Solar Cycle Impact on O+ Abundance Throughout the Magnetosphere

The CLUSTER spacecraft have now been operating for 12 years, from 2001 through the present time, covering almost a full solar cycle. The CODIF instrument has provided a wealth of data over this time, allowing the solar cycle dependence of the ion composition over the energy range 40 eV/e to 40 keV/e throughout the magnetosphere to be determined for the first time. The solar cycle impacts the magnetosphere in a number of ways. The enhanced solar EUV towards solar maximum heats the ionosphere, increasing the ionospheric scale height, which leads to greater outflow. The greater number of CMEs leads to more and larger geomagnetic storms, which also increases the outflow, both in the cusp and in the nightside auroral regions. In addition, the solar cycle impacts not only the ionospheric outflow, but also the transport, through changes in convection. Thus, different regions of the magnetosphere may be impacted in different ways. In this paper we review recent results on how the solar cycle impacts the inner magnetosphere, the ~20 Re plasma sheet, and the lobes. While both increased EUV and magnetospheric activity increase the O+ density and O+/H+ ratio in the inner magnetosphere and plasma sheet, the relative roles are different. In the inner magnetosphere, the impact of EUV dominates, while the contributions are more equal in the plasma sheet. This shows clearly the importance of transport in determining which effect is dominant. This is supported by observations in the lobe, that show that much less cusp-origin O+ reaches the 20 Re plasma sheet during solar minimum than during solar maximum compared to the amount that is observed over the polar cap.