[1] We have used the ion composition data from the CIS/CODIF instrument on the Cluster spacecraft to determine how the H + and O + contribution to the plasma sheet density changes as a function of geomagnetic conditions and solar activity. The Cluster spacecraft are in a polar orbit that cut through the equatorial plasma sheet at ∼19 Re downtail for the first 5 years of the mission. We have restricted the data set to apogee time periods, from 15 to 19 Re, in order to give the composition at a clear position, which can then be used, for example, as a boundary condition for models. The geomagnetic conditions are described using the Kp index, while the solar activity is represented by the use of F10.7 index. Functional forms for these dependencies are provided. The statistical study covers the years from 2001 to 2005, which covers solar maximum, and the declining stage of the solar cycle. We find, as expected, that the O + density in this region depends strongly on both solar EUV and geomagnetic activity. In addition, we find that there is a gradient in the O + /H + density ratio, from the 15 to 19 Re plasma sheet to the 6.6 Re plasma sheet, indicating that there is significant additional entry of O + inside of 15 Re.

While the plasma in the Earth's magnetotail predominantly consists of protons and electrons, there are times when a significant amount of oxygen is present. When magnetic reconnection occurs, the behavior of these heavy ions can be significantly different from that of the protons, due to their larger gyroradius. In this study, we investigate the heavy ion distribution functions in the reconnection ion diffusion region from a 2.5D three‐species particle‐in‐cell numerical simulation and compare those with Cluster observations from the near‐Earth magnetotail. From the simulation results, we find that the heavy ions are demagnetized and accelerated in a larger diffusion region, the heavy ion diffusion region. The ion velocity distribution functions show that, inside the heavy ion diffusion region, heavy ions appear as counterstreaming beams along z in the GSM x‐z plane, while drifting in y, carrying cross‐tail current. We compare this result with Cluster observations in the vicinity of reconnection regions in the near‐Earth magnetotail and find that the simulation predictions are consistent with the observed ion distribution functions in the ion diffusion region, as well as the inflow, exhaust, and separatrix regions. Based on the simulation and observation results, the presence of a multiscale diffusion region model, for O+ abundant reconnection events in the Earth's magnetotail, is demonstrated. A test particle simulation shows that in the diffusion region, the H+ gains energy mainly through Ex, while the O+ energy gain comes equally from Ex and Ey.

The reconnection current layer thickness is expected to scale with either the ion inertial length or the ion gyroradius. Both of these quantities scale with the mass of the ions present. During geomagnetically disturbed times, the reconnection layer in the magnetotail can contain a significant amount of O + . UsingCluster multi-spacecraft measurements, we have studied nine reconnection events in the magnetotail plasmasheet, to determine whether the reconnecting current sheet thickness scales with the ion gyroradius or the ion inertial length, and whether the amount of O + in the plasma sheet has an effect on the current layer thickness. We find that the thickness is equal to the H + gyroradius, even when the plasma sheet has a high O + content. This result is in contrast to Hall MHD expectations and likely results from the multiscale nature of the current sheet.