SSC, University of New Hampshire, Durham

We study features of the January 1997 magnetic cloud and their effect on the optical aurora at 0630-0930 magnetic local time (MLT). WIND data suggest a thin plasma depletion layer (PDL) preceding the cloud, at whose outer edge the magnetic field rotates southwards, the proton temperature has a local maximum, and the dynamic pressure drops by a factor of 2. At the cloud's front boundary there is a further dynamic pressure drop, another localized temperature rise, and a ~30° field shear. Prior to arrival of the southward rotation at Earth, the ~08 MLT aurora is dominated by forms with intense 557.7 nm emission, presumably of boundary plasma sheet origin, located south of zenith (~75° MLAT) and moving eastward. Minutes after the southward rotation reaches Earth, this emission is replaced by an auroral form which encroaches into the field-of-view at ~73° MLAT, expands southward to ~70° MLAT and westward, approaching ~0800 MLT at 0600 UT. Its latitudinal positions satisfy a known relation between cusp latitude and interplanetary B_z, and its morphological and spectral properties are similar to those previously associated with and energy-latitude dispersion signature attributed to plasma transfer at an open low latitude boundary layer. We argue that this auroral precipitation at such far off-noon MLTs may be due to enhanced reconnection occasioned by the strongly southward field in the PLD/cloud. A POLAR pass across the open/closed field line boundary at 66° invariant latitude is consistent with these inferences.

Geophys. Res. Lett., Vol. 25, No. 15, 3051-3054, 1998