Impact of Solar Wind High Speed Streams and the Effect of Solar Wind Dynamic Pressure on the Ionospheric Current System and Field-Aligned Currents

Abstract:

High speed streams (HSS) and associated stream interaction regions (SIR) in the solar wind are one of the major drivers of geomagnetic activity, especially during declining phases of sunspot cycles and near sunspot minimum. We have identified 33 HSS/SIR driven geomagnetic storms that coincide with a Dst drop to less than -50nT during the period 2010-2017 and have investigated their impact on the ionospheric current systems. The events are studied using a superposed epoch analysis with zero epoch centered around the onset of the storm main phase to see the evolution of the current systems with respect to the development of the geomagnetic storm. The events have been analyzed together and in two groups based on the maximum solar wind (SW) dynamic pressure, low and high p_dyn, around the zero epoch. Events with high p_dyn have currents that react earlier to the storm and reach maximum field-aligned current (FAC) and horizontal current on average 1 hour and 10 minutes after the zero epoch. Meanwhile events associated with low p_dyn have more elongated current development that reach maximum activity 5 hours and 50 minutes after zero epoch. The median maximum FAC and westward electrojet in the high p_dyn events are 1.35 and 1.56 times larger than the low p_dyn events, respectively. The development in the storm Dst index differs between the two dynamic pressure groups. Low p_dyn events have a gradual time development of the Dst, while the high p_dyn events have very large -dDst/dt at the hours after the onset of the storm main phase, corresponding to the times of large increase in currents. High correlation between -dDst/dt and the total FAC have been found in both groups and for the individual events, indicating that hours of large plasma injection rate into the ring current corresponds to hours of large FAC activity.