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Doschek, G. A.

Publications and source records attributed to Doschek, G. A..

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

On the rate of energy input in thermal solar flares

The rise phases of solar soft X-ray flares observed by X-ray crystal spectrometers on P78-1 are discussed in terms of the rate of change of X-ray flux as a function of time. It is shown that the flux increases exponentially over most of the rise time. The e-folding time (tau) has a cutoff at approximately 13 s. Soft X-ray flares with smaller values of tau are not observed. It is suggested that this phenomenon is due to the ability of the solar atmosphere to absorb the input energy and convert it into a typical soft X-ray flare, when the value of tau is greater than about 13 s. For energy input rates with tau greater than about 13 s, the temperature attained by the plasma is typically around 2 x 10 to the 7th K, but for values of tau less than 13 s, the gas is heated to much higher temperatures (about 10 to the 8th K), producing a certain class of hard X-ray flares.

Feldman, U.↗
NASA NTRS1983

Si III line ratios in the sun

New atomic data for Si III have been used to predict level populations and emission-line intensity ratios for electron densities and temperatures appropriate to the solar transition region. The electron impact collision rates used here are substantially larger than those previously published owing to delineation of the complex resonance structures in the low-energy collision strengths. This together with small changes in the spontaneous radiative rates produces significant changes in the calculated intensity ratios. Generally good agreement is found with observations obtained using the Naval Research Laboratory slit spectrograph aboard Skylab, electron densities from three Si III ratios and from other methods normally agreeing to 0.2 dex or better for a wide variety of solar features. For a fourth ratio, incorporating lines with a wide wavelength separation, the agreement is less satisfactory, possibly owing to uncertainties in the observational data.

Dufton, P. L.↗
NASA NTRS1983

Lines of Fe XII sensitive to coronal electron density

Lines of Fe XII sensitive to coronal electron density are discussed. The lines appear in solar spectra obtained by the Naval Research Laboratory (NRL) slit spectograph flown on Skylab. These lines are due to transitions between levels of the 3s 2 3p 3 configuration and fall at the wavelengths 1242.03 A, 1349.38 A, 2169.03 A, 2405.71 A, and 2565.99 A. It is shown that the line at 2169.03 A is severely blended by a line of Ni II at heights less than 12 arcsec outside the solar limb. Above 12 arcsec the lines at 2169.03 and 2405.71 A are apparently unblended and can be used to derive electron densities. An average coronal electron pressure of 6 x 10 to the 14th/cu cm K is obtained. However, the emitting path lengths of the Fe XII lines, deduced using the electron densities and absolute intensities, are unrealistically large. The reason for this difficulty is unclear.

Feldman, U.↗
NASA NTRS1983

Solar instruments on the P78-1 spacecraft

The solar experiment package on the P78-1 spacecraft is described. The satellite was launched on 24 February 1979 by the U.S. Air Force and contains high resolution Bragg crystal spectrometers, hard-X-ray proportional counters, and a white light coronagraph. The high resolution spectrometers were built by the Aerospace Corporation and the Naval Research Laboratory. The hard X-ray spectrometers were built by the Aerospace Corporation and the white light coronagraph was supplied by the Naval Research Laboratory. Most of these instruments are still returning data.

Doschek, G. A.↗
NASA NTRS1983

Solar flare X-ray spectra from the P78-1 spacecraft

Results from the high resolution solar X-ray spectrometer experiments on the P78-1 spacecraft are discussed. These results concern physical quantities such as electron temperature and density, turbulence, mass motions, and state of ionization equilibrium, characteristic of the thermal soft X-ray emitting flare plasma, and the time behavior of these quantities during flares. In addition, a brief description of the instruments is given, the plasma diagnostics used in interpreting the spectra are summarized, and the origin of the thermal soft X-ray emitting plasma is discussed in light of the P78-1 results, earlier data, and numerical simulations of magnetic flux tubes heated to solar flare temperatures.

Doschek, G. A.↗
NASA NTRS1983

Numerical simulations of loops heated to solar flare temperatures. I - Gasdynamics. II - X-ray and UV spectroscopy

The NRL's Dynamic Flux Tube Model is used to numerically simulate the dynamic response of a coronal magnetic loop to an energy input of the order encountered in solar flares. The coronal plasma is heated by the deposition of flare energy at the top of the loop to more than 10 million K, yielding a conduction front that moves toward the chromosphere, where the plasma is heated by the large downward conductive flux and ablates upward to the coronal part of the loop at velocities of a few hundred km/sec. The conduction front simultaneously produces chromospheric ablation and compresses the material ahead of it. With the aid of compressional instabilities, the compressed plasma grows throughout the flare heating phase, presenting a possible source of the flare optical continuum emission which is correlated with soft X-ray radiation. The observational consequences of rapidly heated loop gas dynamic processes are discussed. In the second part of this presentation, the dynamical calculation results previously obtained are used to predict the spectral line intensities, profiles and wavelengths of several X-ray lines and the UV line of Fe XXI at 1354.1 A. Three different viewing orientations of the loop are considered.

Cheng, C.-C.↗
NASA NTRS1983

Inner-shell transitions in Fe XIX-XXII in the X-ray spectra of solar flares and Tokamaks

Calculated spectra of the ions Fe XIX-XXII for various densities and temperatures are presented, thereby extending the work begun by Doschek, Feldman, and Cowan (1981). The calculations are based on a code (the Cowan code) that computes both the level structure of an ion and intensity factors for the 1s-2p satellite lines. A comparison is made between the calculated spectra and those observed in solar flares by the P78-1 and SMM instruments. The observed intensities of Fe XX lines, which are the most sensitive to density, are found to agree well with those calculated in the low-density limit. The agreement for lines arising from other ions is also very good. It is also seen that the predicted density variations in Fe XX are confirmed by the higher density Princeton Large Torus plasmas. Thus a possible useful density diagnostic is indicated for tokamak and high-density astrophysical plasmas, perhaps including some solar flares.

Phillips, K. J. H.↗
NASA NTRS1982

Forbidden line emission from highly ionized atoms in tokamak plasmas

Considerable interest in the observation of forbidden spectral lines from highly ionized atoms in tokamak plasmas is related to the significance of such observations for plasma diagnostic applications. Atomic data for the elements Ti Cr, Mn, Fe, Ni, and Kr have been published by Feldman et al. (1980) and Bhatia et al. (1980). The present investigation is concerned with collisional excitation rate coefficients and radiative decay rates, which are interpolated for ions of elements between calcium, and krypton and for levels of the 2s2 2pk, 2s 2p(k+1), and 2p(k+2) configurations, and for the O I, N I, C I, B I, and Be I isoelectronic sequences. The provided interpolated atomic data can be employed to calculate level populations and relative line intensities for ions of the considered sequences, taking into account levels of the stated configurations. Important plasma diagnostic information provided by the forbidden lines includes the ion temperature

Feldman, U.↗
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 NTRS1982

S IV emission-line ratios in the sun

New atomic data are presented for transitions between the five lowest levels in S IV, taking into account the fine-structure rates between the individual J levels together with the electron impact mixing rates within the levels. The values are found to differ significantly from previously published values. Using the atomic data, ionic level populations are deduced for a range of electron temperatures and densities. The results are used to calculate theoretical line intensity ratios for S IV. Excellent agreement is found with intensity ratios for a variety of solar features observed with the NRL normal incidence spectrograph on Skylab.

Dufton, P. L.↗
NASA NTRS1982

Solar transition region response to variations in the heating rate

The response of a numerical model for the upper chromosphere, transition region, and corona to variations in the energy input has been examined. The numerical model solves the set of one-dimensional two-fluid hydrodynamic equations in a simple vertical magnetic flux tube. The atmosphere responds to both the increase and decrease in energy deposition by smoothly readjusting the temperature gradient and the amount of material in the region of peak radiating efficiency to radiate away energy being deposited. At no time during this readjustment is a departure from a thin laminar transition region structure seen. In addition, a time-dependent description of the nonequilibrium ionization of all of the ionization stages of oxygen has been included. This calculation is coupled with the self-consistent calculations of the dynamical variables. It is found that the nonequilibrium ionization balance calculations for both heating and cooling small loops in the quiet sun predict relative ionic abundances which differ substantially from those which would be predicted by an equilibrium calculation

Mariska, J. T.↗
NASA NTRS1982

Observational constraints for a theoretical model describing the soft X-ray flare

High-resolution solar flare X-ray spectra have recently been obtained from X-ray spectrometer experiments flown on an Air Force spacecraft (P78-1) launched on 1979 February 24. Interpretation of the spectra has produced new results concerning the physical conditions and time behavior of the thermal soft X-ray emitting plasma at temperatures near 20,000,000 K. It is argued that soft and hard X-ray events are not causally related to each other, but are simply two different manifestations of flare energy release. They probably occur in different plasma volumes. The source of the preflare plasma appears to be in the cooler parts of the solar atmosphere, perhaps transition region loops with initial temperatures of 100,000 K and densities of 10 to the 11th per cu cm. Continuous energy input, rather than sequential activation of loops, is required to explain the observations. Compression coupled with chromospheric ablation may produce the high densities in coronal flare loops.

Feldman, U.↗
NASA NTRS1982

Doppler wavelength shifts of ultraviolet spectral lines in solar active regions

Doppler shifts are measured for solar UV emission lines formed in the lower transition region of active regions. Doppler shifts in different regions at the same solar location, variations of Doppler shift with position of an active region on the disk, and variations of Doppler shift with time at the same solar location in the same active region were studied. Observations were made with the NRL slit spectrograph on Skylab. Excluding flare and flare-related phenomena, only redshifts are found whose magnitudes correspond to downflow velocities between about 4 and 17 km/s. Shifts are largest for lines formed between about 50,000 and 100,000 K, and are distinctly less for lines formed above 100,000 K. The shifts persist out to the limb, but not above it. There is no obvious change in redshift for lines measured at the same solar location over time intervals of about 20 minutes.

Feldman, U.↗
NASA NTRS1982

Ultraviolet continuum absorption /less than about 1000 A/ above the quiet sun transition region

Lyman continuum absorption shortward of 912 A in the quiet sun solar transition region is investigated by combining spectra obtained from the Apollo Telescope Mount experiments on Skylab. The most recent atomic data are used to compute line intensities for lines that fall on both sides of the Lyman limit. Lines of O III, O IV, O V, and S IV are considered. The computed intensity ratios of most lines from O IV, O V, and S IV agree with the experimental ratios to within a factor of 2. However, the discrepancies show no apparent wavelength dependence. From this fact, it is concluded that at least part of the discrepancy between theory and observation for lines of these ions can be accounted for by uncertainties in instrumental calibration and atomic data. However, difficulties remain in reconciling observation and theory, particularly for lines of O III, and one line of S IV. The other recent results of Schmahl and Orrall (1979) are also discussed in terms of newer atomic data.

Doschek, G. A.↗
NASA NTRS1981

Solar observations and atomic data for the 3s2 1s0-3s3p 3P1 transition in S v

High resolution solar observations of the S v intersystem line at 1199.18 A are available from Skylab. This line is potentially useful as a density diagnostic for high density plasmas expected in solar flares. S v lines are also prominent in solar spectra at wavelengths below 1000 A. Collision strengths and radiative decay rates are calculated for levels of the configurations 3 s2, 3s3p, 3p2, and 3s3d. Level populations for the five lowest energy levels have been calculated as a function of electron density. These calculations are carried out assuming the temperature at which S v is most abundant in solar plasmas. The calculated population of the 3s3p 3P1 level reaches a pseudo-Boltzmann equilibrium at a density which is about five times higher than is derived from solar spectra and previous density determinations. It is believed that the discrepancy is due to a significant contribution from resonances to the 3s2-3s3p 3P excitation rate coefficient. It is suggested that the contribution of the resonances to the S v excitation rate coefficient can be estimated from a comparison of the experimental results and the theoretical calculations.

Feldman, U.↗
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 NTRS1981

On the relationship between soft X-rays and H-alpha-emitting structures during a solar flare

Based on data obtained during a solar flare on March 31, 1979, soft X-ray (SXR) and hard X-ray (HXR) bursts are analyzed and compared with other available data in order to identify structures in H-alpha that may correspond to the SXR-emitting site. Measurements taken with the X-ray telescope and the XUV spectroheliograph flown on Skylab, have shown that the SXR emission from many flares comes from rather small structures of about 10-20 arcsec across. These structures appear to be loops that cross the magnetic neutral line. Understanding of the morphology of SXR was based on data of the solar flare of June 15, 1973, observed from Skylab, and the work of Moore et al., (1980). Dense, highly emissive coronal structures, not suggested to be the X-ray source, were forming, lost energy rapidly by emission and conduction, and finally formed the loops. It is concluded that bright H-alpha loops form rapidly as the SXR emission rises, and the overall decay (cooling rate) of SXR emission is much slower than the formation time of individual loops.

Zirin, H.↗