K. J. H. Phillips

An Observational Overview of Solar Flares

We present an overview of solar flares and associated phenomena, drawing upon a wide range of observational data primarily from the RHESSI era. Following an introductory discussion and overview of the status of observational capabilities, the article is split into topical sections which deal with different areas of flare phenomena (footpoints and ribbons, coronal sources, relationship to coronal mass ejections) and their interconnections. We also discuss flare soft X-ray spectroscopy and the energetics of the process. The emphasis is to describe the observations from multiple points of view, while bearing in mind the models that link them to each other and to theory. The present theoretical and observational understanding of solar flares is far from complete, so we conclude with a brief discussion of models, and a list of missing but important observations.

The X-ray signature of solar coronal mass ejections

The coronal response to six solar X-ray flares has been investigated. At a time coincident with the projected onset of the white-light coronal mass ejection associated with each flare, there is a small, discrete soft X-ray enhancement. These enhancements (precursors) precede by typically ∼20 m the impulsive phase of the solar flare which is dominant by the time the coronal mass ejection has reached an altitude above 0.5 R⊙. We identify motions of hot X-ray emitting plasma, during the precursors, which may well be a signature of the mass ejection onsets. Further investigations have also revealed a second class of X-ray coronal transient, during the main phase of the flare. These appear to be associated with magnetic reconnection above post-flare loop systems.

Evidence for Non-Maxwellian Electron Energy Distributions in the Solar Transition Region: Si III Line Ratios from SUMER

Recent calculations of Si III emission-line strengths are compared with SUMER observations from a quiet solar region, a coronal hole, and an active region. Diagnostic line ratios are used to derive Te and Ne for the emitting plasma in each region, and good agreement between theory and observations is found. A major enhancement in the intensity of the 1313 A emission line is observed, as well as some evidence of a small decrease in the temperature of maximum ionization fraction, as one moves from the coronal hole to the active region. Possible explanations for these effects are discussed, and it is concluded that they may be caused by the presence of nonthermal electrons in the transition region.

The Radial and Angular Variation of the Electron Density in the Solar Corona

We derive, for the first time, electron densities as a function of both radius (R) and position angle (θ) for the southwest quadrant of the off-limb corona, using the density-sensitive Si IX λ349.9/λ341.9 and Si X λ356.0/λ347.7 extreme-ultraviolet line ratios. The observations were made with the coronal diagnostic spectrometer on board the Solar and Heliospheric Observatory over the ranges of 1.00 R☉ < R < 1.20 R☉ and 180° < θ < 270°. Within the south polar coronal hole, the density varies from 2.3 × 108 cm-3 at 1.0 R☉ to 8.3 × 107 cm-3 at 1.20 R☉, while at the equator, the density varies from 6.3 × 108 cm-3 at 1.0 R☉ to 1.6 × 108 cm-3 at 1.20 R☉. The density falloff with height is therefore faster in the equatorial region. We also find that electron densities are, on average, a factor of 2.7 larger in the equatorial regions than in the polar coronal hole at a given radial distance. Finally, we find remarkable agreement between our measured densities as a function of radius and position angle and those predicted by a recent analytic MHD model of the solar wind, strongly supporting its basic premises.

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.

Determination of coronal abundances of sulphur, calcium and iron using the yohkoh bragg crystal spectrometer

Abstract Using spectra from the Bragg Crystal Spectrometer on Yohkoh we have derived coronal abundances of sulphur, calcium and iron during several flares from the ratio of the flux in the resonance line to the nearby continuum. Multi-thermal effects have been taken into account using differential emission measure analysis. We have also determined the abundance of S in cool active regions during a period of very low solar activity. We compare the coronal abundances of S, Ca and Fe with their photospheric values.

Active Region Electron Density and Dimensions from Fe XVII X-Ray Lines

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 1992 January 5 flare at 13.3 UT : observations from Yohkoh

We discuss X-ray spectra and soft X-ray images of an M1.9 flare that occurred on 1992 January 5 near 13.3 UT. These data were obtained with instrumentation on the Japanese Yohkoh spacecraft. They cover the entire rise phase of the flare. To supplement these data we have ground-based magnetograms and Halpha spectroheliograms. We calculate the electron temperature and emission measure of the flare as a function of time during the early rise phase using X-ray spectral line intensities and line ratios. Using spectral line widths, line profile asymmetries, and wavelength shifts due to the Doppler effect, we calculate the dynamical properties of the flare. The time development of the morphology of the flare, as revealed by the soft X-ray images and the Halpha spectroheliograms, and the physical quantities inferred from the X-ray spectra, are compared with chromospheric evaporation models. There is an enhancement of blueshifted emission that is closely correlated with the hard X-ray bursts. Heating of one loop in the flare is consistent with a conduction-evaporation model, but heating is found in several structures that do not appear to be physically associated with each other. No standard evaporation model can adequately explain all of the observations.

High time resolution observations of solar Hα flares II. Search for signatures of electron beam heating

Aims. The H-alpha emission of solar flare kernels and associated hard X-ray (HXR) emission often show similar time variations but their light curves are shifted in time by energy transfer mechanisms. We searched for fast radiative response of the chromosphere in the H-alpha line as a signature of electron beam heating. Methods. We investigate the time differences with sub-second resolution between the H-alpha line emission observed with a Multi-channel Subtractive Double Pass (MSDP) spectrograph on the Large Coronagraph and Horizontal Telescope at Bialkow Observatory, Poland, and HXR emission recorded by the RHESSI spacecraft during several flares, greatly extending our earlier analysis (Paper I) to flares between 2003 and 2005. Results. For 16 H-alpha flaring kernels, observed in 12 solar flares, we made 72 measurements of time delays between local maxima of the RHESSI X-ray and H-alpha emissions. For most kernels, there is an excellent correlation between time variations in the H-alpha line emission (at line centre and in the line wings) and HXR (20-50 keV) flux, with the H-alpha emission following features in the HXR light curves generally by a short time lapse delta t = 1-2 s, sometimes significantly longer (10-18 s). We also found a strong spatial correlation. Conclusions. Owing to our larger number of time measurements than in previous studies, the distribution of delta t values shows a much clearer pattern, with many examples of short (1-2 s) delays of the H-alpha emission, but with some flares showing longer (10-18 s) delays. The former are consistent with energy transfer along the flaring loop legs by non-thermal electron beams, the latter to the passage of conduction fronts.

The origin of the far-ultraviolet continuum in solar and stellar flares

A continuum in the far-ultraviolet region observed in IUE spectra of dMe stars in a flaring state is discussed and compared with that observed in solar flares. Similar continuous emission observed in solar flare far-ultraviolet spectra has been identified with neutral silicon recombination radiation excited by intense lines. This also appears to be the case for stellar flares, as is deduced here from the proportionality of far-ultraviolet line and continuum power for several flares on dMe stars, two flares on an RS CVn star (II Peg), and the spotted active binary BY Dra