Abstract
Abstract. Ion shell distributions are hollow spherical shells in velocity space that can be formed by many processes and occur in several regions of geospace. They are interesting because they have free energy that can, in principle, be transmitted to ions and electrons. Recently, a technique has been developed to estimate the original free energy available in shell distributions from in-situ data, where some of the energy has already been lost (or consumed). We report a systematic survey of three years of data from the Polar satellite. We present an estimate of the free energy available from ion shell distributions on auroral field lines sampled by the Polar satellite below 6 RE geocentric radius. At these altitudes the type of ion shells that we are especially interested in is most common on auroral field lines close to the polar cap (i.e. field lines mapping to the plasma sheet boundary layer, PSBL). Our analysis shows that ion shell distributions that have lost some of their free energy are commonly found not only in the PSBL, but also on auroral field lines mapping to the boundary plasma sheet (BPS), especially in the evening sector auroral field lines. We suggest that the PSBL ion shell distributions are formed during the so-called Velocity Dispersed Ion Signatures (VDIS) events. Furthermore, we find that the partly consumed shells often occur in association with enhanced wave activity and middle-energy electron anisotropies. The maximum downward ion energy flux associated with a shell distribution is often 10mWm-2 and sometimes exceeds 40mWm-2 when mapped to the ionosphere and thus may be enough to power many auroral processes. Earlier simulation studies have shown that ion shell distributions can excite ion Bernstein waves which, in turn, energise electrons in the parallel direction. It is possible that ion shell distributions are the link between the X-line and the auroral wave activity and electron acceleration in the energy transfer chain for stable auroral arcs.
Highlights
Ion shell distributions are spherical shells in velocity space.They can be formed by many processes of which the most widely studied is the pickup process which occurs when the solar wind protons undergo a charge-exchange reaction with almost stationary neutrals
Our analysis shows that ion shell distributions that have lost some of their free energy are commonly found in the plasma-sheet boundary layer (PSBL), and on auroral field lines mapping to the boundary plasma sheet (BPS), especially in the evening sector auroral field lines
Our analysis suggests that the ion shell distributions seen in the Polar/TIMAS 6 RE radial distance in the PSBL form a Velocity Dispersed Ion Signatures (VDIS) when considered in the energylatitude plane
Summary
Ion shell distributions are spherical shells in velocity space. They can be formed by many processes of which the most widely studied is the pickup process which occurs when the solar wind protons undergo a charge-exchange reaction with almost stationary neutrals. Janhunen et al (2003a) provided examples of shell distributions in auroral latitudes at ∼4–5 RE radial distance (geocentric distance) using Polar/TIMAS data and showed with the help of solving the linear dispersion relation numerically and by using a two-dimensional particle simulation that the observed shell distribution is unstable to a broad range of ion Bernstein wave modes. It was shown in the paper that the generated Bernstein waves accelerate. The total-ion downward energy flux is obtained from the differential energy flux by integration over all pitch angles
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