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. Plain Language Summary: The solar wind's dynamic pressure controls the size of the cavity the Earth's magnetic field forms in space. When the pressure increases rapidly, this is known as a solar wind pressure pulse. Pressure pulses can affect currents and magnetic fields within the Earth's space environment, and can have serious space weather implications. In this paper, the differences between pressure pulses that are followed by geomagnetic storms (large releases of energy in the Earth's system), and those that are not are investigated. The state of the Earth's system before the arrival of the pressure pulse is also considered, and its contribution to the resulting space weather effects. It is shown that larger pressure pulses are more likely to trigger a geomagnetic storm. Additionally, analysis presented shows that the Earth's system may already be active prior to the triggering of these storms. Finally, the occurrence and strength of pressure pulse events follows solar activity. Key Points: There is a solar cycle dependence to the occurrence and magnitude of sudden commencements (SCs)Sudden storm commencements (SSCs) are driven by higher magnitude and/or shorter duration pressure pulsesThe magnetosphere is primed by stronger solar wind/interplanetary magnetic field and higher geomagnetic activity for SSCs [ABSTRACT FROM AUTHOR]