A multi-instrument study of the December 1996 magnetic cloud

C. J. Farrugia, B. J. Vasquez, R. B. Torbert, L. Janoo, I. G. Richardson, F. T. Gratton, L. F. Burlaga, K. W. Ogilvie, R. J. Fitzenreiter, R. P. Lepping, V. A. Osherovich, M. Reiner, D. Berdichevsky, A. J. Lazarus, and J. T. Steinberg

Space Science Center, University of New Hampshire, Durham, NH, USA


We study interplanetary magnetic field, proton, electron, cosmic ray energy particles, and wave data acquired by the Wind and IMP8 spacecraft on December 23-26, 1996, a period which contained an ejecta with the characteristic signatures of an interplanetary magnetic cloud. Novel features of the observations include the following. (a) At the start of the interval where criteria identifying the magnetic cloud are satisfied, Wind encounters a magnetic hole of 2 hours' duration across which the total pressure is constant (i.e., a "pressure-balanced structure"). The field and flow vectors satisfy tangential momentum conservation appropriate for a rotational discontinuity. The totality of signatures (e. g., plasma heating), however, indicate the presence of a slow shock. By using various signatures to identify ejecta material, we conclude that the reconnection was between magnetic cloud field lines (i. e., internal to the ejecta). (b) There are impulsive temperature and density changes at field directional discontinuities embedded in regions of oscillatory magnetic field. We suggest that these are Alfvén waves whose properties have been modified by the varying background plasma. (c) Electron temperatures are generally much higher than proton temperatures throughout the 4-day period. Ion acoustic wave emissions are present but are sporadic. Throughout the 4-day interval, the electron temperature and density are anti-correlated. These findings raise a number of issues bearing on our current understanding of the small-scale structure of ejecta.

in The Solar Wind - Magnetosphere System 3, edited by J. K. Biernat, C. J. Farrugia, and D. F. Vogl, p. 99, Verlag der Österreichischen Akademie der Wissenschaften, Austria, 2000