Abstract
AbstractWe present a statistical study of density cavities observed in the inner magnetosphere by the Van Allen Probes during four one‐month periods: February 2013, July 2013, January 2014, and June 2014. These periods were chosen to allow the survey of all magnetic local times. We find that density cavities are a recurrent feature of the density profiles of in situ measurements in the inner magnetosphere. We further investigate the correlation between the density cavities and the enhancement of fluxes of warm ions with energies of 10–100 eV. The results show that warm ion flux enhancements associated with the density cavities were observed more frequently for H+, then for He+ and the least frequently for O+. The occurrences of the associated flux enhancements were increased when considering only the cavities inside the plasmasphere. Possible mechanisms responsible for the observed warm ion flux enhancements and the role of density cavities on these ion flux enhancements are discussed.
Highlights
A variety of magnetospheric missions have surveyed the plasmasphere environment, finding and describing a multitude of density structures
We present a statistical study of such density cavities, observed by the Van Allen Probes mission during four onemonth periods, which cover all magnetic local times
Statistical Results We report results based on four one-month periods (February and July 2013, and January and June 2014) of electron density and ion flux measurements from RBSP-A
Summary
A variety of magnetospheric missions have surveyed the plasmasphere environment, finding and describing a multitude of density structures. Some of the density structures have been described as plasmaspheric plumes (e.g., Goldstein et al, 2014), notches (e.g., Burch et al, 2004; Gallagher et al, 2005), plasmaspheric density troughs or cavities (Carpenter et al, 2000; Horwitz et al, 1990), plasmasphere shoulders (e.g., Pierrard & Cabrera, 2005), and crenulations (e.g., Spasojević, 2003). These structures are determined by the time history of geomagnetic activity, as are the general configuration, size, morphology and composition of the plasmasphere. In contrast to the inner plasmasphere, where densities are high (above ∼100 cm−3), temperatures are cold (∼1 eV) and ions exhibit isotropic and/or pancake pitch angle distributions (PADs, e.g., Horwitz & Chappell, 1979; Nagai et al, 1983), the outer plasmasphere is less dense, contains a warmer plasma component, and ions often exhibit field-aligned PADs
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