Observations and modeling of forward and reflected chorus waves captured by THEMIS

O Agapitov (1, 2) , V Krasnoselskikh (2) , Yu Zaliznyak (2, 3) , V Angelopoulos (4) , Olivier Le Contel (5) , G Rolland (6)

1 Taras Shevchenko National University of Kyiv
2 LPC2E - Laboratoire de Physique et Chimie de l'Environnement et de l'Espace
3 Plasma Theory Department
4 IGPP - Institute of Geophysics and Planetary Physics [San Diego]
5 LPP - Laboratoire de Physique des Plasmas
6 CNES - Centre National d'Études Spatiales [Toulouse]

Discrete ELF/VLF chorus emissions are the most intense electromagnetic plasma waves observed in the radiation belts of the Earth's magnetosphere. Chorus emissions, whistler-mode wave packets propagating roughly along magnetic field lines from a well-localized source in the vicinity of the magnetic equator to polar regions, can be reflected at low altitudes. After reflection, wave packets can return to the equatorial plane region. Understanding of whistler wave propagation and reflection is critical to a correct description of wave-particle interaction in the radiation belts. We focus on properties of reflected chorus emissions observed by the THEMIS (Time History of Events and Macroscale Interactions During Substorms) spacecraft Search Coil Magnetome-ter (SCM) and Electric Field Instrument (EFI) at ELF/VLF frequencies up to 4 kHz at L ≥ 8. We determine the direction of the Poynting flux and wave vector distribution for forward and reflected chorus waves. Although both types of chorus waves were detected near the magnetic equator and have similar , discrete structure and rising tones, reflected waves are attenuated by a factor of 10–30 and have 10% higher frequency than concurrently-observed forward waves. Model-ing of wave propagation and reflection using geometrical optics ray-tracing allowed us to determine the chorus source region location and explain observed propagation characteristics. We find that reflected wave attenuation at a certain spatial region is caused by divergence of the ray paths of these non-ducted emissions, and that the frequency shift is caused by generation of the reflected waves at lower L-shells where the local equatorial gyrofrequency is larger.

Domaines: Planète et Univers [physics]

O Agapitov, V Krasnoselskikh, Yu Zaliznyak, V Angelopoulos, Olivier Le Contel, et al.. Observations and modeling of forward and reflected chorus waves captured by THEMIS. Annales Geophysicae, 2011, 29, pp.541-550. ⟨10.5194/angeo-29-541-2011⟩. ⟨insu-01180066⟩