SM42A-01
Observations of oblique propagation of whistler mode chorus
Whistler-mode chorus is known as an intense wave emission,
naturally occurring in the inner magnetosphere.
It was shown that these waves can play
a role in the process of local acceleration of
electrons in the outer Van Allen radiation belt.
Numerous theoretical studies have been recently published
concerning generation of chorus and its effects, using
both quasi-linear and nonlinear approaches to the description of
wave interactions with energetic electrons. Most of these studies assume
that the waves propagate parallel to the terrestrial magnetic field.
Here we show that this is not always the case.
We examine properties of whistler-mode chorus using data
from the WBD and STAFF-SA instruments on board the Cluster
spacecraft to characterize propagation and
spectral properties of chorus. We identify the source
region of chorus by the Poynting flux analysis using
multicomponent wave measurements of the STAFF-SA instrument
using three magnetic and two electric antennas.
The high-resolution waveforms from the WBD instrument
show that chorus in the source region can be formed by
a succession of discrete wave packets with increasing
or decreasing frequency that sometimes changes into
shapeless hiss with the same propagation characteristics.
Multi-component measurements show that in both these regimes
the wave vectors can be found at large angles
to the terrestrial magnetic field. This has consequences on
wave-particle interactions in the chorus source region.
http://os.matfyz.cz/papers/agu2008/
SM42A-02
Properties of the Dayside Outer Zone (DOZ) chorus relevant to wave-particle interactions
Fine structure and polarization of the chorus waves in the Dayside Outer Zone (DOZ) portion of the Earth magnetosphere are studied using GEOTAIL plasma wave and magnetic field data. Dayside chorus is noted to be composed of ~ 0.1 to 0.5 s rising tone emissions called elements. Chorus rising-tone elements are composed of coherent subelements or packets with durations of ~ 0.005 to 0.01 s. The peak amplitudes within a packet can be ~0.2 nT or greater. The subelement or packet amplitudes are at least an order of magnitude larger than previously-estimated chorus amplitudes obtained by power spectral measurements. Chorus waves are detected propagating both almost along the ambient magnetic field, Bo, and at oblique angles near the Gendrin angle. It is experimentally found that chorus is circularly polarized (to first order) independent of the direction of propagation relative to Bo. We demonstrate that this is what one would expect theoretically. There will be important consequences for wave-particle interactions which we will explore in our talk.
SM42A-03
Drivers of Chorus in the Outer Dayside Magnetosphere
Chorus is an intense whistler-mode wave that permeates the low-density region between the plasmapause and the magnetopause and can be observed on the ground over a range of latitudes. The waves are believed to be generated through the electron cyclotron instability by anisotropic distributions of energetic electrons in the range of 5 to 150 keV. In the inner and middle magnetosphere (L ~eq 4 to 8), the onset of wave generation has been well associated with substorm electron injections. However, there also exists a population of waves in the outer dayside region within a few RE of the magnetopause, the source of which is less well understood. We examine chorus activity from a high invariant latitude ground station (AGO P2, Λ=-69.8) for the first 100 days in 2007 and explore the variability of wave occurrence in terms of solar wind and magnetospheric drivers. We find that when the station is in the dawn local time sector (MLT<10), the onset of waves is clearly linked to substorm injections. However, as the station rotates to noon, wave occurrence favors geomagnetically quiet periods. We interpret our findings in terms of previous spacecraft observations of waves and energetic particles as well as theoretical predictions for drift shell bifurcation near the dayside magnetopause.
SM42A-04 INVITED
Evaluation of Whistler-Mode Chorus Intensification on the Nightside During an Injection Event Observed on the THEMIS Spacecraft
The intensification of the nightside whistler-mode chorus emissions is observed in the low-density region outside the plasmapause during the injection of anisotropic plasmasheet electrons into the inner magnetosphere. THEMIS ESA data of the electron phase space density (PSD) over the energy range between 0.1 keV and 30 keV are used to develop an analytical model for the distribution of injected suprathermal electrons. The path-integrated gain of chorus waves is then evaluated with the HOTRAY code by tracing whistler-mode chorus waves in a hot magnetized plasma. The simulated wave gain is compared to the observed wave electric field and magnetic field from EFI and SCM respectively. The results indicate that lower-energy (<1keV) plasmasheet electrons can penetrate deeper towards the Earth but cause little chorus intensification, while higher-energy (1keV- tens of keV) electrons can be injected at relatively higher L-shells and are responsible for the intensification of lower-band and upper-band whistler-mode chorus. Compared to lower-band chorus, relatively higher electron anisotropy is required to generate upper-band chorus. In addition, higher plasma density results in stronger wave intensity and broader frequency band of chorus waves.
SM42A-05 INVITED
Themis Observations of Long-lived Regions of Large-Amplitude Whistler Waves in the Inner Magnetosphere
The Themis mission, with multiple satellites in near-equatorial orbits, offers an excellent opportunity to observe the large-amplitude whistler waves (>100 mV/m) that can exist in the radiation belts. We use data from the Electric Field Instrument (EFI) to assess several statistical properties of these waves, including the occurrence frequency, spatial extent and longevity of regions of large-amplitude whistlers. We show that the probability distribution of wave activity in the dawn-side radiation belts, especially near L-shells from 3.5 to 5.5, has a significant high-amplitude tail and is hence not well-described by long-term time averages. Regions of enhanced wave activity exhibit four-second averaged wave power above 1 mV/m and sub-second bursts up to several hundred mV/m. These regions are spatially localized to at most several hours of local time azimuthally, but can persist in the same location for several days. These observations of persistent, bursty, large-amplitude waves support the importance of the emerging nonlinear theories of electron acceleration in the radiation belts.
SM42A-06
Large Amplitude Whistlers Observed by Wind-Waves
Stimulated by the discovery of large amplitude whistlers by STEREO S/Waves, (Cattell et al, Geophys. Res. Lett 2008 ) we have searched for similar events in the data from Wind-Waves. Although Wind was primarily intended to monitor the solar wind, the spacecraft spent 47 hours inside 5 RE and 431 hours inside 10 RE during the 8 years that it orbited the earth. Five episodes where found when whistlers had amplitudes comparable to those of Cattell et al. Unlike STEREO, the Waves experiment also had a search coil, so magnetic measurements are available, either only one component, or in rare cases, all three components. Generally, the results confirm the interpretations of Cattell et al, As they found, such whistlers occur near the edge of the plasmasphere, but the Wind-Waves observations show that they occur at a wide range of local times. Large amplitude whistlers are frequently preceded or followed by episodes of large amplitude very narrow solitary waves (100 mV/m), presumed to be electron holes.
SM42A-07
A nonlinear kinetic theory of self-sustaining whistler-mode wave emissions in the inner magnetosphere
We have developed a nonlinear kinetic theory of VLF chorus emissions, taking into account the spatial inhomegeneity of the static magnetic field and the cold plasma density along the magnetic field line in the inner magnetosphere. Based on the detailed analysis of self-consistent simulations reproducing chorus emissions, we have derived theoretical expressions for the nonlinear wave growth and the amplitude threshold for self-sustaining growth of a coherent whistler-mode wave. We assume that the nonlinear wave growth takes place at a specific localized region, where the linear growth rate maximizes. The self-sustaining emissions become possible, when the waves are propagating away from the equator with the increasing inhomegeneity of the static magnetic field and the electron density. The amplitude threshold is tested against observations and self-consistent particle simulations of chorus emissions. The growth rate and the threshold depend critically on the velocity distribution function of energetic electrons. We have obtained a set of differential equations for the wave amplitude and frequency. Solving the equations numerically for various parameters, we can reproduce the frequency variation of various forms of VLF whistler-mode emissions such as rising tones, falling tones, and hooks. The self-sustaining wave growth is due to formation of an electromagnetic electron hole that provides a viable mechanism for very efficient acceleration of relativistic electrons in the radiation belts.