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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 577 records · Page 32
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NASA NTRS1978

Measurement and analysis of magnetic field variation during a class 2b flare

A digital analysis of high-time-resolution videomagnetograms taken during a class 2b flare that occurred at 60 deg east is reported. The data were obtained at the Big Bear Observatory and calibrated by a Mt. Wilson magnetogram. Changes of weak magnetic fields (less than 100 G) with an amplitude of 30% to 100% have been detected over 55% of the optical flare region, apparently taking place at the initial phase of the flare. Statistical considerations suggest a real flare association with most of these changes. H-alpha observations show that large changes took place over the footpoints of heavily inclined structures like penumbral fibrils, while smaller changes took place over the plage region. An apparent polarity reversal was found at the feet of erupted fibrils. Based on force-free field calculations these changes can be reasonably explained as a transformation of the current-carrying fields to potential fields which produced large changes in the field-line inclination and rotation.

Tanaka, K.↗
NASA NTRS1977

Energetic solar flare particles and interplanetary shock waves

Estimates from hard X-ray measurements show that for many flares the bulk of the flare energy is released in the form of approximately 10-100-keV energy electrons. The interaction of these electrons with the solar atmosphere can produce the optical, UV, EUV, and radio emissions observed during the flare impulsive phase. In addition, explosive heating and evaporation of the chromosphere by these electrons can produce the roughly 10 million K soft X-ray plasma. For the large solar flares which produce interplanetary shock waves, the accelerated approximately 10-100-keV electron population may produce the heating and mass motion required for mass ejection and the formation of the shock wave. The shock wave can in turn accelerate ions and electrons to higher energy as it travels through the corona and interplanetary medium.

Lin, R. P.↗
NASA NTRS1978

Umbral flares

H-alpha flare patches usually do not occur in sunspot umbrae. Presented here are cases of a type of umbral flare in which the flare patch originated in, and was confined to, the p spot umbra. All are H-alpha subflares. Two of the four flares were accompanied by type III radio bursts. The simplicity and similarity of the magnetic fields of these regions were striking.

Tang, F.↗
NASA NTRS1978

The EUV continuum emission /1400-1960 A/ in a solar flare observed from Skylab

The total radiative output in the EUV continuum (1400-1960 A) from the 5 September 1973 flare has been obtained from the EUV spectra of the flare observed with the NRL slit spectrograph (SO82B) on Skylab. The radiative energy in the EUV continuum is of the order of 10 to the 29th ergs, which is more than a factor of 2 greater than those radiated in soft X-rays (8-20 A) and in H-alpha for the flare. Thus, the EUV continuum emission is an important radiative energy loss, and should be included in the consideration of the energy balance of the flare.

Cheng, C.-C.↗
NASA NTRS1979

Nucleosynthesis of Li-7 in flares on UV Ceti stars

The possible production of Li-7 by nuclear reactions in UV Ceti flares has been considered. By utilizing solar observations and theory, a relationship is derived between flare energy and production rates for Li-7; approximately 100 erg of total flare energy is found to denote the formation of a Li-7 atom. Based on this value and best estimates of UV Ceti-type flare rates, it is concluded that less than 10% of the Li-7 observed in the intestellar medium may have been produced by this mechanism. Formation of significant amounts of interstellar deuterium by this method is ruled out.

Karpen, J. T.↗
NASA NTRS1979

The evolution of soft X-ray-emitting flare loops

We have constructed a numerical model for a cooling flare loop in which the complete set of single-fluid equations in a one-dimensional geometry (i.e., parallel to the magnetic field) is solved. Both evaporative and static boundary conditions for the chromosphere-corona interface have been developed. This model is used to investigate the effects of initial temperature and density, loop geometry, and boundary conditions on the form of the plasma evolution and the soft X-ray emission. The results are then compared with Skylab S-056 observations of the 1973 August 9 flare. For this comparison, and under the present assumptions, we conclude that even highly compact flares must have a multiloop structure similar to large flares, and that both radiative and conductive cooling are necessary to explain the observations. The data appear to be consistent with the predicted emission from a combination of evaporative cooling loops.

Antiochos, S. K.↗
NASA NTRS1978

Thermal X-rays and deuterium production in stellar flares

The X-ray spectrum of flares is shown to be necessarily thermal up to at least 200 keV because the self-magnetic field of any electron stream required for a thick- or thin-target source is inconsistently large. The resulting flare model can then be related to stellar luminosity convection, and magnetic fields to result in a maximum possible gamma-burst and continuous X-ray flux. One of the most striking isotopic anomalies observed is the extreme enrichment of He-3 in some solar flares and the mysterious depletion of deuterium. The way in which deuterium may be produced and emitted in the largest flares associated with gamma-bursts but in amounts insufficient to support the tentative conclusion of Coleman and Worden (1976) is discussed.

Colgate, S. A.↗
NASA NTRS1979

Study of the post-flare loops on 29 July 1973. II - Physical parameters in the X-ray loops

We use the filter ratio method of analysis to determine spatially resolved values of plasma parameters in the X-ray emitting post-flare loop system which developed on 29 and 30 July 1973. We find that the loops were hotter and had higher plasma pressure at their tops than near their footpoints. The loop tops were at nearly the same temperature at different places 3 hr after the flare maximum and were also at nearly this same temperature 3 and 8 hr later. Variations in brightness transverse to the loops were due to variations in emission measure. We show by consideration of radiative losses alone that energy must have been added to the hottest part of the flare, at the tops of the loops, late in the decay phase of the flare.

Petrasso, R. D.↗
NASA NTRS1979

XUV electron density diagnostics for solar flares

Potential electron-density diagnostics for the high-temperature component of solar flares are studied with reference to the wavelength region from 171 to 630 A. The specific ions discussed include Fe IX through Fe XV, Ni XI through Ni XVII, and ions in the beryllium, boron, carbon, and nitrogen isoelectronic sequences. Line ratios that could be useful as density indicators under solar-flare conditions are indicated, available data for the ions considered are reviewed, and several theoretical intensity ratios are plotted. The results are employed to determine the electron-density distribution as a function of electron temperature for several spectra from two flares. For these flares it is found that the electron density increases from 10 billion to 500 billion per cu cm for a temperature increase from 1 million to 10 million K.

Dere, K. P.↗
NASA NTRS1979

Solar flares

The present paper deals with explosions in a magnetized solar plasma, known as flares, whose effects are seen throughout the electromagnetic spectrum, from gamma-rays through the visible and to the radio band. The diverse phenomena associated with flares are discussed, along with the physical mechanisms that have been advanced to explain them. The impact of solar flare research on the development of plasma physics and magnetohydrodynamics is noted. The rapid development of solar flare research during the past 20 years, owing to the availability of high-resolution images, detailed magnetic field measurements, and improved spectral data, is illustrated.

Rust, D. M.↗
NASA NTRS1980

Flared tube attachment fitting

Tubes can be flared first, then attached to valves and other flow line components, with new fitting that can be disassembled and reused. Installed fitting can be disassembled so parts can be inspected. It can be salvaged and reused without damaging flared tube; tube can be coated, tempered, or otherwise treated after it has been flared, rather than before, as was previously required. Fitting consists of threaded male portion with conical seating surface, hexagonal nut with hole larger than other diameter of flared end of tube, and split ferrule.

Alkire, I. D.↗
NASA NTRS1979

HEAO 1 observations of X-ray emission from flares on dMe stars

The paper reports the detection of two X-ray flares from each of the nearby dMe stars, AT Mic and AD Leo, with the A-2 experiment on board HEAO 1. A spectrum obtained during the brighter AT Mic flare, the first X-ray spectrum of a stellar flare, is well matched by a thermal model with a temperature 3 x 10 to the 7 K and an iron K-alpha emission line. The X-ray luminosities derived are in the range 1.3-16 x 10 to the 30th ergs/s, while emission measures are in the range 1.1-14 x 10 to the 53rd/cu cm. The estimated Lx/Lopt ratios exceed unity and are inconsistent with Mullan's flare model. Several scenarios to explain this discrepancy are proposed.

Kahn, S. M.↗
NASA NTRS1980

Three flares with blue continuum, one with a D3 preflare shell

Three flares with short-lived flashes observed with a 65 A wide filter centered on 3835 A are discussed. Observations at other wavelengths indicated it is a 'blue continuum'; the flashes occur in pairs at the ends of flux loops and result from the precipitation of energetic particles. A shell of D3 absorption was observed in one of the flares, considered to be a compressed sheet of dense, hot material that limits the flare until it breaks out as a surge. It is concluded that the blue continuum may be the same as that seen in flare stars.

Zirin, H.↗
NASA NTRS1980

Densities and mass motions in transition-zone plasmas in solar flares observed from Skylab

The electron densities and bulk motions in the transition-zone plasma of a solar flare are investigated by an analysis of EUV emission line spectra taken on Skylab. Spectra of three flares were obtained with the NRL normal incidence grating slit spectrograph in the ranges 1100-1940 and 1940-3940 A. The line ratios of transition-zone Si III lines and the intensity of the forbidden O IV 1401-A lines during flare maximum indicate electron densities on the order of 10 to the 12th/cu cm, decreasing by a factor of 2 to 3 in the flare decay phase. Line broadening of the transition-zone ions such as Si III, O IV, C IV and N V is noted, and the observed asymmetrical N V line profiles are approximated by a bi-Gaussian model of a stationary and a moving plasma component with a bulk velocity of 12 to 70 km/sec. Turbulent velocities of from 45 to 85 km/sec and from 20 to 40 km/sec are also indicated for the moving and stationary components of the transition-zone plasma, respectively.

Cheng, C.-C.↗
NASA NTRS1980

Flare models

The current status of solar flare modeling is reviewed. Primary and secondary observational features that a proposed flare model should be capable of explaining are discussed, including energy storage and release, particle acceleration, mass ejection, heating of the temperature minimum region, X-ray, EUV, UV, visible and radio emission and mass flow. Consideration is then given to the twisted flux tube paramagnetic recombination model of Gold and Hoyle (1960), the current model of Alfven and Carlqvist (1967), closed current-sheet models such as those of Syrovatskii (1966, 1969, 1977) and Uchida and Sakurai (1977), open-field models such as those of Carmichael (1964) and Barnes and Sturrock (1972), the emerging flux model of Heyvaerts and Priest (1974, 1977, 1978) and the loop-flare models of Spicer (1977) and Colgate (1978). It is noted that no one model can yet account for all the observational features, and that there may turn out to be several types of flare, each requiring its own explanation.

Sturrock, P. A.↗
NASA NTRS1979

Skylab observations of the Al I autoionization lines gamma 1932 and gamma 1936 during a solar flare

Observations are presented of the Al I autoionization lines gamma 1932 and gamma 1936 during the H alpha class SN subflare of 1973 August 9, obtained from the NRL slit spectrograph aboard Skylab. These profiles are compared with observations from an active-region plage and with the quiet-sun observations of McAllister. Line profiles through model flare and quiet-sun atmospheres are computed assuming local thermodynamic equilibrium. The Al I autoionization lines are sensitive to the conditions just below the temperature-minimum region of the model atmosphere, and the observations support evidence that increased heating of this region occurs during flares. It shown, using observations from three additional flares, that excess flare heating down to the T sub min region probably occurs only near loop footpoints. The behavior of Fe and Fe II lines in the 1920 A range is also discussed.

Cook, J. W.↗
NASA NTRS1980

Observations of the O I 1355.6 A and C I 1355.8 A lines in solar flares

The paper presents observations of the intersystem line O I 1355.6 A and the allowed line C I 1355.8 A in solar flares. In flares, the intersystem O I line is weaker than the allowed C I line and the intensity ratio O I/C I is 0.3-0.7. On the other hand, in active regions, O I line is stronger than the C I line and O I/C I is about 1-2, while in quiet sun regions, the O I line is much stronger than the C I line and O I/C I not in excess of 20. The variation of the intensity ratio from quiet sun region to flares may be due to an electron density enhancement of a factor of about 50 in flares.

Cheng, C. C.↗
NASA NTRS1980

The flare of September 7, 1973 - A typical example of a newly recognized class of solar transients

X-ray, extreme-ultraviolet and optical observations of a solar flare are discussed. It is shown that the flare exemplifies a class of transient events characterized by long duration and long decay time and by the development of high systems of loops, generally brighter at the top. In contrast with compact short lifetime events, the distinctive properties of this class of transients are: (1) the disruption of the magnetic configuration at the flare onset, as indicated by prominence eruption or activation and by associated white-light coronal transients; (2) a continuous energy deposition, presumably at the top of loops, during a large fraction of the flare development and well after the intensity peak; (3) a continuous supply of additional material to the top of loops, with subsequent downflows and out-of-hydrostatic equilibrium conditions.

Pallavicini, R.↗