高能电子爆发与绕月卫星表面电位大幅下降的联动效应

"嫦娥"一号、二号绕月飞行经历地球磁尾边界层区域时,分别在2007年11月26日—2008年2月5日和2010年10月3日—2011年2月28日,发现了15次月球轨道0.1~2 MeV电子急剧增加(Bursts of 0.1~2 MeV Energetic Electrons,BEE),卫星周围等离子体离子加速的现象.统计研究表明,这类现象发生在稳定太阳风和弱行星际磁场条件下,且无显著空间环境扰动事件发生时,离子的加速滞后于高能电子爆发,离子能量的变化与高能电子通量的时间演化正相关,地球磁鞘内侧或边界层过渡区域是该类现象的高发区,离子能量增加时卫星表面电位大幅下降可达负几千伏.为了研究高能电子爆发与绕月卫星表面电位变化的关系及其对月球表面电位的影响,本文用电流平衡法建立绕月卫星和月球表面充电模型,并假设能量电子(2eV~2 MeV)满足幂律谱的分布,模拟急剧增加的能量电子对卫星和月球表面电位的影响.模拟结果表明,能量电子急剧增加使得绕月卫星和月球表面电位大幅下降;能量电子总流量1011 cm-2时,绕月卫星和月球表面充电电位可达负上千伏;月球充电到大的负电位的时间仅为卫星充电时间的1/10.鉴于高能电子急剧增加事件的高发生率(~125次/年),能量电子急剧增加使得绕月卫星表面电位大幅下降的发生率应大于实测等离子体离子加速现象的发生率(~25次/年).

Synchronization of energetic electron bursting and lunar orbiter surface charging to negative kilovolts

Fifteen times of ambient plasma ions accelerated on lunar orbit during the bursts of 0.1~2 MeV energetic electrons (BEE) was observed when spacecraft Chang'E-1 and Chang'E-2 flight through the terrestrial magnetosheath or the transition region of the boundaries during 26 November 2007 to 5 February 2008 and 3 October 2010 to 28 February 2011. The 0.1~2 MeV BEE events were observed by High Energetic Particles Detector (HPD) on board Chang'E-1 in a 200 km lunar polar orbit and Chang'E-2 in a 100 km lunar polar orbit. And the ions energy increasing was observed by Solar Wind Ion Detector (SWID) on board Chang'E-1 and 2. These events were found under relatively steady solar wind conditions and weakly IMF values without strong environmental disturbances. The ions acceleration occurred after the energetic electrons bursting, and the accelerated ions energy was correlated positively with the fluxes of the energetic electrons with the spacecraft surface charging to negative kilovolts. We use the current balance equations to simulate the spacecraft and the lunar surface charging during the bursting of the energetic electrons. The observed data of the two lunar orbiters is used to derive the energetic electrons spectrum (>2 eV~2 MeV) with power-low distribution assumption during BEEs. The simulation results indicate that the energetic electron incident of BEE is the dominating cause of the spacecraft surface charging to negative kilovolts. The results also indicate that the lunar and the spacecraft surface will charge to negative kilovolts during the BEEs after the temporal integral of the energetic electrons fluxes reach up to >10¹¹ cm^-2. The balance time for the spacecraft surface charging is about 10 times that the lunar surface charging. It is expected that the occurrence of the synchronization of energetic electrons bursting and surface charging to large negative voltage should be more frequent than the Chang'E-1 and 2 observations though the observed ions acceleration (25 times per year) are fewer than the BEEs (125 times per year).