Ulysses field and plasma observations of magnetic holes in the solar wind and their relation to mirror‐mode structures

Abstract

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 ø 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 ø . The holes occurred preferentially, but not without exception, in the interaction regions on the leading edg es 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 stable for the mirror mode, while the plasma environment of all the holes was significantly closer to mirror instability than was the average solar wind. (3) The plasma containing trains of closely spaced holes was closer to mirror instability than was the plasma containing isolated holes. (4) The near-hole plasma had much higher ion [5 (ratio of thermal to magnetic pressure) than did the average solar wind. (5) Near the holes, T.•/T• • tended to be either >1 or larger than in the average wind. (6) The proton and alpha-particle distribution functions measured inside the holes occasionally exhibited the flattened phase-space-density contours in space found in some numerical simulations of the mirror instability. 130 s, with a median of 50 s, corresponding to thickness in the solar radial direction of-200 proton gyro radii. Nine of the holes showed large angle changes with evidence for sub-Alfv6nic instreaming and field reconnection. Eight of the 28 Paper number 94JA01977. 0148-0227/94/94JA-01977 $05.00 holes, however, exhibited little or no directional change; such structures were named linear holes. The linear holes were observed in regions of high plasma I• = nkT/(B2/8•), and all but one of them occun'ed on or near the leading edges of high-speed streams in the solar wind. Turner et al. suggested that the linear holes, which could not have been caused by reconnection, resulted from the diamagnetic response of the field to local plasma inhomogeneities, but the cause of the inhomogeneities remained an open question. In a follow-on study, Fitzenreiter and Burlaga [1978] analyzed magnetic holes observed by both the IMP 5 and IMP 6 spacecraft. Combination of the data from the two spacecraft demonstrated consisten...

Ion distributions in large magnetic holes in the fast solar wind

“…To obtain additional examples, we carded out an automated search for magnetic holes. The search algorithm was the same as that described by Winterhalter et al [1994] with the exception that the sliding analysis window was increased from 5 min to 1 hr in order to find longer duration magnetic holes. The search covered the year 1994, when Ulysses was continuously in the high speed wind from the southern polar coronal hole as the spacecraft moved from a solar distance of 3.82 AU and a latitude of 48.4øS, to the southerly most point at 2.29 AU and 80.2øS, to 1.56 AU and 44.6øS.…”

Mirror‐mode storms inside stream interaction regions and in the ambient solar wind: A kinetic study

“…[8] For the solar wind, magnetic holes have also been observed at various heliocentric distances either as isolated holes or trains of closely-spaced magnetic holes [Winterhalter et al, 1994;Zhang et al, 2008;Russell et al, 2008]. Recently, using STEREO high-resolution magnetic field data, Russell et al [2009] reported several mirror-mode events of small amplitude trains, which included not only holes, but also peaks that persisted for hours and were referred to as "mirror-mode storms".…”

“…Nevertheless, in situ observations of mirror-mode waves in thermally anisotropic space plasmas are very frequent [Winterhalter et al, 1994;Liu et al, 2006;Génot et al, 2009]. Space plasma environments where nonuniform heating and compression take place, such as the downstream regions of shocks, are excellent sources for temperature anisotropies in the plasma.…”

Magnetic field depressions in the solar wind

“…It has been shown that magnetic field depressions which are bound by directional discontinuities are associated with the excitation of Langmuir waves [Linet al, 1996]. Winterhalter et al [ 1994] studied the occurrence rate and properties of these events in measurements taken by the Ulysses circumsolar spacecraft. They restricted their analysis to length scales below 60 s by using a 300 s sliding window and identified about 2000 events per year with a median duration of 22 s. They further restricted their analysis to holes with a magnetic field rotation of less than 5 ø and a quiet background.…”