Supersubstorms during Storms of September 7-8, 2017

The development of two supersubstorms (i.e., very intense substorms with an amplitude of more than 2000 nT) recorded in the main phase of two consecutive strong magnetic storms with maxima at similar to 0100 UT (Dst similar to -150 nT) and similar to 1300 UT (Dst similar to -115 nT) on September 8, 2017, is analyzed. Data from the SuperMAG global magnetometers network and the Scandinavian profile of IMAGE stations are used. Analysis of spatial distribution maps of ionospheric equivalent electric currents on the Scandinavian meridian derived according to the MIRACLE model and global maps of magnetic field vectors obtained from SuperMAG observations makes it possible to obtain the spatial distribution of planetary-scale disturbances. Both supersubstorms were characterized not only by strong nighttime disturbances at auroral latitudes (similar to-3600 nT and similar to-2600 nT) but also by the simultaneous development of daytime magnetic bays at polar latitudes with amplitudes of similar to-1000 nT and similar to-400 nT, respectively. We assume that the daytime polar disturbances observed simultaneously with the supersubstorms can result from the pull of the westward ionospheric current to the dayside. Our observations support the assumption that the westward electrojet during the supersubstorm develops on a global scale from the evening to the dayside.