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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

Diffusive shock acceleration - Comparison of a unified shock model to bow shock observations

A comparison is made between recent AMPTE/IRM observations of diffuse ions detected upstream of the earth's bow shock when the interplanetary magnetic field was nearly parallel to the solar wind direction and a known collisionless quasi-parallel shock model. These observations, which provide the proton spectrum for all of velocity space, give a direct measure of the shock acceleration efficiency and show how thermal solar winds are injected into the diffusive shock acceleration mechanism. The model accurately describes the proton spectrum; moreover, it predicts the shock structure, the complete particle spectrum, the relative velocity distributions of different ion species in the down stream region, and the enhancement of heavy ions over protons relative to the unshocked solar wind.

Ellison, Donald C.↗
NASA NTRS1981

Energetic particle spectra in finite shocks - The earth's bow shock

Particle acceleration and escape at the earth's bow shock wave are discussed in order to account for reported exponential fast particle spectra. A model is presented of particle acceleration in a finite two-dimensional shock perpendicular to the magnetic field, with particle parallel and perpendicular diffusion coefficients inversely proportional to each other. It is shown that the exponential particle energy per unit charge spectra observed by Ipavich et al. (1979) for the case of a radial solar wind magnetic field may be obtained if the fast particles escape the shock by means of resonant diffusion to unconnected field lines. The calculated e-folding value of the energy/charge ratio is found to be independent of the level of turbulence near the shock and in good agreement with observations. For the case of a nonradial solar wind magnetic field, the model is noted to predict that convection may be the dominant means of escape. It is also pointed out that the parallel and perpendicular diffusion coefficients may be measured indirectly at the bow shock.

Eichler, D.↗
NASA NTRS1981

Three-dimensional interaction of interplanetary shock waves with the bow shock and magnetopause - A comparison of theory with ISEE observations

The reported investigation is concerned with the propagation of the interplanetary shock waves in the solar wind and their three-dimensional interaction with the bow shock and magnetosheath. Formulae are deduced to predict the new position and orientation of the bow shock front after the interaction. To test the understanding of the interplanetary portion of the shock propagation, the obtained results are compared with observations on August 18, 1978, when both ISEE 1 and ISEE 3 were in the solar wind. Two examples of an interplanetary shock wave penetrating into the magnetosphere on October 4, 1978, and August 27, 1978, are examined, taking into account a simple model of the magnetosheath. The results agree with the observed values of the ISEE satellite data within experimental uncertainties.

Zhuang, H. C.↗
NASA NTRS2011

Entropy Generation Across Earth's Bow Shock

Earth's bow shock is a transition layer that causes an irreversible change in the state of plasma that is stationary in time. Theories predict entropy increases across the bow shock but entropy has never been directly measured. Cluster and Double Star plasma experiments measure 3D plasma distributions upstream and downstream of the bow shock that allow calculation of Boltzmann's entropy function H and his famous H-theorem, dH/dt ~ O. We present the first direct measurements of entropy density changes across Earth's bow shock. We will show that this entropy generation may be part of the processes that produce the non-thermal plasma distributions is consistent with a kinetic entropy flux model derived from the collisionless Boltzmann equation, giving strong support that solar wind's total entropy across the bow shock remains unchanged. As far as we know, our results are not explained by any existing shock models and should be of interests to theorists.

Parks, George K.↗
NASA NTRS1984

A comparative review of bow shocks and magnetopauses

Bow shock and magnetopauses formation is discussed. Plasma and magnetic field environments of all the planets from Mercury to Saturn were measured. It was found that all the planets have bow shocks and almost all have a magnetopause. Venus is the only planet with no measurable intrinsic magnetic field and the solar wind interacts directly with Venus' ionosphere. The bow shock characteristics depend on the changing solar wind conditions. The shape of a magnetopause or any obstacle to flow depends on the three dimensional pressure profile that it presents to the solar wind. Jupiter is unusual because of the considerable amount of plasma which is contained in its magnetosphere. Magnetopause boundaries in ecliptic plane projection are modelled by segments of ellipses, matched to straight lines for the magnetotool boundaries or parabolas. Specific properties of known planetary bow shocks and magnetopauses are reviewed.

Lepping, R. P.↗
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

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

Solar and interplanetary control of the location of the Venus bow shock

The Venus bow shock location has been measured at nearly 2000 shock crossings, and its dependence on solar EUV, solar wind conditions, and the interplanetary magnetic field determined. The shock position at the terminator varies from about 2.14 Venus radii at solar minimum to 2.40 Venus radii at solar maximum. The location of the shock varies little with solar wind dynamic pressure but strongly with solar wind Mach number. The shock is farthest from Venus on the side of the planet in which newly created ions gyrate away from the ionosphere. When the interplanetary magnetic field is perpendicular to the flow, the cross section of the shock is quite elliptical. This effect appears to be due to the anisotropic propagation of the fast magnetosonic wave. When the interplanetary magnetic field is aligned with the flow, the bow shock cross section is circular and only weakly sensitive to changing EUV flux.

Russell, C. T.↗
NASA NTRS1976

A summary of observations of the earth's bow shock

The earth's bow shock can be regarded as a fast steady-state collisionless magnetohydrodynamic shock exhibiting a wide range of plasma parameter values or as an example of superalfvenic flow of a MHD fluid past a blunt body. This paper summarizes our knowledge of the bow shock. Satellite crossings of the shock have given some empirical data on the position and shape of the bow shock. Theoretical calculations on position and shape show the critical nature of the Alfven number. The main physical process to be explained with regard to the bow shock is the mechanism of thermalization of the upstream solar wind. Some of the main classes of shocks observed are quasi-parallel shocks, low Mach number laminar shocks, and high beta shocks.

Fairfield, D. H.↗
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 NTRS1990

The solar cycle dependence of the location and shape of the Venus bow shock

The Venus terminator bow shock position is monitored and it is shown that the shock radius increases as the solar cycle approaches a new maximum. It is also shown that the subsolar bow shock changes with the solar cycle, and that these positions are correlated with each other and with solar activity. It is hypothesized that, at solar minimum, the magnetic barrier is weak, and that some absorption of solar wind is to be expected.

Zhang, T.-L.↗
NASA NTRS1974

Structure of the terrestrial bow shock

Preliminary results of a bow shock study which isolates various parameters are reported. The main findings are successful isolation of shock structures by parameter set, and correlated diagnostic behavior. Conventional shock structural nomenclature is first modified to suit the results of spacecraft observations and to provide the terminology needed in the remainder of the paper. Several examples of shock morphology for a wide range of plasma states are then displayed, as seen with various diagnostics. The bow shock is described as a system in the solar wind. Advantages of the bow shock as an object of shock investigation are pointed out, and a few aims of future study are outlined.

Greenstadt, E. W.↗
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 NTRS1985

AOTV bow shock location

Hypersonic bow-shock location and geometry are of central importance to the aerodynamics and aerothermodynamics of aeroassisted orbital transfer vehicles (AOTVs), but they are difficult to predict for a given vehicle configuration. This paper reports experimental measurements of shock standoff distance for the 70 deg cone AOTV configuration in shock-tunnel-test flows at Mach numbers of 3.8 to 7.9 and for angles of attack from 0 deg to 20 deg. The controlling parameter for hypersonic bow-shock standoff distance (for a given forebody shape) is the mean normal-shock density ratio. Values for this parameter in the tests reported are in the same range as those of the drag-brake AOTV perigee regime. Results for standoff distance are compared with those previously reported in the literature for this AOTV configuration. It is concluded that the AOTV shock standoff distance for the conical configuration, based on frustrum (base) radius, is equivalent to that of a sphere with a radius about 35 percent greater than that of the cone; the distance is, therefore, much less than reported in previous studies. Some reasons for the discrepancies between the present and previous are advanced. The smaller standoff distance determined here implies there will be less radiative heat transfer than was previously expected.

Desautel, D.↗