A uniform-twist magnetic flux rope in the solar wind

C. J. Farrugia, L. A. Janoo, R. B. Torbert, J. M. Quinn, K. W. Ogilvie, R. P. Lepping, R. J. Fitzenreiter, J. T. Steinberg, A. J. Lazarus, R. P. Lin, D. Larson, S. Dasso, F. T. Gratton, Y. Lin, and D. Berdichevsky

Los Alamos National Laboratory, Los Alamos, NM 87545


We describe magnetic field, proton, electron, and a -particle observations made by WIND on 24-25 October, 1995 of a structure consisting of a magnetic flux rope containing a plasma of low beta. While the flux rope structure was inferred from the magnetic field data, the particle behavior corroborates the inference. Minimum variance analysis of the magnetic field data indicates an axis highly inclined to the ecliptic plane and pointing away from the sun-Earth line. The diameter of the flux rope is estimated as 0.07 AU. Despite a pronounced overpressure, the structure is not expanding but is rather being convected passively with the ambient flow. An intense antisunward field-aligned flow of heat flux electrons indicates that the flux rope is connected at one end to the Sun. The field variation is suggestive of a magnetic flux rope of constant field line twist, and a least-squares fit of this model to the data confirms this to good approximation. The field line twist per unit length is estimated as ~37 rad AU-1, so that assuming an orientation along the Parker spiral, a given field line has wound ~10 times around the axis from the Sun to 1 AU. In a region inside the tube, the protons, electrons, and alpha particles are hot, the a -to-proton temperature and density ratios are higher than the surroundings, and the a -particles are slower than the protons. Electron and proton anisotropies are negative there (T^ <T| | ). The flux rope does not appear to be related to coronal mass ejections.

American Institute of Physics, 1998