Coronal X-ray activity preceding solar flares

A comprehensive survey of Skylab S-054 soft X-ray images was performed to investigate the characteristics of coronal enhancements preceding solar flares. A search interval of 30 min before flare onset was used. A control sample was developed and tests of the statistical results performed. X-ray images with preflare enhancements were compared with high resolution H images and photospheric magnetograms.The results are as follows: preflare X-ray enhancements were found in a statistically significant number of the preflare intervals, and consisted of one to three loops, kernels or sinuous features per interval. Typically, the preflare feature was not at the flare site and did not reach flare brightness. There was no systematically observed time within the preflare interval for the preflare events to appear and no correlation of preflare event characteristics with the subsequent flare energy. Gas pressures of several preflare features were calculated to be on the order of several dyne cm^–2, typical of active region loops, not flares. These results suggest that observations with both high spatial resolution and low coronal temperature sensitivity are required to detect these small, low pressure enhancements that preceded the smaller flares typical of the Skylab epoch. H brightenings were associated with nearly all of the preflare X-ray enhancements. Changing H absorption features in the form of surges or filament activations were observed in about half of the cases. These results do not provide observational support for models which involve preheating of the flare loop, but they are consistent with some current sheet models which invoke the brightening of structures displaced from the flare site tens of min before onset.     

Evidence for Tether-Cutting Reconnection in a Quadrupole Magnetic Configuration in the April 9, 2001, M7.9 Flare

We studied the M7.9 flare on April 9, 2001 that occurred within a δ-sunspot of active region NOAA 9415. We used a multi-wavelength data set, which includes Yohkoh, TRACE, SOHO, and ACE spacecraft observations, Potsdam and Ondřejov radio data and Big Bear Solar Observatory (BBSO) images in order to study the large-scale structure of this two-ribbon flare that was accompanied by a very fast coronal mass ejection (CME). We analyzed light curves of the flare emission as well as the structure of the radio emission and report the following: the timing of the event, i.e., the fact that the initial brightenings, associated with the core magnetic field, occurred earlier than the remote brightening (RB), argue against the break-out model in the early phase of this event. We thus conclude that the M7.9 flare and the CME were triggered by a tether-cutting reconnection deep in the core field connecting the δ-spot and this reconnection formed an unstable flux rope. Further evolution of the erupted flux rope could be described either by the “standard“ flare model or a break-out type of the reconnection. The complex structure of flare emission in visible, X-ray, and radio spectral ranges point toward a scenario which involves multiple reconnection processes between extended closed magnetic structures.     

Evolution of White-Light Flares Observed by YOHKOH

We consider in detail the evolutionary patterns of few white-light flares observed by Yohkoh. The following data have been used in the analysis: sequences of de-convolved SXT images in X-ray and optical filters, MEM reconstructed HXT images and the other supporting data. The resolution in the de-convolved images is below 1 arc sec. Working with sequences of de-convolved images makes it also possible to investigate the dynamics of these structures with high spatial accuracy. Comparison of the morphology of flare brightenings as observed in hard, soft and optical ranges reveals that these emissions are not co-spatial and are most probably related to different plasma volumes at any instant. These observations cannot be easily accommodated within standard flare scenarios. Traditionally, the hard and optical flare emissions are expected to be co-spatial and the soft X-ray emission is presumed to fill the coronal portion of flaring loop(s) during rise phase. Present observations do not easily fit to such scenario.     

Transient brightenings of interconnecting loops

We study sudden brightenings of coronal loops that interconnect active regions. Such brightenings often occur within one or two days after the birth of a new interconnecting loop, as well as in some old interconnections. The brightenings of young loops are obviously associated with the emergence of new magnetic flux near their footpoints, whereas some enhancements of old loops may be triggered by slowly moving disturbances propagating from other centers of activity. A few loop brightenings are associated with flares, but the loop does not brighten in consequence of energy supply from the flare. Both the flare and the loop brightening are independent consequences of one common agent, presumably newly emerging flux.Temperatures in brightened loops are between 3 and 4 × 10⁶ K and densities are < 2 × 10⁹ cm^–3, probably < 5 × 10⁸ cm^–3 in some old loops. The top part of a loop is the site of the most intense brightening in the initial phase of a loop enhancement. The most frequent lifetime of these brightenings is 6 to 7 hr.Hale Observatories are operated jointly by the Carnegie Institution of Washington and the California Institute of Technology.     

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).     

Relative Timing and Spectra of Solar Flare Hard X-ray Sources

Hard X-ray lightcurves, spectrograms, images, and spectra of three medium-sized flares observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) are presented. Imaging spectroscopy of the 20 February 2002, 11:06 UT flare at 10′′ spatial resolution, comparable to the best previous hard X-ray imaging from Yohkoh, shows two footpoints with an ∼ 8 s delay of peak emission between footpoints. Subsequent imaging at le4′′ shows three sources consistent with two separate loops and simultaneous brightening in connected footpoints. Imaging for the simple two footpoint flare of 2 June 2002 also shows simultaneous footpoint brightening. The more complex 17 March 2002 flare shows at least four different sources during the main peak of the event, and it is difficult to clearly demonstrate simultaneous brightening of connected footpoints. Non-thermal power laws are observed down to ∼ 12–13 keV without flattening in all these events, indicating the energy content in energetic electrons may be significantly greater than previously estimated from assumed 25 keV low energy cutoff. Simultaneously brightening footpoints show similar spectra, at least in the three flares investigated. Double-power-law spectra with a relatively sharp break are often observed. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022469902940     

Preflare conditions,changes and events

Prefiare conditions, changes and events are loosely categorized as distinct, evolutionary or statistical. Distinct preflare phenomena are those for which direct physical associations with flares are implied. Also, they are not known to occur in a like manner during the absence of flares. These include the early stage of filament eruptions within active centers, preflare vortical structures, some transient X-ray emitting features, 5303 Å accelerating coronal arches, and increases in circular polarization at cm wavelengths. Evolutionary preflare changes are considered to be any long-term effect that may be related to the flare build-up even though the same changes may occur in the absence of flares. This category covers the development of current sheets or strongly sheared magnetic fields, evolving magnetic features, emerging flux regions, the development of satellite fields around sunspots, the evolution of reverse polarity field configurations, the merging of adjacent active centers, sunspot motions and the development of velocity patterns. Statistical preflare changes logically include both distinct and evolutionary preflare changes. However, in addition, there are preflare conditions and events that are not necessarily linked to the flare in either a direct physical or indirect evolutionary way. Such parameters or events that may only be statistically significant are certain magnetic field properties, the brightness of active centers at various wavelengths, the previous occurrence of flares and subflares, increased turbulence in filaments and certain radio events.     

Radio and X-ray imaging observations of solar flares and coronal transients

We present a review of selected studies based upon simultaneous radio and X-ray observations of solar flares and coronal transients. We use primarily the observations made with large radio imaging instruments (VLA, BIMA, Nobeyama, and Nançay) along with Yohkoh/SXT and HXT and CGRO experiments. We review the recent work on millimeter imaging of solar flares, microwave and hard X-ray observations of footpoint emission from flaring loops, metric type IV continuum bursts, and coronal X-ray structures. We discuss the recent studies on thermal and nonthermal processes in coronal transients such as XBP flares, coronal X-ray jets, and active region transient brightenings.Dedicated to Cornelis de Jager     

Soft X-ray emission from active regions shortly before solar flares

Detailed examination of the variations in the intensity of soft X-ray emission prior to many solar flares are presented. In addition, these preflare intensity variations are contrasted with the variations typically observed for the same active regions in the absence of a flare. It is shown that a 5–20 min preflare brightening phase is not typically observed. These observations are discussed in context with other complimentary investigations and theoretical models.     

Large-scale active coronal phenomena in Yohkoh SXT images

We discuss Yohkoh SXT observations of stationary giant post-flare arches which occurred on 3–6 May, 1992 and study in detail the last arch, associated with the flare at 19:02 UT on 5 May, which extended above the west limb. The arch was similar to the first giant arch discovered on board the SMM, on 21–22 May, 1980. We demonstrate that the long lifetimes of these structures necessarily imply additional energy input from the underlying active region: otherwise, conduction would cool these arches in less than one hour and even with the unlikely assumption of conduction inhibited, pure radiative cooling would not produce the temperature decrease observed. All arch tops, although varying in brightness, stayed for several days at a fairly constant altitude of 100 000 km, and the arch studied, on 5–6 May, was just a new brightening of the pre-existing decaying structure. The brightening was apparently due to inflow of hot plasma from the flare region. Yohkoh data confirm that these stationary arches are rare phenomena when compared with the rising arches studied in Paper I and with Uchida et al.'s expanding active regions.     

Re-Evaluation of the Flare–Type II–CME Association

We have re-evaluated the association of type II solar radio bursts with flares and/or coronal mass ejections (CMEs) using the year 2000 solar maximum data. For this, we consider 52 type II events whose associations with flares or CMEs were absent or not clearly identified and reported. These events are classified as follows; group I: 11 type IIs for which there are no reports of GOES X-ray flares and CMEs; group II: 12 type IIs for which there are no reports of GOES X-ray flares; and group III: 29 type IIs for which the flare locations are not reported. By carefully re-examining their association from GOES X-ray and H, Yohkoh SXT and EIT-EUV data, we attempt to answer the following questions: (i) if there really were no X-ray flares associated with the above 23 type IIs of groups I and II; (ii) whether they can be regarded as backside events whose X-ray emission might have been occulted. From this analysis, we have found that two factors, flare background intensity and flare location, play important roles in the complete reports about flare–type II–CME associations. In the above 23 cases, for more than 50% of the cases in total, the X-ray flares were not noticed and reported, because the background intensity of X-ray flux was high. In the remaining cases, the X-ray intensity might be greatly reduced due to occultation. From the H flare data, Yohkoh SXT data and EIT-EUV data, we found that ten cases out of 23 might be frontside events, and the remaining are backside events. While the flare–type II association is found to be nearly 90%, the type II–CME association is roughly around 75%. This analysis might be useful to reduce some ambiguities regarding the association among type IIs, flares and CMEs.     

Coronal heating events in high-cadence TRACE data

On March 23rd 1999 a set of TRACE (Transition Region and Coronal Explorer) observations were taken in support of Solar and Heliospheric Observatory (SOHO) / TRACE Joint Observing Program 83. The program is designed to look for coronal heating mechanisms operating at high cadence and to observe their dynamical effects on coronal loop density, temperature and magnetic structure. We present here a study of 27 small, dynamic brightening events seen in this data. These events are seen in the quiet-Sun areas surrounding the active region. The data itself consists of 157 171 Å 512×512 arc sec images at 1 arc sec resolution and 9 second cadence, a previously unavailable combination of cadence, resolution and image size. Three classes of events are introduced: complex, intermediate and simple. All three classes share the property of being dynamic on small time and length scales but differ in the complexity of their behavior. We find that the brightenings across all classes exhibit variations as part of a distribution of time scales (90–400 s) peaked around 228 s. The brightenings are no more than 5 arc sec in diameter. Motions between brightenings occur on time scales from 9 to 90 s and on length scales no greater than 10 arc sec. These motions have velocities estimated to be in the range 89–174 km s^?1. The position of these events in the spectrum of previously observed coronal heating events is discussed.

     

Coronal X-ray enhancements associated with Hα filament disappearances

A survey of soft X-ray images from Skylab has revealed a class of large-scale transient X-ray enhancements in the lower corona which are typically associated with the disappearance of H filaments away from active regions. Contemporary with the H filament disappearance, X-ray emitting structures appeared at or near the filament location with shape and size resembling the filament. Eventually these structures faded, but the filament cavity was no longer obvious. Typically the peak of the X-ray event lagged the end of the filament disappearance by tens of minutes. The durations of the coronal X-ray enhancements were considerably longer than the associated H filament disappearances. Major flare effects, such as chromospheric brightenings, typically were not associated with these X-ray events.One event analyzed quantitatively had a peak temperature between 1.8 and 2.7 × 10⁶ K, achieved a peak density of 10⁹ cm^–3 and resulted in an enhancement in the plasma pressure over the conditions of the preexisting coronal cavity of at least a factor of 7. The mass of the coronal X-ray emitting material was about 10% that of the preexisting filament and the thermal energy of the coronal event was on the order of 10²⁹ erg, about 10% of the mechanical energy of the H filament eruption. The event appeared to cool by radiative losses and not by thermal conduction. It is likely that the coronal enhancements are caused by heating of an excess of previously cooler material, either from the filament itself, or by compression of coronal material by a changing magnetic field.     

Ultraviolet brightenings in active regions as observed from OSO-8

Repeated raster images of solar active regions taken at the line centers of the Si iv and C iv resonance lines using the University of Colorado (CU) ultraviolet spectrometer aboard OSO-8 reveal dramatic transient brightenings of up to factors of 10. These brightenings last several minutes and frequently show a repetitive character. Inspection of simultaneous H flare patrol records show that these transition zone events are often associated with subflare-like brightenings in the chromosphere. These observations indicate that direct excitation or heating of material already at transition zone temperatures caused by non-thermal particle streams is inadequate to explain the degree of brightening of these lines. The measurements suggest that some process that enhances density of material at 10⁵ K is occurring during these events.     

The footpoints of giant arches

We have detected chromospheric footpoints of the giant post-flare coronal arches discovered by HXIS a few years ago. H photographs obtained at Big Bear and Udaipur Solar Observatories show chromospheric signatures associated with 5 sequential giant arch events observed in the interval from 6 to 10 November, 1980. The set of footpoints at one end of the arches consists of enhancements within a plage at the northeast periphery of the active region and the set of footpoints at the other end of the arch consists of brightenings of the chromosphere south of the active region. Both sets of footpoints show very slow brightness variations correlated in time with the brightness variations of the X-ray arches. Current-free modelling of the coronal magnetic field by Kopp and Poletto (1989), based on a Kitt Peak magnetogram, confirms the identification of the two sets of footpoints by showing magnetic field lines connecting them.The brightenings appear as a succession of point-like enhancements whose individual lifetimes are of the time-scale of minutes but which continue to occur for periods of several hours. This behaviour allows us to infer a fine structure in the coronal arches, undetectable in the X-ray images. The discovery of these brightenings and their location at the periphery of the active region also alters our conception of the relationship of the giant arches to the flares that begin concurrently with them. The giant arch phenomenon appears now to be either: (1) a long-lived, semi-permanent, coronal structure which is revived and fed with plasma and energy by underlying dynamic flares, or alternatively (2) a system of high-altitude loops which open at the onset of every such flare and subsequently reconnect over intervals of many hours.     

Deka-keV X-ray emission associated with the onset of radio noise storms

Radio noise storms show that suprathermal electrons (a few tens of keV) are present in the vicinity of active regions during several hours or even a few days. Where and how these electrons are energized is not yet well known. A flare-like sudden energy release in the active region is in general observed at the onset of noise storms, either as a fully developed flare or, more often, as a soft X-ray brightening without conspicuous H signature. In order to investigate to what extent electrons energized in the active region contribute to the noise-storm emission in the overlying coronal structures, we combine radio imaging (Nançay radioheliograph) with X-ray spectral observations at photon energies of a few keV (GOES) and - for the first time - around 10 keV (WATCH/GRANAT). In two of four studied events the WATCH data show a significant excess of the deka-keV count rate above the expectation from an isothermal fit to the GOES fluxes. Although the electron population producing the deka-keV X-ray emission would be energetic enough to power the simultaneous radio noise storm, the much longer duration of the radio emission requires time-extended particle acceleration. The acceleration probably occurs in the corona overlying the X-ray emitting region, triggered by the processes which give rise to the X-ray brightenings.     

Small-Scale Activity Above the Penumbra of a Fast-Rotating Sunspot

High-resolution observations of small-scale activity above the filamentary structure of a fast-rotating sunspot of NOAA Active Region 10930 are presented. The penumbral filament that intrudes into the umbra shows a central dark core and substructures. It almost approached another end of the umbra, like a light bridge. The chromospheric Ca ii H images show many jet-like structures with a bright leading edge above it. These bright jets move across the filament tips and show coordinated up and down motions. Transition region images also show brightening at the same location above the intrusion. Coronal 195 Å images suggest that one end of the bright coronal loop footpoints resides in this structure. The intrusion has opposite polarity with respect to the umbra. Strong downflows are observed at the edges along the length of the intrusion where the opposite-polarity field is enhanced. We also observe a counter-Evershed flow in the filamentary structure that also displays brightening and energy dissipation in the upper atmosphere. This scenario suggests that the jets and brightenings are caused by low-altitude reconnection driven by opposite-polarity fields and convective downflows above such structures.     

The Magnetic Structure and Generation of EUV Flare Ribbons

The `ribbons' of two-ribbon flares show complicated patterns reflecting the linkages of coronal magnetic field lines through the lower solar atmosphere. We describe the morphology of the EUV ribbons of the July 14, 2000 flare, as seen in SOHO, TRACE, and Yohkoh data, from this point of view. A successful co-alignment of the TRACE, SOHO/MDI and Yohkoh/HXT data has allowed us to locate the EUV ribbon positions on the underlying field to within ∼ 2′′, and thus to investigate the relationship between the ribbons and the field, and also the sites of electron precipitation. We have also made a determination of the longitudinal magnetic flux involved in the flare reconnection event, an important parameter in flare energetic considerations. There are several respects in which the observations differ from what would be expected in the commonly-adopted models for flares. Firstly, the flare ribbons differ in fine structure from the (line-of-sight) magnetic field patterns underlying them, apparently propagating through regions of very weak and probably mixed polarity. Secondly, the ribbons split or bifurcate. Thirdly, the amount of line-of-sight flux passed over by the ribbons in the negative and positive fields is not equal. Fourthly, the strongest hard X-ray sources are observed to originate in stronger field regions. Based on a comparison between HXT and EUV time-profiles we suggest that emission in the EUV ribbons is caused by electron bombardment of the lower atmosphere, supporting the hypothesis that flare ribbons map out the chromospheric footpoints of magnetic field lines newly linked by reconnection. We describe the interpretation of our observations within the standard model, and the implications for the distribution of magnetic fields in this active region.     

Analysis of Magnetic Shear in An X17 Solar Flare on October 28, 2003

An X17 class (GOES soft X-ray) two-ribbon solar flare on October 28, 2003 is analyzed in order to determine the relationship between the timing of the impulsive phase of the flare and the magnetic shear change in the flaring region. EUV observations made by the Transition Region and Coronal Explorer (TRACE) show a clear decrease in the shear of the flare footpoints during the flare. The shear change stopped in the middle of the impulsive phase. The observations are interpreted in terms of the splitting of the sheared envelope field of the greatly sheared core rope during the early phase of the flare. We have also investigated the temporal correlation between the EUV emission from the brightenings observed by TRACE and the hard X-ray (HXR) emission (E > 150 keV) observed by the anticoincidence system (ACS) of the spectrometer SPI on board the ESA INTEGRAL satellite. The correlation between these two emissions is very good, and the HXR sources (RHESSI) late in the flare are located within the two EUV ribbons. These observations are favorable to the explanation that the EUV brightenings mainly result from direct bombardment of the atmosphere by the energetic particles accelerated at the reconnection site, as does the HXR emission. However, if there is a high temperature (T > 20 MK) HXR source close to the loop top, a contribution of thermal conduction to the EUV brightenings cannot be ruled out.     

Preflare X-ray morphology of active regions observed with the AS & E telescope on Skylab

Images from the AS & E X-ray telescope on Skylab were examined for evidence of a flare build-up during the 20 minute periods preceding eight small X-ray flares. In two cases no activity could be seen up to 2 min before the flare onset. In no case did a preflare brightening occur in and only in the region which flared.