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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Heating coronal holes and accelerating the solar wind

The special energy requirements of a coronal hole combined with current knowledge of the limited dissipation of Alfven and fast mode MHD waves in the solar corona suggest a unique source of heat for the coronal hole. The near coronal hole requires approximately 3 - 4 x 10 exp 5 ergs/sq cm s, which can come only from the fluid jets, fast particles, and short period MHD waves from the network activity. The high speed streams of solar wind from coronal holes show that there is substantial heating, of 1 - 2 x 10 exp 5 ergs/sq cm s, beyond the sonic point in the wind, which can come only from the dissipation by thermal conduction of long period (approximately equal or greater than 100 sec) MHD waves from subphotospheric convection. Although the Alfven wave flux from the photosphere is generally taken for granted in the literature, we point out that it is a crucial phenomenon that has yet to be established on either a theoretical or observational scientific basis.

Parker, E. N.↗
NASA NTRS1993

Polarimetry of the HI Lyman-alpha for coronal magnetic field diagnostics

We discuss and analyze the possible sources of observational and instrumental uncertainty that can be encountered in measuring magnetic fields of the solar corona through polarimetric observations of the Hanle effect of the coronal Ly-alpha line. The Hanle effect is the modification of the linear polarization of a resonantly scattered line, due to the presence of a magnetic field. Simulated observations are used to examine how polarimetric measurements of this effect are affected by the line-of-sight integration, the electron collisions, and the Ly-alpha geocorona. We plan to implement the coronal magnetic field diagnostics via the Ly-alpha Hanle effect using an all-reflecting Ly-alpha coronagraph/polarimeter (Ly-alphaCoPo) which employs reflecting multilayer mirrors, polarizers, and filters. We discuss here the requirements for such an instrument, and analyze the sources of instrumental uncertainty for polarimetric observations of the coronal Ly-alpha Hanle effect. We conclude that the anticipated polarization signal from the corona and the expected performance of the Ly-alphaCoPo instrument are such that the Ly-alpha Hanle effect method for coronal field diagnostics is feasible.

Fineschi, Silvano↗
NASA NTRS1993

Flux-tube divergence, coronal heating, and the solar wind

Using model calculations based on a self-consistent treatment of the coronal energy balance, we show how the magnetic flux-tube divergence rate controls the coronal temperature and the properties of the solar wind. For a fixed input of mechanical and Alfven-wave energy at the coronal base, we find that as the divergence rate increases, the maximum coronal temperature decreases but the mass flux leaving the sun gradually increases. As a result, the asymptotic wind speed decreases with increasing expansion factor near the sun, in agreement with empirical studies. As noted earlier by Withbroe, the calculated mass flux at the sun is remarkably insensitive to parameter variations; when combined with magnetohydrodynamic considerations, this self-regulatory property of the model explains the observed constancy of the mass flux at earth.

Wang, Y.-M.↗
NASA NTRS1993

How reliable are coronal hole temperatures deduced from observations?

Given the importance of the temperature at the base of the corona for the modeling of the solar wind, we investigate the range of temperatures which have been deduced from remote measurements in coronal holes, within a heliocentric distance of 1.6 solar radii, and the accuracy to which these temperatures have been inferred. Results are presented from an analysis of EUV observations made simultaneously in three spectral lines at the limb in a polar coronal hole, with little contamination from quiet region emission. A temperature range of 7.8-9.3 x 10 exp 5 K is obtained, between 1.02 to 1.07 solar radii for the coronal hole, with a very different temperature range of 9.4 x 10 exp 5 - 1.2 x 10 exp 6 K for the quiet regions bordering it. Inhomogeneities within the coronal hole contribute to a 14 percent variation in inferred temperature. The elemental abundance, which is one of the parameters that influence the temperature inference, can in turn be significantly constrained when intensity ratios from three spectral lines are used.

Habbal, S. R.↗
NASA NTRS1994

UVCS/SOHO capability for determining coronal conditions before, during, and after CME's

The ultraviolet coronal spectrometer (UVCS)/SOHO instrument will provide spectroscopic determinations of plasma parameters describing the evolution of coronal conditions at the site of coronal mass ejections (CME's). Prior to a CME event, UVCS can provide an empirical model of the coronal region where the event originates. The model would include values for the proton density, temperature, and outflow velocity; the electron density and temperature; and minor ion densities, temperatures, and outflow velocities. This information would be determined for heliocentric heights from about 1.5 to 3.5 of the solar radius with a subset of the parameters determined up to heights of 12 of the solar radius. During the event, UVCS would repeatedly sample one or more heights between 1.5 and 3.5 of the solar radius with a time resolution of about 1 min. The goal would be to determine the mass and energy content of the CME as it moves outward. After the event, the conditions in the region would be determined as before. Information about the instrument optical specifications and sensitivity is provided.

Strachan, Leonard↗
NASA NTRS1994

Coronal observations from the soft x-ray telescope on Yohkoh

The Yohkoh payload includes a Soft X-ray Telescope (SXT) which is capable of taking high-resolution (2.5 arcsec) images of the Sun through several X-ray and optical filters. A Charge Coupled Devices (CCD) camera provides images with a low background, and large dynamic range and at a rapid cadence (greater than 2 s), which enables the SXT to observe the effects of flares while continuing to observe the fainter quiet-Sun features. We present the observations from the SXT that illustrate how the faint coronal loops that make up the complex structure of the quiet Sun evolve on a variety of timescales from seconds to months. We describe a variety of coronal structures, such as coronal holes, bright points, the large-scale quiet coronal loops, and active regions, to show the wide range of opportunities that Yohkoh presents to further our understanding of the solar corona.

Strong, K. T.↗
NASA NTRS1994

Coupling of the coronal helium abundance to the solar wind

Models of the transition region-corona-solar wind system are investigated in order to find the coronal helium abundance and to study the role played by coronal helium in controlling the solar wind proton flux. The thermal force on alpha-particles in the transition region sets the flow of helium into the corona. The frictional coupling between alpha-particles and protons and/or the electric polarization field determines the proton flux in the solar wind as well as the fate of the coronal helium content. The models are constructed by solving the time-dependent population and momentum equations for all species of hydrogen and helium in an atmosphere with a given temperature profile. Several temperature profiles are considered in order to very the roles of frictional coupling and electric polarization field in the solar wind, and the thermal force in the transition region. Steady-state solutions are found for coronae with a hydrogen flux at 1 AU of 1.0 x 10(exp 9)/cm(exp 2)/sec or larger. For coronae with lower hydrogen fluxes, the helium flux into the corona is larger than the flux 'pulled out' by the solar wind protons, and solutions with increasing coronal helium content are found. The timescale for forming a helium-filled corona, that may allow for a steady outflow, is long compared to the mixing time for the corona.

Hansteen, Viggo H.↗
NASA NTRS1994

Disruption of coronal magnetic field arcades

The ideal and resistive properties of isolated large-scale coronal magnetic arcades are studied using axisymmetric solutions of the time-dependent magnetohydrodynamic (MHD) equations in spherical geometry. We examine how flares and coronal mass ejections may be initiated by sudden disruptions of the magnetic field. The evolution of coronal arcades in response to applied shearing photospheric flows indicates that disruptive behavior can occur beyond a critical shear. The disruption can be traced to ideal MHD magnetic nonequilibrium. The magnetic field expands outward in a process that opens the field lines and produces a tangential discontinuity in the magnetic field. In the presence of plasma resistivity, the resulting current sheet is the site of rapid reconnection, leading to an impulsive release of magnetic energy, fast flows, and the ejection of a plasmoid. We relate these results to previous studies of force-free fields and to the properties of the open-field configuration. We show that the field lines in an arcade are forced open when the magnetic energy approaches (but is still below) the open-field energy, creating a partially open field in which most of the field lines extend away from the solar surface. Preliminary application of this model to helmet streamers indicates that it is relevant to the initiation of coronal mass ejections.

Mikic, Zoran↗
NASA NTRS1994

Loop models of low coronal structures observed by the Normal Incidence X-Ray Telescope (NIXT)

The X-ray pictures obtained with the Normal Incidence X-Ray Telescope (NIXT), apart from the ubiquitous coronal loops well known from previous X-ray observations, show a new and peculiar morphology: in many active regions there are wide and apparently low-lying areas of intense emission which resemble H alpha plages. By means of hydrostatic models of coronal arches, we analyze the distribution of temperature, density, emission measure, and plasma emissivity in the spectral band to which NIXT is sensitive, and we show that the above morphology can be explained by the characteristics of high pressure loops having a thin region of high surface brightness at the base. We therefore propose that this finding might help to identify high-pressure X-ray emitting coronal regions in NIXT images, and it is in principle applicable to any imaging instrument which has high sensitivity to 10(exp 4) - 10(exp 6) K plasma within a narrow coronal-temperature passband. As a more general result of this study, we propose that the comparison of NIXT observations with models of stationary loops might provide a new diagnostic: the determination of the loop plasma pressure from measurements of brightness distribution along the loop.

Peres, G.↗
NASA NTRS1994

The study of coronal plasma structures and fluctuations with Faraday rotation measurements

We report dual-frequency, polarimetric measurements of Faraday rotation of extragalactic radio sources viewed through the solar corona. The observations were made at the Very Large Array in 1990 during solar maximum. Of the nine observed, an excess rotation measure of -12.6 rad/sq m was detected for one source (0010+005), which was observed at an elongation of about 9 solar radii. This measurement is in fair agreement with an a priori model rotation measure of -8.6 rad/sq m estimated from coronal potential field models and the electron density model of Paetzold et al. (1992). Our measurement provides a value for the coronal magnetic field strength at 9 solar radii given a knowledge of the magnetic field sector structure, of 12.5 +/- 2.3 mG. Rotation measurements of 0010+005 were made approximately once per hour over an 11 hr period. During this interval, a slow change of about 1 rad/sq m/hr in rotation measure was detected. Although we are not absolutely certain that this drift is not unremoved ionospheric Faraday rotation, extensive analysis of data from the other sources suggests that this is not the case (Sakurai & Spangler 1994). The very long timescale for this variation argues against the agency of magnetohydrodynamics (MHD) waves, and we suggest occultation of 0010+005 by relatively static plasma structures in the corona. We filtered our rotation measure time series to search for variations on an hourly timescale, such as those reported by Hollweg et al. (1992), which could be attributed to coronal MHD waves. We were unable to detect such fluctuations and can report only an upper limit to the rms variation of 1.6 rad/sq m. This upper limit is of the same order, but slightly larger than the values typically reported by Hollweg et al. (1982). This upper limit to the rotation measure fluctuations limits the dimensionless wave amplitude (delta B)/B in the corona to be less than 0.7. Using the number, we estimate the MHD wave flux at the coronal base to be less than 1.6 x 10(exp 5) ergs/sq cm/s. This is less than the amount of wave energy flux required by wave-driven models of the solar wind. Finally, we discuss a number of ways in which such observations could be improved in the future.

Sakurai, Takayuki↗
NASA NTRS1994

Latitudinal variability of large-scale coronal temperature and its association with the density and the global magnetic field

In this paper we utilize the latitiude distribution of the coronal temperature during the period 1984-1992 that was derived in a paper by Guhathakurta et al, 1993, utilizing ground-based intensity observations of the green (5303 A Fe XIV) and red (6374 A Fe X) coronal forbidden lines from the National Solar Observatory at Sacramento Peak, and establish it association with the global magnetic field and the density distributions in the corona. A determination of plasma temperature, T, was estimated from the intensity ratio Fe X/Fe XIV (where T is inversely proportional to the ratio), since both emission lines come from ionized states of Fe, and the ratio is only weakly dependent on density. We observe that there is a large-scale organization of the inferred coronal temperature distribution that is associated with the large-scale, weak magnetic field structures and bright coronal features; this organization tends to persist through most of the magnetic activity cycle. These high-temperature structures exhibit time-space characteristics which are similar to those of the polar crown filaments. This distribution differs in spatial and temporal characterization from the traditional picture of sunspot and active region evolution over the range of the sunspot cycle, which are manifestations of the small-scale, strong magnetic field regions.

Guhathakurta, M.↗
NASA NTRS1994

Electron trapping in evolving coronal structures during a large gradual hard X-ray/radio burst

Gradual hard X-ray/radio bursts are characterized by their long duration, smooth time profile, time delays between peaks at different hard X-ray energies and microwaves, and radiation from extended sources in the low and middle corona. Their characteristic properties have been ascribed to the dynamic evolution of the accelerated electrons in coronal magnetic traps or to the separate acceleration of high-energy electrons in a 'second step' process. The information available so far was drawn from quality considerations of time profiles or even only from the common occurrence of emissions in different spectral ranges. This paper presents model computations of the temporal evolution of hard X-ray and microwave spectra, together with a qualitative discussion of radio lightcurves over a wide spectral range, and metric imaging observations. The basis hypothesis investigated is that the peculiar 'gradual' features can be related to the dynamical evolution of electrons injected over an extended time interval in a coronal trap, with electrons up to relativistic energies being injected simultaneously. The analyzed event (26 April. 1981) is particularly challenging to this hypothesis because of the long time delays between peaks at different X-ray energies and microwave frequencies. The observations are shown to be consistent with the hypothesis, provided that the electrons lose their energy by Coulomb collisions and possibly betatron deceleration. The access of the electrons to different coronal structures varies in the course of the event. The evolution and likely destabilization of part of the coronal plasma-magnetic field configuration is of crucial influence in determining the access to these structures and possibly the dynamical evolution of the trapped electrons through betatron deceleration in the late phase of the event.

Bruggmann, G.↗
NASA NTRS1995

Structure and dynamics of coronal plasmas

The Normal Incidence X-ray Telescope (NIXT) obtained a unique set of high resolution full disk solar images which were exposed simultaneously by X-rays in a passband at 63.5 A and by visible light. The perfect alignment of a photospheric visible light image with a coronal X-ray image enables us to present observations of X-ray intensity vs an accurately determined height above the visible limb. The height at which the observed X-ray intensity peak varies from 4000 km in active regions to 9000 km in quiet regions of the sun. The interpretation of the observations stems from the previously established fact that, for the coronal loops, emission in the NIXT bandpass peaks sharply just above the footpoints. Because there is not a sharp peak in the observed X-ray intensity vs off limb height, we conclude that the loop footpoints, when viewed at the limb, are obscured by absorption in chromospheric material along the line of sight. We calculate the X-ray intensity vs height predicted by a number of different idealizations of the solar atmosphere, and we compare these calculations with the observed X-ray intensity vs height. The calculations use existing coronal and chromospheric models. In order for the calculations to reproduce the observed off limb X-ray intensities, we are forced to assume an atmosphere in which the footpoints of coronal loops are interspersed along the line of sight with cooler chromospheric material extending to heights well above the loop footpoints. We argue that the absorption coefficient for NIXT X-rays by chromospheric material is roughly proportional to the neutral hydrogen density, and we estimate an average neutral hydrogen density and scale height implied by the data.

Golub, Leon↗
NASA NTRS1995

Simultaneous Extreme-Ultraviolet Explorer and Optical Observations of Ad Leonis: Evidence for Large Coronal Loops and the Neupert Effect in Stellar Flares

We report on the first simultaneous Extreme-Ultraviolet Explorer (EUVE) and optical observations of flares on the dMe flare star AD Leonis. The data show the following features: (1) Two flares (one large and one of moderate size) of several hours duration were observed in the EUV wavelength range; (2) Flare emission observed in the optical precedes the emission seen with EUVE; and (3) Several diminutions (DIMs) in the optical continuum were observed during the period of optical flare activity. To interpret these data, we develop a technique for deriving the coronal loop length from the observed rise and decay behavior of the EUV flare. The technique is generally applicable to existing and future coronal observations of stellar flares. We also determine the pressure, column depth, emission measure, loop cross-sectional area, and peak thermal energy during the two EUV flares, and the temperature, area coverage, and energy of the optical continuum emission. When the optical and coronal data are combined, we find convincing evidence of a stellar 'Neupert effect' which is a strong signature of chromospheric evaporation models. We then argue that the known spatial correlation of white-light emission with hard X-ray emission in solar flares, and the identification of the hard X-ray emission with nonthermal bremsstrahlung produced by accelerated electrons, provides evidence that flare heating on dMe stars is produced by the same electron precipitation mechanism that is inferred to occur on the Sun. We provide a thorough picture of the physical processes that are operative during the largest EUV flare, compare and contrast this picture with the canonical solar flare model, and conclude that the coronal loop length may be the most important factor in determining the flare rise time and energetics.

Hawley, Suzanne L.↗
NASA NTRS1995

Acceleration of solar wind in polar coronal holes by induction heating

The universal induction heating mechanism supplying with the energy all the processes of coronal heating and the solar wind acceleration is developed. The observed relative 'trembling' of photospheric super-large scale magnetic fields with quasi-periods of 1-4 days amounts 30-40 percent in amplitude. The inductive electric field appears in the corona. The electric currents cause the Joule dissipation. The uneven heating leads to the solar wind acceleration. A model is suggested in which high-speed streams in space are caused by the combination of the enhanced inductive energy flux from the solar coronal active regions; the work against the regular magnetic field; losses from coronal emission. The consideration is made in terms of the dissipative solar wind theory with the finite electrical conductivity of plasma. The leakage of plasma and the energy flux across the magnetic field, caused by the induction heating processes, are taken into account. The polar coronal holes (and the mid-latitude ones) are indicators of energy transfer balance but not direct sources of high-speed streams in the solar wind.

Chertkov, A. D.↗
NASA NTRS1995

Coronal magnetic field diagnostics via the Hanle effect of Lyman series lines

The magnetic field plays a major role in the physics of the solar corona. However, there are no direct measurements of this physical parameter. We describe a method that can provide the most direct determination of the vector magnetic field in the extended corona (i.e., at heliocentric heights between 1.2 R(solar radius) and 2.0 R(solar radius)). The method is based on polarimetric observations of UV lines of the Lyman series, that is, Lyman alpha (Ly-alpha), lambda 1216 A, Lyman beta (Ly-beta), lambda 1025 A, and Lyman gamma (Ly-gamma), lambda 972 A. These lines have a collisional and a resonantly scattered component. Linear polarization is induced in the resonant component by the anisotropy in the chromospheric radiation field that illuminates the corona. Magnetic fields can be suitably determined through the effects that they induce on this resonance polarization (Hanle effect). The Hanle effect of the Ly-alpha is sensitive to field strengths in the 10 - 100 gauss range. The resonance polarization of Ly-beta and Ly-gamma is sensitive, through the Hanle effect, to fields with strengths between 3 - 30 gauss, and 0.3 - 6 gauss, respectively. We describe a new method for separating the resonant from the collisional component of the Ly-beta and Ly-gamma; the method is based on the approximation, valid within 10%, that the collisional component of the Ly-alpha is negligible, in typical coronal conditions. From the intensity and the polarization of the resonant components of these Lyman lines, the strength and direction of coronal fields can be determined. We model the sensitivity of Hanle-effect diagnostics for different coronal structures (e.g., coronal holes and loops).

Fineschi, S.↗
NASA NTRS1995

Exploring the temperature structure of coronal holes with a novel combination of visible Fe lines

The difficulty in establishing the temperature structure and temperature gradient within coronal holes from limb observations is due to the frequent veiling of coronal holes by hotter and denser quiet regions often surrounding them and shaping their boundaries. Nevertheless probing the coronal hole medium itself can be made with a judicious choice of spectral lines. We show how a set of visible forbidden Fe lines, namely Fe IX 3801, Fe X 6374 and Fe Xl 7892 A which are sensitive to plasma temperatures less than or equal to 10(exp 6) K can achieve this purpose. We propose to use these lines in a coordinated manner with coronagraph observations. In addition observations made with the Fe XIV 5303 A line should yield information about any hot material intercepting the line of sight. The proposed combination of these Fe lines offers a very powerful diagnostic tool for coronal hole temperatures and structures.

Habbal, Shadia Rifai↗
NASA NTRS1994

Numerical modeling of coronal mass ejections based on various pre-event model atmospheres

We examine how the initial state (pre-event corona) affects the numerical MHD simulation for a coronal mass ejection (CME). Earlier simulations based on a pre-event corona with a homogeneous density and temperature distribution at lower boundary (i.e. solar surface) have been used to analyze the role of streamer properties in determining the characteristics of loop-like transients. The present paper extends these studies to show how a broader class of global coronal properties leads not only to different types of CME's, but also modifies the adjacent quiet corona and/or coronal holes. We consider four pre-event coronal cases: (1) Constant boundary conditions and a polytropic gas with gamma = 1.05; (2) Non-constant (latitude dependent) boundary conditions and a polytropic gas with gamma = 1.05; (3) Constant boundary conditions with a volumetric energy source and gamma = 1.67; (4) Non-constant (latitude dependent) boundary conditions with a volumetric energy source and gamma = 1.67. In all models, the pre-event magnetic fields separate the corona into closed field regions (streamers) and open field regions. The CME's initiation is simulated by introducing at the base of the corona, within the streamer region, a standard pressure pulse and velocity change. Boundary values are determined using MHD characteristic theory. The simulations show how different CME's, including loop-like transients, clouds, and bright rays, might occur. There are significant new features in comparison to published results. We conclude that the pre-event corona is a crucial factor in dictating CME's properties.

Suess, S. T.↗