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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Physics of the inner heliosphere: Mechanisms, models and observational signatures

The physics of the solar wind acceleration phenomena (e.g. effect of transient momentum deposition on the temporal and spatial variation of the temperature, density and flow speed of the solar wind, formation of shocks, etc.) and the resultant effects on observational signatures, particularly spectroscopic signature are studied. Phenomena under study include: (1) wave motions, particularly spectroscopic signatures are studied. Phenomena under study include:(1) wave motions, particularly Alfven and fast mode waves, (2) the formation of standing shocks in the inner heliosphere as a result of momentum and/or heat addition to the wind and (3) coronal transient phenomena where momentum and/or heat are deposited in the corona to produce transient plasma heating and/or mass ejections. Also included are the theoretical investigation of spectroscopic plasma diagnostics for the inner heliosphere and the analysis of existing Skylab and other relevant data.

Withbroe, G. L.↗
NASA NTRS1985

Evidence for regions of negligible cosmic-ray modulation in the inner heliosphere ( 10 AU)

Gold and Venkatesan report observations of periods during 1974-1976 when extended regions of heliolongitude that emitted lower than average solar wind velocities at 1 AU also exhibited higher than average cosmic ray intensities as measured by the E 35 MeV CPME anti-coincidence scintillator (28 sq cm omnidirectional geometric factor) on IMP-8. Their observations reproduced by a simple model, based on the observed steady solar wind structure, wherein there is little modulation of cosmic rays in the inner heliosphere until they reach the shocked plasma beyond the stream interactions in the outer heliosphere (similar to 5 to 10 AU). Beyond the interaction boundary, the intensity exhibits a constant radial gradient (similar to 2%/AU). The model also offers an explanation for the irregular behavior of the rotation averaged radial gradients observed by inside 10 AU, as well as the significant, but often ephemeral, latitude gradients observed by Voyagers 1 and 2 and IMP-8.

Roelof, E. C.↗
NASA NTRS1985

The large scale dynamics of the outer heliosphere and the long-term modulation of galactic cosmic rays

The network of cosmic ray observatories reaching across the heliosphere has given new insight into the process of solar modulation, establishing that the decreases occur principally in the outer heliosphere and are produced by interplanetary flow systems; that the hysteresis effects appear to be produced by changes in the rigidity dependence of the diffusion coefficient and that the predicted effects on the cosmic ray gradients associated with the reversal of the solar magnetic field polarity are not observed.

Mcdonald, F. B.↗
NASA NTRS1985

The large-scale modulation of cosmic rays in mid-1982: Its dependence on heliospheric longitude and radius

Near solar maximum, a series of large radial solar wind shocks in June and July 1982 provided a unique opportunity to study the solar modulation of galactic cosmic rays with an array of spacecraft widely separated both in heliocentric radius and longitude. By eliminating hysteresis effects it is possible to begin to separate radial and azimuthal effects in the outer heliosphere. On the large scale, changes in modulation (both the increasing and recovery phases) propagate outward at close to the solar wind velocity, except for the near-term effects of solar wind shocks, which may propagate at a significantly higher velocity. In the outer heliosphere, azimuthal effects are small in comparison with radial effects for large-scale modulation at solar maximum.

Pyle, K. R.↗
NASA NTRS1985

The galactic cosmic ray intensity minimum in the inner and outer heliosphere in solar cycle 21

The occurrence of the cosmic ray intensity minimum during solar cycle 21 at widely separated locations in the heliosphere is investigated. The data include in situ measurements by cosmic-ray detectors on the earth-orbiting satellite IMP 8, on the Voyagers 1 and 2 spacecraft, and on the Pioneers 10 and 11 spacecraft, located at distances from 1 to 27 AU. It is noted that Pioneer 10 and the other three spacecraft (Pioneer 11 and Voyagers 1 and 2) are on the opposite sides of the sun, with the latter moving toward the tail of the heliosphere. The data used in the study have been suitably corrected by removing solar flare particle contributions, Forbush decreases, and contamination by the radioisotope thermoelectric generator. It is evident that there exists a time delay in the occurrence of the cosmic ray intensity minimum registered by the IMP 8 and Pioneer 10 detectors, the latter being located at approximately 27 AU. It is shown that the data from the four spacecraft in deep space are on an average consistent with the IMP 8 data at 1 AU when suitably corrected, using 3 percent/AU for the radial gradient and 500 km/s for the average speed of propagation. An overview of the intensity profiles reveals reasonable agreement among them. However, specific intensity features appear at times to propagate with speeds in excess of the average value of about 500 km/s.

Venkatesan, D.↗
NASA NTRS1985

Cosmic-ray picture of the heliosphere

The existing data base on the characteristics of the heliosphere is discussed. It is known that solar gravity is less than necessary to hold all the solar material, and therefore a supersonic solar wind exists. Skylab soft X-ray photographs revealed the existence of coronal holes, which evolve in an 11 yr cycle. It has been proposed that all but the highest energy cosmic rays detected on earth can be attributed to solar and heliospheric origins, a controversial view which requires further empirical and theoretical work on particle acceleration processes and regions of interaction of the solar wind with interplanetary plasma. It is possible that a warped solar current sheet stretches to interplanetary space and organizes the solar magnetic field and thereby guides cosmic rays. An inverse correlation has been identified between the sunspot cycle and cosmic ray intensity. The features and effects of solar flares, subsequent shock waves and high speed particle streams are also discussed.

Venkatesan, D.↗
NASA NTRS1985

Interplanetary conditions during 3-kHz radio-wave detections in the outer heliosphere

Plasma waves detected by the Voyager 1 and 2 spacecraft beyond about 12 AU that may be associated with the turbulence expected at the heliopause are interpreted in terms of the characteristics of the interplanetary medium at large heliocentric distances. The low-energy charged-particle environment in the outer heliosphere during the observations of the unusual plasma-wave signals is addressed. The particle data suggest that the outer heliosphere was unusually stable and free of transient shock and particle events for the roughly eight months during the wave observations.

Lanzerotti, L. J.↗
NASA NTRS1985

A simulation study of distortion of the heliospheric sheet caused by solar activity

During major geomagnetic storms, the interplanetary magnetic field angle, phi, changes often abruptly, either from 135 to 315 or 315 to 135 deg, suggesting that the heliospheric current sheet is pushed upward or downward by disturbed solar wind. The distortion of the heliospheric current sheet by three successive solar flares is simulated to show that such a flapping motion can occur.

Fry, C. D.↗
NASA NTRS1985

A simulation study of two major events in the heliosphere during the present sunspot cycle

Disturbances in the heliosphere which occurred during two of the most active periods of the sun during the present solar cycle, in June and July 1982 and April and May 1978, are qualitatively simulated using the method of Hakamada and Akasofu (1982). A first-order spatial and temporal construction of flare-generated shocks and their multiple interactions with each other and with cororating interaction regions is obtained. A comparison of the results with solar wind observations from Pioneer 10, 11, and 12 suggests that some major flares occurred behind the solar disk during the two periods. The present method provides some qualitative information on how such a series of intense solar flares can greatly disturb both the inner and outer heliospheres. A long-lasting effect on cosmic rays which occurs in conjunction with such disturbances is discussed.

Akasofu, S. L.↗
NASA NTRS1986

Differential measurement and model calculations of cosmic ray latitudinal gradient with respect to the heliospheric current sheet

Simultaneous magnetic field and charged particle measurements from the Voyager spacecraft with heliographic latitude separations of more than 10 deg are used to investigate the distribution of about 1-GeV galactic cosmic ray protons with respect to the heliospheric current sheet in the outer solar system. By comparing the ratio of cosmic ray flux at Voyager 1 to that at Voyager 2 during periods of relatively quiet interplanetary conditions when the spacecraft are either both north or both south of the heliospheric current sheet, an average latitude component of the gradient of the cosmic ray flux on opposite sides of the current sheet is derived under restricted interplanetary conditions of -0.22 + or - 0.03 pct/deg, equivalent to a decrease of about 1 percent/AU away from the current sheet at about 12 AU. The results for these limited periods are in qualitative agreement with propagation models incorporating particle drifts.

Christon, S. P.↗
NASA NTRS1986

The effect of the heliospheric current sheet on cosmic ray intensities at solar maximum - Two alternative hypotheses

The capabilities of two alternative models for accounting for the reversal of the solar magnetic field polarity at the point of solar maximum and for the associated modulation of cosmic ray intensities at 1 AU are assessed. One model posits a continuous increase in the inclination of the heliospheric current sheet to a point of verticality at maximum, when it overturns and the reversal occurs. The alternative view is that the sun sheds the magnetic field of the previous cycle and generates a new field of opposite polarity. Some data do exist for a tilted current sheet which increases its tilt with proximity to the solar maximum. However, coronal data also support the presence of isolated regions of anomalous polarity which spread over the surface of the sun as maximum approaches, a condition commensurate with flux shedding. Both models predict heliospheric current sheet configurations which would produce some cosmic ray modulations observed over the course of the solar cycle.

Thomas, B. T.↗
NASA NTRS1987

Physics of the inner heliosphere: Mechanisms, models and observational signatures

Selected problems concerned with the important physical processes that occur in the corona and solar wind acceleration region, particularly time dependent phenomena were studied. Both the physics of the phenomena and the resultant effects on observational signatures, particularly spectroscopic signatures were also studied. Phenomena under study include: wave motions, particularly Alfven and fast mode waves; the formation of standing shocks in the inner heliosphere as a result of momentum and/or heat addition to the wind; and coronal transient phenomena where momentum and/or heat are deposited in the corona to produce transient plasma heating and/or mass ejection. The development of theoretical models for the inner heliosphere, the theoretical investigation of spectroscopic plasma diagnostics for this region, and the analysis of existing skylab and other relevant data are also included.

Withbroe, George L.↗
NASA NTRS1986

Configurations of corotating shocks in the outer heliosphere

Configurations of corotating shocks in the outer heliosphere are computed on the assumption that the shocks move at constant speeds between collisions. The basic physical process is the interaction of a forward shock with a reverse shock. The points at which this interaction takes place are determined algebraically in terms of the initial shock positions and speeds. Between the points at which the shocks interact, the shocks have the form of Archimedian spirals. Shock configurations are determined for the case of two corotating shock pairs originating at 2.5 AU and for a single shock pair at 2.5 AU. In both cases the heliosphere is divided into several distinct regions distinguished by the number of times the particles have passed through a shock. Since the shock strengths decrease when a forward shock interacts with a reverse shock, corotating shocks are expected to be weaker at larger distances from the sun.

Burlaga, L. F.↗
NASA NTRS1986

The heliospheric energy source

The solar wind and the heliosphere exist as a consequence of the heat input to the corona, particularly the coronal holes. The necessary energy input to coronal holes has been estimated to be 10 to the 6th erg/sq cm sec, requiring Alfven waves with rms fluid velocities of 100 km/sec. Observational upper limits on coronal fluid velocities are of the order of 25 km/sec, which may not apply to the transparent coronal hole. Alternatively it has been suggested that coronal holes may be heated by agitation from neighboring active regions, suggesting that the vigor of a coronal hole depends upon its location. The Ulysses Mission will provide a direct comparison of the strength of the high speed wind from coronal holes at low latitude and coronal holes at high latitude, from which the nature of the presently unknown energy sources of the coronal holes and the resulting structure of the heliosphere may be better judged. The question is fundamental to the dynamics of the windspheres of all stars.

Parker, E. N.↗
NASA NTRS1986

Modulation of galactic cosmic rays in the heliosphere

Observations of the intensity of galactic cosmic rays and anomalous components near earth and in the outer heliosphere are providing important tests for theories of solar modulation. The most recent observations show effects that seem to require that models for modulation include gradient and curvature drifts as well as the conventional processes of convection, adiabatic deceleration, and diffusion. New observations are required to define the interplay of these processes in the three-dimensional heliosphere.

Mckibben, R. B.↗
NASA NTRS1985

Collisionless shocks in the heliosphere: A tutorial review

An update is presented on current knowledge of collisionless shocks in the heliosphere. The individual papers address: a quarter century of collisionless shock research, some macroscopic properties of shock waves in the heliosphere, microinstabilities and anomalous transport, and acceleration of energetic particles.

Stone, Robert G.↗