Investigation of Mass-/Charge-Dependent Escape of Energetic Ions Across the Magnetopauses of Earth and Jupiter

Abstract

We present a continuing investigation of mass-/charge-dependent interactions between energetic ions (greater than tens of kiloelectron volts) and planetary magnetopauses and of the escape of the ions across the boundary. Previous studies at Earth using Magnetospheric Multiscale mission data are refined and advanced showing profound behavior differences between light (H, He) and singly charged heavy ions (O+). We highlight a distinctive feature of oxygen ions: an angular distribution bifurcation providing clear indication of entrainment along the magnetopause in Speiser-like orbits during relatively stable magnetic conditions. This signature, interpreted using a simple kinetic model, suggests that these ions tend to be carried substantial distances along the boundary (even with boundary-normal magnetic fields) in a fashion that impedes their full dayside escape. While large fluctuations and waves can likely sometimes disrupt the observed ordering, the following picture emerges. Energetic particles with gyroradii much smaller than the magnetopause thickness (e.g., electrons and absent boundary-normal magnetic fields) and ions with gyroradii much larger than the thickness (e.g., O+) are impeded from fully escaping across the boundary. However, energetic ions with intermediate-sized gyroradii commensurate with the thickness (e.g., H+, He++, and O6+) can be effectively scattered within the boundary causing them to escape much more readily, with and without boundary-normal fields. This picture is supported by observations from the Juno spacecraft at the near-dawn meridian side of Jupiter's magnetopause. There it is observed that energetic electrons and heavy ions are more strongly contained by the magnetopause than are the energetic protons and helium ions.