Department of Physics and Astronomy, University of Leicester, Leicester, UK

Rapid increases in solar wind dynamic pressure, known as solar wind pressure pulses, compress the Earth's magnetosphere and can rapidly restructure the electrodynamics within. The propagation of pressure pulse effects into the magnetosphere is known as a geomagnetic sudden commencement (SC). SCs can be further subdivided into compressions which are rapidly followed by a geomagnetic storm (a sudden storm commencement, SSC) and those which are not (a sudden impulse, SI). In this paper, SSCs and SIs are compared and contrasted, and we examine in particular the differences between the pressure pulses that drive SSCs/SIs, and explore the physical conditions of the magnetosphere before pressure pulse arrival. Firstly, it is shown that SSCs are more likely to be driven by pressure pulses with higher magnitude and/or shorter rise time. Secondly, the magnetosphere is primed by stronger driving conditions and higher geomagnetic activity prior to SSCs than SIs. Finally, there is a solar cycle dependence in the occurrence and magnitude of solar wind pressure pulses.

Severe ionospheric storms occurred at the end of October 2003. During the evening of 30 October a narrow stream of high electron concentration plasma crossed the polar cap in the antisunward ionospheric convection. A GPS scintillation receiver in the European high arctic, operating at 1.575 GHz, experienced both phase and amplitude scintillation on several satellite-to-ground links during this period. Close examination of the GPS signals revealed the scintillation to be co-located with strong gradients in Total Electron Content (TEC) at the edge of the plasma stream. The gradient-drift instability is a likely mechanism for the generation of the irregularities causing some of the scintillation at L band frequencies during this storm. The origin of the high TEC is explored and the possible implications of the work for scintillation forecasting are noted. The results indicate that the GPS scintillation over Svalbard can originate from traceable ionospheric plasma structures convecting from the American sector.