Kα line emission during solar X-ray bursts

K X-ray line emission from S, Ar, Ca and Fe is calculated for conditions likely to exist in solar flares. We consider both the non-thermal and thermal phases of flares as indicated by X-ray observations. Impulsive non-thermal events seen at the onset of a flare at photon energies > 20 keV generally give rise to small K line fluxes (<250 photons cm^-2 s^-1) on the basis of data presented by Kane and Anderson. The amount of S K radiation in particular depends sensitively on the lower-energy bound of the non-thermal electron distribution giving rise to the impulsive burst, offering a possible means of determining this. Thermal K emission is significant for only Fe ions. For S, Ar and Ca, the temperatures required for a sizeable number of electrons with energies greater than the K-ionization potential will also strip these elements to ionization stages too high for K transitions to be possible. Comparison of thermal K emission from iron during an intense solar flare leads to a very high emission measure on the basis of these calculations, but such a value seems to be compatible with an analysis of the 1–3 Å continuum during the same event.NAS/NRC Resident Research Associate.Visiting Scientist, High Altitude Observatory, NCAR, Boulder, Colo. 80302.     

X-ray and Hα observations of a filament-disappearance flare: An empirical analysis of the magnetic field configuration

A flare event occurred which involved the disappearance of a filament near central meridian on 29 August 1973. The event was well observed in X-rays with the AS & E telescope on Skylab and in H at BBSO. It was a four-ribbon flare involving both new and old magnetic inversion lines which were roughly parallel. The H, X-ray, and magnetic field data are used to deduce the magnetic polarities of the H brightenings at the footpoints of the brightest X-ray loops. These magnetic structures and the preflare history of the region are then used to argue that the event involved a reconnection of magnetic field lines rather than a brightening in place of pre-existing loops. The simultaneity of the H brightening onsets in the four ribbons and the apparent lack of an eruption of the filament are consistent with this interpretation. These observations are compared to other studies of filament disappearances. The preflare structures and the alignment of the early X-ray flare loops with the H filament are consistent with the schematic picture of a filament presented first by Canfield et al. (1974).     

Observed Lα profiles for two solar flares: 14∶12 UT 15 June, 1973 and 23∶16 UT 21 January, 1974

Photographic observations of the time development of the profile of the L line of hydrogen during flares were obtained with the NRL spectrograph on ATM. The profiles for the 15 June, 1973 and 21 January, 1974 flares reported here cover both core and wings of the line. The time sequences begin before flare maximum, and continue well into the decay phase. Careful attention has been given to photometry and absolute calibration. In the case of the 15 June, 1973 flare, data are presented both first-order corrected and uncorrected for incomplete filling of the spectrograph slit by flaring material. Correction of the 21 January, 1974 flare was not possible. We discuss core symmetry and shift, and show that our observations imply integrated flare L/H intensity ratios within a factor of two of unity for these two flares.     

High resolution observation of Hα solar flares and temporal relation between Hα and X-ray,microwave emission

We studied the evolutional characteristics of fine structures in H flare emitting regions and their relation to X-ray and microwave emissions for selected events observed with the 60 cm Domeless Solar Telescope at Hida Observatory, University of Kyoto. The principal conclusions of this investigation are: (1) H kernel consists of some finer bright points or Hflare points whose individual size is less than 1 arc sec. (2) Impulsive brightnenings of H flare points occurred simultaneously with the spikes of the hard X-ray and microwave bursts within the time resolution of our H observations which varied from 1 to 10 s. (3) It is concluded that fast electron beams must be the principal mechanism of heating H flares during the impulsive phase of a flare.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984. Contributions from the Kwasan and Hida Observatories, University of Kyoto, No. 265.     

Observational study of the ‘post’-flare loops on June 4, 1991

A limb, two-ribbon H flare on June 4, 1991, associated with a white-light flare and followed by an emission spray and post-flare loops, is studied. A region of rapidly enhanced brightness at the bottom of the H ribbon above the white-light flare is revealed. The energy released by the white-light flare at _eff = 4100 is estimated to be about 1.5 × 10²⁸ erg s^–1.     

Studies of solar flares using optical,X-ray and radio data

I have studied a number of flares for which good X-ray and optical data were available. An average lag of 5.5 s between hard X-ray (HXR) start and H start, and HXR peak and Ha peak was found for 41 flares for which determination was possible. Allowing for time constants the time lag is zero. The peak H lasts until 5–6 keV soft X-ray (SXR) peak. The level of H intensity is determined by the SXR flux.Multiple spikes in HXR appear to correspond to different occurrences in the flare development. Flares with HXR always have a fast H rise. Several flares were observed in the 3835 band; such emission appears when the 5.1–6.6 keV flux exceeds 5 × 10⁴ ph cm^-2 s^-1 at the Earth. Smaller flares produce no 3835 emission; we conclude that coronal back conduction cannot produce the bright chromospheric network of that wavelength.The nearly simultaneous growth of H emission at distant points means an agent travelling faster than 5 × 10³ km s^-1 is responsible, presumably electrons.In all cases near the limb an elevated Ha source is seen with the same time duration as HXR flux; it is concluded that this H source is almost always an elevated cloud which is excited by the fast electrons. A rough calculation is given. Another calculation of H emission from compressed coronal material shows it to be inadequate.In several cases homologous flares occur within hours with the same X-ray properties.Radio models fit, more or less, with field strengths on the order of 100G. A number of flares are discussed in detail.     

Time delay between Hα and hard X-ray emissions during impulsive solar flares

This investigation shows that statistically there are significant time delays between H and hard X-ray (HXR) emissions during solar flares; most impulsive flares produce HXR emissions up to 1 min before and up to 2 min after the onset of H emission. HXR emissions are also found to be peaked up to 2 min before the H emissions.     

Isophotal photometry and morphological changes in the flares

We report measurements made on the brightness in H of all parts of the flare photographed through a birefringent filter centered on H, using a scanning isodensitometer. From obtained isophotes of the flares we derived some information on the morphological changes in the flare and estimated the total energy in H of the flare.     

Dependence of radio emission in large Hα flares 1967–1970 upon the orientation of the local solar magnetic field

During 1967–1970, the greatness of 90 large flares (H importance 2) was influenced by the orientation of the large-scale ( 100 000 km) magnetic field structure over the flare site. Although the average X-ray and optical emissions are only slightly larger for flares with their overlying fields directed southward, as opposed to northward, the meter-wave-length prompt flux maxima are, on average, an order of magnitude greater for the flares with southward oriented magnetic fields. There is a comparable, but possibly smaller difference in the 10 cm- fluxes. We therefore conclude that, during this period, the orientation of the overlying magnetic field affects the amount of electromagnetic flare energy radiated promptly in the corona (10 cm- and m-), relative to that radiated in the chromosphere (X-ray and optical). We demonstrate that this statistical effect shows some variability in degree during the period, although the trend is consistent throughout.     

The flares of August 1972

We present the analysis of observations of the August flares at Big Bear and Tel Aviv, involving monochromatic movies, magnetograms and spectra. In each flare the observations fit a model of particle acceleration in the chromosphere with emission produced by impact and by heating by the energetic electrons and protons. The region showed inverted polarity and high gradients from birth, and flares appear due to strong magnetic shears and gradients across the neutral line produced by sunspot motions. Post flare loops show a strong change from sheared, force-free fields parallel to potential-field-like loops, perpendicular to the neutral line above the surface.We detected fast (5 s duration) small (1') flashes in 3835 at the footpoints of flux loops in the August 2 impulsive flare at 1838 UT, which may be explained by dumping of > 50 keV electrons accelerated in individual flux loops. The flashes show excellent time and intensity agreement with > 45 keV X-rays. In the less impulsive 2000 UT flare a less impulsive wave of emission in 3835 moved with the separating footpoints. The thick target model of X-ray production gives a consistent model for X-ray, 3835 and microwave emission in the 18:38 UT event.Spectra of the August 7 flare show emission 12 Å FWHM in flare kernels, but only 1 to 2 Å wide in the rest of the flare. The kernels thus produce most of the H emission. The total emission in H in the August 4 and August 7 flares was about 2 × 10³⁰ erg. We belive this dependable value more accurate than previous larger estimates for great flares. The time dependence of total H emission agrees with radio and X-ray data much better than area measurements which depend on the weaker halo.Absorption line spectra show a large (6 km/s^-1) photospheric velocity discontinuity across the neutral line, corresponding to sheared flow across that line.This work has been supported by NASA under NGR 05 002 034, NSF Atmospheric Sciences program under GA 24015, and AFCRL under FI9628-73-C-0085.     

Comparison between some Hα and X-ray flares

In the present paper, H-evolutive curves of chromospheric events are compared with flux evolutive curves of X-ray events observed at the same time in different spectral regions. A correspondence between the emissions E(I _H/I _chr)'s at higher and higher H-intensity levels, and the X-ray fluxes F()'s in harder and harder -ranges is shown. Further, the present observations seem to indicate the existence of a single triggering mechanism during the flash-phase of a flare. It is also shown that these results may be in agreement with Brown's model for chromospheric flares.     

The roots of coronal structure in the Sun's surface

We have compared the structures seen on X-ray images obtained by a flight of the NIXT sounding rocket payload on July 11, 1991 with near-simultaneous photospheric and chromospheric structures and magnetic fields observed at Big Bear. The X-ray images reflect emission of both Mgx and Fexvi, formed at 1 × 10⁶ K and 3 × 10⁶ K, respectively. The brightest H sources correspond to a dying sub-flare and other active region components, all of which reveal coronal enhancements situated spatially well above the H emission. The largest set of X-ray arches connected plages of opposite polarity in a large bipolar active region. The arches appear to lie in a small range of angle in the meridian plane connecting their footpoints. Sunspots are dark on the surface and in the corona. For the first time we see an emerging flux region in X-rays and find the emission extends twice as high as the H arches. Many features which we believe to correspond to X-ray bright points (XBPs) were observed. Whether by resolution or spectral band, the number detected greatly exceeds that from previous work. All of the brighter XBPs correspond to bipolar H features, while unipolar H bright points are the base of more diffuse comet-like coronal arches, generally vertical. These diverge from individual features by less than 30°, and give a good measure of what the canopies must do. The H data shows that all the H features were present the entire day, so they are not clearly disappearing or reappearing. We find a new class of XBPs which we call satellite points, elements of opposite polarity linked to nearby umbrae by invisible field lines. The satellite points change rapidly in X-ray brightness during the flight. An M1.9 flare occurred four hours after the flight; examination of the pre-flare structures reveals nothing unusual.     

Emerging Flux and X-class Flares in NOAA 6555

The active region NOAA 6555 had several locations of highly sheared magnetic field structure, yet, only one of them was the site for all the five X-class flares during its disk passage in March 1991. The pre-flare observations of high-resolution H filtergrams, vector magnetograms and H Dopplergrams of the 2B/X5.3 flare on 25 March 1991 show that the flaring site was characterized by a new rising emerging flux region (EFR) near the highly sheared magnetic field configuration. The polarity axis of the emerging flux was nearly perpendicular to the pre-existing magnetic neutral line. The location of the EFR was the site of initial brightening in H. The post-flare magnetograms show higher magnetic shear at the flare location compared to the post-flare magnetograms, which might indicate that the EFR was sheared at the time of its emergence. As the new EFR coincided with the occurrence of the flare, we suggest that it might have triggered the observed flare. Observations from Big Bear Solar Observatory and Marshall Space Flight Center also show that there was emergence of new flux at the same location prior to two other X-class flares. We find that out of five observed X-class flares in NOAA 6555, at least in three cases there are clear signatures of flare-related flux emergence. Therefore, it is concluded that EFRs might play an important role in destabilizing the observed sheared magnetic structures leading to large X-class flares of NOAA 6555.     

Characteristics of solar coronal source regions producing 3He-rich particle events

We use H, X-ray, and kilometric radio data to examine the solar coronal activity associated with energetic (1 MeV/nucl^–1) ³He-rich particle events observed near Earth. The basis of the study is the 12 ³He-rich events observed in association with impulsive 2 to 100 keV electron events reported by Reames et al. (1985). We find that when H and X-ray brightenings can be associated with ³He/electron events, they have onsets coinciding to within 1 min of that of the associated metric type III bursts. In three or four events we found no associated H or X-ray flares, and in two events even the metric type III bursts were weak or absent. The measured low-energy (2 keV) electron spectra for these events show no evidence of a flattening due to Coulomb collisional losses. These results and several other recent findings are consistent with the idea that the ³He/electron events are due to particle acceleration in the corona well above the associated H and X-ray flares.     

Soft X-ray observations of large-scale coronal active region brightenings

One-hundred fifty-six large-scale enhancements of X-ray emission from solar active regions were studied on full-disk filterheliograms to determine characteristic morphology and expansion rates for heated coronal plasma. The X-ray photographs were compared with H observations of flares, sudden filament disappearances, sprays and loop prominence systems (LPS). Eighty-one percent of the X-ray events were correlated with H filament activity, but only forty-four percent were correlated with reported H flares. The X-ray enhancements took the form of loops or arcades of loops ranging in length from 60 000 km to 520 000 km and averaging 15 000 km in width. Lifetimes ranged from 3 hr to >24 hr. Event frequency was 1.4 per day. X-ray loop arcades evolved from sharp-edged clouds in cavities vacated by rising H filaments. Expansion velocities of the loops were 50 km s^-1 immediately after excitation and 1–10 km s^-1 several hours later. These long-lived loop arcades are identified with LPS, and it is suggested that the loops outlined magnetic fields which were reconnecting after filament eruptions. Another class of X-ray enhanced loops stretched outside active regions and accompanied sprays or lateral filament ejections. H brightenings occurred where these loops intersected the chromosphere. Inferred excitation velocities along the loops ranged between 300 and 1200 km s^-1. It is suggested that these loops outlined closed magnetic fields guiding slow mode shocks from flares and filament eruptions.     

X-rays,filament activity and flare prediction

127 hr of high-resolution H movies of young active regions have been compared with simultaneous 1.5–15 keV X-ray measurements from the Mapping X-Ray Heliometer experiment on OSO-8, with particular attention to preflare periods and to the possibility of X-ray emission associated with filament activity during that time. The period studied included 8 confirmed flares or subflares, 16 unreported events of comparable magnitude, and numerous examples of filament activity. We found no evidence for X-ray emission from areas of enhanced filament activity unless simultaneous brightenings were present in H. In addition, we detected no peculiar behavior of either filaments or X-rays during the period of approximately 20 min preceding these small flares which, even in retrospect, would have allowed them to be predicted.     

Iron Kα-fluorescence in solar flares: A probe of the photospheric iron abundance

The X-ray line at 6.4 keV has been observed from solar flares. It is found that K-fluorescence of neutral iron in the photosphere due to thermal (T 10⁷ K) X-rays of the gradual phase is its dominant production mechanism. For a given flux and energy spectrum of incident X-rays, the flux at 1 AU of iron K-photons depends on the photospheric iron abundance, the height of the X-ray source, and the helio-centric angle between the flare and the observer. Therefore, the flux of iron K-photons, when measured simultaneously with the flux and energy spectrum of the X-ray continuum and the flare location, can give us information on the height of the X-ray source and the photospheric iron abundance. Here we present our Monte Carlo calculations of iron K-fluorescence efficiencies, so that they might be useful for interpretations of future measurements of the 6.4 keV line (e.g., by a detector to be flown on the Solar Maximum Mission).     

The development of X-ray flare onsets near active region filaments

Skylab X-ray images of the early phases of six active region transient brightenings were compared with simultaneous H images to study the spatial relationships between filaments and the X-ray brightenings. When the X-ray loops were roughly perpendicular to the axes of the H filaments, the filaments did not disappear. X-ray loops which appeared nearly parallel to the filaments were generally associated with the disappearances of those filaments. It is suggested that the perpendicular loops correspond to the class I X-ray flares of Pallavicini et al. (1977) while the parallel loops are the early phases of their class II flares characterized in the decay phases by arcades of large loops with low energy densities. Both kinds of X-ray flares can be associated with impulsive phases.     

On the influence of mass loss and convective overshooting on the evolution of massive stars

Evolution of massive stars losing mass with the rateM H L/V C is computed (for =1,2,7). It is shown that observed mass loss rates correspond to 0.3 and, therefore, mass loss by stellar wind cannot play any significant role in the evolution of normal massive stars. However, for several types of massive stars (WR, OH/IR, X-ray sources) enhanced mass loss explains their peculiar features. Computations of evolutionary sequences of massive stars with convective overshooting taken into account (as a formal increase of the convective core) show that a significant broadening of the hydrogen-burning band in the H-R diagram may be obtained.     

Some comments on the east-west solar flare distribution during the 1976–1985 period

We present the results of an analysis of the east-west asymmetry in the solar flare distribution, observed during the years from 1976 to 1985. We conclude that flare events, all type of H flares, are not uniformly spread in heliolongitude over the solar disc when considering events with heliolongitudes greater than 60°, or even closer to central meridian for certain periods. This lack of homogeneity, however, does not have an influence on the definition of east-west asymmetries. Simple random distribution of flares over the solar disc can not account for the asymmetries found, but they can be explained in terms of the transit of active regions in front of the observer's position. Nonetheless, this is not the case for the distribution of flares equal or more intense than importance 1F observed during 1979.