Flaring arches

We discuss first the development of the coronal arch-shaped structure of 57000 km length which was born at or before 08:00 UT on 6 November, 1980 and became the site of 13 quasi-periodic brightenings in hard X-rays from 10:00 to 14:30 UT. The same structure became the site of a series of 17 flaring arches between 15:30 and 24:00 UT on that day. The periodicity of 19 min, defined well for the quasi-periodic variations, seems to be partly retained during the occurrence of the flaring arches.The flaring arch studied in Paper I (called SB arch) was the brightest event of this set of events. This paper presents its extended analysis and also an analysis of three other flaring arches that occurred in this configuration. All these events exhibit similar characteristics and thus demonstrate that the flaring arch is a distinct solar phenomenon with specific characteristic properties.A comparison of H, Ov, and X-ray data for the SB arch essentially confirmed, in a quantitative way, the qualitative interpretation of the flow of emitting plasma through the arch proposed in Paper I. In particular, these data show: (1) a hot conduction front producing X-rays in the least dense plasma ahead, a decelerating more dense plasma bulk seen next in Ov, and still more decelerating very dense plasma eventually visible in emission in H; (2) a gradient of densities from the primary towards the secondary footpoint, by factor 3 in X-rays, one order of magnitude in Ov, and probably more in the densest loops emitting in H; (3) the secondary footpoint with hard X-ray spectrum, predominantly excited by particle streams.Member of the Carrera del Investigador, CONICET, Argentina.     

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.     

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.     

Rates of Flaring in Individual Active Regions

Rates of flaring in individual active regions on the Sun during the period 1981–1999 are examined using United States Air Force/Mount Wilson (USAF/MWL) active-region observations together with the Geostationary Operational Environmental Satellite (GOES) soft X-ray flare catalog. Of the flares in the catalog above C1 class, 61.5% are identified with an active region. Evidence is presented for obscuration, i.e. that the increase in soft X-ray flux during a large flare decreases the likelihood of detection of soft X-ray events immediately following the large flare. This effect means that many events are missing from the GOES catalog. It is estimated that in the absence of obscuration the number of flares above C1 class would be higher by (75±23)%. A second observational selection effect – an increased tendency for larger flares to be identified with an active region – is also identified. The distributions of numbers of flares produced by individual active regions and of mean flaring rate among active regions are shown to be approximately exponential, although there are excess numbers of active regions with low flare numbers and low flaring rates. A Bayesian procedure is used to analyze the time history of the flaring rate in the individual active regions. A substantial number of active regions appear to exhibit variation in flaring rate during their transit of the solar disk. Examples are shown of regions with and without rate variation, illustrating the different distributions of times between events (waiting-time distributions) that are observed. A piecewise constant Poisson process is found to provide a good model for the observed waiting-time distributions. Finally, applications of analysis of the rate of flaring to understanding the flare mechanism and to flare prediction are discussed.     

Fermi-LAT View of Bright Flaring Gamma-Ray Blazars

The Fermi LAT provides a continuous and uniform monitoring of the Universe in the gamma-ray band. During the first year many gamma-ray blazar flares, some unidentified transients and emission by the Sun while in a quiet state were promptly detected. This is mainly due to the design of the mission, featuring a detector, the LAT with a wide field of view, and to the operation of the spacecraft itself, that can cover every region of the sky every 3 hours. Nevertheless, the scientific exploitation of this monitoring is more fruitful when early information about transients reaches a broader community. In this respect, the indefatigable activity of flare advocates, who worked on weekly shifts to validate the results and quickly broadcast information about flares and new detections, was the key to most scientific results.     

Properties of coronal arches

The properties of coronal arches located on the peripheries of active regions, observed during a sounding rocket flight on March 8, 1973, are discussed. The arches are found to overlie filament channels and their footpoints are traced to locations on the perimeters of supergranulation cells. The arches have a wide range of lengths although their widths are well approximated by the value 2.2 × 10⁹ cm. Comparison of the size of the chromospheric footprint with the arch width indicates that arches do not always expand as they ascend into the corona. The electron temperatures and densities of the plasma contained in the arches were measured and the pressure calculated; typical values are 2 × 10⁶ K, 1 × 10⁹ cm^–3, and 2 × 10^–1 dyne cm^–2, respectively. The variation of these parameters with position along the length of the arch indicates that the arches are not in hydrostatic equilibrium.     

Characteristics of Flaring Sites in Solar Active Regions

The properties of the flaring sites in the active region NOAA 6555 and 6659 are presented in this paper. Although, several locations of these active regions displayed high magnetic shear, the X-class flares occurred only at one of them. Our investigations show that these locations are characterized by the `crossing' of magnetic and velocity field neutral lines. Based on a new parameter to define the stressed magnetic fields, we show that some of these flaring locations possessed highly stressed magnetic field which disappeared after the flare. This revised version was published online in July 2006 with corrections to the Cover Date.     

Slow-shock heating in post-flare arches

The heating of a coronal arch, following the occurrence of a dynamic (two-ribbon) flare, is discussed. We investigate whether slow-shock heating, occurring during the reconnection process in the dynamic flare and responsible for the heating of the post-flare loops, is also a workable proposition for the heating of a coronal arch. Contrary to the flare loops, the shock structure in the arch is generally not modified greatly by thermal conduction effects. As a result slow-shock heating may be investigated in terms of the familiar MHD shock jump relations. The observed enhanced arch density with respect to the surrounding corona is explained as a direct consequence of the reconnection process. For a combination of high arch temperatures and low values of coronal magnetic field and density thermal conduction may become important and will lead to an extra density enhancement in the arch. Our interpretation of the arch of 21–22 May, 1980 suggests that the formation of the arch took approximately one hour, and that observed temperature, density and maximum energy content can be consistently explained by the slow-shock heating mechanism.Currently: Center for Astrophysics and Space Sciences, C-011, University of California at San Diego, La Jolla, CA 92093, U.S.A.     

Ca II K emission arches

Arch-like features are often seen in spectrograms of very strong lines near the solar limb when the slit crosses the chromospheric network. We show how earlier kinetic-equilibrium (non-LTE) calculations for Ca ii can be used to predict such features for the K line with two-component atmospheric models.Publication of the Goethe Link Observatory, Indiana University, No. 144Now on leave at the High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado. (NCAR is sponsored by the National Science Foundation.)     

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.     

Magnetography of Solar Flaring Loops with Microwave Imaging Spectropolarimetry

We have developed a general framework for modeling gyrosynchrotron and free–free emission from solar flaring loops and used it to test the premise that 2D maps of source parameters, particularly the magnetic field, can be deduced from spatially resolved microwave spectropolarimetry data. We show quantitative results for a flaring loop with a realistic magnetic geometry, derived from a magnetic-field extrapolation, and containing an electron distribution with typical thermal and nonthermal parameters, after folding through the instrumental profile of a realistic interferometric array. We compare the parameters generated from forward-fitting a homogeneous source model to each line of sight through the folded image data cube both with the original parameters used in the model and with parameters generated from forward-fitting a homogeneous source model to the original (unfolded) image data cube. We find excellent agreement in general, but with systematic effects that can be understood as due to the finite resolution in the folded images and the variation of parameters along the line of sight, which are ignored in the homogeneous source model. We discuss the use of such 2D parameter maps within a larger framework of 3D modeling, and the prospects for applying these methods to data from a new generation of multifrequency radio arrays now or soon to be available.     

Equatorial coronal arches and geomagnetic disturbance

Central-meridian passage of coronal equatorial arches is followed, statistically, after about 7 days by geomagnetic disturbance. Sector-boundary crossings at Earth are most likely to occur 5 days after arch CMP, while metric noise sources cluster at the location of the arches. This agrees with the location of the noise sources relative to sector boundaries described by Sakurai and Stone (1971), rather than with that found by Martres et al. (1970) for 1964 and 1966, and may indicate evolution of the relation between coronal and interplanetary structure. We looked for, but did not find, clustering of coronal arches at a particular phase of the Olsen rotation scheme.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Types of Microwave Quasi-Periodic Pulsations in Single Flaring Loops

Quasi-periodic pulsations (QPP) of microwave emission generated in single flaring loops observed with the Nobeyama Radioheliograph (NoRH) and Nobeyama Radio Polarimeters (NoRP) are studied. Specific features of the time profiles, i.e. the visible presence or absence of QPPs, are not accounted for in the selection. The time evolution of the periods of the QPPs is examined using wavelet and correlation analyses. In ten out of twelve considered events, at least one or more significant spectral components with periods from 5 – 60 s have been found. The quality of the oscillations is rather low: Q=π N, where N is the number of cycles, mostly varies in the range 12 to 40, with an average of 25. We suggest that the detected QPPs can be classified into four types: i) those with stable mean periods (e.g. of 15 – 20 s or 8 – 9 s, the prevailing type); ii) those with spectral drift to shorter periods (mostly in the rise phase of the microwave emission); iii) those with drift to longer periods (mostly in the decay phase); iv)  those with multiple periods showing an X-shaped drift (e.g. in the range from 20 – 40 s in the rise phase).     

Ultraviolet Spectral Behavior of TVCol During and After Flaring Activity

We studied the intermediate polar TVCol during and after its flare in November 1982 observed in the ultraviolet range with the International Ultraviolet Explorer. Two spectra revealing the variations of emission lines at different times are presented. We have estimated a new value of the reddening from the 2200 Å absorption feature, E (B ? V ) = 0.12 ± 0.02, and calculated the line fluxes of C IV and He II emission lines produced in the outer accretion disk. The average ultraviolet luminosity of emitting region during and after the flare is approximately 4 × 10³² erg s^?1 and 9 × 10³⁰ erg s^?1, the corresponding average mass accretion rate is nearly 3 × 10¹⁵ erg s^?1 (4.76 × 10^?11M_⊙ yr^?1) and 5 × 10¹³ erg s^?1 (7.9³ × 10^?13M_⊙ yr^?1), and the average temperature of the emitting region during and after flare is estimated to be of about 3.5 × 10³ K and 2 × 10³ K. We attribute this flare to a sudden increase in the mass accretion rate leading to the outburst activity.     

Ultraviolet Spectral Behavior of UX Ari with Periodic Flaring Activity

Astrophysics - We present International Ultraviolet Explorer (IUE) observations of the luminous, non- eclipsing, double lined spectroscopic binary system UX Ari over the period 1978-1996. Three...     

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.     

Magnetic Relaxation and Particle Acceleration in a Flaring Twisted Coronal Loop

In the present work we aim to study particle acceleration in twisted coronal loops. For this purpose, an MHD model of magnetic reconnection in a linearly unstable twisted magnetic fluxtube is considered. Further, the electric and magnetic fields obtained in the MHD simulations are used to calculate proton and electron trajectories in the guiding-centre approximation. It is shown that particle acceleration in such a model is distributed rather uniformly along the coronal loop and the high-energy population remains generally neutral. It also follows from the model that the horizontal cross-section of the volume occupied by high-energy particles near the loop footpoints increases with time, which can be used as an observational proxy.