Limb Flare Research Articles

Impulsive phase He 10830 spectra of a large solar limb flare of 16 August 1989

We obtained simultaneously He i 10830 A spectra, Hα filtergrams and microwave data of a large limb flare (2N/X20) in 1989. In this paper we characterize He i 10830 spectra in relation to the impulsive phase. All the He i 10830 spectra, except those of the surge, show blue shift or blue asymmetry. The velocities inferred from the spectra range from a few to 160 km s−1, implying that the horizontal motion is very likely present in the structure of this flare at different heights. The He i 10830 profiles of a flare are relatively broad and cannot be simulated by the Doppler broadening mechanism with a uniform flare model atmosphere. It is most likely that these characteristics are related to rapid and localized heating in the low and middle chromosphere. Comparing the SXR and microwave data with the optical data leads to the following scenario: the corona was already heated to some extent before the flare onset, and in the first 2 minutes of the impulsive phase, heat conduction was the main source or, at least, a competitive source, for chromospheric heating. However, the impulsive event, associated with the unusually broadened He i 10830 line (Δλf>20 A) and temporally correlated with a microwave burst, is probably caused by electron-beam heating.

Ultraviolet Events Observed in Active Regions. II. An Interpretation of Flaring Arches and Associated Small Flares

We analyze Hα, UV, and X-ray emissions in and around the spectacular arch system seen in the corona on 1980 March 27 during the Solar Maximum Mission. The flaring of the arch plasma is studied, and its dependence on triggering mechanisms related to the observed small limb flare in the arch footpoint is analyzed. To drive these events, we propose a mechanism in which small electric current circuits and the localized magnetic free energy are continuously generated at a magnetic null by a pressure gradient, which then compress or expand the plasma. This free energy dissipates by Joule effect and upward transport.

Hot and Superhot Plasmas above an Impulsive Flare Loop

We report the discovery of a high-temperature source above the soft X-ray loop of the impulsive limb flare on 1992 January 13 with the Yohkoh Soft X-ray Telescope (SXT). The hot source coincides in position with the loop-top impulsive hard X-ray source and continues to sit above the soft X-ray flare loop throughout the flare. The single high-temperature source in the initial phase evolves to the two high-temperature (15-20 MK) ridge structures in the peak and decay phases, and the compact hard X-ray source appears to be located in between the high-temperature ridges. We assume that the loop-top hard X-ray source is of thermal origin (superhot source). The loop-top hard X-ray source has an effective temperature of 100-150 MK and a total emission measure of ~10-4 of the hot source. The geometrical relationship between the superhot and the hot sources indicates that (1) the hot source is heated by the slow shocks associated with magnetic reconnection and that (2) the superhot source is heated with the fast bow shock owing to the collision of the supersonic downward outflow with the reconnected flux tube. The small emission measure of the superhot source indicates a narrow outflow jet in between the slow shocks. These observations suggest that there is no essential difference between the compact-loop (impulsive) and long-duration event (LDE) flares. The time coincidence of the loop-top hard X-ray source with the footpoint sources indicates that nonthermal electron acceleration is causally related to the formation of the fast shock rather than the slow shocks.

Preliminary Results of Optical Solar Flare Spectra Studies Using a 40-Channel Densitometer

Multi-channel CCD receivers are available for solar spectral research in the Astronomical Observatory of Shevchenko University of Kiev. The observational material considered here are 24 echelle spectrograms of the flare of July 15, 1981 and certain other disk and limb flares. Some temporal aspects of flare development are discussed, as well as properties of metal lines and the structure of the flare region. Disk and limb flare photometry indicate the full spatial coincidence of the emission volumes for all emission lines. No signs of altitude stratification for the lines of H and K Call, hydrogen, and other metals are detected. Metal flare emission is revealed in the cores of Fraunhofer lines, providing that a compact emission source is present in the active region and that broad wings of H and K CaII and hydrogen have formed. No broad wings are observed for metal lines. Two types of lower-intensity strips in the photospheric emission spectrum are detected; the first is the result of the lower temperature in sunspots, while the second is due to the weakening of photospheric radiation after it passes through the flare volume. No new spectral lines are observed in the spectrum of this second type of strip, which may be associated with a condensed layer of relatively cold plasma at the chromospheric level. Four components are always spatially coincident in the spectrum: (1) a compact source of higher intensity with broadened hydrogen and H and K Call line wings; (2) higher intensity in metal lines, but without broadened wings; (3) emission or absorption in helium lines; (4) a lower-intensity strip of photospheric emission due to its weakening after it passes through the flare volume.

3-D MHD SIMULATION OF X-TYPE COALESCENCE OF TWO CURRENT-LOOPS

We present simulation results of X-type coalescence of two current-loops, by using a 3-D resistive MHD code. The results are compared with the limb flare observed by Yohkoh on 2 August, 1993 at 08:30 UT, which is a good example of partial X-type coalescence. It is shown that the maximum temperature enhancement near the cross-point of the two loops, obtained from the simulation, agrees well with the observations when the plasma β is 0.08.

DIFFICULTIES IN OBSERVING CORONAL STRUCTURE

There has developed in recent years a substantial body of evidence to indicate that the temperature and density structure of the corona are far more complicated than had previously been thought. We review some of the evidence and discuss some specific examples: observations of a limb flare, showing that the cool Hα material is cospatial with the hot X-ray emitting material; simultaneous NIXT and Yohkoh SXT observations of an active region, showing that loops seen in one instrument are not seen in the other, and that the effect works in both directions; comparisons of extrapolated magnetic field measurements to the observed coronal structure, indicating that neither potential nor constant-α force-free fits are adequate. We conclude with a description of two new instruments, the TRACE and the TXI, which will help to resolve some of these difficulties.

On the formations connected with the limb flare of November 6, 1979

We analyzed the observation data obtained at Abastumani astrophysical observatory in the form of spectrograms and filtergrams of formations associated with the November 6, 1979 limb flare. We studied the system of flare loops, the late stage of the bright flare loop development, and postflare coronal loops obtained inHα and coronal lines. The physical parameters of the structures observed as well as their dimensions and mutual locations are determined. The Doppler velocities at different altitudes are measured and the prevailing upward direction of the matter motion in the flare loops is revealed.

Hard and Soft X-Ray Observations of Solar Limb Flares

view Abstract Citations (31) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hard and Soft X-Ray Observations of Solar Limb Flares Mariska, John T. ; Sakao, T. ; Bentley, R. D. Abstract Using observations from the Yohkoh Bragg Crystal Spectrometer, hard X-ray telescope, and soft X-ray telescope, we have examined eight limb flares. Four of the flares have the footpoints occulted by the solar limb. We find that the occulted flares generally have softer hard X-ray spectra and smaller peak values of the nonthermal broadening velocity than nonocculted flares. All other physical parameters show no differences between occulted flares and nonocculted flares. The hard X-ray spectra support a model in which the footpoint emission is due to thick-target sources, while the looptop emission is due to thin-target sources. High spectral resolution hard X-ray observations should thus show a break in the hard X-ray spectrum of the looptop source. We can find no obvious explanation for the differences in nonthermal broadening velocity. Publication: The Astrophysical Journal Pub Date: March 1996 DOI: 10.1086/176945 Bibcode: 1996ApJ...459..815M Keywords: SUN: FLARES; SUN: X-RAYS; GAMMA RAYS full text sources ADS |

Eruptive Prominence Associated with Limb Flare of 25 January 1991

AbstractWe have observed an eruptive prominence at the east solar limb on 25 January 1991 which started earlier than 0623 UT and was associated with a limb flare (S16 E90) of class 1B/ X10.0. We have recorded a huge mass ejection in the corona by the limb flare associated eruptive prominence. The eruptive prominence ejected a part of the loop in the corona with maximum speed of about 1280 km/sec. The ejected material attain height upto 150,000 km in the corona and finally faded/disappeared in the corona. During the ascending phase of the prominence material in the corona there was a unscrewing of the loop system associated with the eruptive prominence. The type II, III, and IV radio bursts were also reported by a number of Radio Observatories during observation of the eruptive prominence. The high flux of sudden ionospheric disturbances and the solar radio emissions on fixed frequencies (245-80000 MHz) were also recorded. The eruptive prominence associated with limb flare also shows increased proton flux (>10 MeV) during its occurence. The flare was classified as X10.0 flare. In this paper we have analysed the observed data and compared it with the theoretical model of the solar flare.

Eruptive prominence associated with limb flare of 25 January 1991

We have observed an eruptive prominence at the east solar limb on 25 January 1991 which started earlier than 0623 UT and was associated with a limb flare (S16 E90) of class 1B/ X10.0. We have recorded a huge mass ejection in the corona by the limb flare associated eruptive prominence. The eruptive prominence ejected a part of the loop in the corona with maximum speed of about 1280 km/sec. The ejected material attain height upto 150,000 km in the corona and finally faded/disappeared in the corona. During the ascending phase of the prominence material in the corona there was a unscrewing of the loop system associated with the eruptive prominence. The type II, III, and IV radio bursts were also reported by a number of Radio Observatories during observation of the eruptive prominence. The high flux of sudden ionospheric disturbances and the solar radio emissions on fixed frequencies (245–80000 MHz) were also recorded. The eruptive prominence associated with limb flare also shows increased proton flux (>10 MeV) during its occurence. The flare was classified as X10.0 flare. In this paper we have analysed the observed data and compared it with the theoretical model of the solar flare.

Morphology and Physical Parameters for Two Long-Duration Solar Flares: Observations from YOHKOH

view Abstract Citations (51) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Morphology and Physical Parameters for Two Long-Duration Solar Flares: Observations from YOHKOH Feldman, U. ; Seely, J. F. ; Doschek, G. A. ; Brown, C. M. ; Phillips, K. J. H. ; Lang, J. Abstract We have analyzed the Yohkoh soft X-ray telescope images and Bragg crystal spectrometer spectra of two long-duration X-class events (LDEs), obtained from instruments on the Japanese Yohkoh spacecraft. The two events are a limb flare that occurred on 1992 November 2 near 03 UT and a disk flare that occurred on 1992 February 27 near 10 UT. The spatial resolution of the images is about 2"5, and the time resolution is 2 s. The emission originates from plasma at temperatures ≥ 107 K. The morphology of the LDEs is quite complicated, but the most intense emission consists of a small number of loops (usually one or two prominent loops). The brightest emission regions are located at the tops of loops for most of the flares' duration, as was found in earlier studies of compact flares observed by Yohkoh In the case of the 1992 November 2 limb flare, the brightening at the loop top is visible for a period of 24 hr after flare onset. This implies that the heating mechanism must act over a period of tens of hours for these long-duration events, that the energy is deposited at the top of the loop, and that the hot plasma is confined at the top of the loop. We derive emission measures and temperatures from the soft X-ray telescope and Bragg crystal spectrometer data. Lower limit estimates of electron density of the bright regions at the loop tops are derived from the morphology of the regions and the emission measures. Fractional ion abundances for highly ionized iron are deduced from the spectrometer data. We relate the Yohkoh observations to previous observations from the solar instruments on the Skylab manned space station. Publication: The Astrophysical Journal Pub Date: June 1995 DOI: 10.1086/175844 Bibcode: 1995ApJ...446..860F Keywords: SUN: FLARES; SUN: X-RAYS; GAMMA RAYS full text sources ADS |

Interpreting Yohkoh hard and soft X-ray flare observations

A simple model is presented to account for theYohkoh flare observations of Feldmanet al. (1994), and Masuda (1994). Electrons accelerated by the flare are assumed to encounter the dense, small regions observed by Feldmanet al. at the tops of impulsively flaring coronal magnetic loops. The values of electron density and volume inferred by Feldmanet al. imply that these dense regions present an intermediate thick-thin target to the energised electrons. Specifically, they present a thick (thin) target to electrons with energy much less (greater) thanE c , where 15 keV <E c < 40 keV. The electrons are either stopped at the loop top or precipitate down the field lines of the loop to the footpoints. Collisional losses of the electrons at the loop top produce the heating observed by Feldmanet al. and also some hard X-rays. It is argued that this is the mechanism for the loop-top hard X-ray sources observed in limb flares by Masuda. Adopting a simple model for the energy losses of electrons traversing the dense region and the ambient loop plasma, hard X-ray spectra are derived for the loop-top source, the footpoint sources and the region between the loop top and footpoints. These spectra are compared with the observations of Masuda. The model spectra are found to qualitatively agree with the data, and in particular account for the observed steepening of the loop-top and footpoint spectra between 14 and 53 keV and the relative brightnesses of the loop-top and footpoint sources.

Ongoing partial reconnection in a limb flare

The limb flare on August 2, 1993 08:30 UT, consisting of two crossed loops, shows evidence of partial X-type reconnection. The observations suggest that partial reconnection was an ongoing process during about 10 min until flare ignition.

The microwave and H-alpha sources of the 1992 January 13 flare

We compare X-ray, microwave and H-alpha observations for the 1992 January 13 limb flare. The soft and hard X-ray images of the flare have been studied thoroughly by Masuda et al. (1994) with Yohkoh SXT and HXT images. We find that during the hard X-ray emission peak there is no H-alpha brightening on the disk nor at the limb, so the main ribbons of this flare must be beyond the limb. The microwave source maintains a fixed distance about 10 arcsecs from the optical limb in the frequency range 2.8-14.0 GHz. We interpret this limit in source position as due to the presence of a microwave limb that extends higher than the white-light limb -- to a height of 7300 +/- 1500 km. We believe that the high-frequency microwave emissions are occulted by this extended limb, while the soft and hard X-ray emissions are able to pass through largely unaffected. We also believe, however, that the hard X-ray footpoints are also partially occulted by the photospheric limb, despite the appearance of 'footpoint sources' in HXT data shown by Masuda et al. The smooth X-ray and microwave time profiles, microwave-rich emission relative to hard X-rays, and progressive hard X-ray spectral hardening through the flare peak are all characteristics that we interpret as being a direct result of the occultation of footpoint emission.

A multisource limb flare observed at multiple radio wavelengths

A flare with several radio sources occurred on the solar limb at 2155 UT on 1989 June 20. It was observed by the Very Large Array (VLA) and the Owens Valley Radio Observatory (OVRO). The VLA data consisted of images at 1.4 GHz, while OVRO provided spectral and spatial information over the range 1-15 GHz. We develop a new gyrosynchrotron model to analyze the sources observed at flare peak. This model differs from many previous ones in that it contains spatial variations of both the magnetic field and accelerated particle density. It uses a new gyrosynchrotron approximation which is valid at very low harmonics of the gyrofrequency. For the first time we find that the cause of the change of microwave source size with frequency in the event studied was due primarily to the spatial variation of the accelerated electron density. This is contrary to the common assumption that the variation in size is due to a nonuniform magnetic field. We also investigate a polarized source which brightened later in the flare, finding that it could have been due to plasma radiation or gyrosynchrotron emission.

Miocrowave spectral imaging, H-alpha and hard X-ray observations of a solar limb flare

We compare the microwave, H-alpha, and hard X-ray observations for a west limb C7.3 flare that occurred at 17:10 UT, 1992 June 26. H-alpha movies were obtained at Big Bear Solar Observatory. Before the onset of the flare, overexposed H-alpha images show the complicated flux loop structure above the limb. Material was observed to descend along the loops toward the site where the flare occurred hours later. Using the five-antenna solar array at Owens Valley Radio Observatory, we obtain two-dimensional maps of flare emission from 1.4 to 14 GHz. In all three temporal peaks of the microwave bursts, the maps show the same characteristics. The peak low-frequency emission comes from the top of one bundle of the H-alpha loops and gradually shifts to the foot-point of the loops (the location of H-alpha flare) as the frequency increases. The location of the emission peak shifts 88 sec between 1 and 14 GHz. Seventy percent of the shift occurs between 1 and 5 GHz. The locus of the shift of the emission peak follows the shape of an H-alpha surge that occurred after the flare. For each point along the locus, we create the microwave brightness temperature spectrum and compare the radio-derived electron distribution with that derived from the high-resolution hard X-ray spectra measured with Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO). We find that the peak frequency changes from approximately 3 GHz at the loop top to approximately 7 GHz at the footprint, presumably due to the increase of the magnetic field from approximately 160 GHz at the loop top to approximately 300 G at the footpoint. The high-frequency slope of the microwave power-law spectrum decreases from approximately 10 at the loop top to approximately 5 at the footprint due to a change in the energy distribution of the dominant electrons. The microwave brightness temperature spectral index predicted by the BATSE power-law hard X-ray spectra agrees with the measured value only at the footpoint. At the loop top, the emission may be thermal gyrosynchrotron with a temperature of 3.5 x 10<SUP>7</SUP> K, which is likely to correspond to the superhot component seen in the hard X-ray emission.

Multiple wavelength observations of an off-limb eruptive solar flare

The eruptive prominence and limb flare which occurred at 1454 UT on June 20, 1989 is described and analyzed. This event was observed by many different instruments providing an unusual amount and variety of data: images at 1.4 GHz, 37 GHz, and H-alpha, and spectra in hard X-ray, soft X-ray, and radio frequencies. This array of data makes it possible to explore the relationships between flare and eruptive prominence emissions at different wavelengths. VLA images at 1.4 GHz show changing sources in a set of high (about 10 exp 10 cm) coronal loops associated with the erupting prominence. We use a full gyrosynchrotron code to model a 1.4 GHz source early in the flare as a large coronal loop. The model results lead us to conclude that the initial acceleration occurs in smaller, denser loops which also produce the flare's hard X-ray emission. We also present evidence that a source at 1.4 GHz later in the event is due to second-harmonic plasma emission. This source is adjacent to a leg of the prominence and comes from a dense column of material in the magnetic structure supporting the prominence.

Solar Flares as an Ongoing Magnetic Reconnection Process

AbstractThe soft X-ray images taken by theYohkohSoft X-ray Telescope (SXT) provide a powerful new tool to solve the mechanism of solar flares. In particular, a limb flare that occurred on 1991, December 2 gives us convincing evidence that magnetic reconnection of a neutral sheet formed at the loop top participates in the flare energy release. The neutral sheet appears to be associated with a destabilized rising loop system (filament) sheared with respect to the flaring loop. Similar formations of X-ray arcades are seen in the quiet Sun associated with filament eruptions on much larger spatial dimension and with a much smaller energy scale.

Solar Coronal Magnetic Fields

A short compilation of various radio methods of the determination of magnetic fields in the solar corona is given which, completed by observations in other spectral ranges (e.g. the optical and X-ray ranges), results in a complex picture of the magnetic field. Some topics of interest are the following: (1)Comparison with a standard reference magnetic field in the solar corona,(2)Possible evidence of substantial small-scale fluctuations of the magnetic field (e.g. in active regions),(3)Indication of magnetic fields substantially in excess of the standard distribution (e.g. in limb flare events).

High-temperature flares observed in broadband soft X-rays

Broadband soft solar X-rays monitored by the GOES satellites have been used to detect high-temperature flares (> 25 MK). The data suggest that there are two general categories of high-temperature flares: those that are intrinsically hot and recur repeatedly in particular active regions and those that show enhanced temperatures because of their proximity to the solar limb. Intrinsically hot flares associate with gamma-ray flares and impulsive hard X-ray flares. Hot flares show a small incidence with gradual hard X-ray flares, but those cases are either extremely intense flares or limb flares. The apparently hot flares occur near the visible limb, which suggests the strong thermal stratification of flare plasmas as demonstrated by over-the-limb events; even on the visible disk near the limb, the lower, cooler plasmas are somehow partially occulted.