Abstract
Abstract. Solar wind/magnetosheath plasma in the magnetosphere can be identified using a component that has a higher charge state, lower density and, at least soon after their entry into the magnetosphere, lower energy than plasma from a terrestrial source. We survey here observations taken over 3 years of He2+ ions made by the Magnetospheric Ion Composition Sensor (MICS) of the Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) instrument aboard POLAR. The occurrence probability of these solar wind ions is then plotted as a function of Magnetic Local Time (MLT) and invariant latitude (7) for various energy ranges. For all energies observed by MICS (1.8–21.4 keV) and all solar wind conditions, the occurrence probabilities peaked around the cusp region and along the dawn flank. The solar wind conditions were filtered to see if this dawnward asymmetry is controlled by the Svalgaard-Mansurov effect (and so depends on the BY component of the interplanetary magnetic field, IMF) or by Fermi acceleration of He2+ at the bow shock (and so depends on the IMF ratio BX /BY ). It is shown that the asymmetry remained persistently on the dawn flank, suggesting it was not due to effects associated with direct entry into the magnetosphere. This asymmetry, with enhanced fluxes on the dawn flank, persisted for lower energy ions (below a "cross-over" energy of about 23 keV) but reversed sense to give higher fluxes on the dusk flank at higher energies. This can be explained by the competing effects of gradient/curvature drifts and the convection electric field on ions that are convecting sunward on re-closed field lines. The lower-energy He2+ ions E × B drift dawnwards as they move earthward, whereas the higher energy ions curvature/ gradient drift towards dusk. The convection electric field in the tail is weaker for northward IMF. Ions then need less energy to drift to the dusk flank, so that the cross-over energy, at which the asymmetry changes sense, is reduced.Key words. Magnetospheric physics (magnetospheric configuration and dynamics; magnetopause, cusp, and boundary layers) – Space plasma physics (charged particle motion and acceleration)
Highlights
The mechanisms by which solar wind ions are energised and transported in the geospace environment rely on many timedependent parameters and, as a result, are complicated and not well understood
This study examines the region with invariant latitudes greater than 60◦ containing open field line features such as the cusp and polar cap as well as the open/ closed field line boundary, the low-latitude boundary layer (LLBL - whether on open or closed field lines) and the auroral oval on closed field lines
The cusp is prominent near local noon, centred at about 79◦ invariant latitude; the low-latitude boundary layer (LLBL) is shown ranging from about 68◦ to about 75◦ invariant latitude and extends over a large range of local times
Summary
The mechanisms by which solar wind ions are energised and transported in the geospace environment rely on many timedependent parameters and, as a result, are complicated and not well understood. This study examines the region with invariant latitudes greater than 60◦ containing open field line features such as the cusp and polar cap as well as the open/ closed field line boundary, the low-latitude boundary layer (LLBL - whether on open or closed field lines) and the auroral oval on closed field lines In this region we can infer the nature of global processes of ion injection and transport from the energy and distribution of solar wind ions. Sometimes the POLAR spacecraft has observed intense fluxes of solar wind ions in and around the cusp for prolonged periods These are known as extended cusp-like regions (ECRs) and are sometimes observed during persistent, strong northward IMF BZ. On the 29 May 1996 an ECR event, characterised by high fluxes of solar wind ions over a large range of invariant latitudes and magnetic local times, coincided with a persistent, strong northward IMF.
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