Velocity distribution functions of helium pickup ions around interplanetary shocks

Abstract:

In this study, 2-dimensional velocity distribution functions (VDFs) of He+ pickup ions and He++ solar wind ions are compared for a quasi-perpendicular shock. Ring and shell distributions as well as the reflected ions are clearly separated in the pickup ion VDFs, and the shell forming process is associated with the local Alfvénic activity. On the other hand, solar wind ions kept Maxwellian-like distributions all the time and only heating is seen around the shock. In order to calculate the velocity distributions of helium ions, pulse height information from each ion event in the PLasma And SupraThermal Ion Compostion (PLASTIC) instrument on Solar Terrestrial Relations Observatory (STEREO) was utilized. During each electro-static analyzer energy-per-charge (E/q) step (128 steps, 435.6 ms each), PLASTIC stores 512 raw pulse height events including E/q, time-of-flight, total energy, and arrival direction. This allows us to reproduce partial 3-dimensional distribution functions for various ion species with ~2° angular resolution.

Interplanetary (IP) shocks driven by coronal mass ejections or those associated with corotating interaction regions are thought to be primary sources of energetic ion populations in the heliosphere. Around IP shocks, significant abundance enhancements of He+ pickup ions over He++ ions of solar wind origin provided compelling evidence that the source population for the energetic particle population is not the solar wind itself, but rather the suprathermal ion population that slightly exceeds the velocity range of the solar wind. However, due to the lack of high-resolution measurements of pickup ion phase space densities at these energies, their kinetics is not well understood. Based on the unique set of observations, we discuss characteristic heating, acceleration, and diffusions for helium pickup ions near interplanetary shocks.