Abstract
Dipolarizing flux bundles (DFBs) have been suggested to transport energy
and momentum from regions of reconnection in the magnetotail to the high
latitude ionosphere, where they can generate localized ionospheric
currents that can produce large nighttime geomagnetic disturbances
(GMDs). In this study we identified DFBs observed in the midnight sector
from ~7 to ~10 RE by THEMIS A, D, and E
during days in 2015-2017 whose northern hemisphere magnetic footpoints
mapped to regions near Hudson Bay, Canada, and have compared them to
GMDs observed by ground magnetometers. We found six days during which
one or more of these DFBs coincided within ± 3 min with ≥ 6 nT/s GMDs
observed by latitudinally closely spaced ground-based magnetometers
located near those footpoints. Spherical elementary current systems
(SECS) maps and all-sky imager data provided further characterization of
two events, showing short-lived localized intense upward currents,
auroral intensifications and/or streamers, and vortical perturbations of
a westward electrojet. On all but one of these days the coincident DFB
– GMD pairs occurred during intervals of high-speed solar wind streams
but low values of SYM/H. In some events, in which the DFBs were observed
closer to Earth and with lower Earthward velocities, the GMDs occurred
slightly earlier than the DFBs, suggesting that braking had begun before
the time of the DFB observation. This study is the first to connect
spacecraft observations of DFBs in the magnetotail to intense
(>6 nT/s) GMDs on the ground, and the results suggest DFBs
could be an important driver of GICs.