Multiple-Instrument Observations of Nighttime Magnetic Impulse Events (MIEs) at High Latitudes

Large magnetic impulse events (MIEs) are often observed in high latitude magnetometer arrays. These events can induce large geoelectric fields and ground-induced currents (GICs) that can have harmful effects on electrical power grids. Nighttime MIEs have often been associated with the westward traveling surge of auroral substorms, but details of their generation are not yet fully understood. The installation of the AUTUMNX array in northern Quebec in 2014 provided latitudinal coverage of the auroral zone complementary to the longitudinal coverage at near-cusp and contracted oval latitudes provided by the MACCS array. Here we present observations of four large MIEs observed in eastern Arctic Canada by MACCS and AUTUMNX, supplemented with data from the Canadian CANMOS and CARISMA magnetometer arrays, the Greenland Coastal array, the conjugate AAL-PIP array in Antarctica, THEMIS all-sky imagers, SuperDARN radars, and in two cases, from the Geotail satellite. During the November 11, 2015 0100 UT event, large magnetic deflections (up to 1300 nT) primarily in the westward and field-aligned (Bz) directions were accompanied by derivatives exceeding 20 nT/s in the horizontal direction at two MACCS stations and in the vertical direction at one MACCS and one AUTUMNX station. Vertical deflections and derivatives most often exceeded horizontal ones at the AUTUMNX stations. In all four events the amplitudes of magnetic deflections and dB/dt variations were often largest at MACCS stations, near the poleward boundary of the auroral zone. In three of these cases, auroral streamers at this boundary were observed during the events, indicating that intense ionospheric flow channels were responsible for the ionospheric currents giving the large magnetic deflections.