等离子体片中能量离子的空间分布——Cluster/RAPID观测数据分析

Spatial distribution of energetic ion compositions in the plasma sheet observed by Cluster/RAPID

本文根据Cluster卫星上的粒子成像质谱仪(RAPID)探测器在穿越地球等离子体片过程中的观测数据,统计研究了等离子体片中能量离子能量密度的空间分布(氢离子能量范围从40keV到1500keV,氦离子和氧离子从10keV到1500keV),并且给出了离子能量密度在不同地磁活动时期随GSE-Z向分布的剖面.研究表明能量离子的能量密度以及能量密度的梯度与地磁活动指数Kp之间存在近似线性的关系.观测结果表明形成这种分布变化的主要原因是在地磁活动期间在电流片附近离子能量密度的增加,特别是其中的重离子成分增加更为显著.本文通过一个简化的电流片模型的数值计算,定性地研究了形成能量离子空间分布的机理.计算表明重离子在电流片中可以获得更多的能量,电流片加速可能是形成能量密度分布变化的一种可能的机制.

The RAPID spectrometer （Research with Adaptive Particle Imaging Detectors） onboard Cluster provided us an opportunity to investigate the distribution of energetic ions in the plasmasheet in the energy range of 40 keV - 1500 keV for protons, and 10 keV - 1500 keV for helium and oxygen ions. Totally about 600-hour data have been recorded when the spacecraft cross the plasma sheet in the mid magnetotail at X = （ - 7 - - 19） RE（GSE）. The dependence of ion energy density on different geomagnetic activity levels indicated by Kp indices has been studied. A linear correlation of energy density with Kp has beerr found. The profiles of different ion energy density distributions in GES-Z direction with low Kp and high Kp are presented, respectively. The reason for the positive correlation and the profile may be the fact that the energy density increases near the current sheet, and heavier ions enhance intensely. A simplified current sheet model has been employed to demonstrate that the current sheet acceleration mechanism could be a plausible explanation for these observational results that heavier ions can remain more in the site near the central plasma sheet and obtain more energy at the same time.