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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Solar flare gamma-ray line profiles

Solar gamma-ray lines are produced through collisions of pairs of positive ions whose center of mass energies are above the relevant thresholds for excitation of gamma-ray emitting states. Because of the low density of the solar plasma, prompt gamma rays are emitted by recoiling ions before significant energy loss has occurred. Thus, the lines are expected to be Doppler-broadened to widths of the order of a hundred KeV. Gamma-ray lines resulting from proton and alpha-particle (He-4 ion) beams on carbon and oxygen targets in the laboratory exhibit complex profiles which change rapidly with the gamma-ray observation angle. Earlier work has been limited to modeling laboratory and solar flare gamma-ray profiles produced by proton excitation, but laboratory profiles of the carbon 4.44-MeV line and oxygen 6.13-MeV line from alpha excitation have now been successfully modeled. Solar flare gamma-ray line profiles of the carbon and oxygen lines from heavy ions in the ambient solar medium interacting with representative high-energy proton and alpha particle populations are presented.

Lang, F. L.↗
NASA NTRS2022

Requirements for Progress in Understanding Solar Flare Energy Transport: The Impulsive Phase

Solar flares are a fundamental component of solar eruptive events (SEEs), along with solar energetic particles (SEPs) and coronal mass ejections (CMEs). Flare emission is the first component of a SEE to impact the Earth’s ionosphere which can set the stage for the later effects of the space weather event. Magnetic reconnection drives SEEs by restructuring the solar coronalmagnetic field, liberating a tremendous amount of energy which is partitioned into various physical manifestations: particle acceleration,mass and magnetic-field eruption, atmospheric heating, and the subsequent emission of radiation as solar flares. In this white paper we discuss the observational and theoretical advances required in order to make substantial progress in understanding the physical processes acting during the impulsive phase of a flare. That is, the initial rapid and intense period in which a tremendous amount of energy is released over the span of several minutes, resulting in the dramatic broadband increase to the solar radiative output. A second white paper by us covers the flare’s gradual phase, that is the decay phase where processes occur over longer timescales.

Graham S. Kerr↗
NASA NTRS1978

A phenomenological model of solar flares

The energy of solar flares is derived from the magnetic energy of fields convected to the sun's surface and subsequently converted to heat and energetic particles within the chromosphere. The circumstances of this conversion in most current models is magnetic flux annihilation at a neutral sheet. An analysis is conducted of the constraints of flux annihilation. It is shown that the present evidence of solar cosmic rays, X-rays, gamma-rays, and total energy suggests a choice of annihilation not at a neutral point, but by an enhanced dissipation of a field-aligned current. The field configuration is related both to its origin and to the extensive theory and laboratory experiments concerned with this configuration in magnetic fusion. The magnetic field model is applied to the August 4 flare. It is shown how the plasma heating in the annihilation region balanced by thermal conduction leads to a plasma temperature of about 20 million deg K.

Colgate, S. A.↗
NASA NTRS1983

Secular variations in solar flare proton fluxes

Proton fluences in contemporary solar-flare events (1965-82) are analyzed to obtain values of average flux and characteristic rigidity R0 (MV). Both proton fluences F (E greater than 10MeV) and R0 (in the energy interval 10-30 MeV) values for individual events follow log-normal distribution. Comparison of contemporary average flux and R0 values with long-term averaged values, based on lunar sample data, indicate that the ancient solar-flare proton spectra were harder compared to that observed in contemporary flares. The contemporary and long-term (greater than 10,000 yr) averaged fluxes are similar, although such a comparison may not be meaningful because the contemporary averages suffer from uncertainty due to statistics of a single event. The long-term average data suggests a secular variation in solar-flare activity with enhanced proton fluxes during the last 100,000 years.

Goswami, J. N.↗
NASA NTRS2022

Requirements for Progress in Understanding Solar Flare Energy Transport: The Gradual Phase

Solar flares are a fundamental component of solar eruptive events (SEEs), along with solar energetic particles (SEPs) and coronal mass ejections (CMEs). Flare emission is the first component of a SEE to impact the Earth’s atmosphere which can set the stage for the later arrival of the associated SEPs, CME, and space weather event. Magnetic reconnection drives SEEs by restructuring the solar coronal magnetic field, liberating a tremendous amount of energy which is partitioned into various physical manifestations: particle acceleration, mass and magnetic-field eruption, atmospheric heating, and the subsequent emission of radiation as solar flares. In this white paper we discuss the observational and theoretical advances required in order to make substantial progress in understanding the physical processes acting during the gradual phase of a flare. That is, the decay period, following the initial rapid release of energy during the impulsive phase (see our other white paper). In particular we want to address the unknown processes that sustain the long decay phase of flares and identify the unknown mechanism and magnitude of continued energy injection during the gradual phase.

Graham S Kerr↗
NASA NTRS1971

Longitude distribution of solar flares

Statistical tests of solar flares based on maximum likelihood method, discussing longitude distribution, rigid rotation, planetary effects, etc

Fung, P. C. W.↗
NASA NTRS1972

Charge and energy spectra of particles with E from 0.2 to 30 MeV/nuc in the January 25, 1971 solar flare.

Tracks of 1000 solar particles with charge Z not less than 10 and tracks of about 150 particles with Z equal to 8 have been analyzed in a stack of plastic detectors exposed in a rocket during the solar flare of Jan. 25, 1971. The energy spectra peak at about 1.5 MeV/nuc, with the flux falling to zero at about 0.4 MeV/nuc. Fe, Si, and O appear to have similar energy spectra for energies between 2 and 12 MeV/nuc.

Price, P. B.↗
NASA NTRS1973

Mass motion in solar flares

Mass motions in solar flares are here considered in terms of a previously proposed model. Particle acceleration occurs during reconnection of a current sheet located at coronal heights. The downward component of the particle flux produces an impulsive hard X-ray burst and heats the upper layers of the chromosphere sufficiently to lead to explosive evaporation. Some of the evaporated gas remains trapped in newly closed magnetic field lines and is responsible for the soft thermal component of X-ray emission. Gas which flows along open magnetic field lines subsequently forms a plasmoid which is ejected by magnetic stresses into interplanetary space and may subsequently cause a geomagnetic storm. Analysis of a highly simplified model leads to formulas for the density, temperature, and other parameters of the flare-produced plasma in terms of a length scale and mean magnetic field strength for the flare.

Sturrock, P. A.↗
NASA NTRS1982

Solar flare X-ray spectra from the Solar Maximum Mission Flat Crystal Spectrometer

High-resolution solar X-ray spectra obtained with the Flat Crystal Spectrometer aboard the Solar Maximum Mission from two solar flares and a nonflaring active region are analyzed. The 1-22 A region was observed during the flare on 1980 August 25, while smaller spectral regions were repeatedly covered during the 1980 November 5 flare. Voigt profiles were fitted to spectral lines to derive accurate wavelengths and to resolve blends. During the August 25 flare, 205 lines were found in the range 5.68-18.97 A, identifications being provided for all but 40 (mostly weak) lines. Upper limits to flare densities are derived from various line ratios, the hotter (about 10 to the 7th K) ions giving an electron density of less than 10 to the 12th per cu cm for the August 25 flare. Other observed line ratios (e.g., in Fe XVII and Mg XII) indicate a need for revisions in theoretical calculations.

Phillips, K. J. H.↗
NASA NTRS1992

Energetics and dynamics in a large solar flare of 1989 March

Solar Maximum Mission X-ray observations and National Solar Observatory/Sacramento Peak H alpha spectra are combined in a large (X1.2) solar flare to test predictions of chromospheric heating and evaporation by nonthermal thick-target electrons. It is demonstrated that the ratio of H alpha flare energy flux to the energy flux deposited by thick-target electrons obeys a power-law dependence on electron heating flux, with a slope that is consistent with that predicted by a thick-target electron transport and heating model in a 1D hydrostatic atmosphere. It is concluded that the thick-target model satisfactorily accounts for the observed magnitude of chromospheric H alpha emission, and the amplitudes and timing of oppositely directed plasma motions during the impulsive phase of this X flare.

Wulser, Jean-Pierre↗
NASA NTRS1990

Solar flares

General characteristics of solar flares are discussed. They are a spectacular energy release arising from development of magnetic strain and shear, resulting from eruption of new magnetic flux near old. The release of magnetic energy is channeled into hard electrons (and possibly nucleons), which produce a series of other effects. The ultimate source of the magnetic complexity that produces flares is deep in the sun. A number of examples is given, and the difference between various types of flares is discussed.

Zirin, Harold↗