MAGNETIC HOLES IN THE SOLAR-WIND AND THEIR RELATION TO MIRROR-MODE STRUCTURES

Data obtained by the Ulysses magnetometer and solar wind analyzer have been combined to study the properties of magnetic holes in the solar wind between 1 and 5.4 AU and to 23 degrees south latitude. Although t he plasma surrounding the holes was generally stable against the mirro r instability, there are indications that the holes may have been remn ants of mirror mode structures created upstream of the points of obser vation. Those indications include: (1) For the few holes for which pro ton 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 marginall y stable for the mirror mode, while the plasma environment of all the holes was significantly closer to mirror instability than was the aver age solar wind. (3) The plasma containing trains of closely spaced hol es was closer to mirror instability than was the plasma containing iso lated holes. (4) The near-hole plasma had much higher ion beta (ratio of thermal to magnetic pressure) than did the average solar wind.