Why Are Some Solar Wind Pressure Pulses Followed by Geomagnetic Storms?

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.