Pick-Up Ion Trajectories in a Comet Model

摘要

In-situ measurements at Comets Giacobini Zinner and Halley reveal the presence of energetic cometary ions accelerated to energies at times over 100 keV. This study investigates how the underlying large scale magnetic field and velocity structure of an idealized comet, as represented by an MHD simulation, leads to the acceleration of these particles. Single particle trajectories have been computed using the results from Fedder and coworkers MHD simulation of comets for the background magnetic and convection electric fields. For the present numerical model, it is assumed that the particles are not scattered by fluctuations in the field and plasma. In this scatter free limit, the trajectories of initially cold cometary ions which accelerated due to the large scale convection electric field are more widely dispersed in the plane of the interplanetary magnetic field than in the orthogonal plane. Ions created closets to the nucleus focus into the tail current sheet, while those ions created in the outer coma diverge from the tail axis in response to gradient and curvature drifts. Those ions initially formed close to the nucleus have a lower energy, forming the cold plasma sheet, while the ions created in the outer coma have greater energy and populate the hot wings that extend far from the tail axis. The results suggest that the distinctive double peaked time series seen in the observations of energetic pick up ions arises from control of the particle trajectory by the large scale plasma and field structure of the comet. (Theses)