Large-scale magnetic field inversions at sector boundaries

During the declining phase of the last solar cycle the Wind spacecraft observed a quasi-recurrent pattern of mismatches between sector boundaries identified in suprathermal electron pitch angle spectrograms and in magnetic field data alone. Intervals of mismatch imply the presence of magnetic fields that are locally inverted or turned back on themselves in a way that is intrinsic to the sector boundary. We analyze eight cases of inversion during nine successive solar rotations in 1994-1995. These range in duration from 15 to 53 hours. In most the inversions are incomplete in a systematic way: Rather than pointing opposite to its true polarity along the Parker spiral, the magnetic field hovers at an orientation more nearly orthogonal to it, always in the sense of decreasing azimuth angle. The inversion pattern is consistent with passage through coronal streamer belt loops, in which the polarity of the two legs of each loop matches the sector structure and where one leg has been released from the Sun through interchange reconnection. There are four possible variations of this pattern, depending on the sense of polarity change across the sector boundary and on whether the leading or trailing leg has been released. The latter determines whether the sector boundary or the local field reversal passes first. Three of the four variations are represented in the eight cases. Plasma parameters in the inversions are typical of the slow wind. While some cases display signatures of interplanetary coronal mass ejections, many do not. Thus the inversions may represent the quiet, quasi-steady end of a spectrum of large-scale transient outflows.