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...

High-resolution magnetic field and plasma data gathered by ISEE 3/ICE during several sector boundary crossings are used to investigate the narrow heliospheric current sheet (• 3 x 10 3 km to 10 4 km thick), together with the heliospheric plasma sheet in which it is embedded. The heliospheric plasma sheet region is identified by a significantly enhanced plasma beta caused by density enhancements and diminished magnetic field strength and is about 20 to 30 times the thickness of the current sheet. The thickness of the heliospheric plasma sheet is found to increase exponentially with its average proton density. The heliospheric current sheet is often displaced to one edge or the other of the heliospheric plasma sheet. Further, the point of maximum plasma beta in the plasma sheet, where the magnetic field strength is at a broad local minimum, is not colocated with the heliospheric current sheet. Within the plasma sheet, changes in the magnetic pressure are balanced by corresponding changes in the plasma thermal pressure as expected for a convected solar wind feature. In addition, observations show small pressure differences between the regions upstream and downstream of the plasma sheet, which are interpreted as causing the plasma sheet to move across the spacecraft. 6667 6668 WINTERHALTER ET AL.' HELIOSPHERIC PLASMA SHEET ET AL.' HELIOSPHERIC PLASMA SHEET 6O .• 30 z 20 ET AL.' HELIOSPHERIC PLASMA SHEET 6675 25.0-20.0-15.0-10.0ß 5.o; P'I [] P• = 1.02P 2 -0.38; R = 0.99; Pa = 1.07P2 -0.07; R = 0.99; 0.0

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The magnetometer and electron reflectometer investigation (MAG/ER) on the Mars Global Surveyor spacecraft has obtained magnetic field and plasma observations throughout the near-Mars environment, from beyond the influence of Mars to just above the surface (at an altitude of ∼100 kilometers). The solar wind interaction with Mars is in many ways similar to that at Venus and at an active comet, that is, primarily an ionospheric-atmospheric interaction. No significant planetary magnetic field of global scale has been detected to date (<2 × 10 21 Gauss–cubic centimeter), but here the discovery of multiple magnetic anomalies of small spatial scale in the crust of Mars is reported.

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[1] Solar wind protons detected within Magnetic Holes (MHs) and Magnetic Decreases (MDs) are found to be preferentially heated perpendicular toB 0 . The MHs/MDs are associated with the phase-steepened edges of nonlinear Alfvén waves. The proton anisotropies can lead to the proton cyclotron and mirror mode plasma instabilities. We examine the Ponderomotive Force (PF), a phenomenon due to wave pressure gradients, and show that for this plasma regime and for phase-steepened Alfvén waves, the PF proton acceleration/energization will primarily be orthogonal to B 0 . It is suggested that accelerated ions create the MHs/MDs by a diamagnetic effect.

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