嘉宾(Presenter)：Dr. Xiaojia Zhang
时间（Date & Time）：June 27, 2019, 15:00-16:30（周四）
地点(Venue):闻天楼南楼211, Wentian Building
题目(Title)：Modulation of chorus and ECH wave intensity by compressional ULF waves
Very-low-frequency (VLF) waves play a crucial role in shaping the high-energy electron distributions in the inner magnetosphere and in precipitating electrons to the atmosphere. Generation of these waves are usually attributed to hot plasma sheet electron injections (whistler waves) or the loss-cone instability in electron distributions (electron cyclotron harmonic waves, i.e., ECH waves). Spacecraft observations, however, often demonstrate the presence of whistler waves in marginally stable plasmas. Moreover, these wave emissions can be observed within regions of several Earth radii, RE, in the cross section -- much larger than the spatial scale of localized injections. In this paper, we show that wave generation in marginally unstable plasmas within large spatial domains can be attributed to modulation of hot (resonant) electron distributions by ultra-low-frequency (ULF) waves. The azimuthal electric field of ULF waves transports electrons across ~ 1.5 RE and periodically brings unstable electrons from higher L-shells (presumably from the azimuthally extended flow breaking region) to lower L-shells, where electrons release the stored free energy through whistler (chorus) wave generation. The same oscillatory transport brings electrons with larger loss-cone anisotropy (from the lower L-shell) to higher L-shells, and lead to periodic generation of ECH waves with intensities anti-correlated with chorus wave intensity. This mechanism of VLF wave generation, which leads to periodic VLF emissions and intermittent electron precipitations across a large spatial scale, can be a potential explanation for the pulsating aurora.
Xiaojia Zhang is an assistant researcher at Institute of Geophysics and Planetary Physics of the University of California, Los Angeles (UCLA). She received her PhD with a Space Physics specialty at UCLA. Her research focuses on waves, wave-particle interactions and associated magnetotail dynamics in Earth’s plasma sheet. After completing her first project at UCLA, She turned to another related research topic in Earth’s magnetotail, i.e., examining the excitation of electron cyclotron harmonic (ECH) waves in Earth’s outer magnetosphere and the role ECH waves play in the context of magnetotail dynamics.