Helium in the solar wind
TL;DR: In this article, the Vela 3A and 3B satellites were analyzed to obtain relative helium abundances and plasma properties and the long term average was 0.037, which is lower than the recently reported solar surface ratio of approximately 0.063.
Abstract: Data obtained from electrostatic analyzers on the Vela 3A and 3B satellites from July 1965 to July 1967 have been analyzed to obtain relative helium abundances and plasma properties. Ninety percent of the 10,314 spectra used from this period gave helium to hydrogen density ratios from 0.01 to 0.08. The long term average was 0.037, which is lower than the recently reported solar-surface ratio of approximately 0.063. For the time period covered in this analysis, only 10% of the spectra yielded helium to hydrogen density ratios greater than 0.063. Large variations in the helium to hydrogen density ratio were observed even when averaged over a solar rotation. Sudden increases in the solar wind helium content followed simultaneous occurrences of Forbush decreases and sudden-commencement geomagnetic storms. The magnitude of the helium to hydrogen density ratio, however, was not correlated to the magnitude of the geomagnetic storm, as measured by the Kp index. The ratio of the helium to hydrogen velocities was strongly peaked around 1.0, as might be expected. The distribution of the helium to hydrogen temperature ratios had a full width at half maximum of approximately 2.5 and a mean of approximately 4.0.
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TL;DR: A comprehensive overview of recent observational and theoretical results on solar wind structures and fluctuations and magnetohydrodynamic waves and turbulence, with preference given to phenomena in the inner heliosphere, is presented in this paper.
Abstract: A comprehensive overview is presented of recent observational and theoretical results on solar wind structures and fluctuations and magnetohydrodynamic waves and turbulence, with preference given to phenomena in the inner heliosphere. Emphasis is placed on the progress made in the past decade in the understanding of the nature and origin of especially small-scale, compressible and incompressible fluctuations. Turbulence models to describe the spatial transport and spectral transfer of the fluctuations in the inner heliosphere are discussed, and results from direct numerical simulations are dealt with. Intermittency of solar wind fluctuations and their statistical distributions are briefly investigated. Studies of the heating and acceleration effects of the turbulence on the background wind are critically surveyed. Finally, open questions concerning the origin, nature and evolution of the fluctuations are listed, and possible avenues and perspectives for future research are outlined.
877 citations
TL;DR: In this paper, it was found that a strong anisotropy in the core of proton distributions, with a temperature that is larger perpendicular rather than parallel to the magnetic field, is a persistent feature of high-speed streams, becoming most pronounced in the perihelion, or about 0.3 AU.
Abstract: Such nonthermal features as temperature anisotropies, heat fluxes, and proton double streams have been observed by a Helios solar probe survey of solar wind three-dimensional proton velocity distributions between 0.3 and 1 AU. It is found that a strong anisotropy in the core of proton distributions, with a temperature that is larger perpendicular rather than parallel to the magnetic field, is a persistent feature of high-speed streams, becoming most pronounced in the perihelion, or about 0.3 AU. Isotropic distributions have been detected only close to, and at, magnetic sector boundaries, and the flattest radial temperature profiles are found in high-speed streams. These observations indicate that local heating or proton heat conduction occurs in the solar wind.
723 citations
TL;DR: In this paper, the authors verified the pressure balance relationship between the solar wind dynamic pressure and the location of the subsolar magnetopause by assembling a data set of 1821 magnetopsause crossings.
Abstract: A data set of 1821 magnetopause crossings was assembled. Separate fits to subsets of this data set determine the magnetopause location as a function of solar wind dynamic pressure and interplanetary magnetic field orientation. Solar wind dynamic pressure variations produce self-similar magnetopause motion on time scales of one hour or longer. In this paper, the pressure balance relationship between the solar wind dynamic pressure and the location of the subsolar magnetopause are verified. The relationship between the IMF Bz, region 1 Birkeland current strength, the position of the subsolar magnetopause, and the shape of the dayside magnetosphere is quantified. Cross sections of the dayside magnetopause in planes perpendicular to the earth-sun line are oblate.
513 citations
TL;DR: In this article, a helios solar probe survey of solar wind helium ion velocity distributions and derived parameters between 0.3 and 1 AU is presented, where the role of Coulomb collisions in limiting differential ion speeds and the ion temperature ratio is investigated.
Abstract: A Helios solar probe survey of solar wind helium ion velocity distributions and derived parameters between 0.3 and 1 AU is presented. Distributions in high-speed wind are found to generally have small total anisotropies, with some indication that, in the core part, the temperatures are greater parallel rather than perpendicular to the magnetic field. The anisotropy tends to increase with heliocentric radial distance, and the average dependence of helium ion temperatures on radial distance from the sun is described by a power law. Differential ion speeds with values of more than 150 km/sec are observed near perihelion, or 0.3 AU. The role of Coulomb collisions in limiting differential ion speeds and the ion temperature ratio is investigated, and it is found that collisions play a distinct role in low-speed wind, by limiting both differential ion velocity and temperature.
396 citations
01 Jan 1990
TL;DR: In this paper, it was shown that the solar corona is highly structured and changes its shape enormously during the solar activity cycle, and it was no great surprise when both spatial structure and temporal variability were found to be reproduced in the corona's offspring.
Abstract: From eclipse observations it was well known that the solar corona is highly structured and changes its shape enormously during the solar activity cycle. Hence, it was no great surprise when both these properties (spatial structure and temporal variability) were found to be reproduced in the corona’s offspring, i.e. the solar wind. Even the first continuous observations of the interplanetary plasma performed on board the American Venus probe Mariner 2 in 1962 showed a “series of long-lived, high velocity streams separated by slower moving plasma”, as Neugebauer and Snyder [3.148] phrased it. (For an extensive and very informative review of the early years of solar wind research the interested reader is referred to [3.48]).There is a basic agreement between the slow solar wind parameters (velocity υ p ≈ 300km s-1, proton density η p ≈ 9 cm-3, proton temperature T p ≈ 4 × 104K [3.95]) and current coronal expansion models based on Parker’s theory [3.159]. This led many workers in the field to associate the slow solar wind with a “quiet state” and to regard any fast flow including that in quasistationary high-speed streams (υ ≈ 600 km s-1, η p ≈ 3 cm-3, T p ≈ 105 K [3.56]) more or less as disturbances of this quiet state.
363 citations
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TL;DR: In this paper, the authors summarized the properties of the positive ion component of the solar wind observed during the four months of the Mariner 2 flight to and past Venus in 1962, and calculated the ions' velocity, temperature and density from ∼35,000 energy/charge spectrums by fitting the data to isotropic Maxwell-Boltzmann distributions in a reference frame moving away from the sun at the solar-wind velocity.
Abstract: This paper summarizes the properties of the positive-ion component of the solar wind observed during the four months of the Mariner 2 flight to and past Venus in 1962. The protons' average velocity and temperature were approximately 500 km/sec and 1.7×1050K, respectively. Several streams of hot, high-velocity plasma were observed to recur at 27-day intervals, with peak velocity and temperature values of ∼ 830 km/sec and 9×1050K. One of these streams probably existed for at least 18 months. Between streams the velocity dropped as low as 307 km/sec, while the temperature was ∼ 3×1040K. Near 1 AU the average number density was approximately 5 protons/cm3. The density was usually highest at the leading or western edge of each stream, with a maximum value of ∼80 protons/cm3. Otherwise, the density varied inversely with the plasma velocity. The ions' velocity, temperature, and density were calculated from ∼35,000 energy/charge spectrums by fitting the data to isotropic Maxwell-Boltzmann distributions in a reference frame moving away from the sun at the solar-wind velocity. A model in which the protons and α particles had equal thermal velocities gives a better fit to the observed spectrums than does an equal-temperature model. The spectrums usually had high-energy tails, which became more pronounced at the higher plasma velocities. The velocity, temperature, and high-energy tail were not strongly dependent on distance from the sun, whereas the density varied approximately as the inverse square of this distance.
437 citations
TL;DR: In this article, satellite observations of electrons and protons with E > 100 ev have shown that a sheet of plasma with enhanced energy density stretches across the earth's magnetotail from the dusk to the dawn boundaries of the magnetosphere.
Abstract: Vela satellite observations of electrons and protons with E > 100 ev have shown that a sheet of plasma with enhanced energy density stretches across the earth's magnetotail from the dusk to the dawn boundaries of the magnetosphere. The plasma has been observed at geocentric distances between 15.5 and 20.5 earth radii; this plasma sheet probably extends from the night-side termination of the radiation belt to beyond 31 RE in the antisolar direction. Near the midnight meridian at ∼17 RE the sheet is often ∼4–6 RE thick, while toward the dusk and dawn boundaries the sheet flares out to about twice that thickness. The plane of symmetry of the sheet lies above or below the solar magnetospheric equatorial plane depending on whether the geomagnetic dipole axis tilts toward or away from the sun. The plasma is located in the vicinity of the ‘neutral sheet’ region of magnetic field reversal; inside the plasma sheet the measured kinetic energy densities of the electrons are comparable to the expected magnetic field energy densities, while outside the sheet the kinetic energy densities are much lower. The energy spectrums are quasi-thermal in character, having average energies extending from ∼200 ev to above 12 kev; omnidirectional fluxes extend to above 109 cm−2 sec−1. Both rapid (100–200 sec) and slower changes occur in the average energies of the plasma sheet electrons, resulting in large variations in the flux of energetic electrons (E > 45 kev). The plasma sheet boundaries, which are frequently in motion, are clearly defined by large changes in the electron flux. The average energy of the electron population is higher on the dawn side of the magnetotail than the dusk side, resulting in a more frequent appearance of energetic electrons on the dawn side.
321 citations
142 citations
TL;DR: In this article, the authors measured the ion flux with high resolution in both energy and direction and computed values of the velocity distribution function at points on a plane in velocity space, extended over the whole plane by interpolation and displayed by a contour mapping in this plane.
Abstract: The electrostatic analyzers on the Vela 3 satellites measure the ion flux with high resolution in both energy and direction. From the measured fluxes are computed values of the velocity distribution function at points on a plane in velocity space. The function is extended over the whole plane by interpolation and displayed by a contour mapping in this plane. From this mapping the properties of the function can be visualized, and parameters characterizing the ion component of the plasma can be computed. The distribution functions of solar wind protons observed during the first month the satellites were in orbit are found to be significantly anisotropic in a frame of reference moving with the local plasma bulk velocity. A ‘tail’ of protons with high random energies is usually present; evidence is presented that suggests that this tail is aligned with the interplanetary magnetic field lines, more often pointing away from the sun (along the lines) than toward the sun. The mean random energy or temperature of the protons is highest in this direction. The ratio of the maximum temperature to the average over-all directions ranges from 1.0 to about 2.5, with a value of 1.4 being most common. The existence of this anisotropy can be explained by the invariance of υ⊥²/B as the plasma expands from the sun. Similar anisotropic distribution functions are found for the He++ or α-particle component of the plasma. The ratio of number densities of the α's and protons is found to vary from 0.00 to 0.15, with a mean value of 0.042 during this period.
130 citations
TL;DR: In this article, solar plasma observations during magnetic storms, discussing shocks and tangential discontinuities, geomagnetic variations and geomagnetworks were discussed, as well as He
Abstract: Solar plasma observations during magnetic storms, discussing shocks and tangential discontinuities, geomagnetic variations and He
127 citations