Shell-like configuration in O⁺ ion velocity distribution at high altitudes in the dayside magnetosphere observed by Cluster/CIS

Abstract. We report shell-like configurations seen in O⁺ ion velocity distributions. One case was observed above 8R_E in radial distance in the dayside magnetosphere, presumably in the mantle region, during the observation period of 09:30-10:00 UT on 12 April 2001 by the CIS instrument on board the Cluster satellite. This shell-like configuration was different from so-called "conics" or "beams": the lower energy (cold) population and the higher energy partial shell part were seen together, but there was no obvious signature of heating process. With respect to H⁺ ion velocity distributions observed simultaneously, transverse heating (so-called in "pan-cake" shape) or field-aligned energisation configurations were seen as the result of heating/energisation processes and the upward-going part of the distribution also formed a half spherical thick shell configuration. Concerning O⁺ ion heating in the case of 12 April 2001, it was obviously observed when the spacecraft passed through the mantle region close to the poleward cusp. As the spacecraft moved toward the dayside cusp shell-like (or dome shape) velocity distributions appeared apparently and continued to be observed until the spacecraft reached the magnetopause according to two other different cases (13 February 2001 and 14 April 2001). Two other cases were observed in the Southern Hemisphere and the spacecraft was supposed to pass through the dayside cusp toward the mantle region at higher altitudes (9-11R_E). O⁺ ion velocity distributions in these cases show pre-/post-structured shell-like configurations, depending on the observation sites (mantle or dayside cusp).