Energetic particle fluxes in the exterior cusp and the high-latitude dayside magnetosphere: statistical results from the Cluster/RAPID instrument

Abstract. In this paper we study the fluxes of energetic protons (30–4000 keV) and electrons (20–400 keV) in the exterior cusp and in the adjacent high-latitude dayside plasma sheet (HLPS) with the Cluster/RAPID instrument. Using two sample orbits we demonstrate that the Cluster observations at high latitudes can be dramatically different because the satellite orbit traverses different plasma regions for different external conditions. We make a statistical study of energetic particles in the exterior cusp and HLPS by analysing all outbound Cluster dayside passes in February and March, 2002 and 2003. The average particle fluxes in HLPS are roughly three (protons) or ten (electrons) times larger than in the exterior cusp. This is also true on those Cluster orbits where both regions are visited within a short time interval. Moreover, the total electron fluxes, as well as proton fluxes above some 100 keV, in these two regions correlate with each other. This is true even for fluxes in every energy channel when considered separately. The spectral indices of electron and proton fluxes are the same in the two regions. We also examine the possible dependence of particle fluxes at different energies on the external (solar wind and IMF) and internal (geomagnetic) conditions. The energetic proton fluxes (but not electron fluxes) in the cusp behave differently at low and high energies. At low energies (<70 keV), the fluxes increase strongly with the magnitude of IMF B_y. Instead, at higher energies the proton fluxes in the cusp depend on substorm/geomagnetic activity. In HLPS proton fluxes, irrespective of energy, depend strongly on the K_p and AE indices. The electron fluxes in HLPS depend both on the <K_p index and the solar wind speed. In the cusp the electron fluxes mainly depend on the solar wind speed, and are higher for northward than southward IMF. These results give strong evidence in favour of the idea that the high-latitude dayside plasma sheet is the main source of energetic particles in the exterior cusp. Energetic particles can reach HLPS from the near-Earth tail. The closed field lines of HLPS act as storage for these particles. Direct diffusion (for electrons and high-energy protons) and magnetic reconnection in the high-latitude magnetopause near HLPS (for low energy protons) control the number of particles released into the exterior cusp. Note that this explanation, in contrast to other suggested theories, works both for the energetic protons and electrons in the exterior cusp.

Keywords. Magnetospheric physics (Magnetopause, cusp and boundary layers; Solar wind-magnetosphere interactions) – Space plasma physics (magnetic reconnection)