On Bow Shock Source of Cusp Energetic Ions

Abstract

Energetic ions from ∼10 to 150 keV e -1 are frequently observed by the Polar spacecraft in the magnetic cusp of the Earth's magnetosphere. Composition measurements indicate that they are of solar origin. Their energy spectra resemble those of energetic diffuse ions observed upstream and downstream from the quasi-parallel bow shock where solar wind ions are accelerated via first order Fermi mechanism. These bow shock accelerated ions show characteristic abundance ratios, composition ratios, and spectral dependence on solar wind parameters. Similar features also appear in the cusp energetic ions. Results from a comparison of energetic ions detected in the cusp and in the quasi-parallel magnetosheath and foreshock suggest that the observed cusp ions are very likely accelerated at the quasi-parallel bow shock. When the cusp is magnetically connected to this shock region, bow shock energetic ions can enter the cusp along the interconnected magnetic flux tubes.