Influence of Ionospheric Plasma on Substorm Activity

The issue of the influence of ionospheric plasma on the development of substorms remains controversial. Multi-fluid simulations are used to investigate the sources of plasma during Aug 13, 2001 period of activity that was well observed by Cluster at 18 Re down the tail. It is shown that the ionospheric outflow from the multi-fluid code, which has been documented previously, is able to account for the Cluster observations during the period of several hours of magnetospheric activity, including the changes in the heavy ion density and relative density. The fact that the model is able to account for features both at low altitudes and in the magnetotail suggest that the transport of the plasma within the model is realistic. The influences of the dayside and nightside ion fountains are clearly identified in the model. It is shown that for the smaller substorms that occur during the event, ionospheric H+ plays an important role during substorm onset, while for the more intense substorm, O+ is an important factor. In the latter case relative densities can increase from a few percent to a few tens of percent over an extend region of the magnetosphere. At the inner edge of the plasma sheet, O+ can contribute more than 50% of the local energy density, though its number density in the central plasma sheet remains low. The deposition of this energy at the inner edge of the plasma sheet is an important factor for substorm onset. In addition, while the dayside outflows produce the greatest fluxes to the tail, the nightside outflows are primary involved in substorm onset.