G. A. Doschek

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.

Flows and Nonthermal Velocities in Solar Active Regions Observed with the EUV Imaging Spectrometer on Hinode: A Tracer of Active Region Sources of Heliospheric Magnetic Fields?

From Doppler velocity maps of active regions constructed from spectra obtained by the EUV Imaging Spectrometer (EIS) on the Hinode spacecraft we observe large areas of outflow (20-50 km s -->−1) that can persist for at least a day. These outflows occur in areas of active regions that are faint in coronal spectral lines formed at typical quiet-Sun and active region temperatures. The outflows are positively correlated with nonthermal velocities in coronal plasmas. The bulk mass motions and nonthermal velocities are derived from spectral line centroids and line widths, mostly from a strong line of Fe XII at 195.12 A. The electron temperature of the outflow regions estimated from an Fe XIII to Fe XII line intensity ratio is about -->(1.2–1.4) × 106 K. The electron density of the outflow regions derived from a density-sensitive intensity ratio of Fe XII lines is rather low for an active region. Most regions average around -->7 × 108 cm -->−3, but there are variations on pixel spatial scales of about a factor of 4. We discuss results in detail for two active regions observed by EIS. Images of active regions in line intensity, line width, and line centroid are obtained by rastering the regions. We also discuss data from the active regions obtained from other orbiting spacecraft that support the conclusions obtained from analysis of the EIS spectra. The locations of the flows in the active regions with respect to the longitudinal photospheric magnetic fields suggest that these regions might be tracers of long loops and/or open magnetic fields that extend into the heliosphere, and thus the flows could possibly contribute significantly to the solar wind.

Coronal Plasma Motions near Footpoints of Active Region Loops Revealed from Spectroscopic Observations with Hinode EIS

The solar active region 10938 has been observed from the disk center to the west limb with the Hinode EUV Imaging Spectrometer. In the disk-center observation, subsonic upflow motions of tens of km s -->−1 and enhanced nonthermal velocities have been found near the footpoints of the active region loops assuming a single Gaussian approximation for the emission-line profiles. When the same part of the active region is observed near the limb, both upflows and enhanced nonthermal velocities essentially decrease. There is a strong correlation between Doppler velocity and nonthermal velocity. Significant deviations from a single Gaussian profile are found in the blue wing of the line profiles for the upflows. These suggest that there are unresolved high-speed upflows. We discuss the implications for coronal heating mechanisms.

EUV Emission Lines and Diagnostics Observed with Hinode/EIS

Quiet Sun and active region spectra from the Hinode/EIS instrument are presented, and the strongest lines from different temperature regions discussed. A list of emission lines recommended to be included in EIS observation studies is presented based on analysis of blending and diagnostic potential using the CHIANTI atomic database. In addition we identify the most useful density diagnostics from the ions covered by EIS.

The extreme ultraviolet spectrum of Alpha Aurigae (Capella)

Extreme ultraviolet spectra (λλ 70-740) of the bright spectroscopic binary system, Capella (Alpha Aurigae) obtained with the Extreme Ultraviolet Explorer satellite (EUVE), show a rich emission spectrum dominated by iron emission lines: Fe XV-XXIV. The emission measure for the system reveals a continuous distribution of plasma temperatures between 10 5 and 10 7.8 K, with a clear minimum near 10 6 K and a local maximum at 6×10 6 K. Electron density diagnostics based on Fe XXI indicate N e ≃4×10 11 -10 13 cm 3 at T e =10 7 K

The Bragg Crystal Spectrometer for SOLAR-A

The Bragg Crystal Spectrometer (BCS) is one of the instruments which makes up the scientific payload of the SOLAR-A mission. The spectrometer employs four bent germanium crystals, views the whole Sun and observes the resonance line complexes of H-like Fexxvi and He-like Fexxv, Caxix, and Sxv in four narrow wavelength ranges with a resolving power (λ/Δλ) of between 3000 and 6000. The spectrometer has approaching ten times better sensitivity than that of previous instruments thus permitting a time resolution of better than 1 s to be achieved. The principal aim is the measurement of the properties of the 10 to 50 million K plasma created in solar flares with special emphasis on the heating and dynamics of the plasma during the impulsive phase. This paper summarizes the scientific objectives of the BCS and describes the design, characteristics, and performance of the spectrometers.

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.

The Yohkoh Mission for High-Energy Solar Physics

The Japanese Yohkoh satellite is now in orbit observing the sun with a set of x-ray imagers and x-ray and gamma-ray spectrometers. The data from this successful mission provide new information on solar flares and the suns corona. This paper discusses the Yohkoh observations and presents a sample of the first scientific results from the mission.

Nonthermal velocities in solar active regions observed with the extreme-ultraviolet imaging spectrometer on Hinode

We discuss nonthermal velocities in an active region as revealed by the Extreme-Ultraviolet Imaging Spectrometer (EIS) on the Hinode spacecraft. The velocities are derived from spectral line profiles in the extreme-ultraviolet (EUV) from a strong line of Fe XII at 195.12 A by fitting each line profile to a Gaussian function. We compare maps of the full width at half-maximum values, the Fe XII spectral line intensity, the Fe XII Doppler shift, the electron temperature, and electron density. We find that the largest widths in the active region do not occur in the most intense regions, but seem to concentrate in less intense regions, some of which are directly adjacent to coronal loops, and some of which concentrate in regions which also exhibit relative Doppler outflows. The increased widths can also occur over extended parts of the active region.

Electron Densities in the Solar Polar Coronal Holes from Density-Sensitive Line Ratios of Si VIII and S X

We derive electron densities as a function of height in the north and south polar coronal holes from a forbidden spectral line ratio of Si VIII. Si VIII is produced at about 8 × 105 K in ionization equilibrium. We also derive densities from a similar line ratio of S X (1.3 × 106 K). The spectra were obtained with the Solar Ultraviolet Measurements of Emitted Radiation spectrometer flown on the Solar and Heliospheric Observatory spacecraft. In addition to the primary mechanism of electron impact excitation, the derivation of theoretical level populations for Si VIII and S X includes both proton and resonance capture excitation. We compare the coronal hole results to quiet-Sun coronal measurements obtained outside the east and west limbs. We find for distances of a few arcseconds outside the solar limb that the average line-of-sight electron densities in the coronal holes are about a factor of 2 lower than in quiet-Sun regions. The decrease of density with height is exponential in the polar holes. We also confirm the result known from a variety of earlier observations that the temperature of most of the plasma in coronal holes does not exceed about 106 K.