[P-EM09] Dynamics of Magnetosphere and Ionosphere

[PEM09-05] On the origin of cold-dense plasmas in the dusk magnetotail plasma sheet: MMS and Geotail observations

*Masaki N Nishino¹, Yoshifumi Saito¹, Hiroshi Hasegawa¹, Naritoshi Kitamura², Yukinaga Miyashita^3,4, Tsugunobu Nagai¹, Shoichiro Yokota⁵, Daniel J Gershman⁶, Christopher T Russell⁷, Barbara L Giles⁶ (1.Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 2.Graduate School of Science, The University of Tokyo, 3.Korea Astronomy and Space Science Institute, South Korea, 4.Korea University of Science and Technology, South Korea, 5.Graduate School of Science, Osaka University, 6.Goddard Space Flight Center, NASA, 7.University of California, Los Angeles)

Keywords:Magnetotail plasma sheet, Plasma transport, Northward IMF, Low-latitude boundary layer (LLBL), Magnetospheric Multiscale (MMS) spacecraft, Geotail spacecraft

The near-Earth plasma sheet becomes cold and dense under the northward interplanetary magnetic field (IMF) condition, which suggests entry of solar wind plasma into the magnetosphere across the magnetopause. The cold and dense characteristics of the plasma sheet are more evident in the magnetotail flank regions that are interface between cold solar wind plasma and hot magnetospheric plasma. Several physical mechanisms have been proposed to explain the entry of solar wind plasma across the magnetopause and resultant formation of the cold-dense plasma sheet (CDPS) in the tail flank regions. However, transport path of the cold-dense plasma inside the magnetotail has not been understood yet. Here we present a case study of the CDPS in the dusk magnetotail by Magnetospheric Multiscale (MMS) spacecraft under the conditions of the strongly northward IMF and the high-density solar wind. The ion distribution function consists of high- and low-energy components, and the low-energy one intermittently shows energy dispersion in the directions parallel and anti-parallel to the local magnetic field. Considering the time-of-flight effect of the energy-dispersed low-energy ions, we infer that these ions originate in the region down the tail, and move along the magnetic field toward the ionosphere and then come back to the magnetotail by the mirror reflection. The pitch-angle dispersion analysis gives consistent results on the travelling time and path length of the energy-dispersed ions. Interestingly, Geotail spacecraft detected large magnetic field fluctuations in the low-latitude boundary layer near the dusk terminator, which may be attributed to Kelvin-Helmholtz instability due to the velocity shear between the magnetosheath and the plasma sheet. Based on these observations, we discuss possible generation mechanisms of the energy-dispersed structure of the low-energy ions during the northward IMF.