Global evolution and propagation of electric fields during sudden commencements based on multi-­point observations

Sudden commencements (SCs) are triggered by an abrupt compression of the dayside magnetopause, which causes a fast mode wave propagating toward the Earth in the equatorial magnetosphere across the magnetic field line. The sudden compression also induces the Alfven wave propagation toward the polar ionosphere along magnetic field lines. The latter causes the global transmission of ionospheric electric field at speed of light, and can propagate the influence back to the inner and/or nightside magnetosphere. These general propagation processes have been demonstrated in previous papers using direct observations. We study the spatial and temporal evolution of electric fields and the direction of Poynting fluxes between the magnetosphere and ionosphere associated with SCs. We use multi-point magnetospheric and ionospheric satellites (THEMIS, RBSP, GOES, and C/NOFS) with radars (SuperDARN). An event study on 17 March 2013 shows that the magnetospheric electric field is propagated from dayside to nightside magnetosphere. At the onset time, the magnetospheric magnetic field starts to increase, which indicates that the detected electric field is associated with the compression of the magnetosphere. In the ionosphere, C/NOFS satellite and SuperDARN radar detect the dusk-to-dawn electric field about 1 min after the onset in the magnetosphere. Poynting fluxes evaluated from THEMIS and RBSP data are directed toward the ionosphere along magnetic field lines in both dayside and nightside, which indicates that the Alfven wave launches toward the polar ionosphere at the onset. The spatial evolution of magnetospheric electric fields can be interpreted as follows: First, the fast mode wave propagates from dayside to nightside magnetosphere, and 105-120 s after the onset, the magnetospheric convection becomes stronger. We also find that the spatial distribution of the response time is asymmetric between dawn and dusk, which can be due to the asymmetry of the plasmapause location.