Cluster Observations of Ion Shell Distributions in the Plasma Sheet on 31 March 2001

On 31 March 2001 from 0800--1030 UT the Cluster spacecraft traversed the northern plasma sheet at 2200 local time during a time of violent magnetospheric activity (DST = -350~nT). At a distance of 4--5~R _{E}, the local magnetic field perturbations were as high as 100~nT. Measurements by the Cluster Ion Spectrograph (CIS) experiment revealed intense, energetic (~15 keV) ion distributions, which exhibited a shell structure in velocity space. Although distributions of this form have been reported before, the capabilities of CIS, and the multi-point observations, allow us to significantly improve the characterization of these ions. The CODIF instrument measured 3-D distributions for H⁺, He⁺⁺, He⁺ and O⁺, and showed that during this interval oxygen was dominant. For the first time, 8~s integrations permitted the observation of an apparent evolution of the velocity-space shells; bursts of Earthward-streaming O⁺ beams were seen to spread in pitch-angle and isotropize over intervals < 1~minute. In addition to the energetic shells, the O⁺ had a very cold core, which was seen to stream tailward at low speeds. Both H⁺ and O⁺ at times had as many as three distinct velocity shells. Initial examination of the distributions indicates that the evolution of the shells can be inconsistent with simple adiabatic unfolding in combination with mirroring below the spacecraft; we will consider the role of the observed strong wave power in their production. Towards the end of this plasma sheet crossing, the boundary passed spacecrafts 1, 3 and 4, separated in z by 1000~km, within a span of about 50~s. Each spacecraft recorded nearly-identical non-gyrotropic distributions that appeared to rotate by more than 110^o in gyrophase. These gyrophase-restricted distributions probably indicate remote-sensing of the plasma sheet; we will apply multi-point analyses methods to validate this.