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The Bent Crystal Spectrometer (BCS) on the NASA Solar Maximum Mission spacecraft observed the X-ray spectra of numerous solar flares during the periods 1980 February to November and 1984~--~1989. The instrument, the first of its kind to use curved crystal technology, observed the resonance lines of He-like Ca (\caxix) and Fe (\fexxv) and neighboring satellite lines, allowing the study of the rapid evolution of flare plasma temperature, turbulence, mass motions etc. To date there has not been a solar X-ray spectrometer with comparable spectral and time resolution, while subsequent solar cycles have delivered far fewer and less intense flares. The BCS data archive thus offers an unparalleled resource for flare studies. A recent re-assessment of the BCS calibration and its operations is extended here by using data during a spacecraft scan in the course of a flare on 1980 November~6 that highlights small deformations in the crystal curvature of the important channel~1 (viewing lines of \caxix\ and satellites). The results explain long-standing anomalies in spectral line ratios which have been widely discussed in the past. We also provide an in-flight estimation of the BCS collimator field of view which improves the absolute intensity calibration of the BCS. The BCS channel~1 background is shown to be entirely due to solar continuum radiation, confirming earlier analyses implying a time-variable flare abundance of Ca. We suggest that BCS high-resolution \caxix\ and \fexxv\ line spectra be used as templates for the analysis of X-ray spectra of non-solar sources., Comment: To be published, Astrophysical Journal

ChemiX is a Bragg crystal spectrometer that will fly on the two Interhelioprobe spacecraft due for launch in 2025 and 2026. The spacecraft perihelion will be only 0.3 a.u. and the orbit inclination up to 30°, and so instruments on board will have a close view of solar active regions and flares and regions near each solar pole. The ChemiX X-ray spectrometer, built by a consortium of groups led by the Space Research Centre, Polish Academy of Sciences, will fly on each of the spacecraft, and observe X-ray spectra in the 1.5 - 9 Å range. Spectral lines in this range include resonance lines of helium-like and hydrogen-like ions of elements such as Fe, Ca, Ar, S, and Si, with less abundant elements such as K and Cl represented by weaker lines which the high sensitivity of ChemiX should be able to detect. The free–free and free–bound continua should also be detected since instrumental background will be eliminated. Three of the seven channels of ChemiX will be in a “dopplerometer” arrangement by which spatial and spectral shifts present in flare impulsive stages can be disentangled.

The Bent Crystal Spectrometer (BCS) onboard the NASA Solar Maximum Mission was part of the X-ray Polychromator, which observed numerous flares and bright active regions from February to November 1980, when operation was suspended as a result of the failure of the spacecraft fine pointing system. Observations resumed following the Space Shuttle SMM Repair Mission in April 1984 and continued until November 1989. BCS spectra have been widely used in the past to obtain temperatures, emission measures, and turbulent and bulk flows during flares, as well as element abundances. Instrumental details including calibration factors not previously published are given here, and the in-orbit performance of the BCS is evaluated. Some significant changes during the mission are described, and recommendations for future instrumentation are made. Using improved estimates for the instrument parameters and operational limits, it is now possible to obtain de-convolved, calibrated spectra that show finer detail than before, providing the means to improved interpretation of the physics of the emitting plasmas. The results indicate how historical, archived data can be re-used to obtain enhanced and new, scientifically valuable results., Comment: Figures in B&W but will appear in color when published. Accepted for publication

Diogeness was an uncollimated scanning flat crystal spectrometer observing solar flare X-ray spectra in four narrow wavelength bands in the vicinity of Ca xix, S xv and Si xiii He-like line ‘triplets’ around 3.18 Å, 5.04 Å and 6.65 Å. In two of the spectral channels, emission lines around the Ca xix 3.178 Å resonance line were scanned in opposite directions, being diffracted from precisely adjusted identical Quartz crystals mounted on a common shaft in a so-called Dopplerometer (tachometer) configuration. Observations of solar X-ray spectra made by Diogeness provide a direct diagnostic information on plasma characteristics during the impulsive flare energy release. We present a sample of events which occurred during the Diogeness operation time from August 16, 2001 to September 17, 2001.

We present extreme-ultraviolet (EUV) and X-ray light curves and Doppler velocity measurements for a GOES class M2 solar flare observed in NOAA Active Region 9433 on 2001 April 24 at high time resolution with the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory (SOHO) satellite (9.83 s) and the Bragg Crystal Spectrometer (BCS) and Hard X-Ray Telescope (HXT) on board the Yohkoh satellite (9.00 s). Coordinated imagery with SOHO's Extreme-ultraviolet Imaging Telescope and the Transition Region and Coronal Explorer satellite reveal that the CDS slit was centered on the flare commencement site; coordinated magnetograms from SOHO's Michelson Doppler Imager are consistent with this site being the footpoint of a flare loop anchored in positive magnetic field near the outer edge of a sunspot's penumbra. CDS observations include the preflare quiescent phase, two precursors, the flare impulsive and peak phases, and its slow decline. We find that (1) the average wavelengths of O III, O IV, O V, Ne VI, and He II lines measured during the preflare quiescent phase are equal (within the measurement uncertainties) to those measured during the late decline phase, indicating that they can be used as reference standards against which to measure Doppler velocities during the flare; (2) the EUV lines of O III, O IV, O V, and He II exhibit upflow velocities of ~40 km s-1 during both precursor events, suggestive of small-scale chromospheric evaporation; (3) the Fe XIX EUV intensity rises and stays above its preflare noise level during the second (later) precursor; (4) the maximum upflow velocities measured in Fe XIX with CDS (64 km s-1) and in Ca XIX (65 km s-1) and S XV (78 km s-1) with BCS occur during the flare impulsive phase and are simultaneous within the instrumental time resolutions; (5) the Fe XIX EUV intensity begins its impulsive rise nearly 90 s later than the rise in intensities of the cooler lines; (6) hard X-ray emission arises nearly 60 s after the cool EUV lines begin their impulsive intensity rise; and (7) the EUV lines of O III, O IV, O V, and He II exhibit downflow velocities of ~40 km s-1 during the flare impulsive phase, suggesting momentum balance between the hot upflowing material and the cool downflowing material. Our observations are consistent with energy transport by nonthermal particle beams in chromospheric evaporation theory.

PUBLISHED, Results of the search of the periodic changes of the 530.3 nm line intensity emitted by selected structures of the solar corona in the frequency range 1-10 Hz are presented. A set of 12 728 images of the section of the solar corona extending from near the north pole to the south-west were taken simultaneously in the 530.3 nm ("green") line and white-light with the Solar Eclipse Coronal Imaging System (SECIS) during the 143-seconds-long totality of the 1999 August 11 solar eclipse observed in Shabla, Bulgaria. The time resolution of the collected data is better than 0.05 s and the pixel size is approximately 4 arcsec. Using classical Fourier spectral analysis tools, we investigated temporal changes of the local 530.3 nm coronal line brightness in the frequency range 1-10 Hz of thousands of points within the field of view. The various photometric and instrumental effects have been extensively considered. We did not find any indisputable, statistically significant evidence of periodicities in any of the investigated points (at significance level $\alpha=0.05$)., We thank the Leverhulme Trust (UK) (grant reference F00203/A), Rutherford Appleton Laboratory (Space Science and Technology and Instrumentation Depts.) and the Polish State Committee for Scientific Research (KBN) (grant number is PBZ KBN 054/P03/2001) for financial support. We are grateful to Bulgarian colleagues who arranged the eclipse site at Shabla so well and were particularly helpful in getting our equipment transported in time for the eclipse. We would like to mention specifically Dr Maria Madjarska (now at Armagh Observatory, UK), and particularly the late Prof. Dr Vladimir J. Dermendjiev and the late Prof. Dr Dimitar Mishev (Bulgarian Academy of Sciences, Sofia) for their tireless help before, during and after the eclipse. We also thank the staff of the Bulgarian Army Air Defence Establishment for their help in setting up our base there. We thank Dr Francisco Diego for his help with post-eclipse calibration of the green-line filter. We also thank the several colleagues who gave us technical help, especially Dr Serge Koutchmy, Prof. Jay Pasachoff and Dr Ray Smartt.

Oscillations in network bright points (NBPs) are studied at a variety of chromospheric heights. In particular, the three-dimensional variation of NBP oscillations is studied using image segmentation and cross-correlation analysis between images taken in light of Ca II K3, Hα core, Mg I b2, and Mg I b1 - 0.4 A. Wavelet analysis is used to isolate wave packets in time and to search for height-dependent time delays that result from upward- or downward-directed traveling waves. In each NBP studied, we find evidence for kink-mode waves (1.3, 1.9 mHz), traveling up through the chromosphere and coupling with sausage-mode waves (2.6, 3.8 mHz). This provides a means for depositing energy in the upper chromosphere. We also find evidence for other upward- and downward-propagating waves in the 1.3-4.6 mHz range. Some oscillations do not correspond to traveling waves, and we attribute these to waves generated in neighboring regions.

The spatial variation of chromospheric oscillations in network bright points (NBPs) is studied using high-resolution observations in Ca II K3. Light curves and hence power spectra were created by isolating distinct regions of the NBP via a simple intensity thresholding technique. Using this technique, it was possible to identify peaks in the power spectra with particular spatial positions within the NBPs. In particular, long-period waves with periods of 4-15 minutes (1-4 mHz) were found in the central portions of each NBP, indicating that these waves are certainly not acoustic but possibly due to magnetoacoustic or magnetogravity wave modes. We also show that spatially averaged or low spatial resolution power spectra can lead to an inability to detect such long-period waves.

The {\em DIOGENESS} X-ray crystal spectrometer on the {\em CORONAS-F} spacecraft operated for a single month (25~August to 17~September) in 2001 but in its short lifetime obtained one hundred and forty high-resolution spectra from some eight solar flares with {\em GOES} importance ranging from C9 to X5. The instrument included four scanning flat crystals with wavelength ranges covering the regions of \sixiii\ (6.65~\AA), \sxv\ (5.04~\AA), and \caxix\ (3.18~\AA) X-ray lines and associated dielectronic satellites. Two crystals covering the \caxix\ lines were oriented in a ``Dopplerometer'' manner, i.e. such that spatial and spectral displacements both of which commonly occur in flares can be separated. We describe the {\em DIOGENESS} spectrometer and the spectra obtained during flares which include lines not hitherto seen from spacecraft instruments. An instrument with very similar concept is presently being built for the two Russian {\em Interhelioprobe} spacecraft due for launch in 2020 and 2022 that will make a near-encounter (perihelion $\sim 0.3$ a.u.) to the Sun in its orbit. We outline the results that are likely to be obtained., Comment: Accepted for publication, Solar Physics

The structure and properties of a newly emerged solar active region (NOAA Active Region 7985) are discussed using the Coronal Diagnostic Spectrometer (CDS) and the Extreme-Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory. CDS obtained high-resolution EUV spectra in the 308-381 ? and 513-633 ? wavelength ranges, while EIT recorded full-disk EUV images in the He II (304 ?), Fe IX/X (171 ?), Fe XII (195 ?), and Fe XV (284 ?) bandpasses. Electron density measurements from Si IX, Si X, Fe XII, Fe XIII, and Fe XIV line ratios indicate that the region consists of a central high-density core with peak densities of the order of 1.2 ? 1010 cm-3, which decrease monotonically to ~5.0 ? 108 cm-3 at the active region boundary. The derived electron densities also vary systematically with temperature. Electron pressures as a function of both active region position and temperature were estimated using the derived electron densities and ion formation temperatures, and the constant pressure assumption was found to be an unrealistic simplification. Indeed, the active region is found to have a high-pressure core (1.3 ? 1016 cm-3 K) that falls to 6.0 ? 1014 cm-3 K just outside the region. CDS line ratios from different ionization stages of iron, specifically Fe XVI (335.4 ?) and Fe XIV (334.4 ?), were used to diagnose plasma temperatures within the active region. Using this method, peak temperatures of 2.1 ? 106 K were identified. This is in good agreement with electron temperatures derived using EIT filter ratios and the two-temperature model of Zhang et al. The high-temperature emission is confined to the active region core, while emission from cooler (1-1.6) ? 106 K lines originates in a system of loops visible in EIT 171 and 195 ? images. Finally, the three-dimensional geometry of the active region is investigated using potential field extrapolations from a Kitt Peak magnetogram. The combination of EUV and magnetic field extrapolations extends the core-halo picture of active region structure to one in which the core is composed of a number of compact coronal loops that confine the hot, dense, high-pressure core plasma while the halo emission emerges from a system of cooler and more extended loops.

It has been known for over 50 years that the outer atmosphere of the Sun, the so-called corona, has a temperature of 106 K (~0.1 keV) yet the exact reasons for this are still being debated. The magnetic field that pervades the corona is certainly the source of the heating, but the question is whether the field energy is dissipated via numerous, small, random reconnections (known as `nanoflares', i.e. small energy releases of about 1016 J) or the damping of magnetohydrodynamic (MHD) waves propagating up from the Sun's surface layers (the photosphere). Some new evidence from the SOHO and Yohkoh spacecraft, with instruments on board which image the Sun in the extreme ultraviolet and soft x-rays, are shedding new light on the problem - it appears that the energy spectrum of small flare-like phenomena may be such that nanoflares are sufficiently numerous to supply the corona's energy requirements. However, these new findings do not preclude the possibility of MHD wave motions also supplying significant amounts of energy. Theory suggests very short period (

We report on observations of a large eruptive event associated with a flare that occurred on 27 September 1998 made with the Richard B. Dunn Solar Telescope at Sacramento Peak Observatory (several wave bands including off-line-center Hα), in soft and hard X-rays (GOES and BATSE), and in several TRACE wave bands (including Fe ix/x 171 A, Fe xii 195 A, and C iv 1550 A). The flare initiation is signaled by two Hα foot-point brightenings which are closely followed by a hard X-ray burst and a subsequent gradual increase in other wavelengths. The flare light curves show a complicated, three-component structure which includes two minor maxima before the main GOES class C5.2 peak after which there is a characteristic exponential decline. During the initial stages, a large spray event is observed within seconds of the hard X-ray burst which can be directly associated with a two-ribbon flare in Hα. The emission returns to pre-flare levels after about 35 min, by which time a set of bright post-flare loops have begun to form at temperatures of about 1.0–1.5 MK. Part of the flare plasma also intrudes into the penumbra of a large sunspot, generally a characteristic of very powerful flares, but the flare importance in GOES soft X-rays is in fact relatively modest. Much of the energy appears to be in the form of a second ejection which is observed in optical and ultraviolet bands, traveling out via several magnetic flux tubes from the main flare site (about 60° from Sun center) to beyond the limb.

Calculated intensities of the Fe XVII X-ray lines due to transitions 2p6 − 2p53d lines (near 15 A) and 2p6 − 2p53s lines (near 17 A) are compared with measured line intensities in solar and tokamak spectra. For the solar spectra, temperature Te is obtained from the ratio of the Fe XVII 16.776 A line to a nearby Fe XVIII line. We find excellent agreement for all the major Fe XVII line features in the 15–17 A region except the Fe XVII 15.015 A line, the observed flux of which is less than the theoretical by a factor f. We find that f strongly depends on the heliocentric angle θ of the emitting region, being smallest (0.2) when the region is nearest Sun centre, but nearly 1 near the limb. Attributing this to resonance scattering, we are able to deduce the path length and electron density from the observations. Possible application to stellar active regions is given.

Solar Photometer in X-rays (SphinX) was a spectrophotometer developed to observe the Sun in soft X-rays. The instrument observed in the energy range ≈ 1 – 15 keV with resolution ≈ 0.4 keV. SphinX was flown on the Russian CORONAS–PHOTON satellite placed inside the TESIS EUV and X telescope assembly. The spacecraft launch took place on 30 January 2009 at 13:30 UT at the Plesetsk Cosmodrome in Russia. The SphinX experiment mission began a couple of weeks later on 20 February 2009 when the first telemetry dumps were received. The mission ended nine months later on 29 November 2009 when data transmission was terminated. SphinX provided an excellent set of observations during very low solar activity. This was indeed the period in which solar activity dropped to the lowest level observed in X-rays ever. The SphinX instrument design, construction, and operation principle are described. Information on SphinX data repositories, dissemination methods, format, and calibration is given together with general recommendations for data users. Scientific research areas in which SphinX data find application are reviewed.

Solar active-region temperatures have been determined from the full-Sun spectra of helium-like sulphur (Sxv) observed by the Bragg Crystal Spectrometer on board theYohkoh satellite. The average temperature deduced from Sxv is demonstrated to vary with the solar activity level: A temperature of 2.5 × 106 K is derived from the spectra taken during low solar activity, similar to the general corona, while 4 × 106 K is obtained during a higher activity phase. For the latter, the high-temperature tail of the differential emission measure of active regions is found most likely due to the superposition of numerous flare-like events (micro/nano-flares).

Using calculations of collisional excitation rates in He-like argon (Ar XVII) and intensity factors and other data for nearby dielectronic satellites, X-ray line spectra have been synthesized as a function of electron temperature and density. The Ar XVII excitation rates were obtained from interpolations of R-matrix calculations, and are likely to be the best available at present. The resulting spectra are compared with spectra observed from high-temperature plasmas in the Alcator C tokamak. The agreement between observed spectra and those calculated using measured values of temperature and density illustrates the accuracy of the calculations, though there are small discrepancies which are likely to be connected with wavelengths or intensity factors for the Ar XVI satellites. Thus, argon X-ray line spectra can serve as a means of diagnosing low-density, astrophysical (in particular solar-flare) plasmas for which there is no independent means of measuring temperatures.

The past twenty years or so have seen an enormous increase in our understanding of solar flares. There has been much progress in their observation, both from ground-based telescopes and from spacecraft, and in theoretical interpretation. However, some very important questions remain unresolved, such as the nature of the primary energy release. We review both observational and theoretical evidence and how this is being put together to form an acceptable picture of the phenomenon.

It is proposed that there is a strong connection between the continua formed in the ultraviolet spectra of stellar flares with that seen in solar spectra, and that the mechanism of formation believed to occur in the latter — silicon recombination radiation excited by photo-ionization due to ultraviolet line emission — is the same as in stellar flares.

Introduction In the present chapter we will describe the mechanisms that generate the continuum and line radiation in the solar spectrum in the UV, EUV, and soft X-ray ranges. We will first describe the processes that can excite an atom or ion from an atomic level to another level with higher energy, the decay from which generates a photon, as described in Chapter 3. We will then discuss the processes of ionization or recombination of atoms and ions. We will define the contribution function of an optically thin spectral line and its dependence on key parameters of the plasma itself. We will also describe the processes that generate the free-free (or bremsstrahlung) and free-bound continuum radiation. For simplicity, throughout the remainder of this chapter we will refer to a neutral atom or an ion simply as an ion , regardless of its state of ionization. Collisional excitation and de-excitation of atomic levels One of the main assumptions that we will make in this chapter is that ionization and recombination processes can be decoupled from excitation and de-excitation processes, so that they can be treated separately. This approximation is usually accurate in the solar atmosphere, since the time scales for ionization and recombination are usually longer than those for excitation and de-excitation. However, as we will see in Section 4.9, ionization and recombination effects on excited levels may be non-negligible in several atomic systems, but they can be taken into account using an approximate method that allows us to retain the separation between ionization and recombination on the one hand, and excitation and de-excitation on the other.

The solar atmosphere, above the Sun's surface layers, reaches mega-kelvin temperatures and high levels of dynamic activity through processes involving a pervading magnetic field. This book explores one of the principal means of understanding the solar atmosphere, its ultraviolet and soft X-ray emission. The ultraviolet and X-ray spectra of the Sun's atmosphere provide valuable information about its nature - the heat and density of its various parts, its dynamics, and chemical composition. The principles governing spectral line and continuous emission, and how spectral studies lead to deductions about physical properties, are described, together with spacecraft instrumentation from Skylab, SolarMax, Yohkoh, SOHO, TRACE, and Hinode. With introductions to atomic physics and diagnostic techniques used by solar spectroscopists, a list of emission lines in ultraviolet and soft X-ray regions, and a glossary of terms, this is an ideal reference for graduate students and researchers in astrophysics and solar physics.

Introduction In this chapter we will review the main diagnostic techniques that allow the measurement of the physical properties of optically thin plasmas. These techniques can be used to measure the electron and ion temperatures, electron density, the thermal structure of the plasma, its chemical composition and ionization state, its dynamics, and the velocity distribution of its electrons. Most, but not all, of the techniques we describe assume that the emitting plasma is (i) thermal; (ii) homogeneous and isothermal; and (iii) in collisional ionization equilibrium. The effects of the relaxation of one or more of these assumptions will be discussed when possible, although the effects of departures from ionization equilibrium are difficult to study because of the complexity of evaluating ion abundances in the dynamic environment of out-of-equilibrium plasmas. Diagnostic techniques require atomic databases and spectral codes to be used. These codes are necessary to provide theoretical estimates of line and continuum fluxes as a function of the physical parameters of the plasma, whose comparison with observations provides the measurement of such parameters. In this chapter, we also briefly review the main spectral codes available in the literature. The diagnostic techniques we discuss can be used with either fluxes (radiances) or intensities (irradiances), but for clarity we will mostly use fluxes. Electron density diagnostics Line flux ratios Measuring electron densities using the flux ratio of two spectral lines is the most popular method.

X-ray spectra in the range $1.5-8.5$~keV have been analyzed for 526 large flares detected with the Solar Assembly for X-rays (SAX) on the Mercury {\em MESSENGER} spacecraft between 2007 and 2013. For each flare, the temperature and emission measure of the emitting plasma were determined from the spectrum of the continuum. In addition, with the SAX energy resolution of 0.6 keV (FWHM) at 6~keV, the intensities of the clearly resolved Fe-line complex at 6.7~keV and the Ca-line complex at 3.9~keV were determined, along with those of unresolved line complexes from S, Si, and Ar at lower energies. Comparisons of these line intensities with theoretical spectra allow the abundances of these elements relative to hydrogen to be derived, with uncertainties due to instrument calibration and the unknown temperature distribution of the emitting plasma. While significant deviations are found for the abundances of Fe and Ca from flare to flare, the abundances averaged over all flares are found to be enhanced over photospheric values by factors of $1.66 \pm 0.34$ (Fe), $3.89~\pm~0.76$ (Ca), $1.23~\pm~0.45$ (S), $1.64~\pm~0.66$ (Si), and $2.48~\pm~0.90$ (Ar). These factors differ from previous reported values for Fe and Si at least. They suggest a more complex relation of abundance enhancement with the first ionization potential (FIP) of the element than previously considered, with the possibility that fractionation occurs in flares for elements with a FIP of less than $\sim$7~eV rather than $\sim10$~eV., Accepted for publication, ApJ

Analysis of RHESSI 3--10 keV spectra for 27 solar flares is reported. This energy range includes thermal free--free and free--bound continuum and two line features, at 6.7keV and 8keV, principally due to highly ionized iron (Fe). We used the continuum and the flux in the so-called Fe-line feature at 6.7keV to derive the electron temperature T_e, the emission measure, and the Fe-line equivalent width as functions of time in each flare. The Fe/H abundance ratio in each flare is derived from the Fe-line equivalent width as a function of T_e. To minimize instrumental problems with high count rates and effects associated with multi-temperature and nonthermal spectral components, spectra are presented mostly during the flare decay phase, when the emission measure and temperature were smoothly varying. We found flare Fe/H abundance ratios that are consistent with the coronal abundance of Fe (i.e. 4 times the photospheric abundance) to within 20% for at least 17 of the 27 flares; for 7 flares, the Fe/H abundance ratio is possibly higher by up to a factor of 2. We find evidence that the Fe XXV ion fractions are less than the theoretically predicted values by up to 60% at T_e=25 MK appear to be displaced from the most recent theoretical values by between 1 and 3 MK., To be published, ApJ

Analysis of X-ray spectra from the FCS instrument on SMM has given valuable information about the physical conditions in active regions and flares, notably temperatures and densities. Element abundances can also be derived from some line ratios. This discussion reviews some of the progress made over the past few years.

The Fe XVII 2p61S0-2p53d1P1 X-ray line at 15.015 A is resonance-scattered in solar active region and flare spectra, as has been deduced by comparing theoretical intensities of lines in the Fe XVII 2p6-2p53l (l = s, d) arrays with intensities observed by the flat crystal spectrometer (FCS) on the Solar Maximum Mission. We show that the amount of resonance scattering is strongly dependent on the heliocentric distance of the emitting plasma, disk regions showing much larger scattering than limb regions. Average values for electron density Ne and path length l can be derived for various heliocentric distances using the X-ray line intensities. We find Ne ≈ 109 cm-3 and l ≈ 100,000 km (region on the disk) to Ne ≈ 1011 cm-3 and l ≈ 1000 km (region on the limb) for active region AR 4787, as it rotated across the solar disk to the limb. These lengths appear to be consistent with FCS images. We note, from our findings, that the approximate extent of stellar X-ray active regions could be determined, if high-resolution Fe XVII or equivalent observations were available.

The Sun recently underwent a period of a remarkable lack of major activity such as large flares and sunspots, without equal since the advent of the space age a half century ago. A widely used measure of solar activity is the amount of solar soft X-ray emission, but until recently this has been below the threshold of the X-ray-monitoring Geostationary Operational Environmental Satellites (GOES). There is thus an urgent need for more sensitive instrumentation to record solar X-ray emission in this range. Anticipating this need, a highly sensitive spectrophotometer called Solar Photometer in X-rays (SphinX) was included in the solar telescope/spectrometer TESIS instrument package on the third spacecraft in Russia's Complex Orbital Observations Near-Earth of Activity of the Sun (CORONAS-PHOTON) program, launched 30 January 2009 into a near-polar orbit. SphinX measures X-rays in a band similar to the GOES longer-wavelength channel.

We present three sets of observations of n = 1 to n = 2 lines due to helium-like aluminium (Alxii), made during two solar flares (25 August, 1980 and 19 October, 1986), using the X-Ray Polychromator on the SMM satellite. The observed temperature-sensitive line ratio G is shown to be consistent with the close-coupling calculations of Keenan and McCann (1987), although the ratio R, which is both temperature and density-sensitive for lower-Z elements, is not sufficiently well determined from these data to say more than that the observed values of R are not inconsistent with the theoretical calculations. This region of the spectrum also includes the helium-like magnesium (Mgxi) 11 S - 31 P line, and it is shown that the ratio of this line to the Alxii resonance (11 S - 21 P) line is a more sensitive indicator of electron temperature than are the Alxii G and R ratios. We demonstrate that the three ratios may be used together in order to derive values of emission measure, electron temperature and electron density during these flares.

Description des observations optiques (photometriques et spectrophotometriques) et UV (satellite IUE) simultanees de l'etoile UV ceti effectuees en 1980. Les resultats sont compares avec d'autres observations et l'activite de cette etoile est comparee a celle du Soleil