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.

Ion nose spectral structures observed by the Van Allen Probes

“…The difference in range of L shell penetration for the different ion species, namely, the heavy-ion dominance in the low L shells, has been demonstrated by some previous studies [Lundin et al, 1980;Kistler et al, 1998;Yamauchi et al, 2014;Ferradas et al, 2015]. Early measurements by Lundin et al [1980] using the PROGNOZ-7 spacecraft showed a marked composition boundary near L=4, with the heavy ions dominating inside and H + often (but not always) being dominant outside.…”

Heavy‐ion dominance near Cluster perigees

“…Yamauchi et al . [] also reported observations made by Cluster Ion Spectrometry (CIS)/COmposition DIstribution Function (CODIF), during the early years of the mission, of hot He + band structures (event type 2b in their study) in the inner magnetosphere. They concluded that the He + enhancements were formed as a consequence of long drifts from the tail region and during long quiet periods.…”

“…However, they only reported five of such events, all of them occurring at energies < 1 keV and within the first 13 months of CIS observations. While there have been numerous observations and studies of heavy ions in the magnetosphere, most of them have covered higher energies (typical energies of the ring current, >50 keV) [Keika et al, 2006;Gerrard et al, 2014aGerrard et al, , 2014bKronberg et al, 2015], or lower energies (sub-keV energies) [Yamauchi et al, 2012], or have considered only O + [Sharp et al, 1976;Kistler et al, 1999;Greenspan and Hamilton, 2002;Korth et al, 2002;Mitchell et al, 2003;Keika et al, 2006Keika et al, , 2013Ebihara et al, 2009;Maggiolo and Kistler, 2014;Kronberg et al, 2015], or only He + [Yamauchi et al, 2014], or have focused solely on storm times [Fu et al, 2001;Greenspan and Hamilton, 2002;Ebihara et al, 2009;Forster et al, 2013], or have covered larger geocentric distances (beyond geostationary distance) [Kistler et al, 1990[Kistler et al, , 2006Nosé et al, 2009;Mouikis et al, 2010;Maggiolo and Kistler, 2014]. Overall, there have been more studies on O + than on He + in the inner magnetosphere, probably because O + ions are normally more abundant and contribute more significantly to the plasma sheet and ring current mass density.…”

The Evolution of the Plasma Sheet Ion Composition: Storms and Recoveries

“…Previous work utilizing data from the HOPE instrument has demonstrated that the complex ion composition observations in the inner magnetosphere can be largely understood by simple drift physics and the charge‐exchange losses of the ions along their drift paths (cf. Denton, Henderson, et al, ; Fernandes et al, ; Ferradas et al, ; Henderson et al, ; Nosé et al, ; Yamauchi et al, ; Zhang et al, ). Such work demonstrates that complex features in the distributions can result from drift physics combined with charge‐exchange losses.…”