U. Feldman

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.

Study of the X Rays Produced by a Vacuum Spark

The results of experiments on the x rays emitted by an energetic vacuum spark are presented. X-ray spectroscopic studies show, in addition to the characteristic x-ray lines of the electrode metal, lines originating from very high stages of ionization of the element. In particular, lines of the 1s2–1snp sequence in the helium-like spectra Ti xxi through Ni xxvii, and the 1s–2p transition in Fe xxvi, were observed. Pinhole photography and time-resolved studies of the total x-ray emission indicate that these highly ionized species originate in a small, hot region between the two electrodes, which is produced by a pinch in a plasma of evaporated anode metal.

Vacuum Ultraviolet Source

A low inductance three‐electrode spark source for the vacuum ultraviolet fed by a 20‐kV, 13‐μF capacitor is described. Its main feature is its ability to generate spectra of very high degrees of ionization. A 6‐kV spark produces spectra of CV and CVI as well as FeXVI, while a 17–18‐kV spark yields FeXVIII, AlXII, and AlXIII.

Observation of nonthermal energy distributions during the impulsive phase of solar flares

The Fe XXV resonance line and dielectronic satellite intensities have been measured as functions of time for several flares recorded by the Naval Research Laboratory crystal spectrometer (SOLFLEX) flown on the US Air Force P78-I spacecraft. The intensity ratios of the Fe XXV resonance line, the Fe XXIV n = 2 satellite line j, and the Fe XXIV n = 3 satellite line d13 indicate that nonthermal electron energy distributions occur during the impulsive phase of the flares. For the electron energies at which the j and d13 satellites are formed (4.7 and 5.8 keV, respectively), the electron energy distributions during the impulsive phase are observed to have a bump or to be nearly flat. For all of the flares that were studied, hard X-ray bursts occurred near the time of the nonthermal distributions observed in the SOLFLEX data. 25 references.

High resolution X-ray spectra of solar flares. V - Interpretation of inner-shell transitions in Fe XX-Fe XXIII

We discuss high-resolution solar flare iron line spectra recorded between 1.82 and 1.97 A by a spectrometer flown by the Naval Research Laboratory on an Air Force spacecraft launched on 1979 February 24. The emission line spectrum is due to inner-shell transitions in the ions Fe XX-Fe XXV. Using theoretical spectra and calculations of line intensities obtained by methods discussed by Merts, Cowan, and Magee, we derive electron temperatures as a function for time of two large class X flares. These temperatures are deduced from intensities of lines of Fe XXIII, Fe XXII, and Fe XXIV. Previous measurements by us have involved only lines of Fe XXIV and Fe XXV. We discuss the determination of the differential emission measure between about 12 x 10/sup 6/ K and 20 x 10/sup 6/ K using these temperatures. The possibility of determining electron densities in flare and tokamak plasmas using the inner-shell spectra of Fe XXI and Fe XX is discussed. We also discuss recent theoretical work by Mewe and Schrijver based on atomic data of Grineva, Safronova, and Urnov.

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. The wavelength shifts are measured relative to lines of neutral and singly ionized species formed in the chromosphere. Measurements are made for active regions on the disk and near the limb. Measurements are made over time intervals of about 20 minutes at the same solar location and at different locations in active regions, both on the disk and near the limb. The results are: (1) excluding flare and flare-related phenomena, only redshifts are found; (2) the magnitudes of the shifts correspond to downflow velocities between about 4 and 17 km s/sup -1/; (3) the shifts are largest for lines formed between about 5 x 10/sup 4/ and 10/sup 5/ K, and the shifts are distinctly less for lines formed about 10/sup 5/ K; (4) the shifts presist even out to, but not above, the limb: there is, however, a slight indication that the shifts are somewhat less near the limb than near Sun center; and (5) there is no obvious change in redshift for lines measured at the same solar location over time intervals of about 20 minutes.

Laser-plasma spectra of highly ionized fluorine

Lines between 11.3 and 17.2 A of lithium-like, helium-like, and hydrogen-like fluorine have been observed in spectra of laser-produced plasmas. These lines include nine members of the Lyman series of F IX; eight members of the principal series of F VIII; and satellite lines arising from doubly excited configurations of F VII and F VIII. Similar satellite lines of the abundant solar elements have been identified in soft X-ray spectra of solar flares. A wavelength list of fluorine lines is given, and physical conditions in the plasma are discussed.

Lithium-Like Spectra in Mg, Al, and Si

Using a grazing-incidence spectrometer and a low-inductance 14-μF, 12–15-kV spark source, we have observed spectra of Mg x, Al xi, and Si xii. About 50 new lines are reported between 25 and 65 A, arising from transitions between the lowest 2s and 2p configurations and ns, np, and nd configurations, where n≥3.

High-resolution X-ray spectra of solar flares. VI - Impulsive soft X-ray flares

We discuss soft X-ray spectra of a peculiar class of flares which are characterized by rapid rise and decay times in soft X-rays (<1 minute). At peak X-ray flux the electron temperature and emission measure are quite high, i.e., 18 x 10/sup 6/ and approx.5 x 10/sup 49/ cm/sup -3/, respectively. The spectra were recorded by Bragg crystal spectrometers (SOLFLEX) flown on the P78-1 spacecraft launched by the Air Force on 1979 February 24. The lines we discuss are produced by 1s--2p type transitions in iron and calcium near 1.86 and 3.2 A, respectively. We conclude that the 20 x 10/sup 6/ K plasma cools by in situ radiation, rather than by conduction into the chromosphere and subsequent radiative cooling at chromospheric temperatures. In order to cool within 1 minute, the electron density must be at least 2 x 10/sup 11/ cm/sup -3/. We discuss two events that produced ..gamma..-ray emission detected by other spacecraft. These events show unusual aspects in their spectra during the rise phase, not observed in any other flare examined in the SOLFLEX data base. We comment on the Fe II K..cap alpha.. emission observed for one of these events at the same time that intensemorexa0» hard X-ray bursts were occurring.«xa0less

Spectra of ions in the fluorine I isoelectronic sequence from Sc XIII to Cu XXI.

In a study of the spectra of ions Sc xiii to Cu xxi in the fluorine isoelectronic sequence, lines of the 2p5–2p4 3s and 2p5–2p4 3d arrays have been observed and classified, as well as some lines of the 2s 2p6–2s2p5 3s array. Assignments were made on the basis of extrapolations and an intermediate-coupling calculation. The results are compared with those available in the literature.