Field and flow perturbations in the October 18-19, 1995, magnetic cloud
L. Janoo, C. J. Farrugia, R. B. Torbert, J. M. Quinn, A. Szabo, R. P. Lepping, K. W. Ogilvie, R. P. Lin, D. Larson, J. D. Scudder, V. A. Osherovich, and J. T. Steinberg
Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham
We examine magnetic field and plasma perturbations in the October18-19, 1995, magnetic cloud. Besides the front boundary, the 3-s-averaged magnetic field measurements made by the Magnetic Field Investigation on the Global Geospace Mission spacecraft Wind reveal a further 15 clear magnetic field directional discontinuities (DDs) with field rotations >15° , each lasting for ~1 min. A number of these DDs are clustered near the trailing edge of the cloud. Using 3-s resolution proton data from the Three-Dimensional Plasma and Energetic Particle Experiment on Wind, we find that these DDs are accompanied by perturbations in the flow. We find that except for the front boundary, which is a clear tangential discontinuity, all the others are rotational. Across the DDs the bulk flow speed I sometimes enhanced and sometimes depressed. Changes in proton temperature across the DDs suggest a more elaborate structure, for example, a reconnection layer. In a search for large-scale regularities, we apply minimum variance analysis to determine the normals to contiguous 1-hour-long stretches of the cloud data and find that there are large-scale structures ordering the field and the flow for the first 21 hours of cloud data. Thus we identify three coherent segments of several hours' duration each with a well-defined normal, but the normals to the individual segments are very different from each other. In particular, one segment in the cloud Bz < 0 phase is almost orthogonal to another segment in the Bz < 0 phase. The normals to the DDs within a given segment are closely aligned with the normal to that segment. For the last 6 ours of cloud data, no coherent structure was found since no reliable normals could be determined. Studying flow anisotropies of electrons in the energy range 0.1 - 100 keV and changes in intensity in the >1 keV electrons, Larson et al.  inferred several instances of disconnection of cloud field lines from the Sun, which were attributed to reconnection between adjacent bundles of cloud field lines. Our results are supportive of this interpretation. Furthermore, our results suggest the presence of detailed substructure in, and/or large distortions of, the magnetic cloud which reached 1 AU on October 18-19, 1995.
J. Geophys. Res., 103, 17,249-17,259, 1998