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Doschek, G. A.

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

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2007

The Hinode(Solar-B)Mission: An Overview

The Hinode satellite (formerly Solar-B) of the Japan Aerospace Exploration Agency's Institute of Space and Astronautical Science (ISAS/JAXA) was successfully launched in September 2006. As the successor to the Yohkoh mission, it aims to understand how magnetic energy is transferred from the photosphere to the upper atmospheres and resulting in explosive energy releases. Hinode is an observatory style mission, with all the instruments being designed and built to work together to address the science aims. There are three instruments onboard: the Solar Optical Telescope (SOT), the EUV Imaging Spectrometer (EIS), and the X-ray Telescope (XRT). This paper overviews the mission, including the satellite, the scientific payload and operations. It will conclude with discussions on how the international science community can participate in the analysis of the mission data.

Kosugi, T.↗
NASA NTRS2003

Temperature Measurements in the Solar Transition Region Using N III Line Intensity Ratios

UV emission from B-like N and O ions a rather rare opportunity for recording spectral lines in a narrow wavelength range that can potentially be used to derive temperatures relevant to the solar transition region. In these ions, the line intensity ratios of the type (2s2p(sup 2) - 2p(sup 3)) / (2s(sup 2)2p - 2s2p(sup 2)) are very sensitive to the electron temperature. Additionally, the lines involving the ratios fall within a range of only - 12 A; in N III the lines fall in the 980 - 992 A range and in O IV in the 780 - 791 A range. In this work, we explore the use of these atomic systems, primarily in N III, for temperature diagnostics of the transition region by analyzing UV spectra obtained by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer flown on the Solar and Heliospheric Observatory (SOHO). The N III temperature-sensitive line ratios are measured in more than 60 observations. Most of the measured ratios correspond to temperatures in the range 5.7x10(exp 4) - 6.7x10(exp 4) K. This range is considerably lower than the calculated temperature of maximum abundance of N III, which is approx. 7.6x10(exp 4) K. Detailed analysis of the spectra further indicates that the measured ratios are probably somewhat overestimated due to resonant scattering effects in the 2s(sup 2)2p - 2s2p(sup 2) lines and small blends in the 2s2p(sup 2) - 2p3 lines. Actual lower ratios would only increase the disagreement between the ionization balance calculations and present temperature measurements based on a collisional excitation model. In the case of the O IV spectra, we determined that due to the close proximity in wavelength of the weak line (2s2p(sup 2)-2p3 transitions) to a strong Ne VIII line, sufficiently accurate ratio measurements cannot be obtained. Subject headings: atomic data --- atomic processes --- Sun: transition region --- Sun: U V radiation --- techniques: spectroscopic

Doron, R.↗
NASA NTRS1995

The bright knots at the tops of soft X-ray flare loops: Quantitative results from Yohkoh

Soft X-ray Telescope (SXT) observations from the Japanese Yohkoh spacecraft have shown that confined bright regions are common features at the tops of flare loops throughout most of the duration of the flares. In this paper we present quantitative results for these flare knots, in relation to other flare regions, for four relatively 'simple' flares. Emission measure distributions, electron temperatures, and electron densities are derived from SXT and Yohkoh Bragg Crystal Spectrometer (BCS) observations. The four flares selected are dominated by what appear to be single-loop structures, with bright knots at the loop tops. The flares are neither long-duration nor impulsive events. The spatial distributions of brightness and emission measure in the flares are found to be quite similar for all four events, even though there are significant differences in dynamical behavior between at least two of the events. Temperatures and densities calculated for these flares are consistent with previous results from many solar experiments. An investigation of intensity correlations between adjacent pixels at the tops of the loops suggests the existence of local disturbances in the magnetic loops that occur on spatial scales less than the radii of the loops.

Doschek, G. A.↗
NASA NTRS1994

The determination of electron densities in the solar atmosphere from the 1718.56 A/1486.51 A emission-line ratio in N IV

The theoretical electron density sensitive emission-line ratio R = I(1718.56 a)/I(1486.51 A) in N IV is presented for a range of N(sub e)(approximately equals 10(exp 10) - 10(exp 12)/cu cm) applicable to higher density solar plasmas, such as active regions. A comparison of these calculations with the observed values of R of several solar features obtained with the Naval Research Laboratory's S082B spectrograph on board Skylab reveals general agreement between theory and observation at pointings just above the limb, where line blends with N IV 1718.56 A should be insignificant, which provides experimental support for the accuracy of the line ratio calculations.

Keenan, F. P.↗
NASA NTRS1994

The 1991 November 9 flare at 03.2 UT: Observations from Yohkoh

We discuss X-ray spectra and soft X-ray images of an M1.9 flare that occurred on 1991 November 9 near 03.2 UT. These data were obtained with instrumentation on the Japanese Yohkoh spacecraft. They cover the entire rise phase and peak flare emission, and the beginning of the decay phase. We determine the dynamics, temperature, and emission measure of the flare as inferred from the X-ray line profiles of resonance lines of Fe XXV, Ca XIX, and S XV. We discuss the morphology of the flare as inferred from the soft X-ray images. The November 9 flare is atypical in that a stronger than usual blueshifted emission component (relative to the stationary component) is observed for the resonance lines at flare onset. We discuss several methods for deconvolving the blueshifted component from the stationary component. The X-ray line profiles are consistent with predictions of numerical simulations of chromospheric evaporation. The X-ray images reveal a flare with a complicated loop geometry that is not fully understood. Many of the features in the images are moving upwards at speeds ranging from a few km/s to about 800 km/s. The blueshifted emission begins near the onset of hard X-ray emission, implying that particle acceleration and upflowing plasma have a common energy source.

Doschek, G. A.↗
NASA NTRS1994

The correlation of solar flare hard X-ray bursts with Doppler blueshifted soft X-ray flare emission

We have investigated the temporal correlation between hard X-ray bursts and the intensity of Doppler blueshifted soft X-ray spectral line emission. We find a strong correlation for many events that have intense blueshifted spectral signatures and some correlation in events with modest blueshifts. The onset of hard X-rays frequently coincides to within a few seconds with the onset of blueshifted emission. The peak intensity of blueshifted emission is frequently close in time to the peak of the hard X-ray emission. Decay rates of the blueshifted and hard X-ray emission are similar, with the decay of the blueshifted emission tending to lag behind the hard X-ray emission in some cases. There are, however, exceptions to these conclusions, and, therefore, the results should not be generalized to all flares. Most of the data for this work were obtained from instruments flown on the Japanese Yohkoh solar spacecraft.

Bentley, R. D.↗
NASA NTRS1994

A far-ultraviolet flare on a Pleiades G dwarf

The Hubble Space Telescope/Faint Object Spectrograph (HST/FOS) recorded a remarkable transient brightening in the C IV lambda lambda 1548,50 emissions of the rapidly rotating Pleiades G dwarf H II 314. On the one hand the 'flare' might be a rare event luckily observed; on the other hand it might be a bellwether of the coronal heating in very young solar-mass stars. If the latter, flaring provides a natural spin-down mechanism through associated sporadic magnetospheric mass loss.

Ayres, T. R.↗
NASA NTRS1993

An improved ultraviolet spectral line list for the symbiotic star RR Telescopii

We have remeasured wavelengths and intensities of International Ultraviolet Explorer (IUE) spectra of the symbiotic star, RR Tel. The main work is centered on the long 820 minute exposure high-resolution spectrum obtained on 1983 June 18. The list is intended to serve as a source of improved intensities and wavelengths for the ultraviolet spectrum of this star. A complete line list with intensities based on this exposure has not been published previously. The strongest spectral lines are saturated in the 820 minute exposure, and intensities for these lines are mostly obtained from a 20 minute exposure obtained on the same day. A few intensities are obtained from other exposures if neither the 820 nor the 20 minute exposure is satisfactory. There are 111 lines in our list between 1168 and 1980 A. Some of the very weakest lines may not be real. These are indicated by question marks. We also discuss some of the plasma diagnostics available using spectral lines of O v and O iv.

Doschek, G. A.↗
NASA NTRS1992

The potential for plasma diagnostics from stellar extreme-ultraviolet observations

A description of the lines from the most abundant ions in the EUV spectrum in the range 50-500 A that have special diagnostic significance is presented. These lines are from plasmas in the temperature range from 10 exp 5 to 10 exp 7 K. Their utility for determining temperature, density, and abundances is demonstrated. An objective grating spectrometer, based on the Skylab slitless spectrograph, for making celestial observations in the EUV spectral region with high spectral resolution is also described. Such an instrument, if equipped with a multilayer grating, can achieve efficiencies over limited EUV wavelength ranges that rival much larger telescope-spectrometer instruments for the observation of stars and other celestial objects.

Feldman, U.↗
NASA NTRS1991

Relative abundances in the lower solar transition region

Properties of the lower solar transition region as determined from UV spectral lines recorded by the Naval Research Laboratory HRTS are investigated. The spectra were obtained from a rocket flight in July 1975. Variations of intensity ratios of strong lines of C IV, Si IV, C II, Si III, and O IV over the entire field of view of the HRTS instrument, which extends from disk center to the solar limb are discussed. The largest apparent abundance variations appear to be in the active regions surrounding a sunspot. The C/Si ratio is lower in the active regions than anywhere else in the instrument field of view, implying a lower C/Si abundance ratio in the active regions. There is a possible correlation of line intensity variation in the active region with Doppler shift of the lines; the explanation for this is unclear. The magnitude and reality of intensity ratio variations of all observed solar features are discussed, and monotonic center-to-limb intensity ratio variations due to the opacity of some of the spectral lines are measured.

Doschek, G. A.↗
NASA NTRS1991

The solar spectrum between 914 and 1177 A

A spectral line list with wavelengths and identifications for the 914-1177 A region is presented. The list is based on a Naval Research Laboratory (NRL) solar spectrum obtained from a rocket flight in 1966 and on spectra recorded by the NRL S082-B slit spectrograph flown in 1973 on the Skylab manned space station as part of the Apollo Telescope Mount. Three Skylab spectra were used for this work: a limb spectrum recorded at a position of arcsec outside the white-light limb, and two flare spectra. The wavelength list should be useful in analyzing some spectra to be obtained from the planner NASA Lyman Far Ultraviolet Spectroscopic Explorer mission. A separate table listing observed or predicted forbidden lines that fall in the 914-1177 A range is presented, and some of the plasma diagnostic possibilities for spectral lines in this range are discused.

Feldman, U.↗
NASA NTRS1990

On the dependence of solar flare X-ray spectral line intensity ratios of highly ionized sulfur, calcium, and iron on electron temperature, differential emission measure, and atomic physics

This paper focuses on what can be learned about the emission measure distribution and certain atomic physics parameters from spectral lines of highly ionized ions of sulfur, calcium, and iron that appear in solar flare spectra. The particular lines chosen for analysis allow the electron temperature to be determined independently of the assumption of ionization equilibrium. An attempt is made to find emission measure models based on selected functional dependences of emission measure on temperature that reproduce the observed temperatures deduced from spectral line ratios as well as the relative intensities of resonance lines of different elements.

Doschek, G. A.↗
NASA NTRS1990

Solar coronal Ar/Fe and Ne/Mg abundance ratios derived from ultraviolet forbidden line spectra

An Ar/Fe abundance ratio of 0.031 + or - 35 percent and an upper limit of 0.4 to the Ne/Mg abundance ratio are determined for solar coronal active regions using solar spectra. Forbidden lines occur at 1324.45 A for Mg V, 1349.38 A for Fe XII, 1375.98 A for Ca XV, and 1392.12 A for Ar XI. The unobserved Ne V line is predicted to fall at 1574.68 A. These abundance ratios are consistent with lower abundances of Ne and Ar in the corona relative to low first ionization potential elements.

Doschek, G. A.↗
NASA NTRS1990

Soft X-ray spectroscopy of solar flares - An overview

An overview of the current status of high spectral resolution soft X-ray observations of solar flares is given. The review concentrates primarily on recent results and interpretations of results obtained from orbiting Bragg crystal spectrometers flow during the last solar maximum on the US DoD P78-1 spacecraft, the NASA SMM, and the ISAS Hinotori spacecraft. Results and several key issues regarding interpretation of the spectra are presented. Specifically, the dynamics of coronal flare plasmas as revealed by X-ray line profiles and wavelength shifts are discussed. Recent results concerning the theory of chromospheric evaporation are given. The temperature of coronal flare plasma is discussed within the context of a differential mission measure. Results concerning electron density measurements, nonequilibrium processes, and relative element abundances are also reviewed.

Doschek, G. A.↗
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 NTRS1989

Measurement of wavelengths for inner-shell transitions in Ca XVII-XIX

The wavelengths of transitions in solar flare spectra in the region 3.12-3.24 A have been measured using data recorded by a crystal spectrometer on the Air Force P78-1 spacecraft. Appearing in this wavelength region are inner-shell transitions of the type n = 1-2 in Ca XVII-XIX and resonance transitions in Ar XVII, Ar XVIII, and Fe XXV. The hydrogenic Ar XVIII 1s-1p transition was used to establish an absolute wavelength scale with an estimated uncertainty of 0.3 mA, and the relative wavelengths were measured to an uncertainty that is typically less than 0.1 mA. The presently measured wavelengths are compared with previously measured and calculated wavelengths, and the observed flare spectra are compared with synthetic spectra.

Seely, J. F.↗