J. Sylwester

Multitemperature analysis of solar X-ray line emission

In this paper we propose and test a new method of multitemperature analysis of solar X-ray spectra. The method, which is based on a technique developed by Withbroe (1975), is designed to be used in the interpretation of spectra, to be measured by the X-Ray Polychromator on the Solar Maximum Mission. Various tests of the method on simulated temperature models establish its usefulness, generality, and stability. The possibilities of deriving the relative element abundances are analysed. The results of the present paper extend the possibility of the multitemperature analysis of X-ray spectra as compared with the results of Craig and Brown (1976a, b) and Craig (1977).

Solar Flare Abundances of Potassium, Argon, and Sulphur

The absolute abundance of potassium has been determined for the first time from X-ray solar flare line and continuum spectra. The absolute and relative abundances of Ar and S have also been determined. Assuming that the flare plasma is coronal, and since potassium has the lowest first ionization potential (FIP) of any common element on the Sun, this determination is of importance in the continuing debate concerning the nature of the coronal/photospheric element abundance ratios, which are widely considered to depend on the FIP. The measurements were made with the RESIK crystal spectrometer on the Coronas-F spacecraft. A differential emission measure DEM exp(-βTe) was found to be the most consistent with the data of three models considered. We find that the K/H abundance ratio is (3.7 ± 1.0) × 10-7, a factor of 3 times photospheric. Our measured values of the Ar/H ratio, (2.8 ± 0.2) × 10-6, and of the S/H ratio, (2.2 ± 0.4) × 10-5, are equal to previous coronal and photospheric determinations to within uncertainties. These measurements therefore fit a pattern in which low-FIP elements are enriched in the corona by a factor 3 and in which high-FIP elements (including S) have equal coronal and photospheric abundances.

Soft X-ray coronal spectra at low activity levels observed by RESIK

Context. The quiet-Sun X-ray emission is important for deducing coronal heating mechanisms, but it has not been studied in detail since the Orbiting Solar Observatory (OSO) spacecraft era. Bragg crystal spectrometer X-ray observations have generally concentrated on flares and active regions. The high sensitivity of the RESIK (REntgenovsky Spectrometer s Izognutymi Kristalami) instrument on the CORONAS-F solar mission has enabled the X-ray emission from the quiet corona to be studied in a systematic way for the first time. Aims. Our aim is to deduce the physical conditions of the non-flaring corona from RESIK line intensities in several spectral ranges using both isothermal and multithermal assumptions. Methods. We selected and analyzed spectra in 312 quiet-Sun intervals in January and February 2003, sorting them into 5 groups according to activity level. For each group, the fluxes in selected spectral bands have been used to calculate values of parameters for the best-fit that leads to intensities characteristic of each group. We used both isothermal and multitemperature assumptions, the latter described by differential emission measure (DEM) distributions. RESIK spectra cover the wavelength range (3.3−6. 1A ). This includes emission lines of highly ionized Si, S, Cl, Ar, and K, which are suitable for evaluating temperature and emission measure, were used. Results. The RESIK spectra during these intervals of very low solar activity for the first time provide information on the temperature structure of the quiet corona. Although most of the emission seems to arise from plasma with a temperature between 2 MK and 3 MK, there is also evidence of a hotter plasma (T ∼ 10 MK) with an emission measure 3 orders smaller than the cooler component. Neither coronal nor photospheric element abundances appear to describe the observed spectra satisfactorily.

SphinX soft X-ray spectrophotometer: Science objectives, design and performance

The goals and construction details of a new design Polish-led X-ray spectrophotometer are described. The instrument is aimed to observe emission from entire solar corona and is placed as a separate block within the Russian TESIS X- and EUV complex aboard the CORONAS-PHOTON solar orbiting observatory. SphinX uses silicon PIN diode detectors for high time resolution measurements of the solar spectra in the range 0.8–15 keV. Its spectral resolution allows for discerning more than hundred separate energy bands in this range. The instrument dynamic range extends two orders of magnitude below and above these representative for GOES. The relative and absolute accuracy of spectral measurements is expected to be better than few percent, as follows from extensive ground laboratory calibrations.

Diagnostics of non-thermal distributions in solar flare spectra observed by RESIK and RHESSI

Context. During solar flares an enormous amount of energy is released, and the charged particles, like electrons, are accelerated. These non-thermal electrons interact with the plasma in various parts of solar flares, where the distribution function of electrons can therefore be non-Maxwellian. Aims. We focus on the non-thermal components of the electron distribution in the keV range and analyse high-energy resolution X-ray spectra detected by RESIK and RHESSI for three solar flares. Methods. In the 2‐4 keV range we assume that the electron distribution can be modelled by an n-distribution. Using a method of line-intensity ratios, we analyse allowed and satellite lines of Si observed by RESIK and estimate the parameters of this n-distribution. At higher energies we explore RHESSI bremsstrahlung spectra. Adopting a forward-fitting approach and thick-target approximation, we determine the characteristics of injected electron beams. Results. RHESSI non-thermal component associated with the electron beam is correlated well with presence of the non-thermal n-distribution obtained from the RESIK spectra. In addition, such an n-distribution occurs during radio bursts observed in the 0.61‐ 15.4 GHz range. Furthermore, we show that the n-distribution could also explain RHESSI emission below 5k eV. Therefore, two independent diagnostics methods indicate the flare plasma being a! ected by the electron beam can have a non-thermal component in the 2‐5 keV range, which is described by the n-distribution well. Finally, spectral line analysis reveals that the n-distribution does not occupy the same location as the thermal component detected by RHESSI at 10 keV.

HIGHLY IONIZED POTASSIUM LINES IN SOLAR X-RAY SPECTRA AND THE ABUNDANCE OF POTASSIUM

The abundance of potassium is derived from X-ray lines observed during flares by the RESIK instrument on the solar mission CORONAS-F between 3.53 A and 3.57 A. The lines include those emitted by He-like K and Li-like K dielectronic satellites, which have been synthesized using the CHIANTI atomic code and newly calculated atomic data. There is good agreement between observed and synthesized spectra, and the theoretical behavior of the spectra with varying temperature estimated from the ratio of the two GOES channels is correctly predicted. The observed fluxes of the He-like K resonance line per unit emission measure give log A(K) = 5.86 (on a scale log A(H) = 12), with a total range of a factor 2.9. This is higher than photospheric abundance estimates by a factor 5.5, a slightly greater enhancement than for other elements with first ionization potential (FIP) less than ~10 eV. There is, then, the possibility that enrichment of low-FIP elements in coronal plasmas depends weakly on the value of the FIP which for K is extremely low (4.34 eV). Our work also suggests that fractionation of elements to form the FIP effect occurs in the low chromosphere rather than higher up, as in some models.

Detailed evidence for flare-to-flare variations of the coronal calcium abundance

The analysis of X-ray solar flare spectra obtained by the Bent Crystal Spectrometer on board the Solar Maximum Mission satellite is presented. The ratio of the Ca XIX resonance line intensity to the nearby continuum is used to measure the calcium abundance relative to hydrogen (ACa). A description of the spectroscopic method of determining the absolute calcium abundance is given. Possible instrumental and solar effects that might influence the abundance estimates are evaluated. Over 5000 spectra from more than 100 flares are analyzed. We find a flare-to-flare variation for ACa that is not correlated with flare size, Hα importance, or with several other flare characteristics. For flares observed from two active regions, the observed value of ACa increases as a function of time. The average for all flares is ACa = (5.77 ± 1.41) × 10-6. A discussion of investigated correlations of derived ACa values with several flare characteristics is presented.

Si XII X-Ray Satellite Lines in Solar Flare Spectra

The temperature dependence of the Si XII n = 3 and 4 dielectronic satellite line features at 5.82 and 5.56 A, respectively, near the Si XIII 1s2-1s3p and 1s2-1s4p lines (5.681 and 5.405 A), is calculated using atomic data presented here. The resulting theoretical spectra are compared with solar flare spectra observed by the RESIK spectrometer on the CORONAS-F spacecraft. The satellites, like the more familiar n = 2 satellites near the Si XIII 1s2-1s2p lines, are formed mostly by dielectronic recombination, but unlike the n = 2 satellites, are unblended. The implications for similar satellite lines in flare Fe spectra are discussed.

X-ray emitting hot plasma in solar active regions observed by the SphinX spectrometer

Aims. The detection of very hot plasma in the quiescent corona is important for diagnosing heating mechanisms. The presence and the amount of such hot plasma is currently debated. The SphinX instrument on-board the CORONAS-PHOTON mission is sensitive to X-ray emission of energies well above 1 keV and provides the opportunity to detect the hot plasma component. Methods. We analysed the X-ray spectra of the solar corona collected by the SphinX spectrometer in May 2009 (when two active regions were present). We modelled the spectrum extracted from the whole Sun over a time window of 17 days in the 1.34− 7k eV energy band by adopting the latest release of the APED database. Results. The SphinX broadband spectrum cannot be modelled by a single isothermal component of optically thin plasma and two components are necessary. In particular, the high statistical significance of the count rates and the accurate calibration of the spectrometer allowed us to detect a very hot component at ∼7 million K with an emission measure of ∼2.7 × 10 44 cm −3 . The X-ray emission from the hot plasma dominates the solar X-ray spectrum above 4 keV. We checked that this hot component is invariably present in both the high and low emission regimes, i.e. even excluding resolvable microflares. We also present and discuss the possibility of a non-thermal origin (which would be compatible with a weak contribution from thick-target bremsstrahlung) for this hard emission component. Conclusions. Our results support the nanoflare scenario and might confirm that a minor flaring activity is ever-present in the quiescent corona, as also inferred for the coronae of other stars.

Determination of the calcium elemental abundance for 43 flares from SMM-XRP solar X-ray spectra

Abstract The helium and lithium-like X-ray transitions of Ca XVIII-XIX have been used to make an absolute measurement of the coronal calcium elemental abundance relative to hydrogen (ACa) in solar flares. Cooling phase spectra of 43 flares obtained in channel 1 of the Bent Crystal Spectrometer (BCS) on the Solar Maximum Mission have been analyzed. The abundance is determined from the intensity ratio of the Ca XIX resonance line (1S0 - 1P1) and nearby continuum. A large variation is observed in the values of the derived abundances, ranging up to a factor of 2.5 between the extreme cases. This confirms the earlier results of Sylwester, Lemen, and Mewe [1], who investigated a smaller sample of flares. In addition to the variability of ACa observed between different flares, it was suggested [1] that ACa varies during the heating phase of some flares. We neglect this phenomenon in the present work, and concentrate on the cooling phase during which ACa appears to remain constant for any individual flare. Attempts to correlate the ACa measurements with other observable features are discussed.