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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 37 records · Page 2
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NASA NTRS1980

High-resolution X-ray spectra of solar flares. IV - General spectral properties of M type flares

The spectral characteristics in selected narrow regions of the X-ray spectrum of class M solar flares are analyzed. High-resolution spectra in the ranges 1.82-1.97, 2.98-3.07, 3.14-3.24 and 8.26-8.53 A, which contain lines important for the determination of electron temperature and departure from ionization equilibrium, were recorded by spaceborne Bragg crystal spectrometers. Temperatures of up to 20,000,000 K are obtained from line ratios during flare rise phases in M as well as X flares, while in the decay phase the calcium temperature can be as low as 8,000,000 K, which is significantly lower than in X flares. Large nonthermal motions (on the order of 130 km/sec at most) are also observed in M as well as X flares, which are largest during the soft X-ray rise phase. Finally, it is shown that the method proposed by Gabriel and Phillips (1979) for detecting departures of electrons from Maxwellian velocity distributions is not sufficiently sensitive to give reliable results for the present data.

Feldman, U.↗
NASA NTRS1982

Large scale solar magnetic fields at the site of flares, the greatness of flares, and solar-terrestrial disturbances

Evidence is presented for an intrinsically solar effect which may dominate such solar-terrestrial correlations as that reported by Chertkov (1976), where large H-alpha flares during 1967-1972 in solar active regions with overlying fields on a 100,000 km scale and predominantly north-to-south orientation were more efficient in the production of geomagnetic disturbances than comparable flares in regions whose fields at the flare sites were directed south-to-north. In addition to being responsible for geomagnetic disturbance enhancements, this purely solar effect may cause solar wind velocity and solar flare proton flux enhancements. If the effect can be generalized to other portions of the solar cycle, it could improve present understanding of the flare mechanism and therefore prove useful in the prediction of solar-terrestrial disturbances.

Dodson, H. W.↗
NASA NTRS1982

Study of the post-flare loops on 29 July 1973. IV - Revision of T and n sub e values and comparison with the flare of 21 May 1980

Revised temperature and density estimates are provided in a synthesis of previous studies of the two-ribbon solar flare of July 29, 1973 and results are compared with observations of a similar event by the Solar Maximum Mission on May 21, 1980. Photographs taken through X ray filters permitted determination of the spatial distribution of temperature and density in the loop system and variations over time. The entire growth process of the flare is detailed, including the magnetic field configurations and the enlargement of the Halpha flare ribbon separation distance. The temperature is revised to 8.8 million K during maximum soft X ray emission and 6.8 million K at the appearance of the last Halpha loops. The 1973 flare is noted to have occurred in an old, decaying spotless region, producing higher loops at lower density than the 1980 flare, which displayed ribbons imbedded in a sunspot group.

Svestka, Z.↗
NASA NTRS1982

High-resolution X-ray spectra of solar flares. VI - Impulsive soft X-ray flares

Doschek et al. (1980) and Feldman et al. (1980) have discussed high-resolution soft X-ray spectra of solar flares. The present investigation is concerned with the characteristics of the impulsive events as observed in soft X rays and the cooling of the flare plasma in terms of energy loss processes. A class of intense, short-lived solar flares which appear to be characterized by very high densities at high temperatures is found. For the 1979 November 6 event, the cooling time is so rapid that the density obtained may be a lower limit. The main cooling mechanism for the flares is found to be radiative. Two of the events are associated with strong gamma-ray emission and both of these events are very unusual in their soft X-ray emission. The 1979 November 9 flare shows extremely broad X-ray emission lines during the rise phase, indicative of nonthermal velocities of the order of 300 km/s.

Feldman, U.↗
NASA NTRS1986

High-resolution X-ray spectra of solar flares. VIII - Mass upflow in the large flare of 1980 November 7

The large flare of November 7, 1980 provides a unique opportunity to investigate the upward-moving plasma seen during the early stages of many flares. Soft X-ray spectroscopic data obtained by the Solar Flare X-ray (SOLFLEX) instruments on board the Air Force P78-1 satellite have been used to determine the spatial extent, turbulent velocity, temperature, and emission measure of the blueshifted and stationary plasmas, as well as the upward velocity of the blueshifted component alone. Two geometries are considered in calculating the resultant mass and energy balance. In addition, coincident hard X-ray data was acquired from the HXRBS instrument on board the SMM satellite to determine the relative timing and enertics of the hard and soft X-ray flare plasmas. These results are compared with the predictions of the chromospheric evaporation hypothesis. It is concluded that electron-induced evaporation plays a minor role in this flare, and that another mechanism must account for the observed blueshifted emission.

Karpen, J. T.↗
NASA NTRS1987

On the relationship of flare size and particle anisotropy in solar gamma-ray flares

The effects of radiation directivity on the interpretation of SMM gamma-ray flare observations. If the electron anisotropy is independent of flare size, the size distribution of 300 keV-1 Mev flare is much flatter than that of X-ray flares. This implies that larger flares have, on average, harder electron spectra. Distributions of electrons strongly peaked downward into the solar photosphere are not consistent with the data.

Dermer, Charles D.↗
NASA NTRS1989

Impulsiveness and energetics in solar flares with and without type II radio bursts - A comparison of hard X-ray characteristics for over 2500 solar flares

The hard X-ray characteristics of more than 2500 solar flares are used to study the relative size, impulsiveness, and energetics of flares with and without type II radio bursts. A quantitative definition of the hard X-ray impulsiveness is introduced, which may be applied to a large number of events unambiguously. It is found that the flares with type II bursts are generally not significantly larger, more impulsive, or more energetic than those without type II bursts. Also, no evidence is found to suggest a simple classification of the flares as either 'impulsive' or 'gradual'. Because type II bursts are present even in small flares with relatively unimpulsive energy releases, it is concluded that changes in the ambient conditions of the solar atmosphere causing an unusually low Alfven speed may be important in the generation of the shock wave that produces type II radio bursts.

Pearson, Douglas H.↗
NASA NTRS1990

The beam-driven chromospheric evaporation model of solar flares - A model not supported by observations from nonimpulsive large flares

Most large solar flares exhibit hard X-ray emission which is usually impulsive, as well as thermal soft X-ray emission, which is gradual. The beam-driven chromospheric evaporation model of solar flares was proposed to explain the origin of the soft X-ray emitting flare plasma. A careful evaluation of the issue under discussion reveals contradictions between predictions from the theoretical chromospheric evaporation model and actual observations from a set of large X- and M-type flares. It is shown that although the soft X-ray and hard X-ray emissions are a result of the same flare, one is not a result of the other.

Feldman, U.↗
NASA NTRS1994

Are solar gamma-ray-line flares different from other large flares?

We reevaluate evidence indicating that gamma-ray-line (GRL) flares are fundamentally different from other large flares without detectable GRL emission and find no compelling support for this proposition. For large flares observed by the Solar Maximum Mission (SMM) from 1980 to 1982, we obtain a reasonably good correlation between 4-8 MeV GRL fluences and greater than 50 keV hard X-ray fluences and find no evidence for a distinct population of large hard X-ray flares that lack commensurate GRL emission. Our results are consistent with the acceleration of the bulk of the approximately 100 keV electrons and approximately 10 MeV protons (i.e., the populations of these species that interact in the solar atmosphere to produce hard X-ray and GRL emission) by a common process in large flares of both long and short durations.

Cliver, E. W.↗
NASA NTRS2008

The Flare Irradiance Spectral Model (FISM) and its Contributions to Space Weather Research, the Flare Energy Budget, and Instrument Design

The Flare Irradiance Spectral Model (FISM) is an empirical model of the solar irradiance spectrum from 0.1 to 190 nm at 1 nm spectral resolution and on a 1-minute time cadence. The goal of FISM is to provide accurate solar spectral irradiances over the vacuum ultraviolet (VUV: 0-200 nm) range as input for ionospheric and thermospheric models. The seminar will begin with a brief overview of the FISM model, and also how the Solar Dynamics Observatory (SDO) EUV Variability Experiment (EVE) will contribute to improving FISM. Some current studies will then be presented that use FISM estimations of the solar VUV irradiance to quantify the contributions of the increased irradiance from flares to Earth's increased thermospheric and ionospheric densites. Initial results will also be presented from a study looking at the electron density increases in the Martian atmosphere during a solar flare. Results will also be shown quantifying the VUV contributions to the total flare energy budget for both the impulsive and gradual phases of solar flares. Lastly, an example of how FISM can be used to simplify the design of future solar VUV irradiance instruments will be discussed, using the future NOAA GOES-R Extreme Ultraviolet and X-Ray Sensors (EXIS) space weather instrument.

Chamberlin, Phillip↗
NASA NTRS2023

Solar Flare Ribbon Fronts. I. Constraining Flare Energy Deposition with IRIS Spectroscopy

Spectral lines formed at lower atmospheric layers show peculiar profiles at the "leading edge" of ribbons during solar flares. In particular, increased absorption of the BBSO/GST He I λ10830 line, as well as broad and centrally reversed profiles in the spectra of the Mg II and C II lines observed by the IRIS satellite, has been reported. In this work, we aim to understand the physical origin of such peculiar IRIS profiles, which seem to be common of many, if not all, flares. To achieve this, we quantify the spectral properties of the IRIS Mg II profiles at the ribbon leading edge during four large flares and perform a detailed comparison with a grid of radiative hydrodynamic models using the RADYN+FP code. We also studied their transition region (TR) counterparts, finding that these ribbon front locations are regions where TR emission and chromospheric evaporation are considerably weaker compared to other parts of the ribbons. Based on our comparison between the IRIS observations and modeling, our interpretation is that there are different heating regimes at play in the leading edge and the main bright part of the ribbons. More specifically, we suggest that bombardment of the chromosphere by more gradual and modest nonthermal electron energy fluxes can qualitatively explain the IRIS observations at the ribbon leading front, while stronger and more impulsive energy fluxes are required to drive chromospheric evaporation and more intense TR emission in the bright ribbon. Our results provide a possible physical origin for the peculiar behavior of the IRIS chromospheric lines in the ribbon leading edge and new constraints for the flare models.

Solar flares↗
NASA NTRS1979

A comparison of high-temperature flare models with observations and implications for the low-temperature flare

EUV data from the Harvard College Observatory and Naval Research Laboratory instruments on board the Skylab Apollo telescope mount, together with SOLRAD 9 X-ray data, are analyzed in order to empirically deduce the variation of emission measure with temperature in the atmosphere of a number of solar flares. A 'mean' differential emission measure profile Q(T) for a flare is constructed which is then compared with the profile predicted by a number of theoretical models. It is found that realistic flare models must include both conductive and radiative terms in the energy equation, and that hydrodynamic terms may be important at low temperatures. The implications of the results obtained are discussed for flare models in general and it is shown that the inclusion of the conductive term into models which have hitherto neglected it can perhaps resolve some of the observational difficulties with such models.

Machado, M. E.↗
NASA NTRS1980

High-resolution X-ray spectra of solar flares. III - General spectral properties of X1-X5 type flares

High-resolution X-ray spectra of six class X1-X5 solar flares are discussed. The spectra were recorded by spaceborne Bragg crystal spectrometers in the ranges 1.82-1.97, 2.98-3.07 and 3.14-3.24 A. Electron temperatures derived from dielectronic satellite line to resonance line ratios for Fe XXV and Ca XIX are found to remain fairly constant around 22,000,000 and 16,000,000 K respectively during the rise phase of the flares, then decrease by approximately 6,000,000 K during the decay phase. Nonthermal motions derived from line widths for the April 27, 1979 event are found to be greatest during the rise phase (approximately 130 km/sec) and decrease to about 60 km/sec during decay. Volume emission measures for Fe XXV, Ca XIX and Ca XX are derived from photon fluxes as a function of temperature, and examination of the intensity behavior of the Fe K alpha emission as a function of time indicates that it is a result of fluorescence. Differences between the present and previous observations of temperature variation are discussed, and it is concluded that the flare plasmas are close to ionization equilibrium for the flares investigated.

Doschek, G. A.↗
NASA NTRS1986

A dynamic flare with anomalously dense flare loops

The dynamic flare of November 6, 1980 developed a rich system of growing loops which could be followed in H-alpha for 1.5 hours. Throughout the flare, these loops, near the limb, were seen in emission against the disk. Theoretical computations of b-values for a hydrogen atom reveal that this requires electron densities in the loops to be close to 10 to the 12th per cu cm. From measured widths of higher Balmer lines the density at the tops of the loops was found to be 4 x 10 to the 12th per cu cm if no nonthermal motions were present. It is now general knowledge that flare loops are initially observed in X-rays and become visible in H-alpha only after cooling. For such a high density a loop would cool through radiation from 10 to the 7th K to 10 to the 4th K within a few minutes so that the dense H-alpha loops should have heights very close to the heights of the X-ray loops. This, however, contradicts the observations obtained by the HXIS and FCS instruments on board SMM which show the X-ray loops at much higher altitudes than the loops in H-alpha. Therefore, the density must have been significantly smaller when the loops were formed and the flare loops were apparently both shrinking and becoming denser while cooling.

Svestka, Z.↗
NASA NTRS1987

Flare research with the NASA/MSFC vector magnetograph - Observed characteristics of sheared magnetic fields that produce flares

The present MSFC Vector Magnetograph has sufficient spatial resolution (2.7 arcsec pixels) and sensitivity to the transverse field (the noise level is about 100 gauss) to map the transverse field in active regions accurately enough to reveal key aspects of the sheared magnetic fields commonly found at flare sites. From the measured shear angle along the polarity inversion line in sites that flared and in other shear sites that didn't flare, evidence is found that a sufficient condition for a flare to occur in 1000 gauss fields in and near sunspots is that both: (1) the maximum shear angle exceed 85 degrees; and (2) the extent of strong shear (shear angle of greater than 80 degrees) exceed 10,000 km.

Moore, R. L.↗
NASA NTRS1989

The M8.1 flare of 23 June, 1988. I - Secondary H-alpha brightenings in two-ribbon flares

H-alpha observations of two-ribbon flares often show secondary brightenings which are not directly spatially connected with the main center of activity but which are correlated in time with the primary impulsive flare. A mechanism is presented which explains these secondary brightenings via the reconnection of magnetic loops which are tied to only one of the two ribbons, in contrast with the loops responsible for the main flare which are tied to both ribbons. The distant footpoint is then interpreted as the site of the secondary brightening. The model is applied to the two-ribbon flare of 17:52 UT, June 23, 1988, which started during the rocket flight of the Normal Incidence X-ray Telescope.

Herant, M.↗
NASA NTRS1990

Flare heating and ionization of the low solar chromosphere. II - Observations of five solar flares

Two neutral Mg spectral lines formed in the temperature-minimum region and the low chromosphere, at 4571 and 5173 A, are used to quantify the changes in the atmospheric structure as a function of time during five solar flares. Eight proposed flare heating and ionization mechanisms and predictions of the effects of each on the temperature minimum region are discussed. Two Mg spectral observations made at the National Solar Observatory (Sacramento Peak), along with observations of hard and soft X-rays from the SMM and GOES satellites, are compared to the predictions of the eight proposed mechanisms. The initial effects in all five flares are consistent with backwarming by enhanced Balmer- and Paschen-continuum radiation originating in the upper chromosphere. Extended heating observed in two of the flares is most likely due to UV irradiation. In all cases heating by the dissipation of nonreversed electric currents, collisions with an electron or proton beam, irradiation by soft X-rays, and dissipation of Alfven waves are eliminated.

Metcalf, Thomas R.↗
NASA NTRS1997

EGRET High Energy Capability and Multiwavelength Flare Studies and Solar Flare Proton Spectra

UNH was assigned the responsibility to use their accelerator neutron measurements to verify the TASC response function and to modify the TASC fitting program to include a high energy neutron contribution. Direct accelerator-based measurements by UNH of the energy-dependent efficiencies for detecting neutrons with energies from 36 to 720 MeV in NaI were compared with Monte Carlo TASC calculations. The calculated TASC efficiencies are somewhat lower (by about 20%) than the accelerator results in the energy range 70-300 MeV. The measured energy-loss spectrum for 207 MeV neutron interactions in NaI were compared with the Monte Carlo response for 200 MeV neutrons in the TASC indicating good agreement. Based on this agreement, the simulation was considered to be sufficiently accurate to generate a neutron response library to be used by UNH in modifying the TASC fitting program to include a neutron component in the flare spectrum modeling. TASC energy-loss data on the 1991 June 11 flare was transferred to UNH. Also included appendix: Gamma-rays and neutrons as a probe of flare proton spectra: the solar flare of 11 June 1991.

Chupp, Edward L.↗