar
and rotational energies would be also manifested in the
Jovian magnetospheric disturbances.
[3] The main Jovian auroral oval, which is rather stable
and fixed on the planetary rotational system, would be a
typical manifestation of the steady unloading process of
rotational energy [e.g., Hill, 2001; Clarke et al., 2004, and
references therein]. Some Jovian radio waves are observed
with a high probability of occurrence once or twice a
planetary rotation when observers faced radio sources, as
can be seen from the studies of broadband kilometric
radiation (b-KOM) [e.g., Leblanc, 1988, and references
therein], hectometric radiation (HOM), and decametric
radiation (DAM) [e.g., Menietti et al., 1999, and references
therein] observations. These radio wave emissions would
also represent a steady dissipation process of planetary
rotational energy, although they are sometimes modulated
by solar wind as has been reported by many studies, such as
DAM [e.g., Barrow et al., 1986; Morioka et al., 2002],
HOM [e.g., Desch and Barrow, 1984], and b-KOM [e.g.,
Barrow et al., 1988; Reiner et al., 2000]. In addition to these
steady dissipation processes of rotational energy, some
dynamical energy unloading processes have been demon-
strated. Krupp et al. [1998] and Woch et al. [1998, 1999]
showed dynamical variations of particles and fields on
timescales of a few days from observations with the Galileo
spacecraft. Louarn et al. [1998, 2000] demonstrated from
the Galileo observations that bursts of hectometric radio
emissions, which are possibly related to auroral phenomena
in the polar region, accompany the in situ energetic particle
bursts and magnetic field disturbances similar to terrestrial
substorms. ‘‘Transient particle injection events’’ likely to be
terrestrial substorms were observed in the middle magneto-
sphere [Mauk et al., 1999, 2002]. These phenomena suggest
that a certain intermittent energy release process exists,
which is not well understood but probably related to the
internally driven unloading process. One of our major
concerns in the Jovian magnetosphere involves the store
and release processes of planetary rotational energy: how
the energy is stored in the magnetospheric plasma and
particles, how the stored energy is released, and whether
the release processes are related to the solar wind variations.
In this paper we report newly obtained source characteristics
of Jovian multiple quasi-periodic emissions, which should
lead to a better understanding of the release process of the
Jovian rotational energy.
[4] Quasi-periodic (QP) low-frequency radio bursts in a
frequency range from 5 to 20 kHz with frequency disper-
sion were first reported by Kurth et al. [1989] from Voyager
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, A04223, doi:10.1029/2005JA011563, 2006
1
Planetary Plasma and Atmospheric Research Center, Tohoku Uni-
versity, Sendai, Japan.
2
Kagoshima National College of Technology, Kagoshima, Japan.
3
Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya,
Japan.
4
Department of Physics and Astronomy, University of Iowa, Iowa City,
Iowa, USA.
Copyright 2006 by the American Geophysical Union.
0148-0227/06/2005JA011563$09.00
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observations and labeled Jovian type III bursts because of
their qualitative resemblance to solar type III bursts. After-
ward, Ulysses observations provided a more detailed study
on the spectral characteristics of Jovian low-frequency radio
emissions [Kaiser et al., 1992]. MacDowall et al. [1993]
demonstrated from the Ulysses observations that quasi-
periodic emissions in the very low frequency range (QP
bursts) were categorized into two groups with quasi-periods
of 15 min and 40 min, i.e., QP15 and QP40 bursts. They
also showed a certain correlation between the occurrence of
QP bursts and the solar wind velocity.
[5] The Galileo spacecraft, the first orbiter around Jupiter,
provided a large number of observations of Jovian radio
waves in the magnetosphere. The early studies of the
Galileo plasma wave observations [Kurth et al.,1997]
revealed that QP emissions are not always ‘‘periodic’’ but
are often chaotic and have rather multiple periods as far as
observations are concerned in the equatorial magnetosphere.
The observation of QP radio bursts with the Cassini
spacecraft also reported a much more complicated morphol-
ogy for the bursts and their random periodicity compared
with the Ulysses era [Kaiser et al., 2001]. The high-
resolution spectral study by Menietti et al. [2001] showed
an absence of dispersion in the frequency spectra of QP
bursts detected within the Jovian magnetosphere. This
indicates that the frequency dispersion of the bursts ob-
served in interplanetary space [Kurth et al., 1989] is
produced by propagation through the higher-density
magnetosheath as noticed by Desch [1993]. Hospodarsky
et al. [2004] examined the joint observations of the QP
bursts by the Galileo and Cassini spacecraft and revealed an
important feature of the bursts in that they are beamed in a
strobe light manner and not like a search light rotating with
Jupiter’s magnetic field.
[6] In this paper we investigate the characteristics of these
QP bursts and focus on their group appearance, 10-hour
periodicity, and clock behavior from the Galileo observa-
tions. We then discuss the rotationally controlled energy
dissipation process in the Jovian magnetosphere.
2. Database
[7] The Plasma Wave Subsystem (PWS) [Gurnett et al.,
1992] on board the Galileo spacecraft provides high-quality
observation of electric field spectra in a frequency range
from 5.6 Hz to 5.6 MHz with a time resolution of 18.67 s.
We used the database from the Planetary Plasma Interac-
tions (PPI) node of the Planetary Data System (PDS)
Figure 1. (a) Dynamic spectrogram of radio waves in the Jovian magnetosphere observed by Galileo/
PWS on 21 September 1996. White ovals show ‘‘quasi-periodic burst groups.’’ (b) Magnetic Br
component observed by Galileo/MGA. Positive Br means the outward component from the planet. (c) G02
trajectory of Galileo. Observation period in Figure 1a is indicated by the red line. (d) Expanded spectrogram
in frequency (500 Hz to 100 kHz) and time (1300 –1500).
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provided from the Solar System Exploration Data Services
Office, NASA. The data set of PWS instrument included
electric wave spectra obtained during the period when the
Galileo plasma wave receiver was operated during the
Jupiter orbital mission. The electric field spectral density
is given in units of V
2
/m
2
/Hz. The spectra are binned into
158 logarithmically spaced channels for the full frequency
range of the observation. Data from the Galileo Magnetom-
eter (MAG) [Kivelson et al., 1992] were used to verify the
spatial relation between the current disk and the spacecraft
in the magnetosphere. The data set from the PDS/MAG
covers all Galileo orbits. Data are provided at full sampling
resolution in the spacecraft and several geophysical coordi-
nate systems.
3. Observations
3.1. Grouped Appearance of QP Bursts
[8] Figure 1a is the 24-hour dynamic spectrogram of
radio and plasma waves in a frequency range from 100 Hz
to 5.6 MHz observed by the Galileo/PWS in the Jovian
magnetosphere on day 265 in 1996. The spacecraft was
located in the postmidnight sector of the magnetosphere
(r = 88–91 R
J
, LT = 2 hours), as shown in Figure 1c.
The dynamic spectrogram illustrates typical wave phe-
nomena in the Jovian magnetosphere, which have been
demonstrated in observations since the Voyager explora-
tion [e.g., Warwick et al., 1979, Kaiser et al., 1992, Kurth
et al., 1997]. The emissions above about 1 MHz are
HOM radiation, which show intensity and frequency-band
modulations of about 5 hours. Theses modulations have
been attributed to a corotating emission source with the
planetary magnetic field yielding an effect something like a
rotating search light. Emissions in a confined frequency range
near 100–200 kHz with the duration of about 2 hours are
narrow-band kilometric radiation (n-KOM) [e.g., Warwick et
al., 1979; Kaiser and Desch, 1980]. These emissions are
known to repeat themselves with slightly longer periods (so-
called System IV period) than the planetary rotational period.
The detailed source characteristics of these emissions were
investigated by Reiner et al. [1993] and Ladreiter et al. [1994]
using Ulysses observations. Jovian continuum radiation in a
frequency range from several hundred Hz to a few kHz can
also be seen in Figure 1, being characterized by the sharp
lower cutoff at the local plasma frequency and its continuous
appearance throughout the magnetosphere [e.g., Scarf et al.,
1979; Gurnett et al., 1980; Kaiser et al., 1992]. Kurth et al.
[1986] noted that continuum radiation shows amplitude
variation with periods near both 5 and 10 hours implying a
clock-like modulation.
[9] Overlapping into Jovian continuum radiation, many
quasi-periodic bursts appear in a frequency range from a
few to 20 kHz. An expanded dynamic spectrogram is shown
in Figure 1d during the period from 1300 to 1500 SCET
(spacecraft event time). We can see that the bursts are
independent of the occasional enhancements in continuum
emissions. The spacing between the bursts is not always
periodic but rather irregular with a time interval ranging
from roughly 2 to 30 min, as was pointed out by Hospodarsky
et al. [2004]. We, however, have used the term ‘‘QP burst’’ in
this paper, to conform the precedent nomenclature for the
low-frequency bursty radio emissions observed in the Jovian
magnetosphere. It should be noted that multiple QP bursts
appeared in groups as indicated by the white ovals in
Figure 1a. We can see three groups of QP bursts starting
at about 0100, 1030, and 1940 SCET with roughly 10-hour
intervals. Here, we have called a cluster of multiple QP bursts
a ‘‘QP-burst group.’’ Each burst group has duration of
about 5 hours in the present example. The spacecraft’s
location with respect to the magnetospheric current disk
can be deduced from the magnetic field data. Figure 1b
shows the radial component of the magnetic field (B
r
)
observed by MAG, where B
r
> 0 indicates that the
spacecraft is located in the northern lobe of the magne-
tosphere and vice versa. Note that burst groups are
observed in both northern and southern magnetosphere
across the current sheet.
3.2. Ten-Hour Periodicity of Burst Group and
Clock-Like Behavior
[10] The repeated appearance of the QP-burst group with
an almost planetary rotation period of 10 hours is more
typically illustrated in Figure 2. Figure 2a shows a 5-day
dynamic spectrogram, and Figure 2b shows an integrated
intensity profile of QP bursts in a frequency range from 3 to
20 kHz. The 10-hour periodicity of the QP-burst group
is evident from the figure, although the frequency width
and intensity varied from group to group. This confirms
that the appearance of the QP-burst group is controlled
by the planetary rotation. Figure 2 also reveals that
each burst group appears with a steep rise and slow
decay in both frequency and intensity, exhibiting saw-
tooth variations.
[11] Figure 3 shows a 16-day (from day 132 to day 148 in
1997) dynamic spectrogram when burst group activity
varied during the period. The local time of Galileo on day
132 was 22.3 hours and that on day 148 was 0.9 hours. The
intensity of the burst groups was greatly enhanced and the
frequency band was expanded to the higher frequencies
during the first 4 days, and then their activity ceased for the
Figure 2. (a) Dynamic spectrogram of radio waves
for 5 days from 3 to 8 July 1997. (b) Integrated intensity
of QP-burst groups in frequency band from 3 to 20 kHz.
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next 6 days. After that, new burst groups appeared and
grew larger during the next 3 days. The white scale in the
figure indicates a 9.9-hour period whose origin is at the
start of the first QP-burst group as indicated by the star. It is
surprising that each trigger for a reappearing burst group
(from days 142 to 146) is synchronized with that for the
first activity group (first 4 days). This strongly indicates
that the appearance of QP-burst groups is controlled by
something like a Jovian clock. This is very similar to ‘‘the
clock modulation’’ of Jovian energetic electrons, which are
periodically released into interplanetary space [Chenette et
al., 1974].
3.3. Relationship With the Magnetospheric Activity
[12] From Figure 3, we can also see the relationship
between QP-burst groups and HOM emissions. The HOM
emissions above several hundred kHz have long-term
variations in both frequency and intensity, in addition to
regular rotational modulation of every 5 hours. Around days
132 to 135, HOM-emission intensity became enhanced and
the frequency band expanded to lower sides. The HOM
intensity then declined and the frequency band retreated to
higher frequencies from days 137 to 141. From day 142,
HOM began to intensify again and continued till day 148.
These long-term variations in HOM emissions, which are
considered to represent auroral activity in the Jovian polar
region as was demonstrated by Louarn et al. [2001], seem
to be almost concurrent with QP-burst group activity having
two active periods, i.e., from days 132 to 136 and days 142
to 146 as previously described. It is also noteworthy that the
upper frequency of the QP-burst group expanded when
HOM emissions were the most intense around days 132
and 134. These observations indicate that QP-burst groups
are activated during periods when the Jovian magnetosphere
is disturbed.
[13] Two groups of n-KOM were also detected at a
frequency of around 200 kHz as can be seen in Figure 3.
They arose on days when QP burst activity had almost
maximized, on days 134 and 144. This correspondence
between QP-burst group enhancement and n-KOM appear-
ance suggests that QP bursts are excited by the magneto-
spheric disturbances caused by solar wind because the onset
of n-KOM emissions is known to be strongly related to the
arrival of a solar wind sector boundary at Jupiter as was
clearly presented by Reiner et al. [2000].
3.4. Statistical Survey on Spatial Distribution and
Clock-Like Behavior
[14] We surveyed daily dynamic spectrograms of 14
selected Galileo orbits from August 1996 to January 2002
(G02 to I33 orbit) and chose 364 typical groups of QP
bursts during the observations. Figure 4 shows the distri-
bution of selected QP-burst groups in the magnetosphere.
We can see that typical QP-burst groups were observed at
radial distances greater than 20–25 R
J
and throughout the
whole local times. The fact that QP-burst groups are only
observed in regions more than 20 –25 R
J
may be suggesting
that bursts radiate in the polar region at a certain radiation
cone angle so that the equatorial magnetosphere near the
planet is outside the irradiation area. It is noteworthy that in
the direction finding analysis of QP40 bursts done by
MacDowall et al. [1993], they found their source region
to be a few Jovian radii above the polar region. The lack of
QP burst observations in the morning sector in Figure 4
would partly be because the periapsis of the Galileo orbit
tended to be in the morning sector and the spacecraft was
mostly in regions less than 20 R
J
.
[15] The recurrent appearance of QP-burst groups and
their clock-like behavior was statistically investigated. As
the occurrence of QP-burst groups is characterized by their
sudden commencement as was depicted in Figure 2, we
investigated the start time distribution of QP-burst groups
with respect to (1) the System III longitude (l
III
) of Galileo
and (2) the l
III
of the subsolar point. The broken line in
Figure 5 plots the start time distribution of QP-burst groups
with respect to the l
III
of Galileo. The distribution has a
broad peak at around l
III
= 120. However, the solid line,
which plots the start time with respect to the l
III
of the
subsolar point indicates a sharply focused occurrence peak
at a longitude of around l
III
= 280–300. This strongly
suggests that QP-burst groups are excited when Jupiter has a
particular spin phase angle with respect to the Sun. The
broad distribution seen in the l
III
of Galileo (broken line)
can be understood in the sense that the detection of QP burst
groups was biased around the midnight and in the early
morning sector as seen in Figure 4 so that the l
III
of Galileo
Figure 3. Dynamic spectrogram of radio waves for 16 days from 12 to 28 May 1997. The ticks on the
white scale in the figure indicate interval of 9.9 hours started from the first QP-burst group marked by
star.
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has some loose relation to the subsolar longitude with a
phase difference of roughly 180. We can thus confirm that
the observation of QP-burst groups is not the result of the
emission source rotating with the planetary magnetic field
but the clock-like modulation being caused by the phase
relation between the planetary magnetic field and solar
wind.
[16] Figure 6 plots the statistical duration of QP-burst
groups. It indicates that the duration of QP-burst groups
ranges from 1 to 8 hours and has an occurrence peak with
duration of 4–5 hours. This means that the bursty wave
excitation lasts for half of the planetary rotation on average.
4. Discussion and Conclusions
[17] We presented the occurrence characteristics of mul-
tiple QP bursts observed in the Jovian magnetosphere. They
recurrently appeared in groups with a planetary spin period
of 10 hours. Moreover, QP-burst groups had clock-like
modulations as well as periodic appearances, i.e., the QP-
burst groups were excited when Jupiter had a particular spin
phase angle with respect to the Sun. The subsolar longitude
at the time of excitation was around l
III
= 260–320.We
also found that the activity of QP-burst groups was related
to that of HOM emissions and the excitation of n-KOM
emissions, suggesting that QP bursts are activated during
magnetically disturbed periods. The detection of QP-burst
groups by Galileo was restricted to regions greater than 20 –
25 R
J
suggesting that the waves are emitted in the polar
magnetosphere.
4.1. QP-Burst Group and JAC
[18] Here, let us note some close relationships between
QP-burst groups observed in the Jovian magnetosphere and
intense VLF radio waves observed in interplanetary space
[Kaiser et al., 1992, 1993, 2004; Kaiser, 1998, Morioka et
al., 2004]. The intense VLF radio waves in interplanetary
space are called Jovian anomalous continuum (JAC) or
‘‘reradiated emission’’ [Kaiser et al., 2004]. A typical
dynamic spectrogram of JAC observed in interplanetary
space by Ulysses on 27 –28 October 1991 is shown in
Figure 7a. JAC is characterized by slowly decreasing
frequency in a range from about 7 to 15 kHz and by its
long duration. The radio burst groups that overlapped on the
first half of the JAC spectra could be QP-burst groups
generated in the Jovian magnetosphere. The sequential
JACs that arose every 10 hours are also consistent with
the consecutive appearance of QP burst groups with 10-hour
periodicity. Figure 7b shows the spectrogram of QP-burst
groups observed by Galileo on 4 July 1997. Although both
spectrograms are not simultaneous, we can see by compar-
ing Figures 7a and 7b that the clusters of QP bursts and their
periodic occurrence is quite similar to the bursty component
in the JAC spectrogram.
[19] Quasi-periodic particle accelerations that are almost
synchronized with low-frequency radio bursts (QP bursts)
were discovered in the polar magnetosphere by Ulysses
during a flyby of Jupiter [McKibben et al., 1993]. We
speculate that a cluster of quasi-periodic accelerations
occurs in the polar magnetosphere once a planetary rotation
Figure 4. Distribution for QP-burst group observation in
the magnetosphere. There were 364 typical groups of QP
bursts chosen from selected 14 Galileo orbits from August
1996 to January 2002 (G02 to I33 orbit).
Figure 5. Longitudinal distribution for QP-burst groups.
The solid line indicates longitude of subsolar point at time
of QP-burst group commencements and the broken line
indicates central meridian longitude of Galileo at time of
QP-burst group commencements.
Figure 6. Statistical distribution for duration of QP-burst
group. Used data set is same as in Figure 4.
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and excites multiple quasi-periodic radio bursts (QP-burst
group) and that the cluster of the quasi-periodic electron
beams also excites JAC in the magnetosheath, where JAC
is characterized by long-lasting and slowly frequency-
decreasing spectra. A part of JAC in the magnetosheath
can escape into interplanetary space and is observed as the
free space wave. Two scenarios for the generation and
propagation of JAC are presented elsewhere [Kaiser et al.,
2004; Morioka et al., 2004]. Thus a spacecraft in interplan-
etary space can detect both magnetospheric QP-burst groups
from the polar magnetosphere and JAC radiation from the
magnetosheath simultaneously. The lower-frequency limit
of JAC which is higher than that of QP-burst groups in the
magnetosphere would be due to the cutoff effect at plasma
frequency in the magnetosheath.
4.2. Emission Commencement Synchronized With
Planetary Rotation
[20] The evidence that the periodic commencement of
QP-burst groups takes place at a certain planetary spin
phase is a new finding derived from this study and this is
also consistent with the excitation of JAC [Morioka et al.,
2004]. Figure 8 reproduces the result by Morioka et al.
[2004], which illustrates the System III longitude of the
subsolar point when the commencement of JAC was
detected in interplanetary space (Figure 8a), together with
the occurrence of QP-burst groups derived in this study
(Figure 8b). The very similar distribution of both phenom-
ena with a unique occurrence peak at around l
III
= 260–
320supports that QP bursts and JAC are generated by
‘‘recurrent disturbances’’ once a planetary rotation which
are excited when Jupiter has a particular spin phase with
respect to the solar wind direction (i.e., with respect to the
magnetosphere). The recurrent disturbances assume the
bursty acceleration of energetic electrons and the emission
of QP radio bursts in the polar magnetosphere.
[21] After the discovery of the periodic release of Jovian
energetic electrons into interplanetary space by Chenette et
al. [1974], Simpson et al. [1975], and Schardt et al. [1981]
confirmed that the spectral index of energetic electrons in
the outer magnetosphere and interplanetary space exhibited
a maxima at a subsolar longitude of l
III
= 230±50.Itis
very interesting that the subsolar longitude at the time of the
spectral index maxima of the escaping electrons roughly
corresponds with that at the commencement of QP-burst
groups and JAC demonstrated in this study. To interpret the
Figure 7. Typical dynamic 24-hour spectrograms of (a) Jovian Anomalous Continuum and (b) QP-burst
group. JAC dynamic spectrogram was observed by Ulysses from 27 to 28 October 1991. Dynamic
spectrogram of QP-burst groups was from the Galileo observation on 4 July 1997. Two spectrograms are
properly arranged on time axis to show similar periodicities and time profiles. Note that the frequency
axis is linear in Figure 7a and logarithmic in Figure 7b.
Figure 8. Comparison of occurrence characteristics
between (a) JAC detected from Ulysses observation and
(b) QP-burst groups from Galileo observation. The
horizontal axis is System III longitude of subsolar point
when JAC and QP-burst groups are detected.
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longitudinal asymmetry of Jovian particle escape detected
by Pioneer and Voyager observations, Dessler and
Vasyliunas [1979] and Vasyliunas and Dessler [1981] pro-
posed a magnetic anomaly model, which claimed that the
asymmetry of the surface magnetic field affects the distri-
bution of plasma in the outer magnetosphere. The basic idea
of the magnetic anomaly model is the depressed field region
in the northern hemisphere centered near l
III
= 230, which
rotates once every 10 hours and results in the release of
energetic particles into interplanetary space [Hill et al.,
1974; Hill and Dessler, 1976]. Goertz and Baker [1985]
suggested that in addition to the surface magnetic anomaly,
the periodic energization of energetic particles is caused by
variations in the longitudinally averaged Pedersen conduc-
tivity due to asymmetric solar illumination in the polar
region.
[22] It should be noted that Kurth et al. [1986, 1987]
reported the periodic amplitude variations in Jovian contin-
uum radiation from the Voyager observations and suggested
that the continuum radiation is organized by the clock-
like modulation. Contrary to their continuum radiation
near 3 kHz, we claimed in this study that QP-burst
groups, which overlap into spectra of continuum radia-
tion, have a clock modulation. However, there is a
possibility that QP-burst groups described in this paper
are identical with the ‘‘periodic continuum radiation’’
demonstrated by Kurth et al. [1986], when we consider
differences of both time and frequency resolution between
Galileo and Voyager observations. If it is the case, Kurth et al.
[1986] are the first who mentioned the clock-like behavior of
the ‘‘QP-burst groups’’ in the Jovian magnetosphere.
[23] The present study indicates that quasi-periodic par-
ticle acceleration takes place in the polar ionosphere syn-
chronized with planetary rotation. Energetic particle
emissions from the polar region have been studied in detail
by Karanikola et al. [2004] through the high-latitude
observations with the Ulysses spacecraft. Their results
showed that energetic electron emissions with many varie-
ties of periodicity (15 –80 min) are common in the polar
region, being consistent with our present assumptions.
4.3. Relation With the Magnetospheric and
Solar Wind Disturbances
[24] The occurrence of QP-burst groups accompanying n-
KOM and JAC emissions discussed in this paper indirectly
proves that QP-burst groups arise during the period when
the solar wind dynamic pressure is decreasing because n-
KOM emissions are associated with a depression in the
solar wind dynamic pressure as has been demonstrated by
Reiner et al. [2000], and JAC appears when solar wind
dynamic pressure is declining after a rapid increase [Morioka
et al., 2004]. Theoretical predictions that the kinetic energy of
the rotating magnetosphere is unloaded through ionosphere-
magnetosphere coupling have been proposed and claimed
that Jovian magnetospheric activities are likely to be en-
hanced in conjunction with solar wind pressure variations
[Nishida and Watanabe, 1981; Watanabe and Nishida, 1982;
Southwood and Kivelson, 2001; Cowley and Bunce, 2001].
The occurrence characteristics of QP-burst groups presented
in this paper may be connected with these theoretical pre-
dictions and the assumed ‘‘recurrent disturbances’’ once a
planetary rotation may be a manifestation of the daily
unloading of Jovian rotational energy during a period of
magnetospheric disturbances caused by solar wind pres-
sure variations.
[25]Acknowledgments. The authors would like to thank H. Oya for
his encouragement. We acknowledge the use of PWS data (the Principal
Investigator is D.A. Gurnett) and MAG data (the Principal Investigator is
M. G. Kivelson) in this research. The data used in this study were provided
through the Planetary Plasma Interactions (PPI) Node of the Planetary Data
System (PDS). The PPI Node is located at the Institute of Geophysics and
Planetary Physics at the University of California, Los Angeles. The Ulysses
data were provided from Coordinated Heliospheric Observations (COHO)
database, NSSDC, NASA. The research at the University of Iowa was
supported by NASA through grant NNG05GG98G.
[26]Wolfgang Baumjohann thanks Michael Kaiser and Philippe
Louarn for their assistance in evaluating this paper.
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Y. Miyoshi, Solar-Terrestrial Environment Laboratory, Nagoya Univer-
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H. Nozawa, Kagoshima National College of Technology, Kagoshima,
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