Temperature dependence of emission measure in solar X-ray plasmas. 1: Non-flaring active regions

X-ray and ultraviolet line emission from hot, optically thin material forming coronal active regions on the sun may be described in terms of an emission measure distribution function, Phi (T). A relationship is developed between line flux and Phi (T), a theory which assumes that the electron density is a single-valued function of temperature. The sources of error involved in deriving Phi (T) from a set of line fluxes are examined in some detail. These include errors in atomic data (collisional excitation rates, assessment of other mechanisms for populating excited states of transitions, element abundances, ion concentrations, oscillator strengths) and errors in observed line fluxes arising from poorly - known instrumental responses. Two previous analyses are discussed in which Phi (T) for a non-flaring active region is derived. A least squares method of Batstone uses X-ray data of low statistical significance, a fact which appears to influence the results considerably. Two methods for finding Phi (T) ab initio are developed. The coefficients are evaluated by least squares. These two methods should have application not only to active-region plasmas, but also to hot, flare-produced plasmas

Silicon abundance from RESIK solar flare observations

The RESIK instrument on the CORONAS-F spacecraft obtained solar flare and active region X-ray spectra in four channels covering the wavelength range 3.8 -- 6.1 \AA in its operational period between 2001 and 2003. Several highly ionized silicon lines were observed within the range of the long-wavelength channel (5.00 -- 6.05 \AA). The fluxes of the \sixiv Ly-$\beta$ line (5.217 \AA) and the \sixiii $1s^2 - 1s3p$ line (5.688 \AA) during 21 flares with optimized pulse-height analyzer settings on RESIK have been analyzed to obtain the silicon abundance relative to hydrogen in flare plasmas. As in previous work, the emitting plasma for each spectrum is assumed to be characterized by a single temperature and emission measure given by the ratio of emission in the two channels of GOES. The silicon abundance is determined to be $A({\rm Si}) = 7.93 \pm .21$ (\sixiv) and $7.89 \pm .13$ (\sixiii) on a logarithmic scale with H = 12. These values, which vary by only very small amounts from flare to flare and times within flares, are $2.6 \pm 1.3$ and $2.4 \pm 0.7$ times the photospheric abundance, and are about a factor of three higher than RESIK measurements during a period of very low activity. There is a suggestion that the Si/S abundance ratio increases from active regions to flares.Comment: To be published, Solar Physic

RESIK observations of He-like Ar X-ray line emission in solar flares

The Ar XVII X-ray line group principally due to transitions 1s2 - 1s2l (l=s, p) near 4 Anstroms was observed in numerous flares by the RESIK bent crystal spectrometer aboard CORONAS-F between 2001 and 2003. The three line features include the Ar XVII w (resonance line), a blend of x and y (intercombination lines), and z (forbidden line), all of which are blended with Ar XVI dielectronic satellites. The ratio G, equal to [I(x+y) + I(z)]/I(w), varies with electron temperature Te mostly because of unresolved dielectronic satellites. With temperatures estimated from GOES X-ray emission, the observed G ratios agree fairly well with those calculated from CHIANTI and other data. With a two-component emission measure, better agreement is achieved. Some S XV and S XVI lines blend with the Ar lines, the effect of which occurs at temperatures greater than 8MK, allowing the S/Ar abundance ratio to be determined. This is found to agree with coronal values. A nonthermal contribution is indicated for some spectra in the repeating-pulse flare of 2003 February 6.Comment: Latex file and 3 ps files. Astrophysical Journal Letters (accepted, June 2008

Resonant Scattering of Emission Lines in Coronal Loops: Effects on Image Morphology and Line Ratios

We have investigated the effects of resonant scattering of emission lines on the image morphology and intensity from coronal loop structures. It has previously been shown that line of sight effects in optically thin line emission can yield loop images that appear uniformly bright at one viewing angle, but show ``looptop sources'' at other viewing angles. For optically thick loops where multiple resonant scattering is important, we use a 3D Monte Carlo radiation transfer code. Our simulations show that the intensity variation across the image is more uniform than the optically thin simulation and, depending on viewing angle, the intensity may be lower or higher than that predicted from optically thin simulations due to scattering out of or into the line of sight.Comment: Accepted for publication in Ap

The X-ray spectrum of Fe XVII revisited with a multi-ion model

The theoretical intensities of the soft X-ray Fe XVII lines arising from 2l-3l' transitions are reexamined using a three-ion collisional-radiative model that includes the contributions to line formation of radiative recombination (RR), dielectronic recombination (DR), resonant excitation (RE), and inner-shell collisional ionization (CI), in addition to the usual contribution of collisional excitation (CE). These additional processes enhance mostly the 2p-3s lines and not the 2p-3d lines. Under coronal equilibrium conditions, in the electron temperature range of 400 to 600 eV where the Fe XVII line emissivities peak, the combined effect of the additional processes is to enhance the 2p-3s lines at 16.78, 17.05, and 17.10 A, by ~ 25%, 30%, and 55%, respectively, compared with their traditional, single-ion CE values. The weak 2p-3d line at 15.45 A is also enhanced by up to 20%, while the other 2p-3d lines are almost unaffected. The effects of DR and RE are found to be dominant in this temperature range (400 - 600 eV), while that of CI is 3% at the most, and the contribution of RR is less than 1%. At lower temperatures, where the Fe XVII / Fe XVIII abundance ratio is high, the RE effect dominates. However, as the temperature rises and the Fe XVIII abundance increases, the DR effect takes over. The newly calculated line powers can reproduce most of the often observed high values of the (I17.05 + I17.10) / I15.01 intensity ratio. The importance of ionization and recombination processes to the line strengths also helps to explain why laboratory measurements in which CE is essentially the sole mechanism agree well with single-ion calculations, but do not reproduce the astrophysically observed ratios.Comment: Submitted to Ap

A solar spectroscopic absolute abundance of argon from RESIK

Observations of He-like and H-like Ar (Ar XVII and Ar XVIII) lines at 3.949 Angstroms and 3.733 Angstroms respectively with the RESIK X-ray spectrometer on the CORONAS-F spacecraft, together with temperatures and emission measures from the two channels of GOES, have been analyzed to obtain the abundance of Ar in flare plasmas in the solar corona. The line fluxes per unit emission measure show a temperature dependence like that predicted from theory, and lead to spectroscopically determined values for the absolute Ar abundance, A(Ar) = 6.44 pm 0.07 (Ar XVII) and 6.49 pm 0.16 (Ar XVIII) which are in agreement to within uncertainties. The weighted mean is 6.45 pm 0.06, which is between two recent compilations of the solar Ar abundance and suggest that the photospheric and coronal abundances of Ar are very similar.Comment: 4 figure

The Solar Photospheric-to-Coronal Fe abundance from X-ray Fluorescence Lines

The ratio of the Fe abundance in the photosphere to that in coronal flare plasmas is determined by X-ray lines within the complex at 6.7~keV (1.9~\AA) emitted during flares. The line complex includes the He-like Fe (\fexxv) resonance line $w$ (6.70~keV) and Fe K$\alpha$ lines (6.39, 6.40~keV), the latter being primarily formed by the fluorescence of photospheric material by X-rays from the hot flare plasma. The ratio of the Fe K$\alpha$ lines to the \fexxv\ $w$ depends on the ratio of the photospheric-to-flare Fe abundance, heliocentric angle $\theta$ of the flare, and the temperature $T_e$ of the flaring plasma. Using high-resolution spectra from X-ray spectrometers on the {\em P78-1} and {\em Solar Maximum Mission} spacecraft, the Fe abundance in flares is estimated to be $1.6\pm 0.5$ and $2.0 \pm 0.3$ times the photospheric Fe abundance, the {\em P78-1} value being preferred as it is more directly determined. This enhancement is consistent with results from X-ray spectra from the {\em RHESSI} spacecraft, but is significantly less than a factor 4 as in previous work.Comment: Accepted for publication by MNRA