火星外逸层中的氧原子(O)经光致电离、电荷交换或电子碰撞产生氧离子(O+),新生O+随即被太阳风拾起,进而被对流电场加速.这些O+被称为"拾起"O+.2018年11月17日08:54:00-10:24:00 UT期间,火星大气和挥发物演化任务(Mars Atmosphere and Volatile EvolutioN,MAVEN)卫星从上游太阳风进入磁鞘中,持续观测到背离太阳运动的高能拾起O+(约30 keV),在相空间呈现为初速度不为零的部分环分布.分析表明:这部分高能O+的初始能量较高,回旋半径较大,位于摆线运动的高能区域,穿越火星弓激波进入磁鞘,离子在该范围内能量变化不显著,只是使得相应环分布在穿越弓激波后发生偏转.这些结果有助于进一步认识火星空间高能拾起O+的产生与运动轨迹.
In situ observation of energetic pickup oxygen ions upstream from the Martian bow shock:a case study
The O atoms in the Martian exosphere can be ionized by photoionization,charge exchange,or electron impact.The newborn O+ions are immediately picked up by the solar wind electromagnetic fields,and are then accelerated by the convection electric field.They are called pickup O+ions.During 08∶54:00-10:24:00 UT on 2018 November 17,and Mars Atmosphere and Volatile EvolutioN(MAVEN)observed persistent pickup O+population with energies of～30 keV.They correspond to precipitating O+ions coming from the upstream solar wind.In phase space,they are located near the middle phase of partial ring distribution with an initial velocity(i.e.,near the maximum energy of the partial ring distribution).Our analysis suggests that these precipitating energetic O+can overcome the electrostatic bow shock potential and proceed downstream.Then,they are subject to the enhanced magnetic field in the magnetosheath.The results will help better understand the generation and the motion trajectories of energetic O+ions at Mars.
NETL
NSTL
万方数据
维普
