H+ and O+ content of the plasma sheet at 15-19 Re as a function of geomagnetic and solar activity

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.