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At least 307 records · Page 17

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NASA NTRS1982

Evidence for specularly reflected ions upstream from the quasi-parallel bow shock

Ion velocity distributions in the form of bunches of gyrating particles traveling along helical paths have been observed moving sunward immediately upstream from quasi-parallel parts of the earth's bow shock using Los Alamos/Garching instruments on ISEE-1 and -2. These distributions have characteristics which indicate that they are produced by the nearly specular reflection at the shock of a portion of the incident solar wind ions. In particular, the guiding center motion and the gyrospeeds of the gyrating ions are quantitatively consistent with simple geometrical considerations for specular reflection. These considerations reveal that specularly reflected ions can escape upstream when the angle between the upstream magnetic field and the local shock normal is less than 45 deg but not when the angle is greater than 45 deg. These upstream gyrating ions are an important signature of one of the processes by which solar wind streaming energy is dissipated into other forms of energy at the shock.

Gosling, J. T.↗

NASA NTRS1984

Fast-mode magnetohydrodynamic waves in coronal holes and the solar wind

Fast-mode MHD waves in the solar corona can propagate in any direction relative to the background magnetic field. In coronal holes, they refract into regions of low Alfven speed and are relatively difficult to damp. These characteristics lead to the possibility that fast-mode waves transport energy from magnetically closed coronal regions into coronal holes, that they are refracted into the central regions of coronal holes, and that they deposit most of their energy in the region of supersonic flow of high-speed solar wind streams emanating from coronal holes. To investigate whether this possibility might be realized and fast-mode waves might play a significant role in driving high-speed streams, a parameter study is carried out to examine the propagation and damping of fast-mode waves in various coronal hole models. This study indicates a broad range of coronal hole parameters for which fast-mode waves can play such a role and emphasizes the need for an improved knowledge of large-scale coronal magnetic structure, which is required before any firm conclusions can be drawn.

Fla, T.↗

NASA NTRS1985

Latitude variation of recurrent fluxes in the outer solar system

Recurrent low energy (not less than 0.5 MeV) proton flux enhancements, reliable indicators of corotating plasma inteaction regions in interplanetary space, have been observed on the Voyager 1 and 2 and Pioneer 11 spacecraft in the heliographic latitude range 2 deg S to 23 deg N and the heliocentric radial range 11 to 20 AU. After a period of rather high correlation between fluxes at different latitudes in early 1983, distinct differences develop. The evolution of the fluxes appears to be related to the temporal and latitudinal dynamics of solar coronal holes, suggesting that information about the latitudinal structure of solar wind stream sources propagates to these distances.

Christon, S. P.↗

NASA NTRS1985

White Light Coronograph (WLC) and Ultra-Violet Coronal Spectrometer (UVCS)

The white light coronagraph (WLC) and ultraviolet coronal spectrometer (UVCS) together reveal the corona and the roots of the solar wind from 1.5 to 6 solar radii from Sun center. The WLC measures the plasma density and spatial structure of the corona and coronal mass ejections at a resolution of about 20 arcseconds. The UVCS, in combination with the WLC, measures the temperature and radial outflow speed of the coronal plasma. These instruments will detect mass ejections from active regions and high speed solar wind streams from coronal holes a few days before the source regions rotate onto the face of the Sun, thus giving a week or more of advanced warning for disturbed geomagnetic conditions at Earth.

Moore, R. L.↗

NASA NTRS1985

Low energy proton bidirectional anisotropies and their relation to transient interplanetary magnetic structures: ISEE-3 observations

It is known that the interplanetary medium in the period approaching solar maximum is characterized by an enhancement in the occurrence of transient solar wind streams and shocks and that such systems are often associated with looplike magnetic structures or clouds. There is observational evidence that bidirectional, field aligned flows of low energy particles could be a signature of such looplike structures, although detailed models for the magnetic field configuration and injection mechanisms do not exist at the current time. Preliminary results of a survey of low energy proton bidirectional anisotropies measured on ISEE-3 in the interplanetary medium between August 1978 and May 1982, together with magnetic field data from the same spacecraft are presented.

Marsden, R. G.↗

NASA NTRS1985

Solar wind velocity and daily variation of cosmic rays

Recently parameters applicable to the solar wind and the interplanetary magnetic field (IMF) have become much better defined. Superior quality of data bases that are now available, particularly for post-1971 period, make it possible to believe the long-term trends in the data. These data are correlated with the secular changes observed in the diurnal variation parameters obtained from neutron monitor data at Deep River and underground muon telescope data at Embudo (30 MEW) and Socorro (82 MWE). The annual mean amplitudes appear to have large values during the epochs of high speed solar wind streams. Results are discussed.

Ahluwalia, H. S.↗

NASA NTRS1985

Cosmic ray secular variations in terrestrial records and aurorae

The rediscovery that the Sun and the solar wind can undergo important changes on historical time scales has brought into question the stability of the cyclic behavior of past time series of solar and solar-terrestrial origin. It was found by Vector Fourier analysis that the solar 11 year cycle is present in the series of 10Be, delta 180, in ice cores and of thermoluminescence (TL) in sea sediments during the last Millennia with a frequency modulation, related to the Sun behavior, as tested by comparison with the Sunspot number R sub z series. It was shown that the cyclogram of the series of yearly Aurorae from 1721 to 1979 linear-regression-corrected-for-R sub z is straight for the periodicity zeta=11,1y, which indicates that such periodicity is constant in time corresponding to the only line present in the 11y band. The maxima of this component appear at the same time together with the high speed solar wind streams taking place in coronal holes situated in high heliolatitudes. It is evidenced that the 11 year cycle has undergone frequency oscillations on a time scale of two centuries, although it is very difficult to determine the periodicities with high accuracy.

Attolini, M. R.↗

NASA NTRS1984

Electron precipitation in solar flares - Collisionless effects

A large fraction of the electrons which are accelerated during the impulsive phase of solar flares stream towards the chromosphere and are unstable to the growth of plasma waves. The linear and nonlinear evolution of plasma waves as a function of time is analyzed with a set of rate equations that follows, in time, the nonlinearly coupled system of plasma waves-ion fluctuations. As an outcome of the fast transfer of wave energy from the beam to the ambient plasma, nonthermal electron tails are formed which can stabilize the anomalous Doppler resonance instability responsible for the pitch angle scattering of the beam electrons. The non-collisional losses of the precipitating electrons are estimated, and the observational implication of these results are discussed.

Vlahos, L.↗

NASA NTRS1985

Latitude variation of recurrent MeV-energy proton flux enhancements in the heliocentric radial range 11 to 20 AU and possible correlation with solar coronal hole dynamics

Recurrent low energy (not less than 0.5 MeV) proton flux enhancements, reliable indicators of corotating plasma interaction regions in interplanetary space, have been observed on the Voyager 1 and 2 and Pioneer 11 spacecraft in the heliographic latitude range 2 deg S to 23 deg N and the heliocentric radial range 11 to 20 AU. After a period of rather high correlation between fluxes at different latitudes in early 1983, distinct differences develop. The evolution of the fluxes appears to be related to the temporal and latitudinal dynamics of solar coronal holes, suggesting that information about the latitudinal structure of solar wind stream sources propagates to these distances.

Christon, S. P.↗

NASA NTRS1985

One-dimensional kinematics of particle stream flow with application to solar wind simulation

A simple kinematic method for determining the particle velocity distribution of a model solar wind for which the spatial distribution of particles is given as a function of particle travel time has been developed by Hakamada and Akasofu (1982). Here their method is formalized mathematically and an inverse procedure for determining the particle distribution from a given velocity distribution is derived. This inverse procedure is then applied to a simulated velocity distribution obtained from an MHD finite difference code.

Olmsted, C.↗

NASA NTRS1986

White Light Coronagraph (WLC) and Ultra-Violet Coronal Spectrometer (UVCS)

The WLC and UVS together reveal the corona and the roots of the solar wind from 1.5 to 6 solar radii from sun center. The WLC measures the plasma density and spatial structure of the corona and coronal mass ejections at a plasma density and spatial structure of the corona and coronal mass ejections at a resolution of about 20 arcsec. The UVCS in combination with the WLC measures the temperature and radial outflow speed of the coronal plasma. These instruments will detect mass ejections from active regions and high speed solar wind streams from coronal holes a few days before the source regions rotate onto the face of the Sun, thus giving a week or more of advanced warning for disturbed geomagnetic conditions at Earth.

Moore, R.↗

NASA NTRS1986

Cosmic ray variations and turbulent flow systems - 0.3-1.0 AU; 1977-1980

Possible relationships among the cosmic ray intensity and magnetic fields and flows inside 1 AU are investigated using data gathered by instrumentation on the Helios 1 spacecraft during 1977-80. The data included nearly continuous simultaneous plasma and magnetic field measurements for periods of two months or more at heliocentric radii from 0.3-1 AU. Account was taken of fluctuations of the relative particle counting rates due to spacecraft motion. The passage of transient streams following solar wind shock waves was accompanied by Forbush decreases but not long term modulation. The extent and consistency of the data support a model of shell-like disturbances of diverse shapes and internal structures proceeding outward from the sun. Magnetic field fluctuation spectra (turbulence) calculated for the various types of flow observed permitted establishing power laws relationships between the variations observed and the frequency of the magnetic field. Transient flows were more turbulent than corotating flows. Turbulence was also present in mixed flows, possibly arising from shocks, and was usually confined to the spiral shaped structures of corotation.

Burlaga, L. F.↗

NASA NTRS1986

On the brightness variations of Comet Halley at large heliocentric distances

Sporadic variations of its intrinsic brightness of up to 500 percent, with time scales as short as a few hours, has been exhibited by Halley's comet at large heliocentric distances (11-8 AU). It is shown that many of these brightness enhancements are closely correlated to the encounter of high-speed solar wind streams by the comet. It is proposed that during such periods, the night side of the comet gets charged to numerically large negative electrostatic potentials, with consequent electrostatic levitation and blow-off of fine charged dust grains lying on it. This gives rise to the observed brightness variations.

Flammer, K. R.↗

NASA NTRS1986

Three-dimensional structure of the heliospheric current sheet

Using a modification of the kinematic method of Hakamada and Akasofu (1982), those solar wind streams which originate near the magnetic neutral line on the source surface are followed to map the three-dimensional structure of the heliospheric current sheet (HCS) out to 5 AU for several Carrington rotations. The Stanford potential analysis magnetic field information of Hoeksema et al. (1983) is used to specify the source surface field.

Fry, C. D.↗

NASA NTRS1987

The acceleration of winds by MHD waves in open magnetic structures

It has become increasingly clear in recent years that the available models describing the acceleration of high speed streams in solar coronal holes are inadequate. Thermal models do not result in winds with the proper thermal speed and density. In addition models which assume acceleration by Alfven waves, though promising, are not self-consistent when applied low in coronal holes where the acceleration of the wind takes place. An Alfven wave in the corona with a period of a few hundred seconds will have a wavelength of the order of 10 cm. This is of the order of the diameter of a coronal hole in the low corona, and therefore it is expected that the MHD waves are guided in some sense by the coronal hole in which they propagate. This basic idea led to the development of a leaky waveguide theory to describe MHD wave propagation in coronal holes. Recent efforts were concentrated in two areas: the development of numerical codes to properly describe the hydrodynamics of the flow, and more realistic models of the coronal hole properties such as density gradients and spherical divergence of the flux tube. Some of the important aspects of the theory are discussed, including the effect of density inhomogeneities and smooth transverse density gradients. In addition it is hoped that some early results from the numerical models will be presented. The acceleration of cool winds is a problem of interest in cool supergiants also. Some work was done on the development of wind models for these stars as well.

Davila, Joseph M.↗

NASA NTRS1988

Does solar radio emission trigger SKR?

Using Voyager I data, a preliminary search has been made for evidence of both triggering and enhancement of Saturn Kilometric Radiation (SKR) by solar type III bursts. The interval from September 1, 1980 to September 30,1980 was examined at a frequency near the spectral peak of the SKR (385 kHz). The investigation compared emission levels before and after the arrival of Type III bursts at Saturn by subjecting the data to both superposed epoch analysis and intensity distribution analysis. Strong SKR associated with the arrival of a high-density solar wind stream at Saturn was removed to avoid possible masking of a weak triggering effect. No evidence of triggering or of enhancement of SKR due to the arrival of Type III bursts at Saturn could be found.

Desch, Michael D.↗

NASA NTRS1989

Discrete events and solar wind energization

Based on a multiple-magnetic-reconnection picture, an estimation of the energy flux suggests that small-scale EUV exploding events may contribute a significant amount of energy (of order of 100,000 erg/sq cm sec) to solar atmospheric heating and solar-wind acceleration. Most of the dissipated magnetic energy is converted into thermal energy and plasma turbulence. On a related aspect, a numerical study based on the nonlinear one-fluid hydrodynamic equations shows a self-smoothing effect, whereby a multistream structure of the solar wind formed near the sun can be gradually smoothed during its propagation through interplanetary space. This calculation gives support for the possible contribution of discrete energetic events to high-speed solar wind streams.

Yang, W.-H.↗

NASA NTRS1991

A study of the relationship between micropulsations and solar wind properties

A year-long comparison between daily averages of solar wind parameters obtained from the MIT experiment on IMP-8 and micropulsation measurements made by the Universita dell'Aquila has shown a correlation between solar wind speed and micropulsation power with peaks of the correlation coefficient greater than 0.8 in the period range from 20 to 40 s. Different behavior observed for different period bands suggests that the shorter period activity tends to precede the highest values of the solar wind speed while the longer period activity tends to persist for longer intervals within high velocity solar wind streams. A comparison with simultaneous interplanetary magnetic field measurements supports the upstream origin of the observed ground pulsations.

Yedidia, B. A.↗