PRE-FLARE ACTIVITY AND MAGNETIC RECONNECTION DURING THE EVOLUTIONARY STAGES OF ENERGY RELEASE IN A SOLAR ERUPTIVE FLARE

Joshi, Bhuwan,Veronig, Astrid M.,Lee, Jeongwoo,Bong, Su-Chan,Tiwari, Sanjiv Kumar,Cho, Kyung-Suk IOP Publishing 2011 The Astrophysical journal Vol.743 No.2

<P>In this paper, we present a multi-wavelength analysis of an eruptive white-light M3.2 flare that occurred in active region NOAA 10486 on 2003 November 1. The excellent set of high-resolution observations made by RHESSI and the TRACE provides clear evidence of significant pre-flare activities for similar to 9 minutes in the form of an initiation phase observed at EUV/UV wavelengths followed by an X-ray precursor phase. During the initiation phase, we observed localized brightenings in the highly sheared core region close to the filament and interactions among short EUV loops overlying the filament, which led to the opening of magnetic field lines. The X-ray precursor phase is manifested in RHESSI measurements below similar to 30 keV and coincided with the beginning of flux emergence at the flaring location along with early signatures of the eruption. The RHESSI observations reveal that both plasma heating and electron acceleration occurred during the precursor phase. The main flare is consistent with the standard flare model. However, after the impulsive phase, an intense hard X-ray (HXR) looptop source was observed without significant footpoint emission. More intriguingly, for a brief period, the looptop source exhibited strong HXR emission with energies up to similar to 50-100 keV and significant non-thermal characteristics. The present study indicates a causal relation between the activities in the pre-flare and the main flare. We also conclude that pre-flare activities, occurring in the form of subtle magnetic reorganization along with localized magnetic reconnection, played a crucial role in destabilizing the active region filament, leading to a solar eruptive flare and associated large-scale phenomena.</P>

MAGNETIC RECONNECTION DURING THE TWO-PHASE EVOLUTION OF A SOLAR ERUPTIVE FLARE

Joshi, Bhuwan,Veronig, Astrid,Cho, K.-S.,Bong, S.-C.,Somov, B. V.,Moon, Y.-J.,Lee, Jeongwoo,Manoharan, P. K.,Kim, Y.-H. IOP Publishing 2009 The Astrophysical journal Vol.706 No.2

<P>We present a detailed multi-wavelength analysis and interpretation of the evolution of an M7.6 flare that occurred near the southeast limb on 2003 October 24. Pre-flare images at TRACE 195 angstrom show that the bright and complex system of coronal loops already existed at the flaring site. The X-ray observations of the flare taken from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft reveal two phases of the flare evolution. The first phase is characterized by the altitude decrease of the X-ray looptop (LT) source for similar to 11 minutes. Such a long duration of the descending LT source motion is reported for the first time. The EUV loops, located below the X-ray LT source, also undergo contraction with similar speed (similar to 15 km s(-1)) in this interval. During the second phase the two distinct hard X-ray footpoint (FP) sources are observed which correlate well with UV and Ha flare ribbons. The X-ray LT source now exhibits upward motion as anticipated from the standard flare model. The RHESSI spectra during the first phase are soft and indicative of hot thermal emission from flaring loops with temperatures T > 25 MK at the early stage. On the other hand, the spectra at high energies (epsilon greater than or similar to 25 keV) follow hard power laws during the second phase (gamma = 2.6-2.8). We show that the observed motion of the LT and FP sources can be understood as a consequence of three-dimensional magnetic reconnection at a separator in the corona. During the first phase of the flare, the reconnection releases an excess of magnetic energy related to the magnetic tensions generated before a flare by the shear flows in the photosphere. The relaxation of the associated magnetic shear in the corona by the reconnection process explains the descending motion of the LT source. During the second phase, the ordinary reconnection process dominates describing the energy release in terms of the standard model of large eruptive flares with increasing FP separation and upward motion of the LT source.</P>

FORMATION AND ERUPTION OF A FLUX ROPE FROM THE SIGMOID ACTIVE REGION NOAA 11719 AND ASSOCIATED M6.5 FLARE: A MULTI-WAVELENGTH STUDY

Joshi, Bhuwan,Kushwaha, Upendra,Veronig, Astrid M.,Dhara, Sajal Kumar,Shanmugaraju, A.,Moon, Yong-Jae American Astronomical Society 2017 The Astrophysical journal Vol.834 No.1

<P>We investigate the formation, activation, and eruption of a flux rope (FR) from the sigmoid active region NOAA 11719 by analyzing E(UV), X-ray, and radio measurements. During the pre-eruption period of similar to 7 hr, the AIA 94 angstrom. images reveal the emergence of a coronal sigmoid through the interaction between two J-shaped bundles of loops, which proceeds with multiple episodes of coronal loop brightenings and significant variations in the magnetic flux through the photosphere. These observations imply that repetitive magnetic reconnections likely play a key role in the formation of the sigmoidal FR in the corona and also contribute toward sustaining the temperature of the FR higher than that of the ambient coronal structures. Notably, the formation of the sigmoid is associated with the fast morphological evolution of an S-shaped filament channel in the chromosphere. The sigmoid activates toward eruption with the ascent of a large FR in the corona, which is preceded by the decrease in photospheric magnetic flux through the core flaring region, suggesting tether-cutting reconnection as a possible triggering mechanism. The FR eruption results in a two-ribbon M6.5 flare with a prolonged rise phase of similar to 21 minutes. The flare exhibits significant deviation from the standard flare model in the early rise phase, during which a pair of J-shaped flare ribbons form and apparently exhibit converging motions parallel to the polarity inversion line, which is further confirmed by the motions of hard X-ray footpoint sources. In the later stages, the flare follows the standard flare model and the source region undergoes a complete sigmoid-to-arcade transformation.</P>

Multi-wavelength Analysis of Two Near-limb Solar Flares Associated with Coronal Mass Ejection

B. Joshi,A. Veronig,K.-S. Cho,S.-C. Bong,Y.-J. Moon,P. K. Manoharan 한국천문학회 2007 天文學會報 Vol.32 No.2

LARGE-SCALE CONTRACTION AND SUBSEQUENT DISRUPTION OF CORONAL LOOPS DURING VARIOUS PHASES OF THE M6.2 FLARE ASSOCIATED WITH THE CONFINED FLUX ROPE ERUPTION

Kushwaha, Upendra,Joshi, Bhuwan,Veronig, Astrid M.,Moon, Yong-Jae IOP Publishing 2015 The Astrophysical journal Vol.807 No.1

<P>We investigate evolutionary phases of an M6.2 flare and the associated confined eruption of a prominence. The pre-flare phase exhibits spectacular large-scale contraction of overlying extreme ultraviolet (EUV) coronal loops during which the loop system was subjected to an altitude decrease of similar to 20Mm (40% of the initial height) for an extended span of similar to 30 minutes. This contraction phase is accompanied by sequential EUV brightenings associated with hard X-ray (HXR; up to 25 keV) and microwave (MW) sources from low-lying loops in the core region which together with X-ray spectra indicate strong localized heating in the source region before the filament activation. With the onset of the flare's impulsive phase, we detect HXR and MW sources that exhibit intricate temporal and spatial evolution in relation to the fast rise of the prominence. Following the flare maximum, the filament eruption slowed down and subsequently became confined within the large overlying active region loops. During the confinement process of the erupting prominence, we detect MW emission from the extended coronal region with multiple emission centroids, which likely represent emission from hot blobs of plasma formed after the collapse of the expanding flux rope and entailing prominence material. RHESSI spectroscopy reveals high plasma temperature (similar to 30MK) and substantial non-thermal characteristics (delta similar to 5) during the impulsive phase of the flare. The time evolution of thermal energy exhibits a good correspondence with the variations in cumulative non-thermal energy, which suggests that the energy of accelerated particles is efficiently converted to hot flare plasma, implying an effective validation of the Neupert effect.</P>

PRE-FLARE CORONAL JET AND EVOLUTIONARY PHASES OF A SOLAR ERUPTIVE PROMINENCE ASSOCIATED WITH THE M1.8 FLARE:<i>SDO</i>AND<i>RHESSI</i>OBSERVATIONS

Joshi, Bhuwan,Kushwaha, Upendra,Veronig, Astrid M.,Cho, K.-S. American Astronomical Society 2016 The Astrophysical journal Vol.832 No.2

<P>We investigate the. triggering, activation, and ejection of a solar eruptive prominence that occurred in a multi-polar flux system of active region NOAA 11548 on 2012 August 18 by analyzing data from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, the Reuven Ramaty High Energy Solar Spectroscopic Imager, and the Extreme Ultraviolet Imager/Sun Earth Connection Coronal and Heliospheric Investigation on board the Solar Terrestrial Relation Observatory. Prior to the prominence activation, we observed striking coronal activities in the form of a blowout jet, which is associated with the. rapid eruption of a cool flux rope. Furthermore, the jet-associated flux rope eruption underwent splitting and rotation during its outward expansion. These coronal activities are followed by the prominence activation during which it slowly rises with a speed of similar to 12 km s(-1) while the region below the prominence emits gradually varying EUV and thermal X-ray emissions. From these observations, we propose that the prominence eruption is a complex, multi-step phenomenon in which a combination of internal (tether-cutting reconnection) and external (i.e., pre-eruption coronal activities) processes are involved. The prominence underwent catastrophic loss of equilibrium with the onset of the impulsive phase of an M1.8 flare, suggesting large-scale energy release by coronal magnetic reconnection. We obtained signatures of particle acceleration in the form of power-law spectra with hard electron spectral index (delta similar to 3) and strong HXR footpoint sources. During the impulsive phase, a hot EUV plasmoid was observed below the apex of the erupting prominence that ejected in the direction of the prominence with a speed of similar to 177 km s(-1). The temporal, spatial, and kinematic correlations between the erupting prominence and the plasmoid imply that the magnetic reconnection supported the fast ejection of prominence in the lower corona.</P>

IMPULSIVE ENERGY RELEASE AND NON-THERMAL EMISSION IN A CONFINED M4.0 FLARE TRIGGERED BY RAPIDLY EVOLVING MAGNETIC STRUCTURES

Kushwaha, Upendra,Joshi, Bhuwan,Cho, Kyung-Suk,Veronig, Astrid,Tiwari, Sanjiv Kumar,Mathew, S. K. IOP Publishing 2014 The Astrophysical journal Vol.791 No.1

<P>We present observations of a confined M4.0 flare from NOAA 11302 on 2011 September 26. Observations at high temporal, spatial, and spectral resolution from the Solar Dynamics Observatory, Reuven Ramaty High Energy Solar Spectroscopic Imager, and Nobeyama Radioheliograph observations enabled us to explore the possible triggering and energy release processes of this flare despite its very impulsive behavior and compact morphology. The flare light curves exhibit an abrupt rise of non-thermal emission with co-temporal hard X-ray (HXR) and microwave (MW) bursts that peaked instantly without any precursor emission. This stage was associated with HXR emission up to 200 keV that followed a power law with photon spectral index (gamma) similar to 3. Another non-thermal peak, observed 32 s later, was more pronounced in the MW flux than the HXR profiles. Dual peaked structures in the MW and HXR light curves suggest a two-step magnetic reconnection process. Extreme ultraviolet (EUV) images exhibit a sequential evolution of the inner and outer core regions of magnetic loop systems while the overlying loop configuration remained unaltered. Combined observations in HXR, (E) UV, and Ha provide support for flare models involving the interaction of coronal loops. The magnetograms obtained by the Helioseismic and Magnetic Imager reveal emergence of magnetic flux that began similar to five hr before the flare. However, the more crucial changes in the photospheric magnetic flux occurred about one minute prior to the flare onset with opposite polarity magnetic transients appearing at the early flare location within the inner core region. The spectral, temporal, and spatial properties of magnetic transients suggest that the sudden changes in the small-scale magnetic field have likely triggered the flare by destabilizing the highly sheared pre-flare magnetic configuration.</P>

<i>RHESSI</i>AND<i>TRACE</i>OBSERVATIONS OF MULTIPLE FLARE ACTIVITY IN AR 10656 AND ASSOCIATED FILAMENT ERUPTION

Joshi, Bhuwan,Kushwaha, Upendra,Cho, K.-S.,Veronig, Astrid M. IOP Publishing 2013 The Astrophysical journal Vol.771 No.1

<P>We present Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Transition Region and Coronal Explorer (TRACE) observations of multiple flare activity that occurred in the NOAA active region 10656 over a period of 2 hr on 2004 August 18. Out of four successive flares, three were class C events, and the final event was a major X1.8 solar eruptive flare. The activities during the pre-eruption phase, i.e., before the X1.8 flare, are characterized by three localized episodes of energy release occurring in the vicinity of a filament that produces intense heating along with non-thermal emission. A few minutes before the eruption, the filament undergoes an activation phase during which it slowly rises with a speed of similar to 12 km s(-1). The filament eruption is accompanied by an X1.8 flare, during which multiple hard X-ray (HXR) bursts are observed up to 100-300 keV energies. We observe a bright and elongated coronal structure simultaneously in E(UV) and 50-100 keV HXR images underneath the expanding filament during the period of HXR bursts, which provides strong evidence for ongoing magnetic reconnection. This phase is accompanied by very high plasma temperatures of similar to 31 MK, followed by the detachment of the prominence from the solar source region. From the location, timing, strength, and spectrum of HXR emission, we conclude that the prominence eruption is driven by the distinct events of magnetic reconnection occurring in the current sheet below the erupting prominence. These multi-wavelength observations also suggest that the localized magnetic reconnections associated with different evolutionary stages of the filament in the pre-eruption phase play an important role in destabilizing the active-region filament through the tether-cutting process, leading to large-scale eruption and X-class flare.</P>

Propagation of Interplanetary Coronal Mass Ejections: The Drag-Based Model

Vrš,nak, B.,Ž,ic, T.,Vrbanec, D.,Temmer, M.,Rollett, T.,Mö,stl, C.,Veronig, A.,Č,alogović,, J.,Dumbović,, M.,Lulić,, S.,Moon, Y.-J.,Shanmugaraju, A. Springer-Verlag 2013 Solar physics Vol.285 No.1