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Winterhalter, D.

Publications and source records attributed to Winterhalter, D..

30 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

The heliospheric plasma sheet

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 (approximately equal 3 x 10 (exp 3) km to 10 (exp 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.

Winterhalter, D.↗
NASA NTRS1994

Solar activity and coronal mass ejections on the western hemisphere of the Sun in mid-August 1989: Association with interplanetary observations at the ICE and IMP 8 spacecraft

During the 10-day period from August 12 to 21, 1989, a sequence of coronal mass ejections (CMEs) was observed above the west limb of the Sun by the Solar Maximum Mission (SMM) coronagraph. Most of these CMEs apparently originated in the vicinity of one particularly active region during its passage from near central meridian to behind the west limb of the Sun. We present observations made at 1 AU during this period by the International Cometary Explorer (ICE) (formerly International Sun Earth Explorer-3 (ISEE 3)) and Interplanetary Monitoring Platform (IMP 8) spacecraft which were separated by approximately 75 deg in heliolongitude. Following CMEs on August 12 associated with solar events at approximately W40 deg, IMP 8 (in Earth orbit) detected a strong shock followed by signatures in magnetic field, solar wind plasma, and energetic ion data which suggest that CME-related material ('ejecta') forming the shock driver engulfed the spacecraft. This spacecraft only observed weak shocks, and no ejecta, from later CMEs originating further west of the spacecraft. In contrast ICE, off the west limb at approximately W75 deg, observed the shock from the W40 deg event but failed to encounter the shock driver, whereas clear ejecta signatures were observed following events further west, closer to the spacecraft heliolongitude. The disappearance of these signatures (which include bidirectional energetic ion flows, bidirectional solar wind heat fluxes, quiet, enhanced magnetic fields and anomalously cool plasma) at IMP 8 and their emergence at ICE as the solar source region moved westward supports the association of such signatures with ejecta related to CMEs. The dual-spacecraft observations are also consistent with the conclusion of Richardson and Cane (1993) that ejecta at 1 AU typically extend approximately 50 degs in longitude from the solar source. Some plausible associations between particular intervals of ejecta signatures at ICE and individual CMEs are made. However, these associations are complicated by the large number of CMEs present, by intermittent ICE data coverage, and by uncertainties in the ejecta propagation speeds to the spacecraft.

Richardson, I. G.↗
NASA NTRS1993

(abstract) Ulysses Observations of Magnetic Nulls in the Solar Wind

High time resolution magnetic field measurements (1 vector/s) at radial distances out to 5.3 AU and heliographic latitudes from 0(deg) to > 35(deg) S reveal the presence of solitary pulses lasting tens of seconds in which the field magnitude approaches or reaches zero. The properties of these nulls, their spatial distribution and relation to solar wind structures and to similar-apppearing interplanetary and magnetospheric impulses will be discussed.

magnetic impulses solar wind magnetic nulls Ulysse↗
NASA NTRS1992

A quantitative assessment of the role of the post-shock turbulent region in the formation of Forbush decreases

We present results of a survey of the relation between Forbush decreases, magnetic clouds, and interplanetary shocks during the period August 1978 to November 1982. We have used data from the ISEE-3 study of bidirectional ions associated with magnetic structures or clouds of Marsden et al. (1987), and ground-based observations of Forbush decreases from several neutron monitors. We use the two-step model of a Forbush decrease. We assume that the first step is due to the passage of the postshock turbulent region, and that the second is due to the passage of the magnetic cloud or structure which usually follows the postshock turbulent region. To determine the effectiveness of the postshock turbulent region in causing a Forbush decrease, we have evaluated the radial diffusion coefficient of the postshock turbulent region for the eight largest events during the above period using observations of the magnetic field. We have made a quantitative assessment of the relative importance of the postshock turbulent region in the formation of the Forbush decrease, concluding that the postshock turbulent region alone is not sufficient to cause a Forbush decrease.

Sanderson, T. R.↗
NASA NTRS1989

Observations of large scale spatial gradients in the heliospheric magnetic field

Magnetic field observations by the interplanetary probe Pioneer 11 are used to investigate large-scale spatial gradients in the heliospheric magnetic field. The distance of Pioneer 11 ranges from 1 AU to 24 AU radially, and from -5 deg to + 16 deg heliocentric latitude, providing a view of a small but significant fraction of the three-dimensional heliosphere. To remove the solar cycle variations, the data are normalized using measurements obtained at 1 AU at the corresponding times. To first order, the observations agree with the Parker model for spherically symmetric, radial solar wind flow. However, a second-order deficit in the magnitude and azimuthal component of the magnetic field has been confirmed. Specific issues are addressed which have arisen recently, including an apparent absence of the deficit in the Voyager measurements, the possible influence on the deficit of time and/or latitude variations in the solar wind speed, and the possible effect of latitude asymmetries in the magnetic field strength. This analysis supports the earlier conclusions that the deficit is correlated with radial distance and involves a divergence of magnetic flux away from the equatorial region.

Winterhalter, D.↗
NASA NTRS1988

Interplanetary magnetic field draping about fast coronal mass ejecta in the outer heliosphere

The possibility that the IMF becomes draped around coronal mass ejections (CMEs) propagating rapidly through the quiescent solar wind into the outer heliosphere is investigated theoretically. The results are presented in diagrams and graphs and discussed in detail. It is found that large sunward-directed structures analogous to the Venus and cometary magnetotails should form when the CME velocity exceeds the solar-wind velocity by more than the local Alfven speed; such structures could hang up swept-up IMF flux for as long as several days. Pioneer 11 magnetic-field measurements at 6.9-9.4 AU from three 20-d periods in 1978 are examined and shown to contain some features consistent with CME draping.

Mccomas, D. J.↗
NASA NTRS1983

Magnetic field fluctuations in the Venus magnetosheath

Using a model for the convection pattern of the shocked solar wind flow around the Venus obstacle, Pioneer Venus observations of ultra-low-frequency (about 10-40 s period) magnetic field fluctuations in the magnetosheath have been traced along streamlines to the regions of the quasi-parallel bow shock. The periods and polarizations of the sinusoidal fluctuations are similar to those observed upstream of the quasi-parallel bow shock, where streaming superthermal particles are believed to produce MHD waves by a beam-plasma instability. The results suggest that both disturbances at the ionopause at Venus and the earth's magnetopause may be caused by convection of turbulent magnetic fields from the subsolar bow shock when the interplanetary field direction produces a quasi-parallel shock there.

Luhmann, J. G.↗