Cluster observations of hot He⁺ events in the inner magnetosphere

Abstract

In the inner magnetosphere inside 65° invariant latitude, Cluster Ion Spectrometry detected hot He⁺ events of about a few tens to several hundred eV without the same types of hot H⁺ signature at the same energy. During the 2001-2006 period when the Cluster orbit was almost constant and approximately north-south symmetric at constant local time near the perigee, we found nearly 20 examples in Cluster spacecraft 4. These hot He⁺ events are morphologically classified into two burst types and two dispersed types: (1) Short intensification of He⁺ (1a) without corresponding H⁺ or O⁺ signatures, or (1b) with H⁺ signature at different pitch angles. (2) Energy-latitude dispersed He⁺ stripes that continues for tens of minutes at hundreds to a few thousand eV range (2a) at different drift shell from energy-latitude dispersed H⁺ stripes, or (2b) with very weak H⁺ signature if the energy is constant. While type-1a is observed during or right after substorm activities, type-2b is found after long quiet periods, i.e., after long drift. The relationship with the geomagnetic activity indicates that the plasmasphere can be energized in a mass-dependent way in the evening sector during substorms to form the bursty types (type-1), while the selective He⁺ energization can also take place during quiet periods near the noon. On the other hand, the source of the two dispersed types (type-2) must be remote from the observation point, and the location and the geomagnetic conditions at the time of the He⁺ filtering is an open question.