Dayside Response of the Earth's Magnetosphere to Interplanetary Shocks: Determining the propagation speed of the pulse, ExB flow direction and Ey signature for a variety of shocks using Van Allen Probes, MMS, THEMIS and GOES data

Interplanetary (IP) shocks are abrupt changes in the solar wind velocity and/or magnetic field. When an IP shock impacts the Earth's magnetosphere, it can trigger responses including geomagnetic storms and substorms that affect radiation to satellites and aircraft and ground currents that disrupt the power grid. This impact compresses the magnetosphere, forcing the dayside magnetopause earthward, and generates a signature of the shock we call the pulse, which propagates both radially and azimuthally through the dayside magnetosphere. This pulse is assumed to propagate as a fast mode wave, but experimental verification is needed to better understand the magnetospheric response and improve magnetospheric models.

We have recently compiled a database of 70 sudden impulses from 2012-2016 when multiple satellites (of Van Allen Probes A and B, MMS, THEMIS A-E and GOES 13 and 15) were located in the dayside magnetosphere at impulse times. For a given event, we identified all of the spacecraft on which the pulse is observed, and used cross-correlation to determine the timing of the propagation between the spacecraft and therefore the speed of the response. These speeds were then compared with the plasma fast mode speed and other fundamental speeds of waves in plasma, which are calculated from local plasma parameters.

This is the first statistical study using multiple satellites in the dayside magnetosphere to investigate the response to interplanetary shocks. We find agreement between our calculated propagation speeds and the fast mode speed, and we also found that the initial direction of the ExB velocity following a shock impact was universally in the negative GSE X direction towards the Earth, indicating tailward energy flux. However, during particularly oblique shocks, there is often a significant y-component observed on some spacecraft. Lastly, the E_y signature for a significant percentage of the shocks ( 35%) departed from the typical bipolar (first dawnward, then duskward) signature associated with compression and relaxation of the dayside magnetopause.