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Lepping, R. P.

Publications and source records attributed to Lepping, R. P..

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Simultaneous conjugate observations of dynamic variations in high-latitude dayside convection due to changes in IMF By

Data from two conjugate HF radars currently operating at Goose Bay (Labrador) and the Halley Station (Antarctica), obtained for a single 45-min period about local noon on April 22, 1988, were used to study the near-instantaneous conjugate two-dimensional patterns of plasma convection in the vicinity of the cusp. In particular, the response of these plasma convection patterns to changes in the By component of the IMF was examined. Results indicate that, under quasi-stationary IMF conditions, the conjugate convection patterns are quite similar to the synthesized patterns of Heppner and Maynard (1987) and that the patterns respond rapidly to changes in the IMF By component. Results also show that transitions between convection states begin to occur within minutes of the time that an IMF state change is incident on the magnetospheric boundary, and that the convection reconfigurations expand poleward, completely filling the field of view of an HF radar within 6 min of the time of onset.

Greenwald, R. A.↗
NASA NTRS1990

IMP-8 observations of traveling compression regions - New evidence for near-earth plasmoids and neutral lines

An examination of IMP-8 tail lobe magnetic field measurements has been conducted to determine whether the traveling compression region (TCR) phenomena detected by ISEE-3 in the distant geotail, and believed to be caused by tailward moving plasmoids, are present closer to the earth. The study produced 16 examples of TCRs at distances of X = -31 to -37 R(E). For two events considered in detail, TCRs were observed in close association with substorm growth phase signatures in the lobes. The lengths of these TCRs are estimated to be 8-12 R(E). The conclusion is that the IMP-8 TCR observations provide new evidence that small plasmoids and, hence, multiple reconnection neutral lines can sometimes exist earthward of X = -35 R(E).

Slavin, J. A.↗
NASA NTRS1990

Upstream waves at Uranus

Since the Mach number of the solar wind increases with increasing heliocentric distance, the ratio of thermal to magnetic pressure, or beta, of the Uranian magnetosheath is expected to be much higher than in the terrestrial magnetosheath. Consistent with this expectation, the magnetosheat is observed to be extremely turbulent, and many particles may leak back upstream into the solar wind and/or be scattered from the bow shock. In accord with the expected presence of backstreaming particles, waves of the type associated with terrestrial backstreaming particles are seen outbound along the trajectory of Voyager in the preshock solar wind with frequencies close to 0.001 Hz. The wave frequency is close to that expected for upstream waves based on measurements closer to the sun. Upstream from the bow shock, the magnetic field was found to be much weaker than expected from observations in the inner solar system. The cause of this depression is unlikely to be the upstream particles; rather, the cause is probably intrinsic to the solar wind such as reconnection across the heliospheric current sheet.

Russell, C. T.↗
NASA NTRS1990

Magnetic field line draping in the plasma depletion layer

Simultaneous IMP 8 solar wind and ISEE 1/2 observations for a northern dawn ISEE 1/2 magnetopause crossing on November 6, 1977. During this crossing, ISEE 1/2 observed quasi-periodic pulses of magnetosheathlike plasma on northward magnetic field lines. The ISEE 1/2 observations were originally interpreted as evidence for strong diffusion of magnetosheath plasma across the magnetopause and the Kelvin-Helmholtz instability at the inner edge of the low-latitude boundary layer. An alternate explanation, in terms of magnetic field merging and flux transfer events, has also been advocated. In this paper, a third interpretation is proposed in terms of quasi-periodic magnetopause motion which causes the satellites to repeatedly exit the magnetosphere and observe draped northward magnetosheath magnetic field lines in the plasma depletion layer.

Sibeck, D. G.↗
NASA NTRS1990

Observations of rotational discontinuity - Slow expansion fan structure of the magnetotail boundary

This paper gives the first explicit test of the rotational discontinuity-slow expansion fan structure predicted by MHD models of the open tail boundary. The magnetic field and plasma data collected by IMP 8 during its magnetotail boundary crossings at geocentric distances near 25 Re and high latitudes are analyzed. Many crossings show the particle and field signatures of the paired MHD tail boundary structure. The openness of the tail is revealed by rotational discontinuities. They often show relatively large normal magnetic field components. Other crossings show properties that suggest a tangential discontinuity and reveal regions where, or times when, the tail is closed. Examples of each are given here.

Sanchez, E. R.↗
NASA NTRS1990

Global configuration of a magnetic cloud

A magnetic cloud associated with a 2N flare on January 1, 1978 was observed by IMP-8, Helios A, Helios B, and Voyager 2. The variation of the magnetic field observed at each spacecraft is represented to good approximation by Lundquist's solution for a cylindrically symmetric force-free magnetic field with constant alpha. A least-squares fit of Lundquist's solution to the data from each spacecraft gives the local orientation of the axis of the magnetic cloud. The times of the estimated boundaries of the magnetic cloud at each spacecraft, together with the speeds of the boundaries and the spacecraft position, give the positions of the boundaries at a given time. From these results the magnetic cloud is determined to resemble a flux rope whose minor radius is approximately 0.15 AU at 1 AU, and whose radius of curvature at 1 AU is approximately 1/3 AU.

Burlaga, L. F.↗
NASA NTRS1989

The Uranian magnetopause - Lessons from earth

Magnetic field measurements by Voyager 2 reveal differences between the Uranian magnetopause and the typical terrestrial magnetopause. In particular there are pulsations in the magnetic field at Uranus which may represent partial crossings of the magnetopause, the entry into magnetic flux ropes or the passage through mirror mode waves. Examination of terrestrial data, especially those obtained under conditions of very high plasma beta, suggests that the last possibility is the more likely. The magnetopause crossing itself is also different than those typically observed at 1 AU. It contains a magnetic field rotation of over 270 deg. While this is rare at 1 AU, an example has been found at earth where the field rotation is also greater than 270 deg. In some aspects these crossings resemble slow shocks. This also occurred at a time of high plasma beta. Thus it is concluded that plasma beta is very important in determining the structure of the magnetopause.

Russell, C. T.↗
NASA NTRS1989

Electrostatic waves in the bow shock at Uranus

Electrostatic emissions measured by the Voyager 2 plasma wave detector (PWS) during the inbound crossing of the Uranian bow shock are shown to differ in some aspects from the waves measured during bow shock crossings at Jupiter and Saturn. The wave amplitudes in the foot of the bow shock at Uranus are in general much lower than those detected at the other outer planets due to the unusually enhanced solar wind ion temperature during the crossing. This reduces the effectiveness of wave-particle interactions in heating the incoming electrons. Strong wave emissions are observed in the shock ramp that possibly arise from currents producing a Buneman mode instability. Plasma instrument (PLS) and magnetometer (MAG) measurements reveal a complicated shock structure reminiscent of computer simulations of high-Mach number shocks when the effects of anomalous resistivity are reduced, and are consistent with high ion temperatures restricting the growth of electrostatic waves.

Moses, S. L.↗
NASA NTRS1989

Propagation of magnetosheath hydromagnetic fluctuations into the magnetosphere

A study of the transmission of magnetosheath hydromagnetic fluctuations with f of about 23 mHz and f of about 13 mHz through the magnetopause is reported. The study involves measurements of the fluctuations inside the magnetosphere on the ISEE-2 spacecraft near the equator and on the ground near the boundary flux tube connecting with the spacecraft. The magnetic energy of the fluctuations inside the magnetopause is about 1 percent of those in the magnetosheath. This ratio is consistent with that expected for the transmission of fast-mode waves through a magnetopause with a tangential discontinuity.

Lanzerotti, L. J.↗
NASA NTRS1989

Multisatellite and ground-based observations of transient ULF waves

Transient ULF pulsations associated with variations in solar wind plasma density observed by the IMP 8 satellite are presently studied in light of observations obtained during a fortuitous alignment of the AMPTE and Viking satellites with respect to the EISCAT Magnetometer Cross. It is found that the isolated 10-min oscillation in solar wind plasma density produced magnetic field compression oscillations within the magnetosphere at the same frequency, thereby enhancing resonant oscillations at approximately twice the frequency which were already present. Support is seen for the periodic solar wind density variation's exciting of a tailward-traveling large-scale magnetosphere wave train which excites local field line resonant oscillations.

Potemra, T. A.↗
NASA NTRS1988

Observations of energetic ion enhancements and fast neutrals upstream and downstream of Uranus' bow shock by the Voyager 2 spacecraft

Measurements of energetic ions and electrons upstream and downstream from Uranus obtained during the low energy charged particle experiment on Voyager 2 are discussed. The results indicate that energetic ions were present upstream of Uranus' bow shock and that their charcteristics, in terms of anisotropies and energy spectra, are consistent with an Uranian magnetospheric origin and are inconsistent with predictions of in situ acceleration via the Fermi mechanism. An upper limit to the flux of energetic neutrals escaping the magnetosphere is established, and the volume-averaged neutral hydrogen density inside 5 Uranian radii is constrained.

Krimigis, S. M.↗
NASA NTRS1987

Surface waves on Uranus' magnetopause

Uranus' magnetosphere has a well-developed, thick magnetopause that was fully traversed twice by Voyager 2, once inbound to the planet and once outbound. This boundary appears to resemble earth's magnetopause in approximate shape and even to the extent of supporting surface waves which were observed on the inbound pass at a distance of 18.3 Uranus radii. There were apparently eight partial transitions from the magnetosheath into the current sheet of the magnetopause at this time, followed by a final complete transition to the magnetosphere. Six of the estimated normal vectors to the local boundary show clear evidence of oscillations in the slope with typical angular excursions, from one partial transition to the next, of about 90 deg. The vectors oscillated approximately in a plane that was severely tilted by about 49 deg with respect to Uranus' orbital plane.

Lepping, R. P.↗
NASA NTRS1987

The magnetotail of Uranus

The paper describes the characteristics of Uranus' magnetic tail as deduced from Voyager 2 magnetic field and plasma observations, both low energy in the plasma science investigation and high-energy in the low-energy charged particle investigation. The large-scale geometry of the tail is illustrated and the structure and characteristics of the plasma sheet are considered. Included is a discussion of the geometry of the current sheet embedded in the plasma sheet and an estimate of cross-tail current density in the neutral sheet region.

Behannon, K. W.↗
NASA NTRS1987

Response of the auroral oval precipitation and magnetospheric convection to changes in the interplanetary magnetic field

High-latitude observations on January 18, 1984 during the first GISMOS campaign are examined. An intense substorm occurred after a long period of quiescent geomagnetic activity when the IMF decreased gradually over a 2-hour period from +5 to -8 nT. This present study investigates how the auroral precipitation boundary, the energy flux, and the convection respond to this change from quiet to active conditions.

De La Beaujardiere, O.↗
NASA NTRS1987

Magnetic configuration of the distant plasma sheet - ISEE 3 observations

The influence of the IMF orientation and magnitude and substorm activity on the magnetic configuration of the central plasma sheet at 20-240 earth radii down the geomagnetic tail is investigated on the basis of ISEE-3 data. The results are presented graphically, and high-speed antisolar bulk flows threaded by southward magnetic fields are shown to be present in the distant plasma sheet after periods of substorm activity and southward IMF Bz. The effective dayside reconnection efficiency is estimated as 25 + or - 4 percent, in good agreement with theoretical models.

Slavin, J. A.↗
NASA NTRS1987

Magnetic field and particle pressure in the plasma sheet of Jupiter

The results of an analysis of the energetic particle and magnetic field data acquired by the Voyager 2 spacecraft at distances of about 40-70 Jupiter radii on the nightside of the planet are reported. As in a previous study of similar data at distances of greater than about 80 Jupiter radii, the energy densities of ions (primarily protons) is found to be sufficient to provide the diamagnetic depressions measured in the magnetic field intensity as the spacecraft made successive encounters with the nightside plasma sheet. There is some evidence that the percent contribution of the protons to the energy balance decreases with increasing distance from the planet over this radial interval, although this conclusion is dependent upon the assumption that the proton and heavier ion (oxygen) energy spectra are similar.

Lanzerotti, L. J.↗
NASA NTRS1986

The centrifugal flute instability and the generation of Saturnian kilometric radiation

It is shown that the source region for Saturnian kilometric radiation which originates in the high latitude near-noon dayside ionosphere can be mapped via the Saturn magnetic field to the outer edge of the dayside equatorial plasma sheet. It is shown how the MHD waves observed at the outer boundary of the plasma sheet may be causally related to the field-aligned acceleration of electrons to keV energies. It is suggested that the strong emission peak at a fixed Saturnian longitude is a consequence of locally reduced electron plasma frequency to electron gyrofrequency ratio.

Curtis, S. A.↗