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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

The modelling of latitudinal gradients in the solar wind in the outer solar system

A steady, axisymmetric, quasi-radial, global model is developed for thermally driven stellar winds with embedded magnetic fields. The asymptotic, linear results are presented for 0(1) latitudinal variations in the radial magnetic field, mass-loss rate, and radial velocity of the wind. The MHD equations are solved for the latitudinal dependence of the rotational velocity and magnetic field. They are driven by the meridional flows that develop naturally from internal magnetic stresses. Most flows open flux tubes in the stars equatorial plane, redistributing mass and magnetic flux as a function of stellar latitude. The plasma spins up to conserve angular momentum in fields and plasma.

Nerney, S.↗
NASA NTRS1990

Evidence for the presence of quasi-two-dimensional nearly incompressible fluctuations in the solar wind

Assuming that the slab and isotropic models of solar wind turbulence need modification (largely due to the observed anisotropy of the interplanetary fluctuations and the results of laboratory plasma experiments), this paper proposes a model of the solar wind. The solar wind is seen as a fluid which contains both classical transverse Alfvenic fluctuations and a population of quasi-transverse fluctuations. In quasi-two-dimensional turbulence, the pitch angle scattering by resonant wave-particle interactions is suppressed, and the direction of minimum variance of interplanetary fluctuations is parallel to the mean magnetic field. The assumed incompressibility is consistent with the fact that the density fluctuations are small and anticorrelated, and that the total pressure at small scales is nearly constant.

Matthaeus, William H.↗
NASA NTRSDate not supplied

Impacts of Non-Simultaneous Global Photospheric Magnetic Measurements on Coronal and Solar Wind Modeling

The intensity and arrival time of coronal mass ejections (CMEs) can be significantly influenced by the background solar wind encountered as they propagate outward from the Sun into the interplanetary medium. In addition, solar energetic particles (SEPs) race ahead of CME shock fronts and flare regions along magnetic field lines largely determined by the background solar wind. Predicting the solar wind accurately is therefore critical for improving forecasts of CMEs, SEPs, and high-speed streams. Modeling of the corona and solar wind is challenging in general, as it is highly dependent on global photospheric magnetic field maps, which serve as the boundary conditions to all coronal models that drive solar wind models. Unfortunately, less than half of the Sun’s photospheric magnetic field is reliably measured from any given vantage point and thus it is common for the maps to have highly dated and unreliable measurements in them. While Solar Obiter (SolO) now provides for the first time the opportunity to have simultaneous measurements of nearly the entire surface magnetic field of Sun (e.g., when SolO/PHI measurements are combined with those from SDO/HMI), the required alignment to accomplish this occurs only occasionally. Further, coronal models are extremely sensitive to the strengths of the polar magnetic fields of the Sun, which remain poorly observed. Recently, efforts to mitigate this problem include using flux transport models such as the Air Force Data Assimilative Photospheric Flux Transport (ADAPT) model, which evolves the field forward in time using well known transport processes occurring on the Sun. However, it cannot account for the emergence of new magnetic flux without direct observations. The ESA VIGIL mission will eventually provide continuous observations from the L5, and the proposed SunCHASER mission will likewise do this at the L4 vantage point should it be funded. Ultimately, what is needed is a constellation of spacecraft distributed around the Sun with magnetographs that continuously measure the global surface magnetic field. In this talk, we discuss how the lack of simultaneous global measurements of the photospheric magnetic field adversely impacts the predictive performance of coronal and solar wind models.

C. Nick Arge↗
NASA NTRS2010

Hybrid Model of Inhomogeneous Solar Wind Plasma Heating by Alfven Wave Spectrum: Parametric Studies

Observations of the solar wind plasma at 0.3 AU and beyond show that a turbulent spectrum of magnetic fluctuations is present. Remote sensing observations of the corona indicate that heavy ions are hotter than protons and their temperature is anisotropic (T(sub perpindicular / T(sub parallel) >> 1). We study the heating and the acceleration of multi-ion plasma in the solar wind by a turbulent spectrum of Alfvenic fluctuations using a 2-D hybrid numerical model. In the hybrid model the protons and heavy ions are treated kinetically as particles, while the electrons are included as neutralizing background fluid. This is the first two-dimensional hybrid parametric study of the solar wind plasma that includes an input turbulent wave spectrum guided by observation with inhomogeneous background density. We also investigate the effects of He++ ion beams in the inhomogeneous background plasma density on the heating of the solar wind plasma. The 2-D hybrid model treats parallel and oblique waves, together with cross-field inhomogeneity, self-consistently. We investigate the parametric dependence of the perpendicular heating, and the temperature anisotropy in the H+-He++ solar wind plasma. It was found that the scaling of the magnetic fluctuations power spectrum steepens in the higher-density regions, and the heating is channeled to these regions from the surrounding lower-density plasma due to wave refraction. The model parameters are applicable to the expected solar wind conditions at about 10 solar radii.

Ofman, L.↗
NASA NTRS1994

Latitudinal variation of speed and mass flux in the acceleration region of the solar wind inferred from spectral broadening measurements

Spectral broadening measurements conducted at S-band (13-cm wavelength) during solar minimum conditions in the heliocentric distance range of 3-8 R(sub O) by Mariner 4, Pioneer 10, Mariner 10, Helios 1, Helios 2, and Viking have been combined to reveal a factor of 2.6 reduction in bandwidth from equator to pole. Since spectral broadening bandwidth depends on electron density fluctuation and solar wind speed, and latitudinal variation of the former is available from coherence bandwidth measurements, the remote sensing spectral broadening measurements provide the first determination of the latitudinal variation of solar wind speed in the acceleration region. When combined with electron density measurements deduced from white-light coronagraphs, this result also leads to the first determination of the latitudinal variation of mass flux in the acceleration region. From equator to pole, solar wind speed increases by a factor of 2.2, while mass flux decreases by a factor of 2.3. These results are consistent with measurements of solar wind speed by multi-station intensity scintillation measurements, as well as measurements of mass flux inferred from Lyman alpha observations, both of which pertain to the solar wind beyond 0.5 AU. The spectral broadening observations, therefore, strengthen earlier conclusions about the latitudinal variation of solar wind speed and mass flux, and reinforce current solar coronal models and their implications for solar wind acceleration and solar wind modeling.

Woo, Richard↗
NASA NTRS1974

The interstellar wake of the solar wind

The present work examines theoretically the cooling of the subsonic solar wind by the interstellar hydrogen gas entering the solar system. It is assumed that the interstellar hydrogen gas is distributed uniformly in the space where the solar wind is subsonic, and the solar-wind wake is represented by an idealized column flow of plasma in which the flow velocity is constant and the temperature varies along, not across, the wake. The results are therefore to be taken only in their order of magnitude. In the hot region near the shock sphere the electrons are cooled mainly through electron ionization while the protons cool through charge exchange. In the cooler regions far away from the shock, the protons are still cooled by charge exchange and the electron is cooled by collisions with protons. As the temperature of the solar wind decreases, the magnetic field becomes relatively more important. When the spin axis of the sun is parallel to the direction of the incident interstellar gas flow, the lines of force are in spiral form and the tension causes the plasma to be concentrated along the center of the wake.

Yu, G.↗
NASA NTRS1983

The average solar wind in the inner heliosphere: Structures and slow variations

Measurements from the HELIOS solar probes indicated that apart from solar activity related disturbances there exist two states of the solar wind which might result from basic differences in the acceleration process: the fast solar wind (v 600 kms(-)1) emanating from magnetically open regions in the solar corona and the "slow" solar wind (v 400 kms(-)1) correlated with the more active regions and its mainly closed magnetic structures. In a comprehensive study using all HELIOS data taken between 1974 and 1982 the average behavior of the basic plasma parameters were analyzed as functions of the solar wind speed. The long term variations of the solar wind parameters along the solar cycle were also determined and numerical estimates given. These modulations appear to be distinct though only minor. In agreement with earlier studies it was concluded that the major modulations are in the number and size of high speed streams and in the number of interplanetary shock waves caused by coronal transients. The latter ones usually cause huge deviations from the averages of all parameters.

Schwenn, R.↗
NASA NTRS1995

A study of the solar wind deceleration in the Earth's foreshock region

Previous observations have shown that the solar wind is decelerated and deflected in the earth's upstream region populated by long-period waves. This deceleration is corelated with the 'diffuse' but not with the 'reflected' ion population. The speed of the solar wind may decrease tens of km/s in the foreshock region. The solar wind dynamic pressure exerted on the magnetopause may vary due to the fluctuation of the solar wind speed and density in the foreshock region. In this study, we examine this solar wind deceleration and determine how the solar wind deceleration varies in the foreshock region.

Zhang, T.-L.↗
NASA NTRS1992

The solar wind interaction with Venus

The Pioneer Venus Orbiter (PVO) mission has played a key role in establishing the nature of the solar wind interaction with Venus. Although earlier probes had determined that Venus presented an obstacle much smaller than the size of earth's magnetosphere to the solar wind, they did not carry out in situ measurements pertaining to solar wind interaction studies at low enough altitudes to determine why. They also did not provide datasets of sufficient duration to study the variability of the interaction of both short (one day) and long (solar cycle) timescales. The first 600 of the nearly 5000 orbits of PVO magnetometer data have been used to determine a very low upper limit on the intrinsic dipolar magnetic moment of Venus. The consequence of that low magnetic moment is that the solar wind interacts directly with the upper atmosphere and ionosphere. Relative to a dipolar field obstacle, the ionospheric obstacle is rather incompressible. A bow shock is observed to stand in front of the nearly Venus-sized ionospheric obstacle at a comparatively steady subsolar altitude of approximately 1.5 R(v) (Venus radii). This shock decelerates the supersonic solar wind plasma so that it can flow around the obstacle. It was found to change its average position in the terminator plane from about 2.4 R(v) to 2.1 R(v) as the solar cycle progressed from the 1978 orbit insertion near solar maximum through the 1986-87 solar minimum, and back again during the latest solar activity increase. Between the bow shock and the ionosphere proper, the slowed solar wind plasma flow diverges near the subsolar point and makes its way across the terminator where it reaccelerates and continues anti-Sunward. The solar wind magnetic field, which is in effect frozen into the flowing plasma, is distorted in this 'magnetosheath' region so that it appears to hang up or drape over the dayside ionosphere before it slips around with the flow. These features of the solar wind interaction are also seen when the obstacle is a dipole magnetic field, but there are two important distinctions. In the wake of the Venus obstacle one finds an induced magnetic tail composed of varying interplanetary fields rather than the constant fields of intrinsic origin. This magnetotail is further seen to be populated by Heavy (0+) ions that are evidently escaping from the planet at significant (approximately 10(exp -25) s(exp -1)) rates. These heavy ions are also observed in the dayside magnetosheath. The interpretation is that ions are produced by both photoionization and solar wind electron impact ionization of the upper neutral atmosphere that extends into the magnetosheath.

Luthmann, J. G.↗
NASA NTRS2018

Generation of Kappa Distributions in Solar Wind at 1 au

We examine the generation of kappa distributions in the solar wind plasma near 1 au. Several mechanisms are mentioned in the literature, each characterized by a specific relationship between the solar wind plasma features, the interplanetary magnetic field (IMF), and the kappa index-the parameter that governs the kappa distributions. This relationship serves as a signature condition that helps the identification of the mechanism in the plasma. In general, a mechanism that generates kappa distributions involves a single or a series of stochastic or physical processes that induces local correlations among particles. We identify three fundamental solar wind plasma conditions that can generate kappa distributions, noted as (i) Debye shielding, (ii) frozen IMF, and (iii) temperature fluctuations, each one prevailing in different scales of solar wind plasma and magnetic field properties. Moreover, our findings show that the kappa distributions, and thus, their generating mechanisms, vary significantly with solar wind features: (i) the kappa index has different dependence on the solar wind speed for slow and fast modes, i.e., slow wind is characterized by a quasi-constant kappa index, K (is) approx. 4.3 +/- 0.7, while fast wind exhibits kappa indices that increase with bulk speed; (ii) the dispersion of magnetosonic waves is more effective for lower kappa indices (i.e., further from thermal equilibrium); and (iii) the kappa and polytropic indices are positively correlated, as it was anticipated by the theory.

Livadiotis, G.↗
NASA NTRS1990

Simulation of period doubling of recurrent solar wind structures

Based on satellite observations of a recurrent solar wind structure conducted in 1974, an MHD simulation model, and input functions generated from plasma and magnetic field data, the continuing evolution of the solar wind structure outside 5 AU is studied. The model uses the Rankine-Hugoniot relations to describe the jumps in flow properties across the shocks, and it treats shocks as surfaces of discontinuity with zero thickness. Two interaction processes (the collision and the merging of shocks) play important roles in restructuring the solar wind in the outer heliosphere. The simulation result shows that period doubling occurs between 5 and 10 AU. The recurrent solar wind appears to be a persistent new structure between 10 and 20 AU, and it consists of one merged interaction region per solar rotation.

Whang, Y. C.↗
NASA NTRS1997

Limits to the fractionation of isotopes in the solar wind as observed with SOHO/CELIAS/MTOF

Observations of isotope fractionation in the solar wind for the Mg and Ne elements are reported on the basis of measurements performed by the mass time of flight (MTOF) sensor of the charge, element, and isotope analysis system (CELIAS) onboard the SOHO spacecraft. Their isotopic abundance ratios are shown to vary by (0.8+/-0.6) amu in the solar wind bulk velocity range from 350 km/s to 650 km/s. A good correlation is noticed between the isotope fractionation and the elemental fractionation 4He/1H in the solar wind. The results were discussed in the context of recent theoretical models, which predict isotope fractionation in the solar wind by variable Coulomb friction in several solar wind regimes.

Kallenbach, R.↗
NASA NTRS1975

A model for the origin of solar wind stream interfaces

The basic variations in solar wind properties that have been observed at 'stream interfaces' near 1 AU are explained by a gas dynamic model in which a radially propagating stream, produced by a temperature variation in the solar envelope, steepens nonlinearly while moving through interplanetary space. The region thus identified with the stream interface separates the ambient solar wind from the fresh hot material originally in the stream. However, the interface regions given by the present model are thicker than most stream interfaces observed in the solar wind, a fact suggesting that some additional physical process may be important in determining that thickness. Variations in the density, speed, or Alfven pressure alone appear not to produce streams with such an interface.

Hundhausen, A. J.↗
NASA NTRS2019

Highly Structured Slow Solar Wind Emerging From an Equatorial Coronal Hole

At solar minimum, the solar wind is observed at high solar latitudes as a predominantly fast (> 500 km/s), highly Alfvenic, rarefied stream of plasma originating deep within coronal holes, while near the ecliptic plane it is interspersed with a more variable slow (< 500 kms) wind. The precise origins of the slow wind streams are less certain, with theories and observations supporting sources from the tips of helmet streamers, interchange reconnection near coronal hole boundaries, and origins within coronal holes with highly diverging magnetic fields. The heating mechanism required to drive the solar wind is also an open question and candidate mechanisms include Alfven wave turbulence, heating by reconnection in nanoflares, ion cyclotron wave heating and acceleration by thermal gradients1. At 1 au, the wind is mixed and evolved and much of the diagnostic structure of these sources and processes has been lost. Here we present new measurements from Parker Solar Probe at 36 to 54 solar radii that show clear evidence of slow, Alfvenic solar wind emerging from a small equatorial coronal hole. The measured magnetic field exhibits patches of large, intermittent reversals associated with jets of plasma and enhanced Poynting flux and interspersed in a smoother and less turbulent flow with near-radial magnetic field. Furthermore, plasma wave measurements suggest electron and ion velocity-space micro-instabilities that have been identified with plasma heating and thermalization processes. Our measurements suggest an impulsive mechanism associated with solar wind energization and a heating role for micro-instabilities and provide strong evidence for low latitude coronal holes as a significant contribution to the source of the slow solar wind.

Solar physics↗
NASA NTRS1998

Interpretation of Solar Wind Ion Composition Measurements from Ulysses

The ion compositions measured in situ in the solar wind are important since the ion fractions carry information on the plasma conditions in the inner corona. The conditions in the inner corona define the properties of the solar wind plasma flow. Thus, if the ion fraction measurements can be used to unravel some of the plasma parameters in the inner corona, they will provide a valuable contribution to solving the heating and acceleration problem of the solar wind. The ion charge states in the solar wind carry information on electron temperature, electron density and ion flow speed. They are also sensitive to the shape of the electron distribution function. Through carefully modeling the solar wind and calculating the ion fractions predicted for different solar wind conditions, constraints on the electron temperature and ion flow speeds can be placed if the electron density is measured using polarization brightness measurements.

Esser, Ruth↗
NASA NTRS1992

Charge-exchange born He(+) ions in the solar wind

The effect of charge transfer between solar wind alpha-particles and hydrogen atoms of interstellar origin is revisited. Singly-charged helium ions born in the charge transfer carry important information on processes in the solar wind and the heliosphere. The velocity distribution of such He(+) ions is substantially different from that of He(+) pick-up ions due to ionization of the interstellar helium atoms. Estimates of the expected abundances of the charge-exchange born He(+) in the solar wind are presented, and the possibility of measuring this plasma component on deep space missions is discussed.

Gruntman, Michael A.↗
NASA NTRS2012

Three-Dimensional Magnetohydrodynamic Modeling of the Solar Wind Including Pickup Protons and Turbulence Transport

To study the effects of interstellar pickup protons and turbulence on the structure and dynamics of the solar wind, we have developed a fully three-dimensional magnetohydrodynamic solar wind model that treats interstellar pickup protons as a separate fluid and incorporates the transport of turbulence and turbulent heating. The governing system of equations combines the mean-field equations for the solar wind plasma, magnetic field, and pickup protons and the turbulence transport equations for the turbulent energy, normalized cross-helicity, and correlation length. The model equations account for photoionization of interstellar hydrogen atoms and their charge exchange with solar wind protons, energy transfer from pickup protons to solar wind protons, and plasma heating by turbulent dissipation. Separate mass and energy equations are used for the solar wind and pickup protons, though a single momentum equation is employed under the assumption that the pickup protons are comoving with the solar wind protons.We compute the global structure of the solar wind plasma, magnetic field, and turbulence in the region from 0.3 to 100 AU for a source magnetic dipole on the Sun tilted by 0 deg - .90 deg and compare our results with Voyager 2 observations. The results computed with and without pickup protons are superposed to evaluate quantitatively the deceleration and heating effects of pickup protons, the overall compression of the magnetic field in the outer heliosphere caused by deceleration, and the weakening of corotating interaction regions by the thermal pressure of pickup protons.

Usmanov, Arcadi V.↗
NASA NTRS1976

The August 1972 solar-terrestrial events - Solar wind plasma observations

Solar-wind observations associated with the enhanced levels of solar activity in August 1972 are reviewed with emphasis on recent analyses which more unambiguously characterize the changes in the interplanetary medium. Observations from Pioneer 9 at about 0.8 AU, Pioneer 10 at 2.2 AU, and in the vicinity of earth are reviewed and reinterpreted in the light of new data. Recent calculations of local shock velocities are reviewed, and their implications are discussed. These results indicate that, contrary to previously published observations, there is no significant deceleration of the interplanetary shocks between 0.8 AU and 2.2 AU.

Intriligator, D. S.↗