Magnetic reconnection and coronal transients

Every two-ribbon flare observed during the Skylab period produced an observable coronal transient, provided the flare occurred close enough to the limb. The model presented here treats these two events as a combined process. Transients that occur without flares are believed to involve magnetic fields that are too weak to produce significant chromospheric emission. Adopting the hypothesis that the rising flare loop systems observed during two-ribbon flares are exhibiting magnetic reconnection, a model of a coronal transient is proposed which incorporates this reconnection process as the driving force. When two oppositely directed field lines reconnect a lower loop is created rooted to the solar surface (the flare loop) and an upper disconnected loop is produced which is free to rise. The magnetic flux of these upper loops is proposed as the driver for the transient. The force is produced by the increase in magnetic pressure under the filament and transient.A quantitative model is developed which treats the transient configuration in terms of four distinct parts- the transient itself with its magnetic field and material, the region just below the transient but above the filament, the filament with its magnetic field, and the reconnected flux beneath the filament. Two cases are considered - one in which all the prominence material rises with the transient and one in which the material is allowed to fall out of the transient. The rate of rise of the neutral line during the reconnection process is taken from the observations of the rising X-ray flare loop system during the 29 July, 1973 flare. The MHD equations for the system are reduced to four non-linear ordinary coupled differential equations which are solved using parameters believed to be realistic for solar conditions. The calculated velocity profiles, widths, etc., agree quite well with the observed properties of coronal transients as seen in white light. Since major flares are usually associated with a filament eruption about 10–15 min before the flare and since this model associates the transient with the filament eruption, we suspect that the transient is actually initiated some time before the actual flare itself.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

VERY LARGE ARRAY OBSERVATIONS OF EVOLVING NOISE STORM SOURCES ON THE SUN

The presence of coronal magnetic fields connecting active regions is inferred from decimetric observations of solar noise storms with the Very Large Array (VLA) and from soft X-ray images taken by Yohkoh. Temporal changes in the noise storms appear to be correlated with some soft X-ray bursts detected by both Yohkoh and the GOES satellite. Combined analysis of the radio and X-ray data suggests a re-arrangement of the coronal magnetic field during the onset of impulsive noise storm burst emission. On one day during the combined VLA–Yohkoh–GOES observations, two widely-separated active regions appear to be connected by a faint trans-equatorial 91 cm source as well as two distinct soft X-ray loops. The two active regions show anti-correlated fluctuations in decimetric radio emission. On another day of combined VLA–Yohkoh observations, a series of 91 cm noise storm bursts are observed along the major axis of the associated noise storm continuum. Time sequences of Yohkoh soft X-ray images show a contraction of coronal loops prior to the onset of this series of bursts and a corresponding increase in the X-ray flux in the apparent footpoint of the overarching loop containing the noise storm. These observations imply that energy from a realignment of the magnetic field is being transferred, possibly by accelerated particles, along loops connecting separated active regions on the Sun.     

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     

A blowout surge from the eruption of a miniature filament confined by large coronal loops

By means of Hα, EUV, soft X-ray, hard X-ray, and photospheric magnetic field observations, we report the surge-like eruption of a small-scale filament, called “blowout surge” according to recent observations, occurring on a plage region around AR 10876 on 1 May 2006. Along magnetic polarity reversal boundaries with obvious magnetic cancelations, the filament was located underneath a compact coronal arcade and close to one end of large coronal loops around the AR’s periphery. The filament started to erupt about 8 min before the main impulsive phase of a small two-ribbon flare, which had two Hα blue-wing kernels connected by hard X-ray loop-top sources on the both sides of the filament. After the flare end, the filament further underwent a distant eruption following a path nearly along the preexisting large loops, and thus looked like an Hα surge and an EUV jet. During the eruption, a small coronal dimming was formed near the flare, while weak brightenings appeared around the remote end of the large loops. We interpret these joint observations as the filament eruption being confined and guided by the large loops. The filament eruption, initially embedded in one footpoint region of the large loops, can break away from the magnetic restraint of the overlying compact arcade, but might be still limited inside the large loops. As a result, the eruption took a surge form that can only expand laterally along the large loops rather than erupt radially.     

Coronal changes associated with a disappearing filament

This paper describes Skylab/ATM observations of the events associated with a disappearing filament near the center of the solar disk on January 18, 1974. As the filament disappeared, the nearby coronal plasma was heated to a temperature in excess of 6 × 10⁶K. A change in the pattern of coronal emission occurred during the 11/3 hr period that the soft X-ray flux was increasing. This change seemed to consist of the formation and apparent expansion of a loop-like coronal structure which remained visible until its passage around the west limb several days later. The time history of the X-ray and microwave radio flux displayed the well-known gradual-rise-and-fall (GRF) signature, suggesting that this January 18 event may have properties characteristic of a wide class of X-ray and radio events.In pursuit of this idea, we examined other spatially-resolved Skylab/ATM observations of long-duration X-ray events to see what characteristics they may have in common. Nineteen similar long-lived SOLRAD X-ray events having either the GRF or post-burst radio classification occurred during the nine-month Skylab mission. Sixteen of these occurred during HAO/ATM coronagraph observations, and 7 of these 16 events occurred during observations with both the NRL/ATM slitless spectrograph and the MSFC-A/ATM X-ray telescope. The tabulation of these events suggests that all long-lived SOLRAD X-ray bursts involve transients in the outer corona and that at least two-thirds of the bursts involve either the eruption or major activation of a prominence. Also, these observations indicate that long-lived SOLARD events are characterized by the appearance of new loops of emission in the lower corona during the declining phase of the X-ray emission. However, sometimes these loops disappear after the X-ray event (like the post-flare loops associated with a sporadic coronal condensation), and sometimes the loops remain indefinitely (like the emission from a permanent coronal condensation).Visiting Scientist, Kitt Peak National Observatory, Tucson, Ariz. 85726, U.S.A. operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Presently located at NASA/MSFC, Space Sciences Laboratory, Marshall Space Flight Center, Ala. 35812, U. S.A.     

Emergence of sheared magnetic flux tubes in an active region observed with the SVST and TRACE

The active region NOAA AR 8331 was a target of an international ground-based observational campaign in the Canaries and coordinated with space instruments (TRACE and Yohkoh). We focus our study on observations obtained with the SVST at LaPalma, and with TRACE. On 10 September 1998, arch-filament systems were observed with high spatial and temporal resolution, from the lower to the upper atmosphere of the Sun, during five hours. Flux tubes emerged with increasing shear, which apparently led to energy release and heating in the overlying corona. A model for filament formation by the emergence of U-shaped loops from the subphotosphere, as proposed by Rust and Kumar (1994), is supported by the present observations. The coronal response to these events is visualized by rising, medium-scale loop brightenings. The low-lying X-ray loops show short-lived, bright knots which are thought to result from interaction between different loop systems.     

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.     

The decay of coronal loops brightened by flares and transients

The sizes and shapes of X-ray emitting loops brightened by flares and other coronal transients have been derived from the Skylab S-054 photographs. This information has been combined with estimates of temperature and emission measure derived from the photographs and from Solrad data to compute brightness decay times attributable to various coronal energy loss mechanisms. The computed decay times are compared to those actually observed. Examples are presented of the brightness decay of soft X-ray flare kernels, post-flare loops, and the coronal X-ray enhancement asssociated with an H filament disappearance.The computed decay time due to conductive losses is always found to be much more rapid than that due to radiative losses in the corona. However, the observed soft X-ray brightness decay times are always much longer than those computed from conductive cooling.The role of geometrical inhibition of conduction as discussed by Antiochos and Sturrock (1976a) is examined for these events. It is shown that this mechanism might be adequate to account for the observed results in two of the five cases examined, but it is inadequate in the other three. The possible breakdown of classical collisional thermal conductivity (Forslund, 1970) is examined and it is shown that this mechanism is not applicable to the cases presented here. Confirmation of the existence of the very high conductive fluxes predicted by the coronal flare conductive cooling models is sought from EUV and H observations. No evidence is found which unequivocally demonstrates the presence, at lower levels in the atmosphere, of very high conductive fluxes. The soft X-ray results are consistent with the continuation of evaporation driven by thermal conduction (Antiochos and Sturrock, 1976b) late into the decay phase of the event. In this case, no source of continued magnetic energy dissipation after the initial stages of the flare is required to explain the lifetime of the X-ray emitting loops.     

The flare of September 7, 1973: A typical example of a newly recognized class of solar transients

X-ray, extreme-ultraviolet and optical observations of a solar flare are discussed. It is shown that the flare exemplifies a class of transient events characterized by long duration and long decay time and by the development of high systems of loops, generally brighter at the top. In contrast with compact short lifetime events, the distinctive properties of this class of transients are:

1. The disruption of the magnetic configuration at the flare onset, as indicated by prominence eruption or activation and by associated white-light coronal transients;
2. a continuous energy deposition, presumably at the top of loops, during a large fraction of the flare development and well after the intensity peak;
3. a continuous supply of additional material to the top of loops, with subsequent downflows and out-of-hydrostatic equilibrium conditions.

     

Type II radio emission in coronal transients

Recent observations demonstrate that some type II radio bursts (a) occur below the top of coronal white light loops in the early stages and (b) travel faster than white light transients when both data sources are recorded concurrently. These characteristics are examined with numerical simulations of a coronal transient in combination with the suggestion by Holman and Pesses (1983) that shock drift acceleration may be the originating mechanism for type II emission. The simulated angular relation between the transient shock normal and the upstream magnetic field, along with requirements on this orientation in order that shock drift be effective, lead naturally to the observed spatial relationship (in the lower corona) and relative velocities of white-light transients and type II bursts. The large type II velocities do not directly correspond to either material or shock motion, but are due to the production of emission at different locations along the shock surface. In addition, the model coincides with the hypothesis that the shocks generating the coronal type II emission also produce interplanetary SA (shock-accelerated) events.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Geometrical considerations in imaging the solar corona

X-ray observations of the solar corona show that it is comprised of three-dimensional magnetic structures which appear to be primarily in the form of fluxtubes or loops. Imaging the X-ray corona has led to a greater understanding of the dynamical behaviour of and the energy distribution in these magnetic structures. However, imaging observations, by their very nature, integrate along the line of sight resulting in a two-dimensional representation of the actual three-dimensional distribution. The optically thin nature of the solar corona to X-ray radiation makes the integrated images particularly difficult to interpret. The analysis of the two-dimensional observations must, therefore, inlcude the effect of the orientation of the coronal structure to the line-of-sight direction; a fact which is almost always ignored. In this paper we discuss the effect of loop orientation on the two-dimensional representation and argue that these effects may lead to a misinterpretation of the physics occurring in the structures observed. In particular, we discuss observations taken by the Soft X-ray Telescope (SXT) on board the Yohkoh satellite, taking account of the instrumental thermal response, spatial resolution, and point-spread-function.We test the effect of geometry on the determination of the loop pressure by considering equatorial loops at various longitudes and discuss the implications of this for studies of coronal soft X-ray loops.     

The pre-onset morphology of the 5 September 1973 flare

Simultaneous visible, EUV, and X-ray observations of magnetic structures before and during the onset of the flare of 5 September 1973 are co-registered and interpreted. Ninety minutes before the flare, intense EUV knots fluctuate near the loops which subsequently flare. The pre-flare loop is observed in O IV 554, but not in X-rays, which show instead a parallel structure which is related either to a darkening filament or the subsequent flare kernels. As the full disk X-ray emission increases, first the EUV flare loop appears, then X-ray kernels form at the feet of two EUV loops, one of which overlies the activated filament. The flaring, at any given time, is confined to a single loop (or bundle of loops) whose long axis (barely) crosses the neutral line. As time progresses, the flaring moves to other (probably higher) loops sharing the off-band H footpoints but whose axes are rotated relative to the earlier loops by angles of about 30°. Previous interpretations of single-telescope observations are revised in this joint investigation.     

Observational evidence of continual heating in X-ray emitting coronal loops

A 90 s time resolution study of the soft X-ray emission from three active region loops shows the emission to be constant to about two percent over the half hour period of observation. Soft X-ray observations in two wavebands are used to deduce the temperature and density of these loops. The data unambiguously demonstrate that energy is supplied to each loop during the observations. If heating is due to discrete events, the time interval between events is shown to be less than 10 min, which is short relative to the radiative cooling time of the loops.Skylab Solar Workshop Post-Doctoral Appointee, 1975–1977. The Skylab Solar Workshops are sponsored by NASA and NSF and managed by the High Altitude Observatory, National Center for Atmospheric Research.     

Active-Region Filaments and X-ray Sigmoids

We use Yohkoh soft X-ray telescope data and H full-disk observations to study the evolution of chromospheric filaments and coronal sigmoids in 6 active regions in association with coronal mass ejections (CMEs). In two cases, CMEs are directly observed by the SOHO/LASCO C2 coronagraph. In four cases, other observations (magnetic clouds, geomagnetic storms, sigmoid-arcade evolution) are used as CME indicators. Prior to eruption, each active region shows a bright coronal sigmoidal loop and underlying H filament. The sigmoid activates, erupts and gets replaced by a cusp, or an arcade. In contrast, the H filament shows no significant changes in association with sigmoid eruption and CME. We explain these observations in a framework of the classical two-ribbon flare model.     

He I 10830 observations of the 3N/M4.0 flare of 4 September, 1982

Hei 10830 Å spectroheliograms of a major 3N two-ribbon flare occurring in Boulder Region 3885/3886 early on 4 September, 1982 are discussed and compared with H and soft X-ray observations of the event. This flare, observed for more than 60 hr in Hei 10830, was associated with the eruption of a large filament in the active region complex, the formation of coronal holes, a long-duration soft X-ray event, and was the probable source of a earthward coronal mass ejection and the largest geomagnetic storm of this solar cycle. The results of this study suggest the Hei flare is a chromospheric manifestation of the X-ray coronal loop structures associated with flares.Visitor, National Solar Observatory, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

The Observational Evidence on the Loop–Loop Interaction in a Flare–CME Event on April 15, 1998

The evolution of the soft X-ray and EUV coronal loops related to the April 15, 1998 solar flare–CME event is studied with multiwavelength observations including hard X-rays (BATSE), microwaves (NoRP, CNAO) and magnetograms (SOHO/MDI), as well as images from Yohkoh/SXT and SOHO/EIT at 195 Å. It is shown that: (1) two soft X-ray and EUV loops rose, crossed and turned bright, (2) near one footpoint of these loops, the background magnetic field decreased, (3) there were similar quasi periodic oscillations in the time profiles of hard X-ray and microwave emissions, which characterized the loop–loop coalescence instability, (4) after the loop–loop reconnection, two new loops formed, the small one stayed at the original place, and the large one ejected out as part of the constructed prominence cloud. Based upon these observations, we argue that the decrease of the background magnetic field near these loops caused them to rise and approach each other, and in turn, the fast loop–loop coalescence instability took place and triggered the flare and the CME.     

Counter-streaming Mass Flow and Transient Brightening in Active Region Loops

An active region loop system was observed in a decaying active region for three hours by TRACE and BBSO in a joint campaign on September 27, 1998. Continuous mass motion was seen in Hα offband filtergrams throughout the three hours, and some UV loops were exhibited transient brightenings. We find that: (1) cool material was flowing along the loops at a speed of at least 20 km s^?1. Further, in Hα red and blue wings, we see mass motion along different loops in opposite directions. This is the first report of a counter-streaming pattern of mass motion in an Hα loop system. (2) Transient brightenings of different UV loops at different times were observed at C?iv 1550 Å. These brightened UV loops were located in the same region and at the same altitudes as the Hα loops. The observations show a clear correlation between the transient brightenings of UV loops and mass motion in Hα loops. (3) Both footpoints of the loop system were located in regions of mixed magnetic polarities. Frequent micro-flares at one footpoint of the loops with small-scale brightenings spreading along the loop leg were observed before the brightening and rising of one C?iv loop. Similar to the case of a filament, the continuous mass motion along the loops seems important for maintaining the cool Hα loop system at coronal height. There may be an indication that the mass motion in cool Hα loops and the correlated transient brightening of the active region loops were due to the small-scale chromospheric magnetic reconnection at the footpoint regions of the loop system.

     

Eruptive prominences and coronal transients

The observed interrelationship between coronal transients and eruptive prominences is used as the basis for a theoretical MHD model of these events. The model begins with an equilibrium configuration consisting of a coronal loop or arcade with a filament lying underneath with its axis oriented perpendicular to the overlying field. The lifting of the filament from the solar surface produces an increase in magnetic pressure under the helmet which drives it outward. This increased pressure is associated with the internal field of the filament as well as the field beneath it. The underlying field could be that which produced the filament eruption or, alternatively, reconnected field lines formed by the inward collapse of the legs of the transient towards the neutral line beneath the rising prominence. We do not attempt to explain the filament eruption which may be due to internal forces in the prominence or, alternatively, from forces imposed from beneath as would be produced by emerging flux. In the latter case, the filament is passive and merely acts as a tracer for the more fundamental underlying process.It is shown that the outward force per unit mass produced by the driving magnetic field and the inward restoring forces in the overlying field due to magnetic tension and gravity all decrease with distance at the same rate - namely, as the inverse square of the distance from the solar center. Hence, the ratio of net outward to inward force is independent of radial distance from the Sun. A stability analysis shows that this situation is one of neutral stability.A mathematical model of this physical process is described in which the MHD equations in simplified form, neglecting gas pressure forces, are solved in time for the velocity, width, density, and magnetic field strength of the transient. The solutions show that the velocity increases sharply close to the Sun but quickly approaches a constant value. The width increases linearly with radial distance. Both of these results are in agreement with observations. An examination of the forces exerted on the legs of the transient shows that their motion should be horizontally inward.On leave from the High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colo., U.S.A.     

Type-II Bursts in Meter and Deca – Hectometer Wavelengths and Their Relation to Flares and CMEs: II

Solar coronal holes (CHs) are large regions of the corona magnetically open to interplanetary space. The nearly rigid north?–?south CH boundaries (CHBs) of equatorward extensions of polar CHs are maintained while the underlying photospheric fields rotate differentially, so interchange magnetic reconnection is presumed to be occurring continually at the CHBs. The time and size scales of the required reconnection events at CHBs have not been established from previous observations with soft X-ray images. We use TRACE 195 Å observations on 9 December 2000 of a long-lived equatorial extension of the negative-polarity north polar CH to look for changes of ??5 arcsec to >?20 arcsec at the western CHB. Brightenings and dimmings are observed on both short (≈?5 minutes) and long (≈?7 hours) time scales, but the CHB maintains its quasi-rigid location. The transient CHB changes do not appear associated with either magnetic field enhancements or the changes in those field enhancements observed in magnetograms from the Michelson Doppler Imager (MDI) on SOHO. In seven hours of TRACE observations we find no examples of the energetic jets similar to those observed to occur in magnetic reconnection in polar plumes. The lack of dramatic changes in the diffuse CHB implies that gradual magnetic reconnection occurs high in the corona with large (??10°) loops and/or weak coronal fields. We compare our results with recent observations of active regions at CHBs. We also discuss how the magnetic polarity symmetry surrounding quasi-rigid CHs implies an asymmetry in the interchange reconnection process and a possible asymmetry in the solar wind composition from the eastern and western CHB source regions.     

The Relationship of Green-Line Transients to White-Light Coronal Mass Ejections

We report observations by the Large Angle Spectrometric Coronagraph (LASCO) on the SOHO spacecraft of three coronal green-line transients that could be clearly associated with coronal mass ejections (CMEs) detected in Thomson-scattered white light. Two of these events, with speeds >25 km s^-1, may be classified as ‘whip-like’ transients. They are associated with the core of the white-light CMEs, identified with erupting prominence material, rather than with the leading edge of the CMEs. The third green-line transient has a markedly different appearance and is more gradual than the other two, with a projected outward speed <10 km s^-1. This event corresponds to the leading edge of a ‘streamer blowout’ type of CME. A dark void is left behind in the emission-line corona following each of the fast eruptions. Both fast emission-line transients start off as a loop structure rising up from close to the solar surface. We suggest that the driving mechanism for these events may be the emergence of new bipolar magnetic regions on the surface of the Sun, which destabilize the ambient corona and cause an eruption. The possible relationship of these events to recent X-ray observations of CMEs is briefly discussed. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1004981125702