THEMIS observations of magnetospheric ELF emissions, ULF Pc5 waves and their auroral features

Details THEMIS observations of magnetospheric ELF emissions, ULF Pc5 waves and their auroral features THEMIS observations of magnetospheric ELF emissions, ULF Pc5 waves and their auroral features

May 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agusmsm31b..08l&link_type=abstract

American Geophysical Union, Spring Meeting 2009, abstract #SM31B-08

Other

2704 Auroral Phenomena (2407), 2772 Plasma Waves And Instabilities (2471), 2790 Substorms, 7867 Wave/Particle Interactions (2483, 6984)

Scientific paper

Narrow-banded ELF/VLF emissions at about (n+1/2)fce, where fce is the electron gyrofrequency, are commonly observed in the inner magnetosphere. They are generally believed to originate from an electron-cyclotron harmonic instability due to an interaction between a "cold" (<100 eV) electron plasma and a "hot" (>1 keV) electron population. The hot component provides the local source of free energy for the instability growth, while the cold electrons facilitate the propagation of the instability and control its spatial amplification. Such ELF/VLF emissions are widely considered as a main candidate of driving the pitch-angle scattering and in turn the precipitation of 1-10 keV electrons. In this presentation we report two events of strong ELF emissions near 3/2fce band at ˜L10 in the postmidnight equatorial plasma sheet from recent THEMIS observations. Both events occurred during the late substorm expansion phase and/or recovery phase, and the probes were located at a couple of MLT hours east of the onset meridian determined from the ground auroral observations. In one event the 3/2 fce emissions appeared in forms of discrete bursts modulated by a large-amplitude ULF Pc5 waves with period 5-10 minutes. The Pc5 waves are identified as an eastward-propagating poloidal/compressional waves, and non-FLR in nature. In the other event the 3/2 fce emissions appeared as a somewhat continuous band lasting a few tens of minutes. On a inspection of the electron distribution functions for both events we found that during the event intervals there was a coexistence of cold electron population(˜10 eV), ambient plasma sheet electrons (1-2 keV), and enhanced energetic electrons (>10 keV) presumably drifted from the injection region of the substorm, consistent with the theoretical expectations. Conjugate auroral observations from THEMIS all-sky imagers, CGSM multi-spectral imagers, CGSM meridian scanning photometers, and several higher time resolution white light imagers, reveal distinct features such as eastward-drifting auroral patches and pulsating auroras with period of few seconds during the event intervals. We attempt to relate those drifting patches and pulsating auroras to the in-situ observed ELF emission and its resulting pitch-angle scattering process.

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