Unusual coronal activity following the flare of 6 November 1980

For almost 30 hr after the major (gamma-ray) two-ribbon flare on 6 November 1980, 03:30 UT, the Hard X-Ray Imaging Spectrometer (HXIS) aboard the SMM satellite imaged in > 3.5 keV X-rays a gigantic arch extending above the active region over the limb. Like a similar configuration on 22 May 1980, this arch formed the lowest part of a stationary post-flare radio noise storm recorded at metric wavelengths at Nançay and Culgoora. 6.5 hr after the flare a coronal region below the arch started quasi-periodic pulsations in X-ray brightness, observed by several SMM instruments. These brightness variations had no response in the chromosphere (H), very little in the transition layer (O v), but they clearly correlated with similar variations in brightness at 169 MHz. There were 13 pulses of this kind, with apparent periodicity of about 20 min, until another flare occurred in the active region at 15:00 UT. All the brightenings appeared within a localized area of about 30000 km² in the northern part of the active region, but they definitely did not occur all at the same place.The top of the X-ray arch, at an altitude of 155 000 km, was continuously and smoothly decaying, taking no part in the striking variations below it. Therefore, the area variable in brightness does not seem to be the footpoint of the arch, as we supposed for similar variations on 22 May. More likely, it is a separate region connected directly with the source of the radio storm; particles accelerated in the storm may be dumped into the low corona and cause the X-ray enhancements. The X-ray arch was enhanced by two orders of magnitude in 3.5–5.5 keV X-ray counts and the temperature increased from 7.3 × 10⁶ to 9 × 10⁶ K when the new two-ribbon flare occurred at 15:00 UT. Thus, it is possible that energy is brought into the arch via the upper parts of the reconnecting flare loops - a process that can continue for hours.     

The stationary post-flare arch of May 21/22, 1980

On May 21/22, 1980 the Hard X-Ray Imaging Spectrometer aboard the SMM imaged an extensive coronal structure after the occurrence of a two-ribbon flare on May 21, 20:50 UT. The structure was observed from 22:20 UT on May 21 until its disappearence at 09:00 UT on May 22.At 22:20 UT the brightest pixel in the arch was located at a projected altitude of 95 000 km above the zero line of the longitudinal magnetic field. At 23:02 UT the maximum of brightness shifted to a neighbouring pixel with approximately the same projected altitude. This sudden shift indicates that the X-ray structure consisted of (at least) two separate arches at approximately the same altitude, one of which succeeded the other as the brightest arch in the structure at 23:02 UT.From 23:02 UT onwards the maximum of brightness did not change its position in the HXIS coarse field of view. With a spatial resolution of 32 this places an upper limit of 1.1 km s^-1 on the rise velocity of the arch. Thus, contrary to a similar arch observed on November 6/7, where rise velocities of the order of 10 km s^-1 were measured in the same phase of development, the May 22 arch was a stationary structure at an altitude of 145000 km.The following values were estimated for the physically relevant quantities of the May 21/22 arch at the time of its maximum brightness (23:00 UT): temperature T 6.3 × 10⁶ K, electron density n _e 1.1 × 10⁹ cm^-3, total emitting volume V 5 × 10²⁹ cm³, energy density 2.9 erg cm^–3, total energy contents E 1.4 × 10³⁰ erg, total mass M 9 × 10¹⁴ g.The top of the arch was observed at 145 000 km altitude within 1.5 hr after the flare occurrence. Since it seems unlikely that the structure already existed prior to the flare at 20:50 UT, the arch must have risen to its stationary position with an average velocity exceeding 17 km s^–1 (possibly much faster). We speculate that the arch was formed very fast at the flare onset, when (part of) the active region loop system was elevated within minutes to the observed altitude.     

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.     

Large-scale active coronal phenomena in Yohkoh SXT images

We have found several occurrences of slowly rising giant arches inYohkoh images. These are similar to the giant post-flare arches previously discovered by SMM instruments in the 80s. However, we see them now with 3–5 times better spatial resolution and can recognize well their loop-like structure. As a rule, these arches followeruptive flares with gradual soft X-ray bursts, and rise with speeds of 1.1–2.4 km s^–1 which keep constant for >5 to 24 hours, reaching altitudes up to 250 000 km above the solar limb. These arches differ from post-flare loop systems by their (much higher) altitudes, (much longer) lifetimes, and (constant) speed of growth. One event appears to be a rise of a transequatorial interconnecting loop.In the event of 21–22 February 1992 one can see both the loop system, rising with a gradually decreasing speed to an altitude of 120 000 km, and the arch, emerging from behind the loops and continuing to rise with a constant speed for many more hours up to 240 000 km above the solar limb. In the event of 2–3 November 1991 three subsequent rising large-scale coronal systems can be recognized: first a fast one with speed increasing with altitude and ceasing to be visible at about 300 000 km. This most probably shows the X-ray signature of a coronal mass ejection (CME). A second one, with gradually decreasing speed, might represent very high rising flare loops. A third one continues to rise slowly with a constant speed up to 230 000 km (and up to 285 000 km after the speed begins to decay), and this is the giant arch. This event, including an arch revival on November 4–5, is very similar to rising giant arches observed by the SMM on 6–7 November 1980. Other events of this kind were observed on 27–28 April 1992, 15 March 1993, and 4–6 November 1993, all seen above the solar limb, where it is much easier to identify them.The temperature in the brightest part of the arch of 2–3 November 1991 was increasing with its altitude, from 2 to 4 × 10⁶ K, which seems to be an effect of slower cooling at lower densities. Under an assumption of line-of-sight thickness of 50 000 km, the emission measure indicates densities from 1.1 × 10¹⁰ cm^–3 at an altitude of 150 000 km to 1.0 × 10⁹ cm^–3 at 245 000 km 11.5 hours later. It appears that the arch is composed of plasma of widely different temperatures, and that hot plasma rises faster than the cool component. Thus the whole arch expands upward, and its density gradient increases with time, which explains whyYohkoh images show only the lowest and coolest parts of the expanding structure. The whole arch may represent an energy in excess of 10³¹ erg, and more if conduction contributes to the arch cooling.We suggest that the rise of the arch is initiated by a CME which removes the magnetic field and plasma in the upper corona, and the coronal structures remaining below this cavity begin to expand into the vacuum left behind the CME. However, we are unable to explain why the speed of rise stays constant for so many hours.     

Expanding arch structure of a solar radio outburst

A flare-associated complex outburst was observed on 1968, October 23–24 with the 80 MHz Culgoora radioheliograph. Two harmonic type II bursts were followed by two successive extended sources with arch structure which appeared further beyond the optical limb than the preceding sources. The second arch showed a remarkable expansion with a projected velocity of 1200 km/sec. At its maximum the arch extended to a height of 2R . The height-time plots derived from both the radioheliograph and spectrum observations suggest that two shock waves of different propagation velocities were initiated at the flash phase of the flare: the faster one was responsible for the first type II burst and the first radio-emitting arch; the slower one for the second type II burst and the second arch whose expansion advanced with the shock front.     

The Hα development of an intense limb flare and associated flaring arches

The H analysis of the development of the strong impulsive and faint gradual phase of the June 26, 1983 flare indicates the following: (1) The flare originated from two microprominences on the southeast border of NOAA 4227. Several similar events are summarized in Table II. (2) The main flare structure was a flare cone, which consisted of a bright surge-like stream, elevated above two flare ribbons (located in the cone's base). The flare cone had a height of about 40 × 10³ km and lasted 4 min in H. The upper part of the cone was terminated by a very fine loop, which was bent to the west, where later a chromospheric brightening occurred at the footpoint of a flaring arch. A 300 keV burst and radio spikes were observed during the maximum flare phase. (3) The flaring arch system, with its apex at a height of about 48 × 10³ km, formed the skeleton for the coronal helmet structure (Figure 7(c)). The velocity of the plasma moving along the flaring arch was between 3500 km s^–1} and 6900 km s^–1} during the first brightening (14:07 UT).     

The Queens' flare: Its structure and development; precursors,pre-flare brightenings,and aftermaths

We continue previous research on the limb flare of 30 April, 1980, 20:20 UT, observed in X-rays by several instruments aboard the Solar Maximum Mission (SMM). It is shown quantitatively that the flare originated in an emerging magnetically confined kernel (diameter ～ 20″) which existed for about ten to fifteen minutes, and from which energetic electrons streamed, in at least two injections, into a previously existing complicated magnetic loop system thus forming a less bright but extended and long-lived tongue. The tongue had a length of ～ 35 000 km and lasted ～ 90 min in X-rays (～ 10 keV); at lower energies (～ 0.7 keV) it was larger (～ 80 000 km) and lasted longer. The total number of energetic electrons (≈ 10³⁷) initially present in the kernel is of the same order as the number present in the tongue after the kernel's decline. This gives evidence that the energetic electrons in the tongue originated mainly in the kernel. The electron number densities in the kernel and tongue at maximum brightness were ～ 4.5 × 10¹¹ and ～ 1 × 10¹¹ cm^#X2212;3, respectively. During the first eight minutes of its existence the tongue was hotter than the kernel, but it cooled off gradually. Its decline in intensity and temperature was exponential; energy was lost by radiation and by conduction through the footpoints of the loop system. These footpoints have a cross-section of only ～ 3 × 10⁶ km². This small value, as well as photographs in a Civ UV emission line, suggests a highly filamentary structure of the system; this is further supported by the finding that the tongue had a ‘filling factor’ of ～ 10^#X2212;2. Several faint X-ray brightenings (? 0.005 of the flare's maximum intensity) were observed at various locations along the solar limb for several hours before and after the flare. At ～ 30 min before the flare's onset a faint (? 0.02) flare precursor occurred, coinciding in place and shape with the flare. First the kernel precursor was brightest but the tongue precursor increased continuously in brightness and was the brightest part of the precursor some 10–15 min after the first visibility of the kernel precursor, until the start of the main flare. This suggests (weak) continuous electron acceleration in the tongue during a period of at least 30 min. The main flare was caused by strong emergence of magnetic field followed by two consecutive field line reconnections and accelerations in a small loop system, causing footpoint heating. Subsequently plasma streamed (convectively) into a pre-existing system of larger loops, forming the tongue.     

Hard X-ray images of possible reconnection in the flare of 21 May, 1980

An analysis of the growth of X-ray loops in the flare of 21 May, 1980, observed by HXIS on board SMM spacecraft, has been carried out with high time resolution in six energy channels from 3.5 to 30 keV. This analysis has revealed that the tops of the loops stay for minutes at a given altitude before, quite abruptly, other loop tops begin to appear above them. One of the jumps in altitude, from 27 000 to 45 000 km if the loops extended radially, which occurred quite late in the flare development, is studied in detail. The fact that the tops, of higher loops were first seen in the 22–30 keV energy channel, and only minutes later at lower energies, suggests a new release of energy in a very small volume high in the corona. An initial temperature of at least 50 × 10⁶ K is indicated by the data, inside a volume which may not exceed a few hundred km in diameter. A magnetic reconnection of previously distended field lines appears to be a likely candidate for the observed phenomenon.We also give some revised values of the characteristic parameters of the whole system of loops in this flare which has been the topic of several other studies.     

The magnetic geometry and structure of the giant post-flare arch of 21–22 May, 1980

On 21–22 May, 1980 the HXIS instrument aboard SMM imaged an enormous, more-or-less stationary, X-ray arch structure near the position of a large two-ribbon flare which immediately preceded it in time. As described by vestka et al. (1982), the arch remained visible for up to 10 hours. Previous inferences of the height, orientation, and physical parameters of this feature have been based largely on the X-ray data and on radio observations of the associated stationary Type I noise storm. In the present paper we use the observed photospheric line-of-sight magnetic field distribution to compute, in the current-free approximation, the three-dimensional topology of the coronal field above the flare site. Comparing the HXIS intensity contours of the arch to the projected shapes of the field lines suggests that the arch is indeed aligned with certain coronal flux tubes and allows an independent determination of the geometrical arch parameters to be made. This procedure indicates that the true height of the arch is about 70000 km, i.e., appreciably less than was suggested previously (although it is still certainly to be classified as a giant feature of the post-flare evolution).These results suggest that the arch may be a by-product of magnetic reconnection occurring far above the flare site, analogous to the post-flare loops seen at lower heights. Unlike the latter, however, the field lines undergoing reconnection here link more distant parts of the active region; i.e., they do not represent direct linkages across the magnetic neutral line and thus appear to be topologically quite distinct from those which thread the underlying post-flare loops. In fact, of this group of peripheral field lines, the arch could simply comprise the lowest-lying ones to have been opened up by the flare process (and the first to reconnect again). This would explain why both the arch and the post-flare loops were visible early in the decay phase, being products of separate reconnection processes. Moreover, because of the lower plasma density and longer cooling times of the arch, this feature persisted long after the post-flare loops faded from view. A calculation of the magnetic energy liberated by reconnection shows that this process is easily capable of satisfying the overall energy requirements of the arch (the latter as determined from observations).On leave from Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A.     

Culgoora radio and skylab euv observations of emerging magnetic flux in the lower corona

Detailed comparisons of Culgoora 160 MHz radioheliograms of solar noise storms and Skylab EUV spectroheliograms of coronal loop structures are presented. It is concluded that: (1) there is a close association between changes in large-scale magnetic fields in the corona and the onset or cessation of noise storms; (2) these coronal changes result from the emergence of new magnetic flux at the photospheric level; (3) although new magnetic flux at the photospheric level is often accompanied by an increase in flare activity the latter is not directly responsible for noise storm activity; rather the new magnetic flux diffuses slowly outwards through the corona at rates 1–2 km s^–1 and produces noise storms at 160 MHz 1–2 days later; (4) the coronal density above or in large-scale EUV loop systems is sufficiently dense to account for noise storm emission at the fundamental plasma frequency; (5) the scatter in noise storm positions can be accounted for by the appearance and disappearance of individual loops in a system.     

Revivals of a coronal arch

The giant post-flare arch of 6 November 1980 revived 11 hr and 25 hr after its formation. Both these revivals were caused by two-ribbon flares with growing systems of loops. The first two brightenings of the arch were homologous events with brightness maxima moving upwards through the corona with rather constant speed; during all three brightenings the arch showed a velocity pattern with two components: a slow one (8–12 km^?1), related to the moving maxima of brightness, and a fast one (～ 35 km s^?1), the source of which is unknown. During the first revival, at an altitude of 100000 km, temperature in the arch peaked ～ 1 hr, brightness ～ 2 hr, and emission measure ～ 3.5 hr after the onset of the brightening. Thus the arch looks like a magnified flare, with the scales both in size and time increased by an order of magnitude. At ～ 100000 km altitude the maximum temperature was ?14 × 10⁶K, max.n _e? 2.5 × 10⁹cm^?3, and max. energy density ? 11.2 erg cm^?3. The volume of the whole arch can be estimated to 1.1 × 10³⁰ cm³, total energy ?1.2 × 10³¹ erg, and total mass ?4.4 × 10¹⁵g. The density decreased with the increasing altitude and remained below 7 × 10⁹ cm^?3 anywhere in the arch. The arch cooled very slowly through radiation whereas conductive cooling was inhibited. Since its onset the revived arch was subject to energy input within the whole extent of the preexisting arch while a thermal disturbance (a new arch?) propagated slowly from below. We suggest that the first heating of the revived arch was due to reconnection of some of the distended flare loops with the magnetic field of the old preexisting arch. The formation of the ‘post’-flare loop system was delayed and started only some 30–40 min later. Since that time a new arch began to be formed above the loops and the velocities we found reflect this formation.     

A multiwavelength study of a double impulsive flare

Extensive data from the Solar Maximum Mission (SMM) and ground-based observatories are presented for two flares; the first occurred at 12:48 UT on 31 August, 1980 and the second just 3 min later. They were both compact events located in the same part of the active region. The first flare appeared as a typical X-ray flare: the Caxix X-ray lines were broadened ( 190±40 km s^-1) and blue shifted ( 60±20 km s^-1) during the impulsive phase, and there was a delay of about 30 s between the hard and soft X-ray maxima. The relative brightness of the two flares was different depending on the spectral region being used to observe them, the first being the brighter at microwave and hard X-ray wavelengths but fainter in soft X-rays. The second flare showed no significant mass motions, and the impulsive and gradual phases were almost simultaneous. The physical characteristics of the two flares are derived and compared. The main difference between them was in the pre-flare state of the coronal plasma at the flare site: before the first flare it was relatively cool (3 × 10⁶ K) and tenuous (4 × 10⁹ cm^-3), but owing to the residual effects of the first flare the coronal plasma was hotter (5 × 10⁶ K) and more dense (3 × 10¹¹ cm^-3) at the onset of the second flare. We are led to believe from these data that the plasma filling the flaring loops absorbed most of the energy released during the impulsive phase of the second flare, so that only a fraction of the energy could reach the chromosphere to produce mass motions and turbulence.A simple study of the brightest flares observed by the SMM shows that at least 43% of them are multiple. Thus, the situation studied here may be quite common, and the difference in initial plasma conditions could explain at least some of the large variations in observed flare parameters. We draw a number of conclusions from this study. First, the evolution of the second flare is substantially affected by the presence of the first flare. Secondly, the primary energy release in the second event is in the corona. Thirdly, the flares occur in a decaying magnetic region, probably as a result of the interaction of existing sheared loops; there is no evidence of emerging magnetic flux. Also, magnetic structures of greatly varying size participate in the flare processes. Lastly, there is some indication that the loops are not symmetrical or stable throughout the flares, i.e. the magnetic field does not act as a uniform passive bottle for the plasma, as is often assumed in flare models.NOAA/Space Environment Laboratory, currently at NASA/MSFC, Ala., U.S.A.Now at Sacramento Peak Observatory, Tucson, Ariz., U.S.A.     

A statistical study of moving type IV bursts based on Culgoora radioheliograph observations

Thirty-one moving type IV (IV(M)) bursts recorded with the Culgoora radioheliograph are examined to deduce their characteristic features, such as spatial distribution, projected velocity, etc., and their relation to other phenomena. The distribution of the projected velocity suggests that less than 15% of the total IV(M) bursts have fast velocities (>1000 km s^–1), almost equal to MHD shock velocity, and that the remaining IV(M) bursts have slower velocities (400 km s^–1) and are probably not associated with MHD shock waves. Most of the slow IV(M) bursts (and 70% of the total IV(M) bursts) are of an isolated plasmoid type. Even if they are associated with minor H flares, IV(M) bursts of the isolated-plasmoid type have 10³¹ ergs in the form of magnetic energy. They are in many cases closely associated with extended flare-continuum sources; this seems plausible if the flare continuum is interpreted as an interaction of a plasmoid with a large-scale magnetic arch.The association of IV(M) bursts with energetic proton events seems to be poor - contrary to expectation.     

Second-stage acceleration in a limb-occulted flare

We analyze hard and soft X-ray, microwave and meter wave radio, interplanetary particle, and optical data for the complex energetic solar event of 22 July 1972. The flare responsible for the observed phenomena most likely occurred 20° beyond the NW limb of the Sun, corresponding to an occultation height of 45 000 km. A group of type III radio bursts at meter wavelengths appeared to mark the impulsive phase of the flare, but no impulsive hard X-ray or microwave burst was observed. These impulsive-phase phenomena were apparently occulted by the solar disk as was the soft X-ray source that invariably accompanies an H flare. Nevertheless essentially all of the characteristic phenomena associated with second-stage acceleration in flares - type II radio burst, gradual second stage hard X-ray burst, meter wave flare continuum (FC II), extended microwave continuum, energetic electrons and ions in the interplanetary medium - were observed. The spectrum of the escaping electrons observed near Earth was approximately the same as that of the solar population and extended to well above 1 MeV.Our analysis of the data leads to the following results: (1) All characteristics are consistent with a hard X-ray source density n _i 10⁸ cm^–3 and magnetic field strength 10 G. (2) The second-stage acceleration was a physically distinct phenomenon which occurred for tens of minutes following the impulsive phase. (3) The acceleration occurred continuously throughout the event and was spatially widespread. (4) The accelerating agent was very likely the shock wave associated with the type II burst. (5) The emission mechanism for the meter-wave flare continuum source may have been plasma-wave conversion, rather than gyrosynchrotron emission.     

Pre- and post-flare X-ray variations in active regions

Extremely low background noise of the HXIS experiment aboard the SMM made it possible to detect > 3.5 keV X-ray emissions from non-flaring active regions which are 10³–10⁴ times weaker than the X-ray flux from flares. Short-lived X-ray bursts and long-lived X-ray enhancements of various intensities seem to characterize active regions in different phases of their development. After major two-ribbon flares, giant X-ray arches are seen in the corona, slowly decaying for many hours after the flare end. Associated with these arches appear to be quasi-periodic flare-like variations of purely coronal nature.     

Observation of the impulsive phase of a simple flare

We present a broad range of complementary observations of the onset and impulsive phase of a fairly large (1B, M1.2) but simple two-ribbon flare. The observations consist of hard X-ray flux measured by the SMM HXRBS, high-sensitivity measurements of microwave flux at 22 GHz from Itapetinga Radio Observatory, sequences of spectroheliograms in UV emission lines from Ov (T ≈ 2 × 10⁵ K) and Fexxi (T ≈ 1 × 10⁷ K) from the SMM UVSP, Hα and Hei D₃ cine-filtergrams from Big Bear Solar Observatory, and a magnetogram of the flare region from the MSFC Solar Observatory. From these data we conclude:

1. The overall magnetic field configuration in which the flare occurred was a fairly simple, closed arch containing nonpotential substructure.
2. The flare occurred spontaneously within the arch; it was not triggered by emerging magnetic flux.
3. The impulsive energy release occurred in two major spikes. The second spike took place within the flare arch heated in the first spike, but was concentrated on a different subset of field lines. The ratio of Ov emission to hard X-ray emission decreased by at least a factor of 2 from the first spike to the second, probably because the plasma density in the flare arch had increased by chromospheric evaporation.
4. The impulsive energy release most likely occurred in the upper part of the arch; it had three immediate products:

1. An increase in the plasma pressure throughout the flare arch of at least a factor of 10. This is required because the Fexxi emission was confined to the feet of the flare arch for at least the first minute of the impulsive phase.
2. Nonthermal energetic (～ 25 keV) electrons which impacted the feet of the arch to produce the hard X-ray burst and impulsive brightening in Ov and D₃. The evidence for this is the simultaneity, within ± 2 s, of the peak Ov and hard X-ray emissions.
3. Another population of high-energy (～100keV) electrons (decoupled from the population that produced the hard X-rays) that produced the impulsive microwave emission at 22 GHz. This conclusion is drawn because the microwave peak was 6 ± 3 s later than the hard X-ray peak.

     

Dynamic evolution of recurrent mass ejections observed in Hα and Civ lines

During a coordinated SMY program, the consecutive formation of two new active centers merging together within AR 2646 was observed from 28 August, to 5 September, 1980. The two preceding spots compressed an inverse polarity spot on 1 September 1980, causing recurrent ejecta of matter with time intervals around 10 min. The observations of the MSDP spectrograph operating in H at the Meudon Solar tower and of the UVSP spectrometer on SMM in the Civ 1548 Å line show that cold and hot material had the same projection, although the upward Civ velocity structure was more extended than the H one. We present evidence that observed contrasts of the H absorbing structure can be interpreted in terms of a dynamic cloud model overlying the chromosphere. H matter follows a magnetic channel with upward velocity around 20–30 km s^–1 in the first phase of the event and with downward velocity ( - 40 km s^–1) in the second phase. The stored energy is not sufficient to trigger a flare, nor even to propulse matter along the full length of an arch, because of the periodic reorganisation of the magnetic field.     

21 May 1980 flare review

A review is given of observations and theories relevant to the solar flare of 21 May, 1980, 20 ∶ 50 UT, the best studied flare on record. For more than 30 hr before the flare there was filament activation and plasma heating to above 10 MK. A flare precursor was present ≥6 min before the flare onset. The flare started with filament activation (20 ∶ 50 UT), followed by thick-target heating of two footpoints and subsequent ablation and convective evaporation involving energies of 1 to 2 × 10³¹ erg. Coronal explosions occurred at 20 ∶ 57 UT (possibly associated with a type-II burst) and at 21 ∶ 04 UT (associated with an Hα spray?). Post-flare loops were first seen at 20 ∶ 57 UT, and their upward motion is interpreted as a manifestation of successive field-line reconnections. A type-IV radio burst which later changed into a type-I noise storm was related to a giant coronal arch located just below the radio noise storm region. Some implications and difficulties these observations present to current flare theories are mentioned.     

Thermal structures associated with post-flare coronal arches

Shortly after the dynamic flare of 14 44 UT on 6 November, 1980, which initiated the second revival in the sequence of post-flare coronal arches of 6–7 November, a moving thermal disturbance was observed in the fine field of view of HXIS. From 15 40 UT until about 18 UT, when it left the field of view, the disturbance rose into the corona, as indicated by a projected velocity of 7.4 km s^-1 in the south-east direction. The feature was located above the reconnection region of the dynamic flare and was apparently related to the revived coronal arch. Observations in the coarse field of view after 18 UT revealed a temperature maximum in the revived arch, rising with a velocity of 7.0 km s^-1 directly in continuation of the thermal disturbance. The rise velocity of the disturbance was initially (at least until 17 20 UT) very similar to the rise velocities observed for the post-flare loop tops of the parent flare. This suggests that the rise of the reconnection point, in the Kopp and Pneuman (1976) mechanism responsible for the rise of the loop tops, also dictates the rise of the disturbance. From energy requirements it follows that in this phase the disturbed region is still a separate magnetic island, thermally isolated from the old arch structure and the post-flare loops. After 18 UT the rise of the post-flare loop tops slowed down to 2 km s^-1, which is significantly slower than the rise of the brightness and temperature maxima of the revived arch in the coarse field of view. Thus in this phase the Kopp and Pneuman mechanism is no longer directly responsible for the rise of the thermal structure and the rise possibly reflects the merging of the old and the new arch structures.A similar thermal disturbance was observed after the dynamic flare of 07: 53 UT on 4 June, 1980. On the other hand, the confined flare of 17 25 UT on 6 November, 1980, did not show this phenomenon. Apparently this type of disturbance occurs after dynamic flares only, in particular when the flare is associated with an arch revival.     

Post-flare coronal arches observed with the SMM/XRP Flat Crystal Spectrometer

The phenomenon of post-flare coronal arches, initially discovered with the Hard X-Ray Imaging Spectrometer (HXIS), was investigated using observations made with the SMM Flat Crystal Spectrometer (FCS) on 20 through 23 January, 1985. Since these observations were made with a different type of instrument from HXIS, they provide independent information on the physical characteristics of the arch phenomenon and extend our knowledge to lower coronal temperatures.Conspicuous arch activity was observed after three flares and after a disturbance which could not be identified. (1) A dynamic flare starting on 20 January at 20: 39 UT was responsible for the formation of the primary arch structure. (2) An arch revival, showing characteristics very similar to those of the arch revivals observed with HXIS, took place after the dynamic flare starting on 21 January at 23: 50 UT. The most conspicious difference relates to the moving thermal disturbance observed very shortly after the onset of the parent flare, in particular to its propagation velocity. This difference in the arch revival is probably related to the different range of plasma temperatures covered by the FCS observations (3 × 10⁶ K through 6 × 10⁶ K) and the HXIS observations (>10⁷ K) and the consequently more important effects of radiative cooling in the FCS arch revival. (3) More arch activity was observed after a (possibly dynamic) flare starting at 03: 40 UT on 21 January and (4) after an unidentified event with estimated time of occurrence near 23: 00 UT on 22 January. Similar to the arch revival, this activity was primarily characterized by the energization of (i.e., input of energy to) a pre-existing arch structure. The activity after the unidentified event suggests the existence of a mode of arch activation which is different from the typical flare-associated revival and is characterized by the absence of significant activity at chromospheric levels.