Geomagnetic Disturbances and Midlatitude Airglow During the 20 December 2015 Magnetospheric Storm

We report on the novel features of stormtime, midlatitude PiB/PiC geomagnetic pulsations, ionospheric and field‐aligned currents, and oxygen, O 1 S and O 1 D, emissions at 557.7 and 630.0 nm, respectively. Those were observed during the main phase of the 20 December 2015 storm with significant variations of the solar wind dynamic pressure, P d , and interplanetary magnetic field B z . Intensifications of the shortest period, Т < 10 s, Pi1B pulsations and oxygen emissions marked the substorm onsets. The distinct characteristic of the strong substorm (SME = 2,910 nT) was the presence of bay‐like geomagnetic variations with the X and Z components with the opposite signs in the northern and southern sections of the IMAGE chain near 18 MLT. Using the magnetogram inversion technique of our institute, we obtained the MLT‐MLAT distribution (map) of equivalent and field aligned currents revealing an additional westward electrojet to the north of the eastward current. We have shown that such a current system provides the observed distribution of geomagnetic variations along the 18 MLT meridian during the 20 December 2015 strong substorm, including at the high‐latitude half of the IMAGE chain of stations. These conclusions were clearly validated by our simple current system model. We also revealed a localized geomagnetic event during the SME decrease interval between two substorm activations, when the magnitudes of the H/X geomagnetic component, PiB/PiC pulsations, and oxygen emissions at mid latitudes were more than twice greater than during the strong substorm.