Sector boundary transformation by an open magnetic cloud

Abstract

A magnetic cloud observed by the Wind spacecraft on February 8, 1995, was remarkable for its impact on the interplanetary sector structure. The magnetic field data imply that the cloud occurred in the middle of a sector and that the arrival of the following sector boundary on February 10 coincided with the arrival time predicted from the corresponding source surface map. The electron heat flux data, however, give incontrovertible evidence that instead the cloud brought the sector boundary, well ahead of the predicted arrival time. The electron heat flux data show little counterstreaming within the cloud, indicating predominantly open helical field lines. Under the assumption that the cloud originally had the form of a closed flux rope loop with legs rooted to the Sun, observational constraints dictate that the sector boundary was displaced not because it was pushed aside by the cloud but because reconnection in the leading leg opened field lines there, creating a topological change spanning 45° of heliographic longitude. The solar source of the cloud was deduced from an associated eruptive arcade event extending northeastward from an active region in Yohkoh soft X ray data on February 4. On February 8, the same active region was the source of impulsive energetic electron events observed at Wind during a brief counterstreaming interval, consistent with magnetic connection in the leading leg at that time. The cloud's helicity matches that predicted from the skew of the arcade fields in the February 4 X ray event, but the predicted alignment of the arcade and cloud axes was off by 35°. We use an MHD model with boundary conditions derived from solar magnetograms to illustrate the tilted arcade configuration in the corona that gave rise to the magnetic cloud and the lesser tilt of the heliospheric current sheet stemming from it.