Acceleration of interstellar helium in the inner heliosphere

Abstract

The heliosphere, the volume inflated by the solar wind, is impenetrable to inter-stellar plasma, except for high-energy galactic cosmic rays. Neutral components of the local interstellar medium (LISM), however, do enter at the speed of Sun's relative motion to LISM. On their journey through the heliosphere, interstellar neutrals are subject to ionization by solar wind protons and electrons, solar photons, and also the gravitational pull of the Sun. Once ionized, the newly created ions, called "pickup ions" (PUIs), are swept out by the solar wind toward the termination shock (TS). Helium atoms, having a higher ionization potential, penetrate deeper into the solar system, with their trajectories gravitational focused to form a cone of high concentration of LISM He in the wake of the Sun's motion through LISM. This He cone provides a rich source of He⁺ PUIs. Recent theories suggest that PUIs are the seed particles for the detected anomalous cosmic rays (ACRs)--singly-charged ions highly enriched in He, N, O, and Ne, and with energies < 50 MeV/nuc after solar modulation. To reach such high energies, it would be necessary that the PUIs be pre-accelerated before reaching the TS, where the ions are finally accelerated to become ACRs. This thesis investigates two aspects of the PUIs life cycle by using the data gathered by Solar Wind Composition Spectrometer (SWICS) on the Advance Composition Explorer (ACE). The first is a study of pre-acceleration of He⁺ PUIs up to 100 KeV, by shocks generated by coronal mass ejecta. Our result implies that quasi-parallel shocks are more efficient at accelerating He⁺ PUIs in this energy interval than quasi-perpendicular shocks. The second is a study of time-variability of the gravitationally focused He cone. Our results suggest that the most likely cause of observed variability is due to the changing ionization rate of He over the solar cycle.