A Significant Finding of New Morphological Type of Transpolar Arc:
Nightside Distorted Transpolar Arc
Abstract
Based on a large database of Wideband Imaging Camera (WIC), which is a
part of Far Ultraviolet (FUV) instrument, onboard the Imager for
Magnetopause-to-Aurora Global Exploration (IMAGE) satellite during 5
years between 2000 and 2005, we found a new morphological type of
transpolar arcs (TPAs), which are identified as “nightside distorted
TPAs”. The nightside ends of the dawnside (duskside) TPAs got distorted
toward pre- (post-) midnight sector; These TPAs look like the shape of
an alphabetical letter of “J” or “L”. We identified 17 nightside
distorted TPAs from our database. The 12 events out of 17 nightside
distorted TPA events were the dawnside TPA with the nightside end
distorted toward the pre-midnight sector (“J”-shaped TPA), and the
TPAs in the duskside, whose nightside parts got distorted toward the
post-midnight sector, were found in the remnant 5 events (“L”-shaped
TPA). Statistically, the nightside distorted TPAs can dominantly be
found under the northward Interplanetary Magnetic Field (IMF)
conditions. Furthermore, when the IMF-By component pointed to the
dawnward (duskward) direction, the “J” (“L”)-shaped TPAs were
dominantly observed, suggesting that the relation between the IMF-Bz and
By orientations, and the locations of the nightside distorted TPAs is
consistent with that between the IMF conditions and regular TPA
previously reported. We also followed the time sequence of the nightside
distorted TPA evolution with the IMAGE FUV-WIC imager and additional
Polar UVI data. In most cases, the “J” and “L”-shaped TPAs started
to grow from the nightside main auroral oval, and protruded to the
dayside region with being distorted. In this presentation, we will
introduce several selected cases of the nightside distorted TPA among
our database, and discuss why and how the nightside end of the TPA
became distorted toward pre- or post-midnight sectors based on the
in-situ satellite nightside plasma sheet observations and the electron
drift velocity distribution calculated utilizing a simple
electromagnetic field model in magnetotail, together with the auroral
imager data.