Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsm13b2047k&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SM13B-2047
Physics
[2720] Magnetospheric Physics / Energetic Particles: Trapped, [2772] Magnetospheric Physics / Plasma Waves And Instabilities
Scientific paper
We perform a case study of whistler-mode chorus waves in the dayside outer (L>7) magnetosphere under quiet geomagnetic conditions. We use simultaneous conjugate observations made at 1230-1930 UT on 26 July 2008 by three THEMIS spacecraft and ground-based VLF receivers at two automatic geophysical observatories (AGO) in Antarctica supported by the US Polar Experiment Network for Geospace Upper-atmosphere Investigations (PENGUIn) project. Solar wind dynamic pressure was constant and weak (~1 nPa); the Dst index was around -10 nT; and the AE index showed weak activity (<200 nT). VLF wave intensification was observed by THEMIS A, D, and E. Waveform data from THEMIS A show that the waves were intensified in a frequency range of 500-800 Hz (0.3-0.4 fce) and circularly right-handed polarized. Filter bank data show that all three THEMIS probes observed wave intensification in the 287-1240 Hz range around noon and 6-9 RE near the equatorial plane. VLF signals at the AGO P2 station (AP2: MLAT = -76.6 deg.; MLT = UT-3.5h) were intensified in the frequency range of 500-1000 Hz. At the AGO P3 station (AP3: MLAT = -83.63 deg., MLT = UT-2h), VLF signals were intensified in the same frequency range; the increase rate was smaller than at AP2. The VLF wave intensification peaked around noon. The AGO AP2 and AP3 stations are mapped along field lines to the outer magnetosphere; the mapped locations in the equatorial plane are 9-10 RE and 10-11 RE around noon, respectively. We also confirm, from fluxgate magnetometer data, that AP2 and AP3 were equatorward of the open-closed boundary. The observed chorus waves were intensified in narrow ranges of MLT and radial distance. We conclude that the localized intensification continued for at least 1.5 hours. We examine the configuration of magnetic field lines in which the THEMIS spacecraft and the AGO stations reside during the wave intensification, and find that the intensification occurred when field lines have small gradient along a field line (dB/ds) in a wide range of magnetic latitude. We suggest that chorus wave growth in flattened field lines is responsible for the continuous intensification of dayside chorus waves during quiet geomagnetic conditions. We discuss about two possible generation mechanisms: non-linear wave growth due to greater electron trapping and linear growth in a uniform medium for an extended length.
Angelopoulos Vassilis
Bortnik Jacob
Gerrard Andrew J.
Keika Kunihiro
Li Wangrong
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