ULYSSES FIELD AND PLASMA OBSERVATIONS OF MAGNETIC HOLES IN THE SOLAR-WIND AND THEIR RELATION TO MIRROR-MODE STRUCTURES

The term ''magnetic hole'' has been used to denote isolated intervals when the magnitude of the interplanetary magnetic field drops to a few tenths, or less, of its ambient value for a time that corresponds to a linear dimension of tens to a few hundreds of proton gyro-radii. Data obtained by the Ulysses magnetometer and solar wind analyzer have been combined to study the properties of such magnetic holes in the solar wind between 1 AU and 5.4 AU and to 23 degrees south latitude. In order to avoid confusion with decreases in field strength at interplanetary discontinuities, the study has focused on linear holes across which the field direction changed by less than 5 degrees. The holes occurred preferentially, but not without exception, in the interaction regions on the leading edges of highspeed solar wind streams. Although the plasma surrounding the holes was generally stable against the mirror instability, there are indications that the holes may have been remnants of mirror-mode structures created upstream of the points of observation. Those indications include the following: (1) For the few holes for which proton or alpha-particle pressure could be measured inside the hole, the ion thermal pressure was always greater than in the plasma adjacent to the holes. (2) The plasma surrounding many of the holes was marginally table for the mirror mode, while the plasma envitonmen?t of all the holes was significantly closer to mirror instability than was the average salar wind. (3) The plasma containing trains of closely spaced holes was closer to mirror instability tl;an was the plasma containing isolated holes. (4) The near-hole plasma had much higher ion beta (ratio of thermal to magnetic pressure) than did the average solar wind. (5) Near the holes, T-perpendicular to/T-II tended tb be either >1 or larger than in the average wind. (6) The proton and alpha-particle distribution function's measured inside the holes occasionally exhibited the flattened phase-space-density contours in nu(perpendicular to)-nu(II) space found in some numerical simulations of the mirror instability.