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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Planetary bow shocks

Planetary bow shocks provide insight into both the behavior of collisionless shocks and the nature of the planetary obstacle responsible for creating those bow shocks. This review paper first presents a survey of the microstructure of planetary bow shocks using data obtained at Mercury, Venus, the earth, Mars, Jupiter, and Saturn. When it examines the correspondence of the behavior of the shock jumps with that predicted by theory, and the differences between gas dynamic and magnetohydrodynamic solutions. Finally, it examines the information available from observations of the location of the bow shock about the nature of the solar wind-planetary interaction.

Russell, C. T.↗
NASA NTRS1987

Heliospheric shocks (excluding planetary bow shocks)

A summary of advances made during 1983-1985 in the theory of collisionless heliospheric shocks is presented (with reference to the latest observations of slow shocks in the geomagnetic tail) together with a summary of recently developed simulations of shock structure and shock propagation. Consideration is also given to the theoretical basis for the production of energetic particles by shocks in the solar wind and the corona. Finally, satellite-based observations related to interplanetary and coronal shocks and to heliospheric boundary shock are discussed.

Mihalov, J. D.↗
NASA NTRS1992

Voyager energetic particle observations at interplanetary shocks and upstream of planetary bow shocks - 1977-1990

The Voyager 1 and 2 vehicles include instrumentation that makes comprehensive electron and ion measurements in several energy channels with good energy, temporal, and compositional resolution. Data gathered from 1977 to 1988, including observations downstream and upstream of four planetary bow shocks (earth, Saturn, Uranus, Jupiter) and numerous interplanetary shocks to about 30 AU, are analyzed in the context of the Fermi and shock drift acceleration models. Overall results indicate that electrons and ions observed upstream of planetary bow shocks have their source inside the parent magnetosphere, with first order Fermi acceleration playing a secondary role at best.

Krimigis, S. M.↗
NASA NTRS1988

Subcritical dispersive shock waves upstream of planetary bow shocks and at Comet Giacobini-Zinner

The nonlinear evolution of ULF (magnetosonic) waves is studied using electromagnetic simulations. The waves were observed upstream of the planetary bow shocks at Comet Giacobini-Zinner. It is shown that as ULF waves generated by the resonant ion beam instability steepen, their polarization becomes linear. A high-frequency wave packet is generated by the steepening process. The steepened wave has a structure and a behavior similar to a subcritical dispersive shock. It is suggested that Comet Giacobini-Zinner did not have a single bow shock. It is concluded that the comet had a transition region consisting of a series of shocks which were convected by the solar wind.

Omidi, N.↗
NASA NTRS1992

Shock drift acceleration of energetic protons at a planetary bow shock

Results are presented of numerical orbit integrations of the interaction of suprathermal charged particles (protons) with a planetary bow shock, performed in order to investigate the effect of the changing geometry of the shock, due to its curvature, on the kinematics of the particle/shock interaction. Results obtained for both the 2D and 3D bow-shock geometries show that the introduction of shock curvature to the shock in the simulations leads to an increase of the reflection probability of incident suprathermal protons. It was also found that the curvature of the planetary bow shock allows protons to enter at a given value of theta(Bn) (where theta(Bn) is the angle between the local shock normal and the incident magnetic field) and to exit at a different theta(Bn) value. The result has consequences in the characteristics of the energetic charged particle environment in the planetary foreshock.

Giacalone, Joe↗
NASA NTRS1982

Overshoots in planetary bow shocks

The parametric variation with solar wind conditions in the overshoot in magnetic field strength observed in planetary bow shocks and believed to be associated with the ion reflection process is examined based on both terrestrial and planetary bow shock data. The combined data from Venus, earth, Jupiter and Saturn cover a wider range of solar wind densities, magnetic field strength, electron and proton temperatures, Mach numbers, beta, spiral angle, and scale lengths than observable from earth alone. The bow shock structure, particularly the magnitude of the post-shock field strength overshoot, is found to depend principally on plasma beta and the magnetosonic Mach number with a continuously increasing overshoot strength associated with increasing beta and Mach number.

Russell, C. T.↗
NASA NTRS1986

Energetic ions upstream of planetary bow shocks - Fermi acceleration or leakage?

Observations of energetic ions upstream of earth, Jupiter, and Saturn are examined. The velocity dispersions, energy spectra, and ion compositions for the three planets are described. Fermi acceleration and leakage are analyzed as the potential mechanism for the presence of energetic particles upstream of planetary bow shocks. It is noted that energetic ions upstream of planetary bow shock originate from within the planetary magnetosphere, and leakage is the mechanism for the energetic particles.

Krimigis, S. M.↗
NASA NTRS1986

Energetic ions upstream of planetary bow shocks: Fermi acceleration or leakage

Spacecraft observations of Jupiter, Saturn, and Earth planetary bow shocks are assessed. For Jupiter, the unique composition of magnetospheric plasma, where oxygen and sulfur ions are major components, and the simultaneous presence of MeV electrons enable identification of the upstream particles as originating from within the magnetosphere. The observations at Saturn are phenomonologically similar, but no unique tracer element exists which can identify the origin. Earth observations show that substantial energetic particle activity occurs within the Earth's plasma sheet when upstream ions and electrons are observed in the interplanetary medium. Observations of the three planets are discussed to compare in-situ acceleration in the foreshock region vs. leakage of already accelerated ions from the parent magnetosphere. It is concluded that the evidence favors magnetospheric leakage.

Krimigis, S. M.↗
NASA NTRS1995

The location of planetary bow shocks: A critical overview of theory and observations

A bow shock (BS has been observed in the collisionless solar wind upstream of every planet except Pluto, which has yet to be visited by a spacecraft. They are all of similar character, but their size relative to the planet varies widely, e.g., the planeto-centric distance to the BS nose ranges from about 1.4 R(sub V) for Venus to 88 R(sub J) or more for Jupiter. Comparisons are reviewed that show its location may be represented satisfactorily by a gasdynamic (GD) model, provided the properties of the solar wind and planetary magnetic field and ionosphere are known and used as input in the application. Factors that determine the location are discussed, and examples are presented to illustrate effects of their variation, including which part of a BS is influenced by a local variation of the magneto/ionopause (MIP) shape. The interplanetary magnetic field (IMF) has no influence on the BS location in the GD model, but is shown to have a small effect in corresponding solutions of the basic MHD model from which the GD model is derived as the limit for weak IMF. Nearly all GD and MHD solutions are for steady flow, but a solution for unsteady flow associated with the passage of an interplanetary shock is also presented. It shows that the BS moves rapidly from its initial to final location, e.g., in about minute for the earth. Since many changes in the solar wind occur over longer intervals, these results help explain the success of quasi-stationary solutions in modeling the BS in time-varying solar wind flows.

Spreiter, J. R.↗
NASA NTRS1981

On the nature of ULF waves upstream of planetary bow shocks

The ULF electromagnetic waves associated with the earth's foreshock appear in two discrete frequency ranges, designated the low frequency waves at 0.01 - .05 Hz and the high frequency waves at 0.4 - 1.0 Hz. Falling within this second class are both the 0.4 Hz discrete wave packets and the slightly higher frequency wave trains commonly found just preceding the bow shock. Similar waves have now also been observed upstream of, but clearly associated with, the bow shocks of Mercury, Venus and Jupiter. Those observations are reviewed along with original recent work using the two point measurements made possible by the launch of ISEE 1 and 2 to further characterize the terrestrial waves.

Hoppe, M.↗
NASA NTRS1981

Plasma wave turbulence at planetary bow shocks

Voyager 1 observations of plasma wave turbulence at Saturn's bow shock are discussed and compared with corresponding data from Jupiter, earth, and Venus. The results suggest that the plasma instabilities that develop at the lower Mach number bow shocks of the terrestrial planets differ from those found at the high Mach number bow shocks of the outer planets.

Scarf, F. L.↗
NASA NTRS1982

Particle acceleration at planetary bow shock waves

One property of the collisionless shocks that may be studied through a comparison of their behavior in a variety of plasma conditions at several different planets is the occurrence of MHD waves, associated with particle beams accelerated at these shocks and flowing backward to the sun. Mercury, Venus, earth and Jupiter observations of one of these wave classes show that (1) the empirical relationship between interplanetary field strength and wave frequency in the observer's rest frame is approximately true at all the planets considered, and (2) the observed frequencies are consistent with resonance with beams of ions of the same energy at each of the planets. This is, in turn, in keeping with the Sonnerup (1969) geometrical model of ion reflection at collisionless shocks. It is suggested that this ion acceleration mechanism may occur in astrophysical systems similar to the solar system, providing a source of acceleration for cosmic rays.

Hoppe, M. M.↗
NASA NTRS1992

Remote radio observations of solar wind parameters upstream of planetary bow shocks

Radio emission is frequently produced at twice the electron plasma frequency 2fp in the foreshock region upstream of the terrestrial bow shock. Observations of this emission provide a remote diagnostic of solar wind parameters in the foreshock. Using ISEE-3 radio data, we present the first evidence that the radio intensity is proportional to the kinetic energy flux and to other parameters correlated with solar wind density. We provide a qualitative explanation of this intensity behavior and predict the detection of similar emission at Jupiter by the Ulysses spacecraft.

Macdowall, R. J.↗
NASA NTRS1988

Multipoint measurements of upstream waves

Two-wave MHD populations are seen at collisionless shocks: precursor waves standing in the shock ramp which form an integral part of the shock and upstream waves which are usually attempting to propagate upstream but are carried back toward the shock by the solar wind flow. Both types of waves are observed at interplanetary shocks and planetary bow shocks. The difficulty in studying interplanetary shocks is that the shock normal is hard to determine accurately but multiple spacecraft measurements are of some assistance in this regard. Two types of multispacecraft studies have been used, closely spaced ones such as with ISEE-1 and -2 and more distantly separated ones such as with ISEE and UKS. These studies suggest that the paradigm proposed here for the evolution of large amplitude or 'fully developed' turbulence needs some revision.

Russell, C. T.↗
NASA NTRS1993

Plasma waves as indicators of the termination shock

The plasma wave receivers on the Voyager spacecraft will likely provide indicators of both the actual crossing of the termination shock as well as precursors of the shock crossing. Since the electron foreshock can extend considerable distances upstream of the termination shock, the detection of these waves can provide as many as several weeks warning that a crossing of the termination shock is imminent. Electrostatic turbulence associated with planetary bow shocks themselves is also an expected feature of the solar wind termination shock and will provide an important signature with which to identify the shock and to provide information on its thickness and fundamental processes. Both upstream Langmuir waves and electrostatic wave turbulence can often be found in conjunction with interplanetary shocks, although the generally weaker nature of these shocks often leads to weaker plasma wave signatures than observed at planetary bow shocks. We demonstrate with Voyager observations that the amplitudes expected for each of these phenomena are well within the range of detectability by the Voyager plasma wave receiver even for termination shock distances exceeding 100 AU.

Kurth, W. S.↗
NASA NTRS1990

Steepening of kinetic magnetosonic waves into shocklets - Simulations and consequences for planetary shocks and comets

The generation and the nonlinear evolution of oblique low-frequency electromagnetic (kinetic magnetosonic) waves which were observed upstream of planetary bow shocks and at the Giacobini-Zinner comet, and referred to as shocklets, were investigated using an electromagnetic hybrid code. The observations show that the waves, which have a sinusoidal form when their amplitude is small, become steepened and linearly polarized as they grow in amplitude. The results of simulations show the original small-amplitude elliptically polarized wave grows and steepens, so that its polarization changes and becomes somewhat linear. The steepening process is associated with the coherent generation of a broad spectrum of waves on the magnetosonic whistler branch, which propagate at various phase and group velocities. It is shown that the presence of shocklets upstream of a planetary bow shock can modify its local structure by changing the solar wind Mach number and temperature, or by colliding with the shock.

Omidi, N.↗