D. Winterhalter et al., ULYSSES FIELD AND PLASMA OBSERVATIONS OF MAGNETIC HOLES IN THE SOLAR-WIND AND THEIR RELATION TO MIRROR-MODE STRUCTURES, J GEO R-S P, 99(A12), 1994, pp. 23371-23381
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. Dat
a obtained by the Ulysses magnetometer and solar wind analyzer have be
en combined to study the properties of such magnetic holes in the sola
r wind between 1 AU and 5.4 AU and to 23 degrees south latitude. In or
der to avoid confusion with decreases in field strength at interplanet
ary discontinuities, the study has focused on linear holes across whic
h the field direction changed by less than 5 degrees. The holes occurr
ed preferentially, but not without exception, in the interaction regio
ns on the leading edges of highspeed solar wind streams. Although the
plasma surrounding the holes was generally stable against the mirror i
nstability, there are indications that the holes may have been remnant
s of mirror-mode structures created upstream of the points of observat
ion. Those indications include the following: (1) For the few holes fo
r 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 wa
s marginally table for the mirror mode, while the plasma envitonmen?t
of all the holes was significantly closer to mirror instability than w
as the average salar wind. (3) The plasma containing trains of closely
spaced holes was closer to mirror instability tl;an was the plasma co
ntaining isolated holes. (4) The near-hole plasma had much higher ion
beta (ratio of thermal to magnetic pressure) than did the average sola
r wind. (5) Near the holes, T-perpendicular to/T-II tended tb be eithe
r >1 or larger than in the average wind. (6) The proton and alpha-part
icle distribution function's measured inside the holes occasionally ex
hibited the flattened phase-space-density contours in nu(perpendicular
to)-nu(II) space found in some numerical simulations of the mirror in
stability.