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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 451 records · Page 25
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NASA NTRS1972

Solar flares in the EUV observed from OSO-5.

Solar flares in three broad EUV spectral bands have been observed from OSO-5 with a grating spectrophotometer. Results are given for three large flares of Mar. 12, Mar. 21 and Apr. 21, 1969. In general, the time dependence of the flare intensity in each band is characterized by a slowly varying component with impulsive bursts superimposed. Bands 2 (465 to 630 A) and 3 (760 to 1030 A) are quite similar in their time variations, but band 1 (280 to 370 A) shows less impulsive structure, and declines more slowly. Absolute EUV intensities for the flares are estimated, and a comparison is made with the 2800 mc/sec radio emission. A flare model is proposed to account for the EUV time variations during a large flare.

Kelly, P. T.↗
NASA NTRS1976

The relevance of solar flares to astrophysics

The physical mechanisms associated with solar flares are reviewed. The relevance of flare mechanisms to other astrophysical phenomena is discussed. In this context, specific models of quasars and radio galaxies, Sco X-1 and gamma-ray bursts are examined.

Sturrock, P. A.↗
NASA NTRS1982

Fluorescent excitation of photospheric Fe K-alpha emission during solar flares

The Bent Crystal Spectrometer on the NASA Solar Maximum Mission satellite provides high spectral and temporal resolution observations of the Fe K-alpha lines. Analyses have been conducted of spectra from almost 50 solar flares that occurred during 1980. These data strongly support fluorescent excitation of photospheric iron by photons of E greater than 7.11 keV emitted by the hot coronal plasma produced during the flare. After comparison of the data with a model, the observed K-alpha line widths are discussed along with estimates of the size of the emitting region, the height of the coronal source and the photospheric iron abundance.

Parmar, A. N.↗
NASA NTRS1985

The propagation of solar flare particles in a turbulent coronal loop

Energetic solar flare particles, both electrons and protons, must survive the turbulent environment of a flaring loop and propogate to the lower corona or chromosphere in order to produce hard X-ray and gamma ray bursts. This plasma turbulence, often observed in soft X-ray line widths to be in excess of 100 km/s, is presumably capable of efficiently scattering the fast flare particles. This prevents to some degree the free streaming of accelerated particles and depending on the amplitude of the turbulence, restricts the particles to diffusive propagation along the length of the loop to the target chromosphere. In addition this turbulence is capable of performing additional acceleration of the fast particles by the second order Fermi mechanism. For compact flares with rise times 10s, the acceleration effect is small and the propagation of the particles is governed by spatial diffusion and energy loss in the ambient medium. The solution of the time dependent diffusion equation with velocity dependent diffusion and energy loss coefficients yields for the case of nonrelativistic protons particle precipitation rates which are necessary for calculating thick target gamma ray emission and also yields the total thin target emissivity.

Ryan, J. M.↗
NASA NTRS1985

Time extended production of neutrons during a solar flare

The most energetic neutral emissions expected from solar flares are gamma rays (10 MeV) from relativistic primary and secondary electron bremsstrahlung,from approx 0 meson decay, and from neutrons ( 50 MeV). Bremsstrahlung photon energies extend to that of the highest energy electron present, but the shape of the pi sup 0 gamma ray spectrum, peaking at 69 MeV, does not depend strongly on the proton spectrum above threshold, which is approx. 292 MeV for meson production on protons. The highest energy neutrons observed indicate directly the highest energy ions which interact at the Sun, and the presence or absence of anergy cutoff in the acceleration process. The high-energy proton spectrum shape can be determined from the neutron spectrum.

Chupp, E. L.↗
NASA NTRS1985

Isotopic overabundances and the energetic particle model of solar flares

According to the energetic particle model of solar flares particles are efficiently accelerated in the magnetic field loop of an active region (AR) by hydromagnetic turbulence. It is demonstrated that the isotopic overabundances observed in some flares are not a consequence of the flare itself but are characteristic of the plasma in the AR. Only when a flare releases the plasma into the interplanetary space it is possible to observe this anomalous composition at spacecraft locations.

Perez-Enriquez, R.↗
NASA NTRS1985

The coalescence instability in solar flares

The nonlinear coalescence instability of current carrying solar loops can explain many of the characteristics of the solar flares such as their impulsive nature, heating and high energy particle acceleration, amplitude oscillations of electromagnetic and emission as well as the characteristics of two-dimensional microwave images obtained during a flare. The plasma compressibility leads to the explosive phase of loop coalescence and its overshoot results in amplitude oscillations in temperatures by adiabatic compression and decompression. It is noted that the presence of strong electric fields and super-Alfvenic flows during the course of the instability play an important role in the production of nonthermal particles. A qualitative explanation on the physical processes taking place during the nonlinear stages of the instability is given.

Tajima, T.↗
NASA NTRS1986

Investigations of turbulent motions and particle acceleration in solar flares

Investigations of X-raya spectra of solar flares show that intense random (turbulent) motions are present in hot flare plasma. Here it is argued that the turbulent motions are of great importance for flare development. They can efficiently enhance flare energy release and accelerate particles to high energies.

Jakimiec, J.↗
NASA NTRS1988

Filament eruptions and the impulsive phase of solar flares

Filament motion during the onset of the solar flare impulsive phase is examined. The impulsive phase onset is established from profiles of about 30 keV X-ray fluxes and the rapid flare brightenings characteristic of the H-alpha flash phase. The filament motion begins several minutes before the impulsive or flash phase of the flare. No new accleration is observed in the motion of the filament during the onset of the impulsive phase for at least two of the four flares. The most common H-alpha brightenings associated with the impulsive phase lie near the magnetic inversion line roughly centered under the erupting filament. Filament speeds at the onset of the impulsive or flash phase lie in the range 30-180 km/s. These characteristics indicate that the filament eruption is not driven by the flare plasma pressure, but instead marks an eruption of magnetic field driven by a global MHD instability of the field configuration in the region of the flare.

Kahler, S. W.↗
NASA NTRS1991

Soft X-ray emission from electron-beam-heated solar flares

Using time-dependent numerical simulations and Solar Maximum Mission observations of a solar flare on 1985 January 23, a study is conducted of the ability of an electron-beam-heating model to reproduce the rise phase of a flare as observed in soft X-ray lines of Ca XIX. The electron beam is parameterized by a peak flux, a low-energy cutoff, and a spectral index, and has a time dependence similar to the observed hard X-ray burst. For a spectral index of 6, only models with a low-energy cutoff of 20 keV reproduce the observed peak emission in the Ca XIX line complex. All models with a low-energy cutoff of 15 keV produce too much emission, while all models with a 25-keV cutoff too little emission. None of the models reproduces the temporal behavior of the soft X-ray emission. The electron-beam-heated component is theorized to only represent a small fraction of the energy released in the impulsive phase of this flare.

Mariska, John T.↗
NASA NTRS1992

Direct electric field heating and acceleration of electrons in solar flares

We show that the observed properties of solar flare X-ray and microwave emission can be explained through the Joule heating and electric field acceleration of runaway electrons in current channels. The global properties of the flaring region required for this are presented. We have fit a hybrid thermal/nonthermal electron distribution, consisting of hot, isothermal electrons with a nonthermal tail of runaway electrons, to high-resolution hard X-ray and microwave spectra and have obtained excellent fits to both. The hybrid model relaxes the electron number and energy flux requirements for the hard X-ray emission over those of a purely nonthermal model. The model also provides explanations for several previously unexplained aspects of the high-resolution microwave spectra. The fit parameters can be related to physical properties (such as the electric field strength in the current channels) of the acceleration region.

Holman, Gordon D.↗
NASA NTRSDate not supplied

Correlated Spatiotemporal Evolution of Extreme-Ultraviolet Ribbons and Hard X-rays in a Solar Flare

We study the structure and evolution of solar flare ribbons in the chromosphere to infer properties of magnetic reconnection that occurs in the corona. We analyze the imaging observations of the M7.3 SOL2014-04-18T13 flare obtained by IRIS in both the near and far ultraviolet passbands and by SDO/AIA in the 1600A passband. Two flare ribbons are observed to spread away from the magnetic polarity inversion line as the flare progresses. Using the high-resolution IRIS observations, we measure the width of the newly brightened ribbon front along the extension of the ribbon, which maps the feet of magnetic field lines reconnecting in the current sheet in the corona. We find that the width of the ribbon front is highly structured, possibly reflecting corresponding structure in the coronal current layer. Further the ribbon grows most rapidly with time in regions where non-thermal hard X-ray (HXR) emission is concentrated (Brosius, et al. 2015). The light curve of the ultraviolet emissions in this region, as measured by IRIS and AIA, also well matches the HXR light curve at photon energies above 25 keV. In contrast, the ribbon-width evolution and light curves in other regions along the ribbons do not correlate well with the HXR emission. These results suggest that there is a strong connection between the production of non-thermal electrons and locally enhanced perpendicular extent of flare ribbon fronts, which reflect the inhomogeneous structure and/or reconnection dynamics of the flare current sheet in the corona.

Stephen Naus↗
NASA NTRS1979

Empirical studies of solar flares: Comparison of X-ray and H alpha filtergrams and analysis of the energy balance of the X-ray plasma

The physics of solar flares was investigated through a combined analysis of X-ray filtergrams of the high temperature coronal component of flares and H alpha filtergrams of the low temperature chromospheric component. The data were used to study the magnetic field configuration and its changes in solar flares, and to examine the chromospheric location and structure of X-ray bright points (XPB) and XPB flares. Each topic and the germane data are discussed. The energy balance of the thermal X-ray plasma in flares, while not studied, is addressed.

Moore, R. L.↗
NASA NTRS1983

Protons from the decay of solar flare neutrons

Fluxes of energetic protons in interplanetary space are observed which are interpreted as the decay products of neutrons generated in a solar flare on 1982 June 3 at 11:42 UT. Because of the particular geometry of this event the spectrum of neutrons escaping from the sun can be constructed with great accuracy in the kinetic energy range 10-100 MeV. The resulting spectrum places stringent constraints on the free parameters used in previously published calculations of neutron production in solar flares. An estimate is made of the diffusion mean free path of charged particles in the interplanetary medium in a new way.

Evenson, P.↗
NASA NTRS1987

A preliminary summary of the observations of the 16 February 1984 solar flare (STIP Interval XV, 12-21 February 1984)

The solar flare on 16 Feb. 1984 (0900 UT) and the associated photon and particle emissions were perhaps the most interesting solar and interplanetary phenomena during STIP Interval XV, 12 to 21 Feb. 1984. The X-ray and microwave radio emissions, as observed from the Earth, were relatively weak and no optical flare was reported. However, the hard X-ray and low energy gamma-ray observations made with the Pioneer Venus Orbiter spacecraft behind the west limb of the Sun indicate that the flare was, in reality, very intense. There is evidence that the flare was located approx 40 deg behind the west limb of the Sun and hence, for instruments located near the Earth, the most intense parts of the X-ray and microwave radio sources were occulted by the photosphere. However, the effect of occultation on the metric type II, type III, and type IV and decimetric (type DCIM) radio sources appeared to be relatively small. Following the flare, a large increase in the counting rates was recorded by several ground level neutron monitors and energetic particle detectors located in interplanetary space. A preliminary analysis of the 16 Feb. 1984 flare observations follows.

Kane, S. R.↗
NASA NTRS1972

X-ray line emission associated with solar flares.

The characteristics of X-ray spectra associated with solar flare activity are examined on the basis of observation data obtained by means of OSO-III and OSO-V. Solar spectra immediately before and during flares are compared, and variation of X-ray line emission with time during an X-ray burst is studied for two emission lines of iron. These and other observations suggest that at any instant of time only a fraction of the eventual amount of flare plasma is being heated and ionized in a highly localized volume, and that the ionization takes place nearly instantaneously.

Neupert, W. M.↗