Xu Liu

Dynamic plasmapause model based on THEMIS measurements

This paper presents a dynamic plasmapause location model established based on 5u2009years of Time History of Events and Macroscale Interactions during Substorms (THEMIS) measurements from 2009 to 2013. In total, 5878 plasmapause crossing events are identified, sufficiently covering all 24 magnetic local time (MLT) sectors. Based on this plasmapause crossing database, we investigate the correlations between plasmapause locations with solar wind parameters and geomagnetic indices. Input parameters for the best fits are obtained for different MLT sectors, and finally, we choose five input parameters to build a plasmapause location model, including 5u2009min-averaged SYM-H, AL, and AU indices as well as hourly-averaged AE and Kp indices. two out-of-sample comparisons on the evolution of the plasmapause is shown during two magnetic storms, demonstrating good agreement between model results and observations. Two major advantages are achieved by this model. First, this model provides plasmapause locations at 24 MLT sectors, still providing good consistency with observations. Second, this model is able to reproduce dynamic variations of the plasmapause on timescales as short as 5u2009min.

Spectral properties and associated plasma energization by magnetosonic waves in the Earth's magnetosphere: Particle‐in‐cell simulations

In this paper, we perform a 1-D particle-in-cell (PIC) simulation model consisting of three species, cold electrons, cold ions and energetic ion ring, to investigate spectral structures of magnetosonic waves excited by ring distribution protons in the Earths magnetosphere, and dynamics of charged particles during the excitation of magnetosonic waves. As the wave normal angle decreases, the spectral range of excited magnetosonic waves becomes broader with upper frequency limit extending beyond the lower hybrid resonant frequency, and the discrete spectra tends to merge into a continuous one. This dependence on wave normal angle is consistent with the linear theory. The effects of magnetosonic waves on the background cold plasma populations also vary with wave normal angle. For exactly perpendicular magnetosonic waves (parallel wave number k||=0), there is no energization in the parallel direction for both background cold protons and electrons due to the negligible fluctuating electric field component in the parallel direction. In contrast, the perpendicular energization of background plasmas is rather significant, where cold protons follow unmagnetized motion while cold electrons follow drift motion due to wave electric fields. For magnetosonic waves with a finite k||, there exists a non-negligible parallel fluctuating electric field, leading to a significant and rapid energization in the parallel direction for cold electrons. These cold electrons can also be efficiently energized in the perpendicular direction due to the interaction with the magnetosonic wave fields in the perpendicular direction. However, cold protons can be only heated in the perpendicular direction, which is likely caused by the higher-order resonances with magnetosonic waves. The potential impacts of magnetosonic waves on the energization of the background cold plasmas in the Earths inner magnetosphere are also discussed in this paper.

Quasilinear Analysis of Saturation Properties of Broadband Whistler Mode Waves

Saturation properties of parallel propagating broadband whistler mode waves are investigated using quasilinear theory. By assuming that the electron distribution stays bi-Maxwellian, we combine the previously obtained energy equation of quasilinear theory with wave equation to self-consistently model the excitation of broadband whistler waves. The resulting evolution profile of wave intensity, spectrum, and electron temperature are consistent with those from particle-in-cell (PIC) simulations. We obtain the inverse relation between the saturation temperature anisotropy (A) and parallel plasma beta (β∥) directly from quasilinear theory. Our A-β∥ relation agrees very well with previous results from observation and PIC simulation. We also demonstrate that it might be possible to predict the wave amplitude from the initial maximum linear growth rate alone, and show that the peak frequency and spectrum width are well defined functions of the final β∥ at saturation, but not of the initial β∥.