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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 289 records · Page 16
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NASA NTRS1985

Formation of ion acoustic solitary waves upstream of the earth's bow shock

The turbulent plasma development of Lee and Parks is applied to the solar wind approaching the earth's bow shock region. The ponderomotive force contribution is due to ion acoustic waves propagating in the direction of the ambient magnetic field. In this case, the envelope of the ion acoustic wave is shown to satisfy the cubic Schroedinger equation. Modulational instabilities exist for waves in the solar wind, thereby predicting the generation of solitary waves. This analysis further identifies that the ion acoustic waves which exhibit this instability have short wavelengths.

Pangia, M. J.↗
NASA NTRS1987

Waves in the magnetic field and solar wind flow outside the bow shock at comet P/Halley

The existence of hydromagnetic waves in the mass-loaded solar wind upstream from the bow shock of a comet is well-established both for comet Giacobini-Zinner and for comet Halley. Whereas previous reports have been concerned either with the magnetic field observations or with plasma observations, here observations of the magnetic field with the solar wind proton and alpha particle distributions are combined. This allows the three possible modes of propagation for these waves to be separated. The magnetic component is predominantly transverse to the magnetic field and linearly polarized. The flow vector also has a substantial amount of power parallel to the magnetic field. An examination of the pressure variations shows that slow magnetosonic waves are more common than the fast mode.

Johnstone, A.↗
NASA NTRS1990

Observations of the flank of earth's bow shock to -110 R(E) by ISEE 3/ICE

The ISEE 3/ICE spacecraft made over 100 crossings of earth's bow shock between x(SEC) = -20 and -110 R(E) most of them further away than -60 R(E) while resetting its course for comet Giacobini-Zinner. The parameters of the crossings are tabulated and some of their magnetic, plasma wave, and spectral profiles at low magnetosonic Mach numbers are displayed. Loci of crossings are compared with the extrapolation of a model of earlier statistical success, showing that the model remains useful in the far flank.

Greenstadt, E. W.↗
NASA NTRS1983

Ions upstream of the earth's bow shock - A theoretical comparison of alternative source populations

The trajectories of ions reflected or leaked upstream from the earth's bow shock and subject solely to the Lorentz force in a steady interplanetary magnetic field B and the V x B electric field are studied theoretically. Expressions are obtained for the guiding center motion and gyromotion in a frame (the Hoffman-Teller frame) moving parallel to the shock surface with sufficient speed to transform the incident solar wind velocity into motion entirely along the interplanetary magnetic field. Equations are derived which transform these motions back to the observer's frame. The predicted upstream motions for four different source models for upstream ions are compared using these expressions: magnetic moment-conserving reflection of solar wind ions, specular reflection of solar wind ions, magnetic moment-conserving leakage of magnetosheath ions, and leakage of magnetosheath ions parallel to the shock normal.

Schwartz, S. J.↗
NASA NTRS1985

Electron plasma waves upstream of the earth's bow shock

Electrostatic waves are observed around the plasma frequency fpe in the electron foreshock, together with electrons backstreaming from the bow shock. Using data from the sounder aboard ISEE 1, it is shown that this noise, previously understood as narrow band Langmuir waves more or less widened by Doppler shift or nonlinear effects, is in fact composed of two distinct parts: one is a narrow band noise, emitted just above fpe, and observed at the upstream boundary of the electron foreshock. This component has been interpreted as Langmuir waves emitted by a beam-plasma instability. It is suggested that it is of sufficiently large amplitude and monochromatic enough to trap resonant electrons. The other is a broad band noise, more impulsive than the narrow band noise, observed well above and/or well below fpe, deeper in the electron foreshock. The broad band noise has an average spectrum with a typical bi-exponential shape; its peak frequency is not exactly equal to fpe and depends on the Deybe length. This peak frequency also depends on the velocity for which the electron distribution has maximum skew. An experimental determination of the dispersion relation of the broad band noise shows that this noise, as well as the narrow band noise, may be due to the instability of a hot beam in a plasma.

Lacombe, C.↗
NASA NTRS1985

Gyrating ions and large-amplitude monochromatic MHD waves upstream of the earth's bow shock

Episodes of nearly monochromatic, low-frequency (0.03 Hz) hydromagnetic waves are occasionally observed upstream of the earth's bow shock. High time resolution (3 s) measurements of two-dimensional ion distributions during two nearly monochromatic wave events reveal that the ion distributions asociated with these waves are 'gyrating ions.' Such distributions consists of suprathermal ions with parallel and perpendicular velocities confined to a fairly narrow range of (nonzero) values. The ions are also often confined to a fairly narrow range of gyrophase angle ('gyrophase bunched'). In one of the two cases, the observed frequency of the waves agrees quite well with the Doppler shifted resonance frequency of waves in right-hand resonance with the observed gyrating ions. In the second case, the observed frequency is lower than the predicted frequency by a factor of 1.5-2.

Thomsen, M. F.↗
NASA NTRS2010

Fiber-Based Measurement of Bow-Shock Spectra for Reentry Flight Testing

We demonstrated a fiber-based approach for obtaining optical spectra of a glowing bow shock in a high-enthalpy air flow. The work was performed in a ground test with the NASA Ames Aerodynamic Heating Facility (AHF) that is used for atmospheric reentry simulation. The method uses a commercial fiber optic that is embedded in the nose of an ablating bluntbody model and provides a line-of-sight view in the streamwise direction - directly upstream into the hot post-shock gas flow. Both phenolic impregnated carbon ablator (PICA) and phenolic carbon (PhenCarb 28) materials were used as thermal protection systems. Results show that the fibers survive the intense heat and operate sufficiently well during the first several seconds of a typical AHF run (20 MJ/kg). This approach allowed the acquisition of optical spectra, enabling a Boltzmann-based electronic excitation temperature measurement from Cu atom impurities (averaged over a line-of-sight through the gas cap, with a 0.04 sec integration time).

Schott, Timothy D.↗
NASA NTRS1972

Observation of nonuniform structure of the earth's bow shock correlated with interplanetary field orientation.

Explorer 33 and 35 magnetometers, on the western and eastern flanks of the earth's bow shock, respectively, observed the boundary concurrently between 0130 and 0430 UT, Oct. 30, 1968. Contrasting shock structures were recorded. Explorer 35 saw a quiet abrupt shock, whereas Explorer 33 saw an irregular noisy boundary with much upstream wave activity. The interplanetary field was roughly in the average archimedean spiral angle and was therefore approximately tangent to the shock at Explorer 35 and normal to the shock at Explorer 33. Gross motions and variable tilting of the aberrated shock probably contributed to the peculiar sequence of shock crossings at the two spacecraft. The observations support a model of the shock in which perpendicular and oblique collisionless structures coexist and form a nonuniform magnetosheath outer boundary.

Greenstadt, E. W.↗
NASA NTRS1987

A kinetic study of solar wind mass loading and cometary bow shocks

The kinetic processes associated with solar wind mass loading due to pickup of cometary ions, and the formation of cometary bow shocks were investigated numerically using a hybrid simulation code described by Leroy et al. (1981), in which solar wind protons and heavy cometary ions are treated kinetically, but the electrons are treated as a massless fluid. It was found that the solar wind decelaration and pickup of cometary ions take place through both the macroscopic electromagnetic fields embedded in the solar wind and the microscopic field associated with low-frequency electromagnetic waves that are generated by the unstable velocity distribution function of the cometary ions. The results of the main simulation runs at various cone angles are described and compared with the recent observations of comets Giacobini-Zinner and Halley.

Omidi, N.↗
NASA NTRS1990

Comparison of picked-up protons and water group ions upstream of Comet Halley's bow shock

The similarities and differences between the picked-up cometary protons and water-group (WG) ions upstream of the bow shock of Comet Halley are examined using measurements obtained by the ion mass spectrometer and plasma analyzer experiments on board Giotto. It was found that the dependencies of the pitch angle and the energy diffusion rates of the cometary protons and WG ions on the ion densities and on the angle alpha between the interplanetary field and the solar wind velocity vector were very different. This finding could not be explained in terms of presently available theories and models.

Neugebauer, 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 NTRS1984

The structure of oblique subcritical bow shocks - ISEE 1 and 2 observations

ISEE 1 and 2 spacecraft magnetic field data are used to determine the scale lengths of various elements of shock structure, as well as wavelengths and wave polarizations, in a study of structural elements which include shock ramps and precursor wave trains over a series of oblique, low Mach number terrestrial bow shocks. Dissipative processes are reflected in the damping of the precursors, and dissipative scale lengths are of 200-800 km, or several times greater than shock thicknesses. The source of dissipation in the shocks does not appear to be the wave-wave decay of the whistlers, for which no evidence is found. The interaction of the whistler itself with upstream electrons is suggested as a simple and self-consistent explanation for the observed wave train damping.

Mellott, M. M.↗
NASA NTRS1972

Study of waves in the earth's bow shock.

Results of an examination of the perturbation vectors of waves upstream and downstream from the region of maximum compression in the bow shock on Ogo 5 under particularly steady solar-wind conditions. The polarization of the upstream waves was right-hand circular, and that of the downstream waves left-hand elliptical in the spacecraft frame. By observing that the polarization of the waves remained unchanged as the shock motion swept the wave structure back and forth across the satellite three times in eight minutes, it was found that the waves were not stationary in the shock frame. A study of the methods of determining the shock normal indicates that the normal estimated from a shock model should be superior to the normal based on magnetic coplanarity. The propagation vectors of the waves examined did not coincide with the shock-model normal, the average magnetic field, or the plasma-flow velocity. However, the major axis of the polarization ellipse of the downstream wave was nearly parallel to the upstream propagation vector.

Holzer, R. E.↗
NASA NTRS1973

Directional diffusion coefficients of solar protons inside and outside the bow shock.

The directional diffusion coefficients of low-energy (greater than or equal to 0.3 MeV) solar protons inside and outside the bow shock are examined during the solar flare event of Jan. 24, 1969. The data are derived from simultaneous observations obtained by Explorer 33 inside the magnetosheath and by Explorer 35 in the interplanetary medium. Although the gross properties of the spin-averaged intensities on a diffusion-type plot appear to be the same in both media, the directional intensities show significant variations. It is shown that directional intensities of low-energy protons can be described reasonably well by anisotropic diffusion with an associated diffusion coefficient. Directional diffusion coefficients are found to differ by a factor of as much as three among different directions in space, and from the spin-averaged diffusion coefficient. This suggests that anisotropic diffusion does indeed take place and that so called 'isotropic' diffusion coefficients derived in the past from spin-averaged intensities may actually be directional diffusion coefficients in cases where substantial anisotropies (greater than 50%) exist.

Verzariu, P.↗
NASA NTRS1980

Energization of solar wind ions by reflection from the earth's bow shock

The existence of ion beams with energies a few times the solar wind energy and streaming outward from the earth's bow shock has been known for some time. To explain the observed ion energies, a simple reflection model has been proposed in which the particles gain energy by displacement parallel to the interplanetary electric field. In this model the energy gained in the reflection can be described as a function of the angles between the interplanetary magnetic field, the solar wind velocity, and the local shock normal. Ion beams under widely varying conditions have been observed in ISEE 1 and 2. For 18 cases, with beam energies ranging from approximately 1.4 to 30 times the solar wind energy, a comparison between the observed and the predicted beam energies has been made.

Paschmann, G.↗
NASA NTRS1987

Ion beams and the ion/ion acoustic instability upstream from the earth's bow shock

This paper considers the generation of enhanced ion acoustic fluctuations by field-aligned ion beams upstream from the earth's bow shock. Steep slopes in the beam distribution parallel and possibly strongly oblique to the magnetic field are correlated with bursts of ion acoustic noise in the upstream region. Linear theory shows that it is the slope of the beam distribution at oblique angles to the magnetic field that determines the growth rate of the ion/ion acoustic instability. Because of instrumentation limitations, it is suggested but not confirmed that enhanced ion acoustic fluctuations in the upstream region and in the presence of field-aligned beams are driven by such steep-sided distributions.

Fuselier, S. A.↗
NASA NTRS1996

Magnetic Field Orientation Effects on the Standoff Distance of Earth's Bow Shock

Three-dimensional, global MHD simulations of solar wind flow onto a prescribed magnetopause obstacle are used to show that a bow shock's nose location a(sub s), and the relative subsolar magnetosheath thickness Delta(sub ms)/a(sub mp) are strong functions of the IMF cone angle theta (between v(sub sw) and B(sub sw)) and the Alfven Mach number M(sub A). For a given M(sub A) the shock is more distant for higher theta (restricted to the interval 0-90deg by symmetries), while a(sub s)/a(sub mp) and Delta(sub ms/a(sub mp) increase with decreasing M(sub A) for theta greater than or approximately 20deg but decrease with decreasing M(sub A) for theta approximately Odeg. Large differences in Delta(sub ms/a(sub mp) are predicted between theta = Odeg and 90deg at low M(sub A), with smaller differences remaining even at M(sub A) approximately 10. The theta = Odeg results confirm and extend the previous work of Spreiter and Rizzi [1974]. The simulations show that successful models for the subsolar shock location cannot subsume the dependences on M(sub A) and theta into a sole dependence on M(ms). Instead, they confirm a recent prediction [Cairns and Grabbe, 1994] that a(sub s)/a(sub mp) and Delta(sub ms)/a(sub mp) should depend strongly on theta and M(sub A) for M(sub A) less than or approximately 10 (as well as on other MHD variables). Detailed comparisons between theory and data remain to be done. However, preliminary comparisons show good agreement, with distant shock locations found for low M(sub A) and large theta greater than or approximately 45deg and closer locations found for theta less than or approximately 20deg even at M/A approximately 8.

Cairns, Iver H.↗