Unusual features of the January 1997 magnetic cloud and their effect on optical dayside auroral signatures

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\u27s 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 BZ, and its morphological and spectral properties are similar to those previously associated with an 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 PDL/cloud. A POLAR pass across the open/closed field line boundary at 66° invariant latitude is consistent with these inferences