Search

The partial eruption of a filament channel with bifurcated substructures is investigated using data sets obtained from both ground-based and space-borne facilities. Small-scale flux reconnection/cancellation events in the region triggered the pile-up of ambient magnetic field, observed as bright extreme ultraviolet (EUV) loops in close proximity to the filament channel. This led to the formation of a V-shaped cusp structure at the site of interaction between the coalesced EUV loops and the filament channel, with the presence of distinct plasmoid structures and associated bidirectional flows. Analysis of imaging data from SDO /AIA further suggests vertical splitting of the filament structure into two substructures. The perturbed upper branch of the filament structure rose up and erupted with the onset of an energetic GOES M1.4 flare at 04:30 ut on 2015 January 28. The estimated twist number and squashing factor obtained from non-linear force free-field extrapolation of the magnetic field data support the vertical split in the filament structure with high twist in the upper substructure. The loss in equilibrium of the upper branch due to torus instability implies that this is a potential triggering mechanism for the observed partial eruption. [ABSTRACT FROM AUTHOR]

We present the multiwavelength observations of a flux rope that was trying to erupt from NOAA AR 11045 and the associated M-class solar flare on 12 February 2010 using space-based and ground-based observations from TRACE, STEREO, SOHO/MDI, Hinode/XRT, and BBSO. While the flux rope was rising from the active region, an M1.1/2F class flare was triggered near one of its footpoints. We suggest that the flare triggering was due to the reconnection of a rising flux rope with the surrounding low-lying magnetic loops. The flux rope reached a projected height of ≈0.15 R with a speed of ≈90 km s while the soft X-ray flux enhanced gradually during its rise. The flux rope was suppressed by an overlying field, and the filled plasma moved towards the negative polarity field to the west of its activation site. We found the first observational evidence of the initial suppression of a flux rope due to a remnant filament visible both at chromospheric and coronal temperatures that evolved a couple of days earlier at the same location in the active region. SOHO/MDI magnetograms show the emergence of a bipole ≈12 h prior to the flare initiation. The emerged negative polarity moved towards the flux rope activation site, and flare triggering near the photospheric polarity inversion line (PIL) took place. The motion of the negative polarity region towards the PIL helped in the build-up of magnetic energy at the flare and flux rope activation site. This study provides unique observational evidence of a rising flux rope that failed to erupt due to a remnant filament and overlying magnetic field, as well as associated triggering of an M-class flare. [ABSTRACT FROM AUTHOR]

We present a multiwavelength analysis of a long-duration, white-light solar flare (M8.9/3B) event that occurred on 04 June 2007 from AR NOAA 10960. The flare was observed by several spaceborne instruments, namely SOHO/MDI, Hinode/SOT, TRACE, and STEREO/SECCHI. The flare was initiated near a small, positive-polarity, satellite sunspot at the center of the active region, surrounded by opposite-polarity field regions. MDI images of the active region show a considerable amount of changes in the small positive-polarity sunspot of δ configuration during the flare event. SOT/G-band (4305 Å) images of the sunspot also suggest the rapid evolution of this positive-polarity sunspot with highly twisted penumbral filaments before the flare event, which were oriented in a counterclockwise direction. It shows the change in orientation, and also the remarkable disappearance of twisted penumbral filaments (≈35 – 40%) and enhancement in umbral area (≈45 – 50%) during the decay phase of the flare. TRACE and SECCHI observations reveal the successive activation of two helically-twisted structures associated with this sunspot, and the corresponding brightening in the chromosphere as observed by the time-sequence of SOT/Ca ii H line (3968 Å) images. The secondary, helically-twisted structure is found to be associated with the M8.9 flare event. The brightening starts six – seven minutes prior to the flare maximum with the appearance of a secondary, helically-twisted structure. The flare intensity maximizes as the secondary, helically-twisted structure moves away from the active region. This twisted flux tube, associated with the flare triggering, did not launch a CME. The location of the flare activity is found to coincide with the activation site of the helically-twisted structures. We conclude that the activation of successive helical twists (especially the second one) in the magnetic-flux tubes/ropes plays a crucial role in the energy build-up process and the triggering of the M-class solar flare without a coronal mass ejection (CME). [ABSTRACT FROM AUTHOR]

We present Hα observations from ARIES (Nainital) of a compact and impulsive solar flare that occurred on March 10, 2001 and which was associated with a CME. We have also analyzed HXT, SXT/ Yohkoh observations as well as radio observations from the Nobeyama Radio Observatory to derive the energetics and dynamics of this impulsive flare. We coalign the Hα, SXR, HXR, MW, and magnetogram images within the instrumental spatial-resolution limit. We detect a single HXR source in this flare, which is found spatially associated with one of the Hα bright kernels. The unusual feature of HXR and Hα sources, observed for the first time, is the rotation during the impulsive phase in a clockwise direction. We propose that the rotation may be due to asymmetric progress of the magnetic reconnection site or may be due to the change of the peak point of the electric field. In MW emission we found two sources. The main source is at the main flare site and another is in the southwest direction. It appears that the remote source is formed by the impact of accelerated energetic electrons from the main flare site. From the spatial correlation of multiwavelength images of the different sources, we conclude that this flare has a three-legged structure. [ABSTRACT FROM AUTHOR]

We present Hα CCD observations of three small-to-medium-size two-ribbon flares observed in the giant AR 9433 on 24 April 2001. Flare observations at other associated wavelengths (e.g., soft X-rays (SXR), hard X-rays (HXR), microwaves (MW)) obtained from archives are also presented and compared. We have tested the Neupert effect for the most energetic third flare. The flare observations are in agreement with the thick-target model. In the case of this flare the HXR emitting electrons appears to be the heating source of SXR and Hα emissions. The flares are also studied in EUV and UV emissions using TRACE data. We discuss the complexity of the magnetic field using SOHO/MDI magnetograms. The flares are observed to occur in both (f/p) polarity regions in highly sheared magnetic field with emerging flux regions and MMFs. [ABSTRACT FROM AUTHOR]

In this paper we present the analysis of M 1.8 class (GOES SXR) limb flare and associated post flare loops of 2 May, 2001 which erupted on the west limb in NOAA 9433 (N15, W88). The flare started at around 00:32 UT from a small mound of twisted loops. When we started Ha observations at 00:52 UT the flare was in decay phase enveloped by highly sheared/twisted post flare loops which we call as primary post flare loops. The unique feature of this flare was that a new system of loops were developed when the primary loops were almost decaying. We call these newly formed loops as secondary loops. The main aim of the paper is to study the dynamics of the Ha flare/post flare loops and to understand how the magnetic field gets relaxed from highly sheared/twisted stage to a simplified potential field. We have discussed the nature of temporal behavior of the temperature and emission measure of the flare. The post flare condensation and formation of secondary loops has also been studied in this paper. The flare was also associated with SXR, HXR and MW emissions. [ABSTRACT FROM AUTHOR]

We present a multi-wavelength study of recurrent surges observed in Hα, UV (Solar and Heliospheric Observatory (SOHO)/EIT), and Radio (Learmonth, Australia) from the super-active region NOAA 10484 on 2003 October 25. Several bright structures visible in Hα and UV corresponding to subflares are also observed at the base of each surge. Type III bursts are triggered and RHESSI X-ray sources are evident with surge activity. The major surge consists of bunches of ejective paths forming a fan-shaped region with an angular size of (≈65°) during its maximum phase. The ejection speed reaches up to ∼200 km s–1. The SOHO/Michelson Doppler Imager magnetograms reveal that a large dipole emerges from the east side of the active region on 2003 October 18-20, a few days before the surges. On 2003 October 25, the major sunspots were surrounded by “moat regions” with moving magnetic features (MMFs). Parasitic fragmented positive polarities were pushed by the ambient dispersion motion of the MMFs and annihilated with negative polarities at the borders of the moat region of the following spot to produce flares and surges. A topology analysis of the global Sun using Potential Field Source Surface shows that the fan structures visible in the EIT 171 Å images follow magnetic field lines connecting the present active region to a preceding active region in the southeast. Radio observations of Type III bursts indicate that they are coincident with the surges, suggesting that magnetic reconnection is the driver mechanism. The magnetic energy released by the reconnection is transformed into plasma heating and provides the kinetic energy for the ejections. A lack of a radio signature in the high corona suggests that the surges are confined to follow the closed field lines in the fans. We conclude that these cool surges may have some local heating effects in the closed loops, but probably play a minor role in global coronal heating and the surge material does not escape to the solar wind. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents statistical analysis of Soft X-ray (SXR) flares during the period January 1976 to December 2007 covering solar cycles (SCs) 21, 22 and 23. We have analysed north–south (N–S) and east–west (E–W) asymmetry of SXR flares at total (1–90°), low (1–40°) and high (50–90°) latitudes and center meridian distances (CMDs). We have also presented the N–S and E–W asymmetry of different intensity classes (B, C, M and X) during the period of investigation. A slight southern and eastern excess is found after analysis during SCs 21, 22 and 23. N–S distribution shows that the SXR flare events are most prolific in the 11–20° latitude band in the northern and southern hemispheres whereas E–W distribution does not show any prolific band. We found that the N–S and E–W asymmetry which often peaks near the activity minimum is in agreement with the theoretical dynamo models. Our results show that N–S asymmetry is statistically more significant than E–W asymmetry. It is revealed that the SXR flare activity (M and C class flares) during SC 23 is low as compared to the SC 22, indicating the violation of Gnevyshev–Ohl rule. The B class flare activity is higher for SC 23 whereas C, M and X class activities are higher for SC 21. We have also analysed the flare evolution parameters, i.e. duration, rise time, decay time and event asymmetry for total SXR as well as for different classes during last three SCs. The duration, rise time and decay time increase with increasing intensity class. The increase is more pronounced for the duration and decay time than for the rise time for SCs 21 and 22 while for SC 23 it is the same for duration, rise time and decay time. On analyzing event asymmetry indices, we found more positive values during SC 21 and SC 22 , but for SC 23 we have more negative values . Our study shows that during SC 23 we have more SXR flare events having shorter decay time than the rise time as compared to SCs 21 and 22. [Copyright &y& Elsevier]