Engineering topics

Barnes, A.

Publications and source records attributed to Barnes, A..

At least 55 records · Page 3

NASA NTRS1984

Shock acceleration of nuclei and electrons in the heliosphere beyond 24 AU

Radially propagating shocks of solar flare origin are analyzed using plasma and charged particle measurements on Pioneer 10 to show that protons and helium nuclei are continuously accelerated to energies of up to roughly 70 MeV per nucleon. The discovery that electrons are simultaneously accelerated to relativistic energies places new constraints on models for acceleration by these quasi-perpendicular shocks in the outer heliosphere.

Pyle, K. R.↗

NASA NTRS1984

Rapid expansion of polytropes

An extremely massive unevolved star may be idealized as an n = 3 polytropic sphere supported by radiation pressure. Such a polytrope is subject to explosive, supersonic expansion, whose later stages can be described by similarity solution of the dynamical equations. This picture is confirmed by numerical hydrodynamic simulation, which shows that an n = 3 polytrope, initially at rest near equilibrium, undergoes an explosive expansion which approaches the similarity solution at large times.

Barnes, A.↗

NASA NTRS1984

Corotating interplanetary streams and associated ionospheric disturbances at Venus and earth

During the summer of 1979, solar corona structure was such that a sequence of recurrent regions produced a corresponding sequence of corotating solar wind streams with pronounced downstream signatures. One of these stream events passed earth on July 3 and was observed later at Venus late on July 11th, with similar characteristics. Corresponding in situ measurements at earth from the Atmospheric Explorer-E satellite and at Venus from the Pioneer Venus Orbiter are examined for evidence of comparable perturbations of the planetary ionospheres. The passage of the stream shock front is found to be associated with pronounced fluctuations in n(O+) which appear as pronounced local depletion of ion concentrations in both ionospheres. The ionosphere disturbances appear to be closely associated with large variations in the solar wind momentum flux. The implied local ionospheric depletions observed at each planet are interpreted to be the consequence of plasma redistribution, rather than actual depletions of plasma.

Taylor, H.↗

NASA NTRS1983

Spatial dependences in the distant solar wind: Pioneers 10 and 11

Pioneer 10, 11 observations of the solar wind and magnetic field between 1 and 20 AU are reviewed. Spatial dependences, which are emphasized, must be inferred in the presence of large temporal variations including solar cycle effects. The separation of spatial and temporal dependences is achieved principally through the use of multipoint observations including baseline measurements at 1 AU. Measurements of the solar wind parameters (radial speed, flux, proton temperature) and of the magnetic field magnitude and components are compared with two theories, the Parker theory which assumes radial, azimuthally symmetric flow and the Goldstein-Jokipii theory which includes effects associated with stream-stream interactions. The observed radial gradients in the proton density and velocity and the magnetic field are consistent with the Parker model. A qualitative dependence of field magnitude on heliomagnetic latitude, i.e., referred to the observed location of the heliospheric current sheet, was derived. The field strength was found to decrease with distance from the current sheet.

Smith, E. J.↗

NASA NTRS1983

An unusual interplanetary event - Encounter with a comet?

The possibility that the slow rise to a sharp maximum and then decay surrounding a strong current sheet observed in the Venus magnetic field 0.72 AU from the sun was caused by passage of Venus through the wake of an active comet is examined. Data were also gathered by the ISEE 3 satellite magnetometer at 0.99 AU 25 h, 20 min later, a delay corresponding to the transit time for the solar wind. No shock structures bounded the phenomenon. The data indicate the presence of a small body in a much larger field of interaction and the magnetometer, solar wind probe, and electron temperature probe support a behavior similar to a planetary magnetosheath. The observed He structure in interplanetary space ruled out a solar wind source, while consideration of the magnetic equator and magnetic pole suggest that an unknown comet passed through the region between the sun and Venus at a distance where the effects would not be detected at earth.

Russell, C. T.↗

NASA NTRS1983

Factors controlling the location of the Venus bow shock

The location of the Venus bow shock determined from magnetic field measurements during the first and third years of Pioneer Venus orbiter operation is examined and compared with nearly simultaneously obtained interplanetary solar wind data to determine those factors that control the size of the Venus bow shock. The location of the intersection of the bow shock with the terminator that is best determined by the data does not vary significantly between years 1979 and 1981 and is only 16 percent more distant than the Venera 9 and 10 shock when account is taken of solar wind aberration. Alfvenic Mach number and magnetosonic Mach number affect the size of the bow shock significantly. Solar wind dynamic pressure has a lesser effect. No significant asymmetries in the shock shape were found either as a result of the orientation of the clock angle of the IMF in the terminator plane or the angle of the IMF relative to the shock normal.

Tatrallyay, M.↗

NASA NTRS1983

Large-amplitude hydromagnetic waves in collisionless relativistic plasma - Exact solution for the fast-mode magnetoacoustic wave

An exact nonlinear solution is found to the relativistic kinetic and electrodynamic equations (in their hydromagnetic limit) that describes the large-amplitude fast-mode magnetoacoustic wave propagating normal to the magnetic field in a collisionless, previously uniform plasma. It is pointed out that a wave of this kind will be generated by transverse compression of any collisionless plasma. The solution is in essence independent of the detailed form of the particle momentum distribution functions. The solution is obtained, in part, through the method of characteristics; the wave exhibits the familiar properties of steepening and shock formation. A detailed analysis is given of the ultrarelativistic limit of this wave.

Barnes, A.↗

NASA NTRS1983

Hydromagnetic waves, turbulence, and collisionless processes in the interplanetary medium

An extended discussion is conducted concerning the origin and evolution of interplanetary hydromagnetic waves and turbulence, and their influence on the large scale dynamics of the solar wind. The solar wind is at present the preeminent medium for the study of hydromagnetic waves and turbulence, providing an opportunity for advancement of understanding of the most fundamental processes of the astrophysical plasmas. All interplanetary fluctuations whose time scale is observed to be greater than 1 sec can be regarded as hydromagnetic fluctuations. It has been found to be simplest, and generally very satisfactory, to model interplanetary variations as fluctuations in an MHD fluid. Attention is given to the classification of wave modes, geometrical hydromagnetics, Alfven wave pressure, rugged invariants, and the kinetic theory of collisionless processes.

Barnes, A.↗

NASA NTRS1982

The distant interplanetary wake of Venus - Plasma observations from Pioneer Venus

Observations by the Pioneer Vernus Orbiter plasma analyzer during passages through the Venus solar wind wake in June 1979 are reported. The spacecraft traveled 8-12 Venus radii behind the planet from the sun, detecting frequent, episodic disappearances of the plasma. The disappearance occurred inside the magnetotail, indicating a tattered, filamentary extension of the solar wind cavity. Positive oxygen ions were observed inside and outside of the magnetotail on some days, and were absent on others. The data indicate that the cavity contracts during high solar wind dynamic pressure, although the presence of 0(+) ions is not correlated well with the solar wind dynamic pressure. The most intense bursts originated from a flux of 10 million ions/sq cm per sec, with an 0(+) kinetic energy estimated to be 100,000-1,000,000 K.

Mihalov, J. D.↗

NASA NTRS1982

Pioneer Venus observations of plasma and field structure in the near wake of Venus

Ionospheric plasma density depletions or 'holes' are observed by the Pioneer Venus orbiter in association with radial magnetic fields in the near wake of Venus. This report presents examples of the collected observations of these unexpected features of the Venus nightside ionosphere obtained by the Langmuir probe, magnetometer, ion mass spectrometer, retarding potential analyzer, plasma analyzer, and electric field experiments. The connection between plasma density depletions and temperature changes, changes in ion composition, plasma wave emissions, and magnetic fields with a substantial radial component is illustrated. Mechanisms that may be responsible for the formation and maintenance of holes are suggested.

Luhmann, J. G.↗

NASA NTRS1982

Disappearing ionospheres on the nightside of Venus

Instruments on the Pioneer Venus Orbiter have detected a substantial ionosphere on the nightside of Venus during most orbits. However, during some orbits the nightside ionosphere seems to have almost disappeared, existing only as irregular patches of low-density plasma. The solar wind dynamic pressure on these occasions is greater than average. Data from several instruments (Langmuir probe, ion mass spectrometer, retarding potential analyzer, magnetometer, and plasma analyzer) have been correlated for a number of orbits during which the nightside ionosphere had disappeared. The magnetic field tends to be coherent, horizontal, and larger than usual, and the electron and ion temperatures are much larger than they usually are on the nightside. Mechanisms are suggested which might explain the reasons for the disappearance of the ionosphere when the solar wind dynamic pressure is large.

Cravens, T. E.↗

NASA NTRS1982

Test particle study of Landau damping of steepening magnetosonic waves

A test particle study of Landau damping of steepening large-amplitude magnetosonic waves is made. Motions of test particles in a model of a steepening large-amplitude magnetosonic wave are traced. The kinetic energy change of the ensemble of test particles is computed to estimate the effective Landau damping rate of the magnetosonic wave. The numerical results are compared with the linear kinetic theory of Landau damping and interpreted in terms of a simple physical picture for particle trapping.

Matsumoto, H.↗

NASA NTRS1982

Modulation of dayside on and neutral distributions at Venus Evidence of direct and indirect solar energy inputs

The details of solar variability and its coupled effects on the Venusian dayside are examined for evidence of short-term perturbations and associated energy inputs. Ion and neutral measurements obtained from the Orbiter Ion Mass Spectrometer and Orbital Neutral mass Spectrometer are used to show that the dayside concentrations of CO2(+) and the neutral gas temperature are smoothly modulated with a 28-day cycle reasonably matching that of the solar F(10.7) and EUV fluxes. Earlier measurements show less pronounced and more irregular modulations and more conspicuous short-term day-to-day fluctuations in the ions and neutrals, as well as relatively large enhancements in the solar wind, which appear consistent with differences in solar coronal behavior during the two periods. It is suggested that the solar wind variations cause fluctuations in joule heating, producing the observed short-term ion and neutral variations.

Taylor, H. A., Jr.↗

NASA NTRS1981

Evidence for the acceleration of ionospheric O/+/ in the magnetosheath of Venus

Plasma spectra from 12 orbits of the Pioneer-Venus Orbiter are reported which suggest the presence of ionospheric material in the magnetosheath plasma near and sunward of the terminator plane. Each spectrum shows a high E/q peak, consistent with O(+) moving at a speed lower than or comparable to that of the ambient magnetosheath plasma. It is pointed out that even though the data set is limited, the most intense heavy-ion fluxes clearly occur at altitudes less than a few thousand km. With the identification of the energetic component at O(+), the data reveal a trend for the speed of O(+) near the planet to be lower than the local magnetosheath speed, whereas the two speeds are comparable at altitudes above a few thousand km. These data, in combination with related observations that indicate O(+) moving at ambient speed in the distant wake, suggest that ionospheric material is swept up by the magnetosheath, accelerated to ambient speed within a few thousand km, and carried back through the wake region.

Mihalov, J. D.↗

NASA NTRS1981

Radio wave scattering observations of the solar corona First-order measurements of expansion velocity and turbulence spectrum using Viking and Mariner 10 spacecraft

Radio wave scattering data were collected at 3.6 and 13 cm wavelengths by means of the radio link between the Viking orbiters and the earth during the Nov. 25, 1976 solar conjunction of Mars, which occurred near the beginning of solar cycle 21; Mariner 10 solar activity observations during 1974 are also used. It is found that the temporal frequency variance spectrum of amplitude fluctuations is useful for characterizing the bulk motion of the plasma, and the spectral index of electron density turbulence is obtained. The measurements of solar wind velocity and spectral index cover 78 days for Viking and 49 days for Mariner 10 and show the combined effects of changing heliocentric distance, solar latitude, and solar longitude as well as solar activity. It is concluded that the observational velocity profile differs significantly from the theoretical profiles in two ways: (1) the theoretical profile does not show the abrupt change in velocity at about 15 solar radii, and (2) the observational profile shows acceleration at larger radial distances than the model profiles. The observational profiles indicate velocities of less than about 150 km/sec out to 15 solar radii.

Tyler, G. L.↗

NASA NTRS1981

Interplanetary Alfvenic fluctuations - A stochastic model

The concept of minimum variance is investigated for nonplanar interplanetary Alfvenic fluctuations in which the field direction varies randomly. The theory of the random wandering of a vector of constant length is developed as a representation of the magnetic field, and it is found that the minimum variance tends to coincide with the mean field directions over the correlation time of the fluctuations. The Fokker-Planck limit of the theory is then developed and used to analyze the statistic distribution of field directions with and without a reflecting barrier. Results suggest that the tendency of the Alfvenic fluctuations to have a direction of minimum variance statistically aligned with the mean magnetic field may be purely a consequence of the randomness of the fluctuation and not imply that the fluctuations are necessarily plane waves. Extensive statistical studies of the observed directional variations of the interplanetary magnetic field are necessary to test this hypothesis.

Barnes, A.↗

NASA NTRS1979

Theory of the large-amplitude plane magnetoacoustic wave propagating transverse to the magnetic field in a hot collisionless plasma

An exact solution of the kinetic and electromagnetic equations for a large-amplitude plane magnetoacoustic wave propagating transverse to the magnetic field in a hot collisionless plasma is presented. The solution gives simple relations among the magnetic-field strength, density, stress tensor, and plasma velocity, all of which are measurable in the interplanetary plasma. These relations are independent of the electron and ion velocity distributions, subject to certain restrictions on 'high-velocity tails.' The magnetic field of the wave is linearly polarized. The wave steepens to form a shock much as the analogous waves of MHD theory do.

Barnes, A.↗

NASA NTRS1979

Physics of the solar wind for the 1975-1978 IUUG Quadrennial Report

The paper surveys topics related to the origin, expansion, and acceleration of the solar wind and the plasma physics of the interplanetary medium. The study of the relationship between coronal holes and solar-wind streams, and the associated revision of ideas about solar wind acceleration and heating are reviewed. In addition, topics of hydromagnetic waves and turbulence, and interplanetary electrons, as items of particular importance during the past quadrennium, are discussed. While the research discussed was concerned with data taken near solar minimum, further solar-wind studies will concentrate on observations from the rising and maximum phases of the solar cycle.

Barnes, A.↗