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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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NASA NTRS1985

The structure of high-temperature solar flare plasma in non-thermal flare models

Analytic differential emission measure distributions have been derived for coronal plasma in flare loops heated both by collisions of high-energy suprathermal electrons with background plasma, and by ohmic heating by the beam-normalizing return current. For low densities, reverse current heating predominates, while for higher densities collisional heating predominates. There is thus a minimum peak temperature in an electron-heated loop. In contrast to previous approximate analyses, it is found that a stable reverse current can dominate the heating rate in a flare loop, especially in the low corona. Two 'scaling laws' are found which relate the peak temperature in the loop to the suprathermal electron flux. These laws are testable observationally and constitute a new diagnostic procedure for examining modes of energy transport in flaring loops.

Emslie, A. G.↗

NASA NTRS1986

Narrow-band, slowly varying decimetric radiation from the dwarf M flare star YZ Canis Minoris

Observations of slowly varying radiation from the dwarf M star YZ Canis Minoris with a maximum flux density of 20 mJy and narrow-band frequency structure at frequencies near 1465 MHz are presented. Possible explanations for this radiation are examined. Thermal gyroresonant radiation would require impossibly large coronal loops and magnetic field strengths. The narrow-band structure cannot be explained by continuum emission processes such as thermal bremsstrahlung, thermal gyroresonant radiation, or nonthermal gyrosynchrotron radiation. Coherent burst mechanisms seem to be required.

Lang, K. R.↗

NASA NTRS1988

A partially collisional model of the Titan hydrogen torus

Occasional collision effects are encompassed by the present numerical model of atomic hydrogen densities in Titan's hydrogen torus, which is noted to be azimuthally symmetric; the torus density peaks sharply at Titan's orbit, and rapidly decreases in the both radially outward and perpendicular directions. Collisions of Saturnian atoms with the torus population produce a corona. Without this coronal population, the torus model would not reproduce the Voyager 2 Lyman-alpha intensities. These observations may nevertheless be modelled by a torus-plus-corona model that also reconciles the Pioneer 11 torus observations with those of the two Voyager spacecraft.

Hilton, Douglas A.↗

NASA NTRS1972

Solar-wind structure determined by corotating coronal inhomogeneities. II - Arbitrary perturbations.

The problem of the solar-wind structure resulting from long-lived inhomogeneities in the solar corona has been extended within the framework of a linearized hydrodynamic approach to allow for arbitrary perturbations in the plasma parameters, namely, velocity, density, and temperature. By using a Parker model for the zero-order flow speed, solutions are given for perturbations imposed at the zero-order critical point. Using the restriction that the solutions must be regular at the critical point, it is possible to express all situations as a linear sum of the solutions for pure velocity, density, and temperature perturbations at the critical point. The results of the integrations for three inner-boundary conditions can be used as the elements of matrices that map arbitrary perturbations at one heliocentric distance to any other distance. One interesting feature of the solutions is that the amplitudes at earth for the pure initial temperature situation are larger than for the other two situations.

Siscoe, G. L.↗

NASA NTRS1983

Conductive damping of coronal motions

Time-dependent motions in the solar corona are subject to modification through conductive transport of thermal energy away from compression regions and into rarefaction regions. The evolution depends on how the energy is introduced - by an essentially thermal perturbation or by a density perturbation. The net effect of thermal conduction is to convert local kinetic energy in the transient into widely distributed thermal energy, which then may in turn be partially converted into kinetic energy of the bulk flow. Local motion in the transient is thereby damped. These effects are modeled here using a numerical solution of the time-dependent solar wind equations for single-fluid thermally conductive flow. The simulations of transient phenomena are made for ordinary collisional thermal conduction and compared with similar simulations done using polytropic flow. Between one and five solar radii, the gross character in the two cases is very similar. However, there are effects with thermal conduction that cannot be modeled with polytropic flow, illustrated here by temperature and velocity forerunners and a delayed and large-amplitude trailing velocity low. If thermal conduction is artificially inhibited by simple reduction of the thermal conduction coefficient, relative velocity amplitudes remain larger in approximate proportion to the amount the coefficient is reduced.

Suess, S. T.↗

NASA NTRS1981

Propagation of magnetically guided acoustic shocks in the solar chromosphere

The propagation of a train of acoustic shocks guided by diverging magnetic fields through a static model of the solar chromospheric network and transition region is investigated. For initial flux densities of 1,000,000 ergs/sq cm/sec in the lower chromosphere, the local efficiency of acoustic transmission into the corona can be much higher than that calculated for a plane parallel atmosphere. Acoustic energy will tend to be deposited at higher chromospheric levels in diverging magnetic fields, and magnetic guiding may influence the temperature profile of the network and plages. The total flux that can be transmitted into the corona along such diverging fields is severely limited since the magnetic elements occupy a small fractional area of the photosphere and the transmission efficiency is a rapidly decreasing function of initial flux density. Diverging magnetic fields and a varying ratio of specific heats are not likely to allow high frequency shocks to dissipate high enough in a static atmosphere to contribute significantly to the coronal energy balance.

Foukal, P.↗

NASA NTRS1993

A two-dimensional MHD global coronal model - Steady-state streamers

We describe a two-dimensional time-dependent, numerical, magnetohydrodynamic model for the determination of the physical properties of coronal streamers from the top of the transition zone (solar radius = 1) to 15 solar radii. Four examples are given: for dipole, quadrupole, and hexapole initial field topologies. The computed parameters are density, temperature, velocity, and magnetic field. In addition to the properties of the solutions, their accuracy is discussed. We use the model as the basis for a general discussion of the way boundary conditions are specified in this and similar simulations.

Wang, A.-H.↗

NASA NTRS2019

Near-Sun Observations of an F-Corona Decrease and K-Corona Fine Structures

Remote observations of the solar photospheric light scattered by electrons (K-corona) and dust (F-corona/Zodiacal light) have been made from the ground during eclipses and from space at 1 AU and as close as 0.3 AU. Previous observations of dust scattering have not confirmed the existence of a theoretically-predicted dust free zone near the Sun. The transient nature of the corona has been well characterized for large events, but questions still remain (e.g. initiation, production of solar energetic particles) and for small events even the structure is uncertain. Here we report on imaging the solar corona, from the Parker Solar Probe spacecraft, during the first two perihelion passes (0.16-0.25 AU), each of ten days duration. The view from these distances is qualitatively similar to the historical view, but there are some significant differences in the details. We have uncovered at short elongations a decrease in the intensity of the F-coronal intensity, which is suggestive of the long-sought dust free zone. Also we have resolved the plasma structure of very small eruptions, which are being frequently ejected from the Sun. They take two forms - the commonly observed magnetic flux ropes or the predicted, but not yet observed, magnetic islands arising from the tearing mode instability in the current sheet. Our observations of the coronal streamer evolution confirm the large-scale topology of the solar corona, but they also reveal that, as recently predicted, streamers are composed of yet smaller sub-streamers channeling continual density fluctuations at all visible scales.

Astronomy and astrophysics↗

NASA NTRS2016

The FIELDS Instrument Suite for Solar Probe Plus Measuring the Coronal Plasma and Magnetic Field, Plasma Waves and Turbulence, and Radio Signatures of Solar Transients

NASA's Solar Probe Plus (SPP) mission will make the first in situ measurements of the solar corona and the birthplace of the solar wind. The FIELDS instrument suite on SPP will make direct measurements of electric and magnetic fields, the properties of in situ plasma waves, electron density and temperature profiles, and interplanetary radio emissions, amongst other things. Here, we describe the scientific objectives targeted by the SPP/FIELDS instrument, the instrument design itself, and the instrument concept of operations and planned data products.

Solar Probe Plus↗

NASA NTRS1982

The growth of filaments by the condensation of coronal arches

A model of filament formation based on the condensation of coronal arches is described. The condensation results from initiating the radiative instability within an arch by superimposing a transient energy supply upon the steady state heating mechanism. The transient energy supply increases the density within the arch so that when it is removed the radiative losses are sufficient to lead to cooling below the minimum in the power loss curve. Times from the initial formation of the condensation to its temperature stabilization as a cool filament have been calculated for various initial conditions. They lie in the range 10,000-100,000 s with the majority of the time spent above a temperature of 1 x 10 to the 6th K. Under the assumption that the condensation of a single arch forms an element of the filament, a complete filament requires the condensation of an arcade of loops. Using experimentally derived parameters, filament densities of 10 to the 11th to 10 to the 12th per cu cm can be obtained.

Davis, J. M.↗

NASA NTRS1986

Electron density diagnostics in the 10-100 A interval for a solar flare

Electron density measurements from spectral-line diagnostics are reported for a solar flare on July 13, 1982, 1627 UT. The spectrogram, covering the 10-95 A interval, contained usable lines of helium-like ions C V, N VI, O VII, and Ne IX which are formed over the temperature interval 0.7-3.5 x 10 to the 6th K. In addition, spectral-line ratios of Si IX, Fe XIV, and Ca XV were compared with new theoretical estimates of their electron density sensitivity to obtain additional electron density diagnostics. An electron density of 3 x 10 to the 10th/cu cm was obtained. The comparison of these results from helium-like and other ions gives confidence in the utility of these tools for solar coronal analysis and will lead to a fuller understanding of the phenomena observed in this flare.

Brown, W. A.↗

NASA NTRS1986

Small-scale structures and the density irregularity of the inner corona

The observational evidence is considered that the electron density irregularity factor is much greater than unity in the inner corona, in particular, evidence derived from the photometric comparison of the K-corona emission p Beta with the EUV emission from coronal ions. A simple mathematical model was developed for the irregularity having a minimum number of parameters. This model was used to explore some implications of the observations and to show that well-known resolved structures such as polar plumes and coronal loops as presently understood cannot alone explain the irregularity.

Orrall, F. Q.↗

NASA NTRS1992

A two-dimensional MHD global coronal model - Steady-state streamers

A 2D, time-dependent, numerical, MHD model for the simulation of coronal streamers from the solar surface to 15 solar is presented. Three examples are given; for dipole, quadrupole and hexapole (Legendre polynomials P1, P2, and P3) initial field topologies. The computed properties are density, temperature, velocity, and magnetic field. The calculation is set up as an initial-boundary value problem wherein a relaxation in time produces the steady state solution. In addition to the properties of the solutions, their accuracy is discussed. Besides solutions for dipole, quadrupole, and hexapole geometries, the model use of realistic values for the density and Alfven speed while still meeting the requirement that the flow speed be super-Alfvenic at the outer boundary by extending the outer boundary to 15 solar radii.

Wang, A.-H.↗

NASA NTRS1997

IPS observations of the solar wind velocity and the acceleration mechanism

Coronal holes are well know sources of high speed solar wind, however, the exact acceleration mechanism of the wind is still unknown. Interplanetary scintillation (IPS) observations indicate that the fast solar wind reaches an average velocity of 800 km s(exp -1) within several solar radii with large velocity fluctuations. However, the origin of the IPS velocity spread below 10 solar radii is unclear. A previously developed coronal home model with a more realistic initial state is applied, and time-dependent, nonlinear, resistive 2.5-DMHD equations are numerically solved. It is found that nonlinear solitary-like waves with a supersonic phase speed are generated in coronal holes by torisonal Alfven waves in the radial flow velocity. The outward propagating nonlinear waves are similar in properties to sound solitons. When these waves are present, the solar wind speed and density fluctuate considerably on a time scale of an hour and on spatial scales of several solar radii in addition to the Alfvenic fluctuations. This is in qualitative agreement with the IPS velocity observations beyond 10 solar radii.

Ofman, L.↗

NASA NTRS1999

Corona Structure and Heating Constraints Determined from SERTS Observations

Intensities of the Extreme-ultraviolet (EUV) spectral lines were measured as a function of radius off the solar limb by two flights of the Goddard's Solar Extreme-ultraviolet Rocket Telescope and Spectrograph (SERTS) for three quiet sun regions. Density scale heights were determined for the different spectral lines. Limits on the filling factor were determined. In the one case where an upper limit was determined it was much less than unity. Coronal heating above 1.15 solar radii is required for all three regions studied. For reasonable filling factors, local heating is needed.

Falconer, D. A.↗

NASA NTRS2008

Jet-Front Speed and the Origin of Jets in Polar Coronal Holes

The area-average strength of the open magnetic field in the polar coronal holes can be estimated from the radial component of the magnetic field measured by Ulysses in the solar wind, the fraction of the solar sphere covered by the polar coronal holes, and the fraction of the heliosphere filled by the fast solar wind from the polar coronal holes. For the present minimum phase of the solar cycle, the estimated strength is approximately 10 G. Using this strength for the ambient open field in the standard reconnection model for jets in coronal holes, we obtain for any given jet-front speed a lower bound on the initial temperature of the expanding jet-front plasma, and an upper bound on the ambient plasma density at the reconnection site. These two bounds indicate the following. For jet-front speeds of approximately 1000 km/s, (1) the reconnection site has to be in the low corona or upper transition region (n(e) is less than 10(exp 9) cm(exp -3)), not in the lower transition region or chromosphere, (2) the jet-front plasma is initially heated to T greater than approximately 10(exp 7) K, and (3) hence a compact X-ray flare is produced at the base of the jet. For jet-front speeds less than approximately 100 km/s, (1) the jet can be produced by reconnection in the lower transition region (approximately 10(exp 9) less than n(e) less than approximately 10(exp 10) cm(exp-3)) or upper chromosphere (approximately 10(exp 10) less than n(e) less than approximately 10(exp 12) cm-3), (2) the initial temperature of the jet-front plasma can be less than 10(exp 6) K, and (3) hence some EUV and H(alpha) jet-type macrospicules may be produced with no detectable X-ray emission.

Moore, Ron↗

NASA NTRS1975

A study of coronal structures in a non-magnetic star

Convective instability in the solar corona growth, especially linear growth, was studied. An investigation was also made of nonlinear instability development on a computer. The computer was used to follow a growing perturbation from a small local enhancement in the density of the solar wind to a large increase in the density which would look like one of the rays seen in the sun's corona.

Source record↗

NASA NTRS2005

Coronal Current Sheet Evolution in the Aftermath of a CME

We report on SOHO-UVCS observations of coronal restructuring following a Coronal Mass Ejection (CME) on November 26, 2002, at the time of a SOHO-Ulysses quadrature campaign. Starting about 3 hours after the CME, which was directed towards Ulysses, UVCS began taking spectra at 1.7 solar radii, covering emission from both cool and hot plasma. Observations continued, with occasional gaps, for more than 2 days. Emission in the 974.8 Angstrom line of [Fe XVIII], indicating temperatures above 6x10(6) K, was observed throughout the campaign in a spatially limited location. Comparison with EIT images shows the [Fe XVIII] emission to overlie a growing post-flare loop system formed in the aftermath of the CME. The emission most likely originates in a current sheet overlying the arcade. Analysis of the [Fe XVIII] emission allows us to infer the evolution of physical parameters in the current sheet over the entire span of our observations: in particular, we give the temperature vs. time in the current sheet and estimate the density. Ulysses was directly above the location of the CME and intercepted the ejecta. High ionization state Fe was detected by SWICS throughout the magnetic cloud associated with the CME, although the rapid temporal variation suggests bursty, rather than smooth, reconnection in the coronal current sheet. Both the remote and in situ observations are compared with predictions of theoretical CME models.

Bemporad, A.↗