Long term changes in the cross polar cap potential observed using an array of ground magnetometers.

We present changes in the cross polar cap potential and convection patterns observed over seasonal and solar-cycle time scales using an array of ground magnetometers. The findings are based on the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique run for a period of 12 years (1991-2002). It was found that the variation between summer and fall of the cross polar cap potential (CPCP) is on the order of 15%. On the solar-cycle scale, the CPCP shows a 20% change with respect to annual values between solar min and max, with the minimum cross polar cap values occurring at solar minimum. The maximum values for the period occurred in 1994, which we believe is associated with a significant number of high-speed-stream events in that year. Spatially we noted that the patterns show a strong dependence on IMF By, and are seasonally consistent with previous work but with little change over the solar cycle. It was found however that the distance between minimum and maximum in the potential cells increases 10-15% from solar minimum to solar maximum. We discuss the issues encountered in using a multi-station array over a long time period and the effect of changing data inputs on the results.