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.     

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.     

Mass motions associated with Hα active region arch structures

We have studied mass motions associated with active region arch structures from observations of a developing active region near the center of the solar disk. We present a method for the computation of the line-of-sight velocity from photographs at H ± 0.5 under the assumption of Beckers' cloud model and reasonable assumptions about the Doppler width and optical depth of the arches. Some arches show motions typical to arch filaments (the material moves towards the observer near the apex of the arch and away from the observer near the footpoints), while in others the velocity field is more complex. Assuming a symmetric loop, we reconstructed the velocity vector along an arch filament. The results are consistent with the picture where material is draining out of the filament while the whole structure is ascending with a velocity near that of the apex, which does not exceed 10 km s^–1. The motion is systematically slower than expected from a free-fall model.     

Line-of-Sight Velocity Fluctuations of a Quiescent Prominence

Series of H spectra and slit-jaw H filtergrams of a quiescent prominence taken at Pic du Midi Observatory on 7 November 1977 are studied. The observations have been digitized by means of an automated Joyce Loebl microdensitometer. The image processing of the H filtergrams reveals an internal structure of the prominence consisting of several arches. Series of high-resolution H spectra obtained with the slit position located on a selected part of one of the prominence arches have been chosen for Doppler-shift analysis. The obtained time series of the line-of-sight velocity reveal large velocity variations near the periphery of the arch and a strong dependence of the velocity (in sign and magnitude) on the position along the slit. The prominence arch shows also cyclic displacements along the line-of-sight direction implying Alfvén string-mode oscillations.     

Multiwavelength analysis of a well observed flare from SMM

We describe and analyse observations of an M1.4 flare which began at 17: 00 UT on 12 November, 1980. Ground based H and magnetogram data have been combined with EUV, soft and hard X-ray observations made with instruments on-board the Solar Maximum Mission (SMM) satellite. The preflare phase was marked by a gradual brightening of the flare site in Ov and the disappearance of an H filament. Filament ejecta were seen in Ov moving southward at a speed of about 60 km s^–1, before the impulsive phase. The flare loop footpoints brightened in H and the Caxix resonance line broadened dramatically 2 min before the impulsive phase. Non-thermal hard X-ray emission was detected from the loop footpoints during the impulsive phase while during the same period blue-shifts corresponding to upflows of 200–250 km s^–1 were seen in Ca xix. Evidence was found for energy deposition in both the chromosphere and corona at a number of stages during the flare. We consider two widely studied mechanisms for the production of the high temperature soft X-ray flare plasma in the corona, i.e. chromospheric evaporation, and a model in which the heating and transfer of material occurs between flux tubes during reconnection.     

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.     

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.     

Siphon flows in the solar corona

The flows in a coronal magnetic arch associated with a pressure difference between the footpoints are investigated. Steady flows are of different types: always subsonic; subsonic in one branch of the arch, supersonic in the second; subsonic-supersonic with stationary shocks which adjust the flow to the boundary conditions in the second footpoint. The large velocity increase along the loop in subsonic-supersonic flows is associated with a large density decrease. A velocity drop and a density jump occur across the shock. The emission of such arches in coronal lines (625 of Mg x and 499 of Si xii) is calculated. It is suggested that the intensity drop along the axis observed in some UV loops is due to the density drop associated with subsonic-supersonic flows.     

X-ray bright surges

We present evidence of X-ray emission from surges that are bright in H. These surges have many features common to flaring arches of Martin and vestka (1988); the basic difference between the two is that in flaring arches cold and hot plasma are injected into clearly defined closed magnetic loops, while in the surges the injection goes into large-scale magnetic field structures of which the second footpoint is usually unknown. Because of the steep density gradient in such large-scale structures, the X-ray visibility of bright surges is limited to a few tens of seconds only. A series of repetitive surges, some of them bright and emitting X-rays, occurred on 8 July, 1980 from footpoints of two large-scale coronal structures, which might have been the legs of an enormous arch at least 600 Mm long.     

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.     

Measurement and analysis of magnetic field variation during a class 2b flare

We report a digital analysis of high-time-resolution videomagnetograms taken during a class 2b flare that occurred at 60° east. The data were obtained at the Big Bear Observatory and calibrated by a Mt. Wilson magnetogram. Changes of weak magnetic fields (less than 100 G) with an amplitude from 30 to 100% have been detected over 55% of the optical flare region, apparently taking place at the initial phase of the flare. Statistical considerations suggest a real flare association with most of these changes.H observations show that large changes took place over the footpoints of heavily inclined structures like penumbral fibrils, while smaller changes took place over the plage region. An apparent polarity reversal was found at the feet of erupted fibrils.Based on force-free field calculations these changes can be reasonably explained as a transformation of the current-carrying fields to potential fields which produced large changes in the field line inclination and rotation.Visiting Associate, Summer 1977.     

Flaring arches

We show detailed observations in X-rays, UV lines, and H of an extended arch, about 300000 km long, which developed as a consequence of a compact subflare. This subflare occurred in an included magnetic polarity of relatively low magnetic field strength (compared to that of the sunspots). The apparition of this big arch was preceded by that of a smaller arch, about 30000 km long, which masked the polarity inversion line filament in the early phase of the subflare. The big arch which developed later, around the time of the main X-ray and UV spike of the subflare, connected the included polarity and the main leading sunspot of the region, and became fully developed in a few minutes. The fact that both arches were simultaneously observed in all spectral domains as well as their fine structure in H can only be explained by considering the arch as composed of several unresolved portions of material having widely different temperatures. The H observations can be interpreted as showing the appearance of this cool material as a result of condensation, but a more appealing interpretation is that there was almost simultaneous ejection of superhot (10⁷ K), hot (10⁶ K), mild (10⁵ K), and cool (10⁴ K) material from the subflare site along previously existing magnetic tubes of much lower density. The termination of the subflare was marked by a rather hard X-ray and UV spike which appeared to originate in a different structure than that of the main spike. The material in the arch gradually cooled and drained down after the end of the subflare.Member of Carrera del Investigador, CONICET, Argentina.     

Large-scale brightenings associated with flares

Flare-associated large-scale (>10¹⁰ cm) X-ray brightenings, the so-called giant arches in the nomenclature of vestka and co-workers, were discovered in images obtained by the SMM Hard X-ray Imaging Spectrometer hours after the onset of two-ribbon flares. The apparent correlation between both phenomena suggested that they could be interpreted in the framework of the same model.In this paper we show that large-scale loop brightenings, of sizes similar to the giant arches, occur also in association with confined flares in complex active regions. In these cases, the relation between the large-scale structure and the underlying flare is clearly given by the magnetic field topology. We also show that energization of these structures can be partially due to the injection of suprathermal particles that are accelerated at the separator region.We discuss the implications of these results within the framework of the interacting loops picture of flares and of the giant arch phenomenology.Member of the Carrera del Investigador Científico, CONICET, Argentina.     

Energy balance in a magnetically confined coronal structure observed by OSO-7

A model of a coronal region of enhanced Fexv and Fexvi emission is developed and its energy balance is examined using extreme ultraviolet observations from OSO-7 together with calculations of possible force-free coronal magnetic field configurations. The coronal emissions overlying the photospheric boundary between regions of opposite magnetic polarity are found to be associated with generally non-potential (current-carrying) magnetic fields in the forms of arches with footpoints in regions of opposite polarity. The orientation of these arches relative to the neutral line changes with degree of ionization of the emitting ion (which we infer from our limb observations to be a function of height) and may be evidence of differing electric currents along various field lines. The appearance of a coronal arch, seen side-on, can conveniently be represented by a parabola and a detailed analysis (Appendix) shows this to be a realistic approximation that should be generally useful in analyzing two-dimensional pictures of coronal structures. Applying this analysis to the most prominent coronal region observed in the radiations of Fexv and Fexvi, we find a maximum in the electron temperature, T _e , of 2.6 × 10⁶K at the top of arches whose heights are 20000–40000 km and whose footpoints are separated by ≈ 100000 km. A temperature gradient of ▽T _e ≈5 × 10^-5K cm^-1 is found in this coronal structure. Radiative losses are typically fifteen times greater than conductive losses and the energy deposition required to maintain the coronal feature is nearly uniformly distributed along its length.     

Arch Filament Systems Associated with X-Ray Loops

Using multi-wavelength observations obtained with the Tenerife telescopes (VTT and GCT) and with the Yohkoh satellite, we observed new emerging flux with an associated arch filament system (AFS) in the chromosphere and bright X-ray loops in the corona. We observed the change of connectivity of the X-ray loop footpoints which may be at the origin of the occurrence of a subflare. Densities, gas and magnetic pressures of cold AFS and hot loops were derived and discussed. The extrapolation of the photospheric magnetic field observed with the GCT in a linear force-free field assumption (constant ) shows that this region, in spite of having roughly a global potential configuration, consists of two systems of arch filaments. We found these two systems best fitted with two sheared magnetic topologies of opposite values of ± 0.1 Mm^-1     

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.     

Flaring arches

Flaring arches is a name assigned to a particular component of some flares. This component consists of X-ray and H emission which traverses a coronal arch from one to the other of its chromospheric footpoints. The primary footpoint is at the site of a flare. The secondary footpoint, tens of thousands of kilometers distant from the source flare, but in the same active region, brightens in H concurrent with the beginning of the hard X-ray burst at the primary site. From the inferred travel time of the initial exciting agent we deduce that high speed electron streams travelling through the arch must be the source of the initial excitation at the secondary footpoint. Subsequently, a more slowly moving agent gradually enhances the arch first in X-rays and subsequently in H, starting at the primary footpoint and propagating along the arch trajectory. The plasma flow in H shows clearly that material is injected into the arch from the site of the primary footpoint and later on, at least in some events, a part of it is also falling back.Thus a typical flaring arch has three, and perhaps four consecutive phases: (1) An early phase characterized by the onset of hard X-ray burst and brightening of the secondary footpoint in H. (2) The main X-ray phase, during which X-ray emission propagates through the arch. (3) The main H phase, during which H emitting material propagates through the arch. And (4) an aftermath phase when some parts of the ejected material seem to flow in the reverse direction towards the primary site of injection.An extensive series of flaring arches was observed from 6 to 13 November, 1980 at the Big Bear Solar Observatory and with the Hard X-Ray Imaging Spectrometer (HXIS) on board the SMM in a magnetically complex active region. The two most intense arches for which complete H and X-ray data are available and which occurred on 6 November at 17 21 UT (length 57000 km) and on 12 November at 16 57 UT (length 263 000 km) are discussed in this paper.     

Giant solar arches and coronal mass ejections in November 1980

Using data from the SOLWIND coronagraph and photometers aboard HELIOS-A we examine coronal mass ejections from an active region which produced a series of giant post-flare coronal arches. HXIS X-ray observations reveal that in several cases underlying flares did not disrupt these arch structures, but simply revived them, enhancing their temperature, density and brightness. Thus we are curious to know how these quasi-stationary X-ray structures could survive in the corona in spite of recurrent appearances of powerful dynamic flares below them. We have found reliable evidence that two dynamic flares which clearly revived the preexisting giant arch were not associated with any mass ejection. After two other flares, which were associated with mass ejections, the arch might have been newly formed when the ejection was over. In one of these cases, however, the arch had typical characteristics of a revived structure so that it is likely that it survived a powerful mass ejection nearby. In a magnetic configuration of the arch which results from potential-field modelling (Figure 1(b)) such a survival seems possible.     

DOUBLE-LOOP CONFIGURATION OF SOLAR FLARES

We analyzed several flares, which are presumed to be caused by interactions between an emerging loop and an overlying loop. We call such a basic combination of loops a double-loop configuration, and we reveal its topology on the basis of the microwave and soft X-ray observations of the flares and the magnetograms. In many cases, the magnetic field of the flare loops shows a bipolar + remote unipolar structure, rather than a quadrapole structure. The footpoints of two loops are distributed in three magnetic patches, and two of the footpoints of the loops, one from the emerging loop and the other from the overlying loop, are included in a single magnetic polarity patch. Therefore, the two loops form a three-legged structure, and the two loops are not anti-parallel as assumed in the traditional reconnection models. Typically, the emergence of a parasitic polarity near the major preceding-polarity region or the following one in an active region creates this configuration, but, in one of the analyzed flares, two active regions are involved in the configuration. Not only the flares, but various other active phenomena – microflares, thermal plasma flows like jets, and surges – occur in the same magnetic configuration. Hence, the interaction between two loops, which forms the three-legged structure, is an important source of the various types of activity.     

Coronal interconnection of two active regions observed in 3.5–8.0 keV X-rays

Using HXIS data, we have studied the further development of the coronal arch extending towards the SE above active region No. 17255 in November 1980. The arch, studied originally by vestka (1984) disappeared on 10 November (after pronounced revival), but since 9 November HXIS revealed another arch-like structure towards the SW. We have studied the development of this new structure which appeared to be most likely an arch interconnecting AR 17255 with AR 17251, located nearly 30° to the west. This interconnection revived many times during the following days with intensity varying with the activity in both active regions. We have estimated the physical characteristics in this coronal structure and compared them with observations of interconnecting loops made at lower energies by Skylab in 1973. The temperature (maximum values 7.5–14 × 10⁶ K) and the density (1.1–5.0 × 10⁹ cm^–3) are found to be higher than in the Skylab loops (a result that could be expected because the HXIS energy range was harder than that of Skylab) and similar to the values deduced for the earlier arch system extending to the SE. However, much shorter decay times of the brightness variations indicate the presence of conduction in contrast to the SE arch in which conduction was clearly inhibited. This supports the assumption that the SE and SW coronal structures were two different phenomena.Presently at Delft Institute of Technology, Landbergstraat 3, 2628 CE Delft, The Netherlands.