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NASA NTRS1998

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

The accomplishments of the participation in the Compton Gamma Ray Observatory Guest investigator program is summarized in this report. The work involved the study of Energetic Gamma Ray Experiment Telescope (EGRET)/Total Absorption Shower Counter(TASC) flare data. The specific accomplishments were the use of the accelerator neutron measurements obtained at the University of New Hampshire to verify the TASC response function and to modify the TASC fitting program to include a high energy neutron contribution, and to determine a high energy neutron contribution to the emissions from the 1991 June 11, solar flare. The next step in the analysis of this event was doing fits to the TASC energy-loss spectra as a function of time. A significant hardening of the solar proton spectrum over time was found for the flare. Further data was obtained from the Yohkoh HXT time histories and images for the 1991 October 27 flare. The results to date demonstrate that the TASC spectral analysis contributes crucial information on the particle spectrum interacting at the Sun. The report includes a paper accepted for publication, a draft of a paper to be delivered at the 26th International Cosmic Ray Conference and an abstract of a paper to be presented at the Meeting of the American Physical Society.

Chupp, Edward L.↗
NASA NTRS1992

The energy spectra of solar flare hydrogen, helium, oxygen, and iron - Evidence for stochastic acceleration

The time-integrated differential energy spectra of H, He, O, and Fe measured in 10 large flare events observed at 1 AU over the energy range of 0.3-80 MeV/nucleon showed consistent patterns in their spectral shapes: particles with larger mean mass-to-charge ratios were generally less abundant at higher energies. A steady state model of stochastic particle acceleration with rigidity-dependent diffusion coefficients fit the spectra best; spectra representative of diffusive shock acceleration also described the spectra of some events with the same number of free parameters, but often fell off faster in energy above 30 MeV per nucleon than the observations. The two model predictions differed most at energies near 0.1 MeV per nucleon, below the lowest energies observed in this study. The stochastic model quantitatively described the observed spectral ordering with less efficient acceleration of species with larger mean mass-to-charge ratios.

Mazur, J. E.↗
NASA NTRS1989

High-resolution X-ray spectra of solar flares. IX - Mass upflow in the long-duration flare of 1979 June 5

Blueshifted X-ray spectral line components in Fe and Ca spectra of a large long-duration flare observed on June 5, 1979. It is found that blueshifted emission exists for a time interval of at least 28 minutes indicating upflowing plasma at about 250 km/s. Emission measures for both the blueshifted and stationary plasma are derived and the results are interpreted in terms of chromospheric evaporation. The total amount of hot upflowing plasma during the flare rise time exceeds the amount of stationary plasma contained in the loop close to the time of the peak of the flare. This result contradicts the simplest version of the evaporation model. Evaporation can account for the observations only if some of the upflowing plasma cools on time scales much shorter than the rise time of the event, which was about 40 minutes.

Doschek, G. A.↗
NASA NTRS1985

Observations of solar flare photon energy spectra from 20 keV to 7 MeV

Solar flare photon energy spectra in the 20 keV to 7 MeV range are derived from the Apr. 1, Apr. 4, apr. 27 and May 13, 1981 flares. The flares were observed with a hard X-ray and a gamma-ray spectrometers on board the Hinotori satellite. The results show that the spectral shape varies from flare to flare and the spectra harden in energies above about 400 keV. Effects of nuclear line emission on the continuum and of higher energy electron bremsstrahlung are considered to explain the spectral hardening.

Yoshimori, M.↗
NASA NTRS1981

X-ray spectra of solar flares obtained with a high-resolution bent crystal spectrometer

Preliminary results obtained for three solar flares with the bent crystal spectrometer on the SMM are presented. Resonance and satellite lines of Ca XIX and XVIII and Fe XXV and XXIV are observed together with the Fe XXVI Lyman-alpha line. Plasma properties are deduced from line ratios and evidence is presented for changes of line widths coincident with the occurrence of a hard X-ray impulsive burst. Fe K-alpha spectra from a disk center and a limb flare agree with the predictions of a fluorescence excitation model. However, a transient Fe K-alpha burst observed in a third flare may be explained by the collisional ionization of cool iron by energetic electrons.

Culhane, J. L.↗
NASA NTRS1985

High-resolution X-ray spectra of solar flares. VII - A long-duration X-ray flare associated with a coronal mass ejection

It has been recognized that very long duration X-ray events (lasting several hours) are frequently associated with coronal mass ejection. Thus, Sheeley et al. (1983) found that the probability of the occurrence of a coronal mass ejection (CME) increases monotonically with the X-ray event duration time. It is pointed out that the association of long-duration, or long-decay, X-ray events (LDEs) with CMEs was first recognized from analysis of solar images obtained by the X-ray telescopes on Skylab and the Naval Research Laboratory (NRL) slitless spectroheliograph. Recently high-resolution Bragg crystal X-ray spectrometers have been flown on three spacecraft, including the Department of Defense P78-1 spacecraft, the NASA Solar Maximum Mission (SMM), and the Japanese Hinotori spacecraft. In the present paper, P78-1 X-ray spectra of an LDE which had its origin behind the solar west limb on November 14, 1980 is presented. The obtained data make it possible to estimate temperatures of the hottest portion of the magnetic loops in which the emission arises.

Kreplin, R. W.↗
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 NTRS2020

Spectral Evidence for Heating at Large Column Mass in Umbral Solar Flare Kernels. I. IRIS Near-UV Spectra of the X1 Solar Flare of 2014 October 25

The GOES X1 flare SOL2014-10-25T17:08:00 was a three-ribbon solar flare observed with the Interface Region Imaging Spectrograph (IRIS)in the near-UV(NUV)and far-UV. One of the flare ribbons crossed a sunspot umbra, producing a dramatic, ∼1000% increase in the NUV continuum radiation. We comprehensively analyze the UV spectral data of the umbral flare brightenings, which provide new challenges for radiative−hydrodynamic modeling of the chromospheric velocity field and the white-light continuum radiation. The emission line profiles in the umbral flare brightenings exhibit redshifts and profile asymmetries, but these are significantly smaller than in another, well-studied X-class solar flare. We present a ratio of the NUV continuum intensity to the FeIIλ2814.45intensity. This continuum-to-line ratio is a new spectral diagnostic of significant heating at high column mass (log m/[gcm−2]>−2) during solar flares because the continuum and emission line radiation originate from relatively similar temperatures but moderately different optical depths. The full spectral readout of these IRIS data also allow for a comprehensive survey of the flaring NUV landscape: in addition to many lines of Fe II and Cr II, we identify a new solar flare emission line, HeIλ2829.91(as previously identified in laboratory and early-type stellar spectra). The Fermi/GBM hard X-ray data provide inputs to radiative−hydrodynamic models (which will be presented in Paper II) in order to better understand the large continuum-to-line ratios, the origin of the white-light continuum radiation, and the role of electron beam heating in the low atmosphere.

Adam F Kowalski↗
NASA NTRS1974

Solar flare proton rigidity spectra deduced from cosmic ray neutron monitor observations

The solar flare proton rigidity spectra for several flares occurring between 1967 and 1972 have been deduced from the ground level cosmic ray neutron monitor observations. To obtain consistent agreement for all the ground level events (GLEs) analyzed, the specific yield functions of Lockwood and Webber (1967) must be reduced slightly below P = 1.6 GV. The typical spectral indices of solar cosmic rays deduced for GLEs vary from 4 to 6 if the differential spectrum is represented by KP to the minus gamma power. Only occasionally is the spectrum as steep as 8 or 9. The observed spectral index is independent of the magnitude of the integral solar proton flux above 1 GV.

Lockwood, J. A.↗
NASA NTRS2009

Elemental Composition of 433 Eros: New Calibration of the NEAR-Shoemaker XRS Data

We present a new calibration of the elemental-abundance data for Asteroid 433 Fros taken by the X-ray spectrometer (XRS) aboard the NEAR-Shoemaker spacecraft. (NEAR is an acronym for "Near-Earth Asteroid Rendezvous,") Quintification of the asteroid surface elemental abundance ratios depends critically on accurate knowledge of the incident solar X-ray spectrum, which was monitored simultaneously with asteroid observations. Previously published results suffered from incompletely characterized systematic uncertainties due to an imperfect ground calibrations of the NEAR gas solar monitor. The solar monitor response function and associated uncertainties have now been characterized by cross-calibration of a large sample of NEAR solar monitor flight data against. contemporary broadband solar X-ray data from the Earth-orbiting GOES-8 (Geostationary Operational Environmental Satellite). The results have been used to analyze XRS spectra acquired from Eros during eight major solar flares (including three that have not previously been reported). The end product of this analysis is a revised set of Eros surface elemental abundance ratios with new error estimates that more accurately reflect the remaining uncertainties in the solar flare spectra: Mg/Si=.753 +0.078/-0.055, Al/Si=0.069 +/-0.055, S/Si=0.005+/-0.008, Ca/Si=0.060+0.023/-0.024, and Fe/Si= 1.578+0.338/-0.320. These revised abundance ratios are consitent within cited uncertainties with the results of Nittler et al. [Nittler, L.R., and 14 colleagues, 2001. Meteorit Planet. Sci 36, 1673-1695] and thus support the prior conclusions that 433 Eros has major-element composition simular to ordinary chondrites with the exception of a stong depletoin in sulfur, most likely caused by space weathering.

Lim, Lucy F.↗
NASA NTRS1981

A discussion of theoretical ionization equilibrium calculations based on solar flare X-ray spectra

Several sets of ionization equilibrium calculations exist for use in interpreting X-ray and EUV spectra of astrophysical plasmas. In particular, the calculations of Jordan (1969, 1970), Jacobs et al. (1977, 1978) and Summers (1974) are well known. The temperatures of maximum fractional abundance calculated by Summers for the more highly ionized and heavier elements such as iron are about a factor of two higher than the temperatures calculated by Jordan and Jacobs et al. By the use of recently obtained X-ray spectra of solar flares, it is shown that the temperatures calculated by Summers (1974) for iron are incorrect. The temperatures calculated by Jordan or Jacobs et al. should be used until further improvements become available.

Feldman, U.↗
NASA NTRS1993

Ion acceleration in impulsive solar flares

Nonrelativistic spectra of protons and ions accelerated in impulsive solar flares are derived using more realistic turbulence power spectra. The calculation is based on a particle transport equation extracted from a second step acceleration model containing stochastic acceleration. The turbulence model is generalized to waves with a small angle to the magnetic field vector and to turbulence power spectra with spectral indices s smaller than 2. Due to the occurrence of impulsive flares at low coronal heights, Coulomb losses at the dense coronal plasma and diffusive particle escape are taken into account. The ion spectra show deviations from long-duration spectra near the Coulomb barrier, where the losses become maximal. The Z-squared/A-dependence of the Coulomb losses leads to spectral variations for different ions. We present a method to estimate the turbulence parameters and injection conditions of the flare particles using ion ratios like Fe/O of impulsive flares.

Steinacker, Jurgen↗
NASA NTRS2010

A Test of Thick-Target Nonuniform Ionization as an Explanation for Breaks in Solar Flare Hard X-Ray Spectra

Solar nonthermal hard X-ray (HXR) flare spectra often cannot be fitted by a single power law, but rather require a downward break in the photon spectrum. A possible explanation for this spectral break is nonuniform ionization in the emission region. We have developed a computer code to calculate the photon spectrum from electrons with a power-law distribution injected into a thick-target in which the ionization decreases linearly from 100% to zero. We use the bremsstrahlung cross-section from Haug (1997), which closely approximates the full relativistic Bethe-Heitler cross-section, and compare photon spectra computed from this model with those obtained by Kontar, Brown and McArthur (2002), who used a step-function ionization model and the Kramers approximation to the cross-section. We find that for HXR spectra from a target with nonuniform ionization, the difference (Delta-gamma) between the power-law indexes above and below the break has an upper limit between approx.0.2 and 0.7 that depends on the power-law index delta of the injected electron distribution. A broken power-law spectrum with a. higher value of Delta-gamma cannot result from nonuniform ionization alone. The model is applied to spectra obtained around the peak times of 20 flares observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI from 2002 to 2004 to determine whether thick-target nonuniform ionization can explain the measured spectral breaks. A Monte Carlo method is used to determine the uncertainties of the best-fit parameters, especially on Delta-gamma. We find that 15 of the 20 flare spectra require a downward spectral break and that at least 6 of these could not be explained by nonuniform ionization alone because they had values of Delta-gamma with less than a 2.5% probability of being consistent with the computed upper limits from the model. The remaining 9 flare spectra, based on this criterion, are consistent with the nonuniform ionization model.

Holman, gordon↗
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

A hybrid thermal/nonthermal model for the energetic emissions from solar flares

We present an alternative interpretation of the high-resolution solar flare spectra obtained in 1981 by Lin et al. In our interpretation electron heating and acceleration are simultaneous and physically linked, rather than heating being a secondary effect of particle acceleration. We show that the observed properties of solar flare X-ray emission can in general be explained through the Joule heating and electric field acceleration of runaway electrons in current channels. We have fitted a hybrid thermal/nonthermal electron distribution, consisting of hot isothermal electrons with a nonthermal tail of runaway electrons, to representative spectra obtained by Lin et al. 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 low-energy 'cutoff' to the nonthermal part of the X-ray spectrum is determined by the critical velocity in the electron distribution above which runaway acceleration occurs. We relate the fit parameters to the physical properties (such as the electric field strength in the current channels) of the acceleration region.

Holman, Gordon D.↗
NASA NTRS1990

Stochastic Fermi acceleration in solar flares

Proton spectra, valid from non- to ultra-relativistic energies, resulting from stochastic Fermi acceleration in solar flares are calculated. These spectra were obtained by numerically solving the Fokker-Planck equation, in which the escape of the particles from the acceleration region is characterized by an energy-independent escape time. In addition to equilibrium spectra, time-dependent energy spectra showing the approach to equilibrium are also presented. These numerical equilibrium spectra are compared with previous results which were obtained either by Monte Carlo simulations or approximate analytical treatments. There are no analytic solutions valid in the transrelativistic regime, which is very important for the production of pions and neutrons in solar flares. The acceleration efficiency is related to physical parameters, in particular the energy density in either magnetosonic or Alfven waves, and a lower limit is placed on either of these energy densities from acceleration times implied by gamma-ray observations. Also discussed is the physical interpretation of the escape time.

Miller, James A.↗
NASA NTRS1979

An investigation of the 1.9 A feature in solar-flare X-ray spectra

The 1.9 A feature, observed in the X-ray spectra of three solar flares with the Columbia University and Lockheed Palo Alto Research Laboratory spectrometers on the OSO 8 satellite, is shown to be due to a blend of 1s-2p transitions in a range of Fe ions. In the temperature range 9-16 x -10 to the 6th K, the feature has a mean wavelength of 1.900 + or - 0.009 A and is 0.04 A wider than a single line, indicating that the main contributors are Fe XIX-Fe XXII. Most of the emission originates from the dielectronic recombination process, and when inner-shell excitation is included together with normal collisional excitation, the observed intensity of the feature can be accounted for adequately. For these events, if the electron density is below approximately 10 to the 12th/cu cm, deviations from ionization equilibrium will be significant for ions more highly ionized than Fe XXI.

Parkinson, J. H.↗