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Thomsen, M. F.

Publications and source records attributed to Thomsen, M. F..

At least 55 records · Page 3
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NASA NTRS1990

Hot flow anomaly formation by magnetic deflection

Hot flow anomalies (HFAs) are localized plasma structures observed in the solar wind and magnetosheath near the earth's quasi-parallel bow shock. This paper presents one-dimensional hybrid computer simulations illustrating a formation mechanism for HFAs in which the single hot ion population results from a spatial separation of two counterstreaming ion beams. The higher-density cooler regions are dominated by the background (solar wind) ions, and the lower-density hotter internal regions are dominated by the beam ions. The spatial separation of the beam and background is caused by the deflection of the ions in large-amplitude magnetic fields which are generated by ion/ion streaming instabilities.

Onsager, T. G.↗
NASA NTRS1990

Observational test of a hot flow anomaly formation mechanism

The hot-flow anomalies (HFAs) observed in the vicinity of the earth's bow shock, whose high-temperature plasma is strongly deflected relative to the solar wind and flanked by density and magnetic field enhancements, are presently addressed by a model in which the coupling of ions reflected off the shock with the solar wind would convert the relative streaming energy between the reflected and solar wind ions into thermal energy; the hot plasma would then expand to form the HFA. Attention is given to a simple observational test of this model, which compares the measured temperature and density with the values expected immediately after the assumed coupling and after expansion.

Onsager, T. G.↗
NASA NTRS1990

Plasma flow reversals at the dayside magnetopause and the origin of asymmetric polar cap convection

Events observed in a fast plasma experiment, where the y-component of the plasma flow within the low latitude boundary layer and magnetopause current layer was oppositely directed to that in the adjacent magnetosheath, are examined. The observations are shown to be qualitatively and quantitatively consistent with previous observations of accelerated flows at the magnetopause and with models of magnetic reconnection, with reconnection occurring at low latitudes near the GSE XY plane, independently of the magnitude or the sign of the y-component ot the local magnetosheath magnetic field. Local magnetic shears at the magnetopause for these events (in 60-180 deg range) and the fact that these events occur at low latitudes do not support the antiparallel merging hypothesis. The observations of B(y)-dependent flow reversals demonstrate how the asymmetric polar cap convection and related phenomena, such as the Svalgaard-Mansurov effect, originate in magnetic reconnection at the dayside magnetopause.

Gosling, J. T.↗
NASA NTRS1990

Two-state ion heating at quasi-parallel shocks

This paper documents the alternating occurrence of two different ion states of the ion distributions downstream from several quasi-parallel shocks: a cooler (and denser) core/shoulder type and a hotter (less dense) and more Maxwellian type. Three separate lines of evidence are presented to show that the two states are not related in an evolutionary sense, but that both are produced alternately at the shock: (1) the asymptotic downstream plasma parameters (density, ion temperautre, and flow speed) are intermediate between those characterizing the two different states closer to the shock, suggesting that the asymptotic state is produced by a mixing of the two initial states; (2) examples of apparently interpenetrating distributions are found during transitions from one state to the other; and (3) examples of both types of distributions are found at actual crossings of the shock ramp.

Thomsen, M. F.↗
NASA NTRS1990

Specularly reflected He(2+) at high Mach number quasi-parallel shocks

The first observations of near-specularly reflected He(2+) upstream from high Mach number quasi-parallel shocks are presented. The He(2+) observations are compared with simultaneous suprathermal proton observations to determine the density ratio of specularly reflected He(2+) to protons. The presence of specularly reflected He(2+) at high Mach number quasi-parallel shocks suggests that it may be a seed population for diffuse He(2+) seen in the same region.

Fuselier, S. A.↗
NASA NTRS1990

Survey of coherent ion reflection at the quasi-parallel bow shock

This paper presents the results of a survey of ion beam observations near the earth's quasi-parallel bow shock. Particular attention is given to three issues: (1) the spatial locations of the cold coherent ion beams relative to the shock, (2) the velocity space locations of the ion beams relative to that expected for specular reflection, and (3) the parameter regimes over which the beams are observed. Evidence is found that coherent ion reflection occurs commonly in the quasi-parallel regime, over essentially the full range of upstream plasma parameters normally found at the earth's bow shock. It was also observed that the cold coherent reflected beams spread rapidly in the velocity space upstream from the shock. The observations presented serve as an extension to those presented by Gosling et al. (1989) and further strengthen the conclusion that specularly reflected ions are contributing directly to the downstream thermalization at the earth's quasi-parallel bow shock.

Onsager, T. G.↗
NASA NTRS1990

Noncoplanar component of the magnetic field at low Mach number shocks

The component of the magnetic field that deviates from the plane defined by the shock normal and the upstream magnetic field is examined for low Mach number bow shocks. The integrated value of this noncoplanar component is compared to the predictions of Jones and Ellison (1987). A test of this relationship was first reported by Gosling et al. (1988) who found good agreement only at the two low Mach number shocks that were included in their study. Analysis of a more extensive collection of low Mach number shocks confirms the Jones and Ellison relationship at very low Mach numbers as well as its deterioration for higher Mach numbers. However, there also is an indication that the relationship may break down for shocks that are nearly perpendicular.

Friedman, M. A.↗
NASA NTRS1990

Magnetic pulsations at the quasi-parallel shock

The plasma and field properties of large-amplitude magnetic field pulsatins upstream from the quasi-parallel region of the earth's bow shock are examined in high time resolution using data from ISEE 1 and 2. The relative timing of the magnetic field profiles observed at the two spacecraft shows that some of the pulsations are convecting antisunward across the spacecraft while others are brief out/in motions of bow shock across the spacecraft. Pulsations with both timing signatures are the site of slowing and heating of the solar wind plasma. The ions tend to be only weakly heated in the convecting pulsations, while within the out/in pulsations the ion heating can be quite substantial but variable. This variation occurs not only from pulsation to pulsation but also from point to point within a given pulsation. In general, the hottest distributions within the out/in pulsations tend to occur in regions of lower density and field strength. Magnetic pulsations bear a number of similarities to previously identified hot diamagnetic cavity events as well as to more durable crossings of the quasi-parallel shock itself. These various phenomena may be different manifestations of the same basic physical processes, in particular the coupling of coherently reflected ions to the solar wind beam.

Thomsen, M. F.↗
NASA NTRS1989

Suprathermal electrons at earth's bow shock

Results are presented on electron measurements near the earth's bow shock, carried out with the fast-plasma experiments on ISEE 1 and 2, with emphasis placed on the suprathermal population of electrons present near the earth's bow shock. The pattern found for the suprathermal electrons at the bow shock suggests that a portion of the solar wind electron population is commonly accelerated to relatively high energy as the solar wind convects across quasi-perpendicular portions of the bow shock. The results are interpreted in terms of the global magnetic field geometry of the bow shock environment.

Gosling, J. T.↗
NASA NTRS1989

Ion reflection and downstream thermalization at the quasi-parallel bow shock

Using the results of ISEE 2 plasma and magnetic field measurements, two features related to the ion thermalization process at high-Mach-number (M above 2) quasi-parallel collisionless shocks are discussed. These are the presence of a coherent secondary beam of ions within the shock layer which is considered to be produced by reflection, and downstream ion distributions which contain both a relatively cold core of directly transmitted ions and a hotter 'shell' of ions, which appear to result from the disruption and scattering of ions initially reflected at the shock. Evidence is presented that coherent ion reflection is an important element of the ion energy dissipation process at high-Mach-number quasi-parallel shocks.

Gosling, J. T.↗
NASA NTRS1989

On the source of diffuse, suprathermal ions observed in the vicinity of the earth's bow shock

ISEE 2 data from the Fast Plasma Experiment were used to search for upstream 'diffuse' suprathermal ion events observed at times when the local IMF was nearly radial and the spacecraft was positioned at local times greater than 1530. For these observing conditions, which correspond to a local theta-Bn of about 45 deg, the field lines sampled should be at least 4 earth radii from the earth's magnetopause. A number of diffuse ion events which satisfy these conditions were found, and two of them are described in detail. The results show that the events are observed in association with large-amplitude low-frequency magnetic field fluctuations; the suprathermal particle velocity distributions are roughly isotropic in the solar wind frame, and the spectra extend smoohtly to energies beyond 40 keV. It is suggested that diffuse ion events observed in the vicinity of the bow shock at energies below 40 keV are primarily caused by shock processes.

Gosling, J. T.↗
NASA NTRS1988

On the origin of hot diamagnetic cavities near the earth's bow shock

The origin of hot diamagnetic cavities (HDCs) observed occasionally upstream from the earth's bow shock is investigated by examining the results of November 16, 1977, observation, when four of these events occurred on a single day, as well as plasma and field data from that day. The results suggest that HDCs may form as a result of an unusually strong interaction between shock-reflected ions and the incoming solar wind. It is proposed that this interaction stems from a temporary and localized reflection of a larger-than-normal fraction of the incident ions, which is stimulated by sudden changes in the upstream field orientation; the consequences of such a temporary overreflection are found to be consistent with many of the observed features of HDCs, including the strong slowing, deflection, and heating of the flow, as well as the localization, internal recoveries, and occasional formation upstream from the shock itself.

Thomsen, M. F.↗
NASA NTRS1988

Noncoplanar magnetic fields at collisionless shocks - A test of a new approach

The expressions derived by Jones and Ellison (1987) (JE) for the spatial integral of the noncoplanar field component in a fast-mode collisionless shock and for the magnitudes of the magnetic-field rotation and potential drop difference are evaluated on the basis of ISEE 1 and 2 observations in the earth bow shock and numerical simulations using the hybrid code of Leroy et al. (1981 and 1982). The data and simulation results are presented graphically and shown to be consistent with the JE hypothesis, that the rotation and drop difference are due to unequal electron and ion masses. The rotations and potential drop differences predicted by the JE equations, however, are found to be accurate only for subcritical shocks at low Mach numbers; the underestimation of the values for hypercritical shocks is attributed to neglect of the ion current from reflected gyrating ions.

Gosling, J. T.↗
NASA NTRS1988

Interplanetary magnetic field orientations associated with bidirectional electron heat fluxes detected at ISEE 3

A statistical survey of interplanetary magnetic field orientations associated with bidirectional electron heat fluxes observed at ISEE 3 in orbit about the Sunward Lagrange point indicates that magnetic connection of the spacecraft to the earth's shock was frequently the source of the bidirectionality. When the interplanetary magnetic field was oriented within 5 deg of the earth-spacecraft line, backstreaming electrons from the bow shock were clearly observed approximately 18 percent of the time, and connections apparently occurred for angles as large as about 30-35 deg.

Stansberry, J. A.↗
NASA NTRS1988

Multi-spacecraft observations of collisionless shocks

The ability to make simultaneous, multipoint measurements of field and plasma properties has substantially increased knowledge of the structure and physics of collisionless shocks in space. In addition to allowing the determination of the large-scale macroscopic structure of such shocks as well as reliable estimates of shock normal direction, multipoint measurements have made it possible to measure shock thicknesses and speeds under a variety of upstream conditions. This ability has enabled a number of quantitative analyses of shock structure, including the confirmation of the dispersive whistler-mode character at low Mach numbers, the assessment of the relative importance of magnetic and electrostatic forces within the shock, and a detailed examination of possible dissipation mechanisms.

Thomsen, M. F.↗
NASA NTRS1988

ISEE studies of the quasi-parallel bow shock

ISEE-1 and-2 plasma and magnetic field observations are used to examine several encounters with brief magnetic pulsations in the quasi-parallel region of the earth's bow shock. The two-spacecraft timing of the magnetic field signature is examined to see if the pulsation encounters are nested (as for back and forth motion of the shock over the spacecraft) or simply time-shifted (as for a structure convected anti-sunward across the spacecraft). Examples of both types of signatures are found, some within minutes of each other, suggesting that at least some pulsations probably originate from the growth and steepening of upstream waves but may eventually be able to stand in the upstream flow, perhaps becoming part of the shock surface itself. The possibility that some of the pulsations may simply be brief encounters with the moving shock is also not ruled out.

Thomsen, M. F.↗
NASA NTRS1988

An ISEE-1/2 spacecraft study of an unusual flux transfer event

Using the ISEE-1/2 spacecraft, an unusual FTE was studied. The perturbation in the direction normal to the magnetopause reached a maximum size of 64 nT, exceeding the field amplitude in the adjacent magnetosphere. The observation was made at 1500 Local Time in the Southern Hemisphere (10 deg below the magnetic equator). From two point observations, it was found that the structure has a scale length of 2R(E) in the north-south direction in boundary normal coordinates. The velocity of the structure in that direction was about 50 km/sec. The dimension and the velocity in the east-west direction could not be established. The FTE signature recurred after about 9 minutes.

Zhu, X. M.↗