POLOIDAL MONOCHROMATIC PULSATIONS IN THE Pc4-Pc5
RANGE OBSERVED IN THE EARTH MAGNETOSPHERE
Belakhovsky V.1, Pilipenko V.2, Samsonov S.3, Klimushkin D.4, and Mager P.4
1
2
3
4
Polar Geophysical Institute, Apatity, Russia
Institute of the Earth Physics, Moscow, Russia
Institute of Cosmophysical Research and Aeronomy, Yakutsk, Russia
Institute of Solar-Terrestrial Physics, Irkutsk, Russia
Abstract. In this study we examine the spatial and polarization structure of monochromatic pulsations in the Pc5
frequency range, detected by a fleet of satellites (GOES, LANL, THEMIS) in the morning sector of the
magnetosphere on Sept. 09, 2007. These pulsations are excited during low geomagnetic activity. The satellite
observations show that these pulsations are a poloidal-type fundamental mode of Alfven field line oscillations with
small scale in the azimuthal direction (m~25). These pulsations practically cannot be seen on the ground CARISMA
magnetometers. The observed Pc5 waves are accompanied by simultaneous pulsations of the fluxes of energetic
electrons and protons, as seen by LANL-1994 satellite. The modulation depth in the fluxes of energetic particles is
larger than the modulation depth in geomagnetic field. A several-fold increase of the electron density in the
magnetosphere is found before the onset of poloidal Pc4-5 waves. As seen by THEMIS satellites, these pulsations
propagate in the morning sector in the sunward direction, i.e. in the direction of electron drift. We suppose that the
injection of energetic electrons may be responsible for the excitation of the poloidal Pc5 pulsations at the morning
flank of the magnetosphere via the "ship wave" mechanism.
1. Introduction. There are many types of ULF waves in the Earth magnetosphere, which differ by their periods
(Pc3-5, Pg, Pi2-3), waveforms (broadband or monochromatic), polarization structure (poloidal, toroidal, or
compressional), etc. One of the most puzzling type is the poloidal pulsations [Anderson, 1993]. In this study we try
to examine this type of pulsations with new data facilities and compare observations with recent theoretical ideas. In
particular, we try to answer the following questions: What is the wave mode responsible for these pulsations? Is the
monochromatic wave form of these pulsations related to the occurrence of specific wave resonator? What is the
driving factor of these pulsations during very a quiet period?
Fig. 1. The location of the GOES, LANL, and THEMIS spacecraft in
XY plane of the GSM coordinate system during the event Sept. 9,
2007 at 12.00 UT.
2. Data. The geomagnetic field data with the high time resolution (0.512 sec) from the GOES-12 (MLT=UT-5),
GOES-10 (MLT=UT-4), GOES-11 (MLT=UT-9) geostationary spacecrafts was used. The particle dynamics was
detected by the LANL-01 (MLT=UT), LANL-02 (MLT=UT+4.5), LANL-89, LANL-97 (UT+7), LANL-94
(MLT=UT-3.5) geostationary spacecraft. We have used the moments computed from measurements of the MPA
instrument (ions ~130eV/e - 45keV/e and electrons ~30eV - 45keV), and SOPA instrument (electrons and protons
>50-75 keV).
THEMIS probe data have been used for observations of the magnetic and electric field variations, and energetic
particle fluxes. CARISMA network stations are used for the registration of geomagnetic field variations in conjugate
to the satellite regions. NORSTAR riometers are used for the control of the energetic electron precipitation into the
ionosphere.
3. The event September 9, 2007
3.1. GOES geomagnetic field observations. The monochromatic pulsations are well evident on the GOES-10 and
GOES-12 geostationary spacecraft during ~5 hours (Fig. 2). The pulsations are stronger, up to ~8 nT, in the radial
component he. Weaker pulsations can be seen also in the field-aligned component hp and in the module of the
geomagnetic field ht. This polarization indicates on the poloidal-type transverse wave structure. Whereas these
poloidal pulsations are clearly evident at near-by spacecraft GOES-10 and GOES-12 (MLT difference is ~1 h), they
cannot be seen at more distant geostationary GOES-11 spacecraft, located on the night side.
Fig. 2. The magnetic field variations at GOES-10 (left-hand panel) and GOES-12 (right-hand panel) spacecraft.
Magnetograms from the GOES-10, -12 spacecraft demonstrate a very monochromatic waveform of these waves.
The spectral analyze reveals a narrow spectral maximum (not shown) at f~5.4 mHz (T~3 min), that is in the nominal
Pc5 range.
Though the poloidal pulsations are clearly evident simultaneously on both GOES-10 and GOES-12
spacecraft, the coherence between them is low. This does not enable us to determine the wave phase delay between
these spacecraft.
3.2. Ground observations. The considered pulsations cannot be seen on the ground CARISMA magnetometers (not
shown), even at stations near the conjugate points of the GOES-10 and GOES-12 spacecraft. This fact indicates on a
small-scale transverse structure of these pulsations.
3.3. Geomagnetic activity. These pulsations are observed during a low geomagnetic activity, contrary to majority
of other types of ULF waves. The SYM-H index is about -10 nT, and AE-index is ~60-160 nT. According to the
OMNI database, the solar wind speed is V ≈ 380 km/s, and density is N ≈ 4 cm-3. Just before the onset of these
pulsations, the AE-index has increased from ~30 nT to ~80 nT. We suppose that the source of these pulsations is
associated with local processes inside the magnetosphere.
3.4. LANL energetic particles observations. The considered pulsations are accompanied by simultaneous
pulsations in the fluxes of energetic electrons and protons (Fig. 3) with the same period, as seen from the
observations on LANL-1994 geostationary spacecraft. The GOES-10 and LANL-1994 spacecraft are located close
to each other (Fig. 1). At the same time no periodic pulsations are observed in the riometer data from the
NORSTAR network.
The modulation depth (ratio of the pulsation amplitude to the background level) for the proton fluxes
reaches ΔJp/Jp ≈ 60% (in the 50-75 keV energy channel). The modulation depth in electron fluxes is lower, ΔJe/Je ≈
12%. Surprisingly, the modulation depth of the magnetic variations is lower than that of particles, Δht/ht ≈ 1.3%,
and Δhe/he ≈ 10%. Thus, the ΔJe/Je and Δhe/he are quite comparable, but ΔJp/Jp >> Δhe/he.
Fig. 3. The comparison of the magnetic field variation (he-component) on GOES-10 spacecraft (MLT=UT-4) with
the fluxes of energetic electrons (left-hand panel) and protons (right-hand panel) in various energy channels on
LANL-1994 spacecraft (MLT=UT-3.5).
Before the onset of Pc5 waves on the GOES-10, the increase of the magnetospheric electron density has
been detected by the LANL-1994 spacecraft (Fig. 4). We suppose that this cloud of energetic electrons may be
responsible for the pulsation excitation. Approximately 1 hour after the appearance of Pc5 pulsations, a substantial
increase of the magnetospheric electron temperature Te has been observed by LANL-1994 (Fig. 4). This
temperature growth may be associated with the electron heating by enhanced Pc5 waves.
3.5. THEMIS observations. The poloidal pulsations in the morning sector have been also detected by THEMIS-E,
-B, and -C spacecraft (Fig. 5). According to time delay between signals on different probes, we find that these Pc5
waves propagate in the sunward direction, i.e. in the same direction as drifting electrons. The pulsation phase
velocity in XY plane is ~100 km/s, i.e. much smaller than the typical Alfven speed VA. The time delay between
THB-THC-THE probes gives the azimuthal wave number m≈24.6. Thus, these pulsations are indeed small-scale
disturbances in the azimuthal direction.
The frequency of the poloidal pulsations on THE is lower f~4.5 mHz, than that at GOES-10, f~5.4 mHz.
Because THE probe is located on a larger L-shell than GOES-10 does (see Fig. 1), this frequency difference may
indicate on resonant properties of these pulsations, i.e. a frequency decrease with an increase of L-shell.
The magnetometer (GOES-10) and particle detector (LANL-1994) are shifted azimuthally by ~0.5 h MLT.
As a result the phase difference of small-scale magnetic pulsations and periodic particle oscillations is not steady
and cannot be reliably measured.
The cross-phase between the azimuthal Ey electric component (Fig. 5) and radial magnetic component Bx
is close to 90°. So, these oscillations are standing Alfven waves along a field line.
4. Discussion. The poloidal Pc5 pulsations under examination are quite similar to Pg pulsations: both have a very
monochromatic waveforms, and are excited during low geomagnetic activity. However, contrary to Pg pulsations,
the observed poloidal pulsations cannot be seen by ground-based magnetometers. This distinction may be caused by
a higher value of m-value of the poloidal Pc5 waves as compared with typical Pg pulsations (m~15) [Takahashi et
al., 2011].
Fig. 4. Electron density (upper panel), temperature
(mid-panels) at LANL-1994, and radial magnetic
component at GOES-10 (bottom panel).
Fig. 5. Magnetic and electric field variations at the
THEMIS-E in the GSM coordinate system.
It may be supposed that the observed poloidal pulsations are generated by a kinetic instability of the ‘hot’
electrons. However, such instabilities require a finite value of parameter β for their effective excitation. According
to LANL-1994 and GOES-10 data, during the considered event е is very low, ~0.015. Therefore, these poloidal
pulsations can hardly be generated due to development of kinetic instabilities. We suggest that generation of these
waves by energetic electrons may occur in a non-resonant way, via the "ship waves" mechanism [Mager and
Klimushkin, 2008]. The poloidal Alfven wave is supposed to be emitted by a non-steady electric current created by a
drifting electron cloud. The generated "ship waves" should propagate in the direction of the electron drift, e.g.
eastward. The azimuthal wave number is determined as m=A/d, where A is the eigenfrequency of the standing
poloidal Alfv´en wave, and d is the particle drift frequency. The amplitude of the generated wave is estimated as
B / B0  2eNLVA / cB0 , where N is the density of drifting energetic particles, L is the field-aligned wave scale.
This relationship shows that this generation mechanism may be efficient even in a low- (<<1) plasma. Therefore,
the "ship wave" mechanism could interpret the excitation of poloidal Pc5 waves by energetic electrons injected to in
the morning flank of the magnetosphere.
5. Conclusions. Very monochromatic poloidal pulsations in the Pc5 range in the morning sector of the
magnetosphere have been found in the satellite magnetometer data. Observed features of these waves match the
notion on the standing Alvenic waves with small scale in the transverse direction. These waves cause a strong
modulation of the fluxes of the energetic electrons and protons, whereas the particle modulation depth is larger than
that of the magnetic field. The cloud of energetic electrons may be the source of the considered pulsations.
Acknowledgment. This study was supported by the grants of RFFI 12-05-00273, 12-05-98522, and Program of the
Presidium of RAS № 4.
References
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waves // Ann. Geophys. 11, 128-143, 1993.
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a moving plasma inhomogeneity // Ann. Geophys. 26. 1653–1663. 2008.
- Takahashi K., et al., Multisatellite observations of a giant pulsation event // J. Geophys. Res., 116, A11223, 2011.