arXiv subjects

Bhuwan Joshi

Publications and source records attributed to Bhuwan Joshi.

At least 55 records · Page 3Linked to original sources

arXiv2018

Generalisation of the Magnetic Field Configuration of typical and atypical Confined Flares

Atypical flares cannot be naturally explained with standard models. To predict such flares, we need to define their physical characteristics, in particular, their magnetic environment, and identify pairs of reconnected loops. Here, we present in detail a case-study of a confined flare preceded by flux cancellation that leads to the formation of a filament. The slow rise of the non-eruptive filament favours the growth and reconnection of overlying loops. The flare is only of C5.0 class but it is a long duration event. The reason is that it is comprised of three successive stages of reconnection. A non-linear force-free field extrapolation and a magnetic topology analysis allow us to identify the loops involved in the reconnection process and build a reliable scenario for this atypical confined flare. The main result is that a curved magnetic polarity inversion line in active regions is a key ingredient for producing such atypical flares. A comparison with previous extrapolations for typical and atypical confined flares leads us to propose a cartoon for generalizing the concept

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arXiv2018

Successive flux rope eruptions from $δ$-Sunspots region of NOAA 12673 and associated X-class eruptive flares on 2017 September 6

In this paper, we present a multi-wavelength analysis of two X-class solar eruptive flares of classes X2.2 and X9.3 that occurred in the sigmoidal active region NOAA 12673 on 2017 September 6, by combining observations of Atmospheric Imaging Assembly and Helioseismic Magnetic Imager instruments on board the Solar Dynamics Observatory. On the day of the reported activity, the photospheric structure of the active region displayed a very complex network of $δ$-sunspots that gave rise to the formation of a coronal sigmoid observed in the hot EUV channels. Both X-class flares initiated from the core of the sigmoid sequentially within an interval of $\sim$3 hours and progressed as a single "sigmoid--to--arcade" event. Differential emission measure analysis reveals strong heating of plasma at the core of the active region right from the pre-flare phase which further intensified and spatially expanded during each event. The identification of a pre-existing magnetic null by non-force-free-field modeling of the coronal magnetic fields at the location of early flare brightenings and remote faint ribbon-like structures during the pre-flare phase, which were magnetically connected with the core region, provide support for the breakout model of solar eruption. The magnetic extrapolations also reveal flux rope structures prior to both flares which are subsequently supported by the observations of the eruption of hot EUV channels. The second X-class flare diverged from the standard flare scenario in the evolution of two sets of flare ribbons, that are spatially well separated, providing firm evidence of magnetic reconnections at two coronal heights.

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arXiv2017

Observational and model analysis of a two-ribbon flare possibly induced by a neighbouring blowout jet

In this paper, we present unique observations of a blowout coronal jet that possibly triggered a two-ribbon confined C1.2 flare in a bipolar solar active region NOAA 12615 on 2016 December 5. The jet activity initiates at chromospheric/transition-region heights with a small brightening that eventually grows in a larger volume with well developed standard morphological jet features, viz., base and spire. The spire widens up with a collimated eruption of cool and hot plasma components, observed in the 304 and 94 A channels of AIA, respectively. The speed of the plasma ejection, which forms the jet's spire, was higher for the hot component (~200 km/s) than the cooler one (~130 km/s). The NLFF model of coronal fields at pre- and post-jet phases successfully reveal opening of previously closed magnetic field lines with a rather inclined/low-lying jet structure. The peak phase of the jet emission is followed by the development of a two-ribbon flare that shows coronal loop emission in HXRs up to ~25 keV energy. The coronal magnetic fields rooted at the location of EUV flare ribbons, derived from the NLFF model, demonstrate the pre-flare phase to exhibit an "X-type" configuration while the magnetic fields at the post-flare phase are more or less parallel oriented. The comparisons of multi-wavelength measurements with the magnetic field extrapolations suggest that the jet activity likely triggered the two-ribbon flare by perturbing the field in the interior of the active region.

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arXiv2017

Trigger of successive filament eruptions observed by SDO and STEREO

Using multi-wavelength observation from SDO and STEREO, we investigated the mechanism of two successive eruptions (F1 & F2) of a filament in active region NOAA 11444 on 27 march, 2012. The filament was inverse `J' shaped and lying along a quasi-circular polarity inversion line (PIL). The first part of the filament (F1) erupted at ~2:30UT on 27 March 2012, the second part of the filament (F2) erupted at around 4:20 UT on the same day. A precursor/pre-flare brightening was observed below filament's main axis about 30 min prior to F1. The brightening was followed by a jet-like ejection below filament, which triggered the eruption. Before the eruption of F2, the filament seems to be trapped within the overlying arcade loops almost for $\sim$1.5~hr before its successful eruption. Interestingly, we observed simultaneously contraction (~12km/s) and expansion (~20km/s) of arcade loops in the active region before F2. HMI magnetograms show the converging motion of the opposite polarities resulting in flux cancellation near PIL. We suggest that flux cancellation at PIL resulted jet-like ejection below filament's main axis, which triggered the eruption F1 similar to tether-cutting process. The eruption F2 was triggered by removal of the overlying arcade loops via reconnection process. Both filament eruptions produced high speed (~1000km/s) CMEs.

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arXiv2017

Evolutionary aspects and north-south asymmetry of soft X-ray flare index during solar cycles 21, 22, and 23

Aims. In this paper, we investigate the temporal evolution and north-south (N-S) asymmetry in the occurrence of solar flares during cycle 21, 22, and 23, and compare the results with traditional solar activity indices. Methods. The flare activity is characterized by a soft X-ray (SXR) flare index, which incorporates information about flare occurrences during a selected interval along with the peak intensity of individual events. Results. The SXR flare index correlates well with other conventional parameters of solar activity. Further, it exhibits a significantly higher correlation with sunspot area over sunspot number, which suggests the variations in sunspot area to be more closely linked with the transient energy release in the solar corona. The cumulative plots of the flare index indicate a slight excess of activity in the northern hemisphere during cycle 21, while a southern excess clearly prevails for cycles 22 and 23. The study reveals a significant N-S asymmetry, which exhibits variations with the phases of solar cycle. The reliability and persistency of this asymmetry significantly increases when the data is averaged over longer periods, while an optimal level is achieved when data is binned for 13 Carrington rotations. The time evolution of the flare index further confirms evolution of dual peaks in solar cycles during the solar maxima and violation of Gnevyshev-Ohl rule for the pair of solar cycles 22 and 23. Conclusions. The SXR flare index in the northern and the southern hemispheres of the Sun exhibits significant asymmetry during the evolutionary phases of the solar cycle, which implies that N-S asymmetry of solar flares is manifested in terms of the flare counts as well as the intensity of flare events.

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arXiv2017

Formation and eruption of a flux rope from the sigmoid active region NOAA 11719 and associated M6.5 flare: A multi-wavelength study

We investigate the formation, activation and eruption of a flux rope from the sigmoid active region NOAA 11719 by analyzing E(UV), X-ray and radio measurements. During the pre-eruption period of ~7 hours, the AIA 94 A 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 flux rope in the corona and also contribute toward sustaining the temperature of the flux rope higher than 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 ascend of a large flux rope in the corona which is preceded by the decrease of photospheric magnetic flux through the core flaring region suggesting tether-cutting reconnection as a possible triggering mechanism. The flux rope eruption results in a two-ribbon M6.5 flare with a prolonged rise phase of ~21 min. 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 HXR footpoint sources. In the later stages, the flare follows the standard flare model and the source region undergoes a complete sigmoid-to-arcade transformation.

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arXiv2016

Pre-flare coronal jet and evolutionary phases of a solar eruptive prominence associated with M1.8 flare: SDO and RHESSI observations

We investigate 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 AIA on board SDO, RHESSI, and EUVI/SECCHI on board STEREO. Prior to the prominence activation, we observed striking coronal activities in the form of a blowout jet which is associated with rapid eruption of a cool flux rope. Further, 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 ~12 km/s 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 ~ 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 ~177 km/s. 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.

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arXiv2016

On the role of repetitive magnetic reconnections in evolution of magnetic flux-ropes in solar corona

Parker's magnetostatic theorem extended to astrophysical magnetofluids with large magnetic Reynolds number supports ceaseless regeneration of current sheets and hence, spontaneous magnetic reconnections recurring in time. Consequently, a scenario is possible where the repeated reconnections provide an autonomous mechanism governing emergence of coherent structures in astrophysical magnetofluids. In this work, such a scenario is explored by performing numerical computations commensurate with the magnetostatic theorem. In particular, the computations explore the evolution of a flux-rope governed by repeated reconnections in a magnetic geometry resembling bipolar loops of solar corona. The revealed morphology of the evolution process, including onset and ascent of the rope, reconnection locations and the associated topology of the magnetic field lines, agrees with observations, and thus substantiates physical realisability of the advocated mechanism.

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arXiv2015

Large-scale contraction and subsequent disruption of coronal loops during various phases of the M6.2 flare associated with the confined flux rope eruption

We present a detailed multi-wavelength study of the M6.2 flare which was associated with a confined eruption of a prominence using TRACE, RHESSI, and NoRH observations. The pre-flare phase of this event is characterized by spectacular large-scale contraction of overlying extreme ultraviolet (EUV) coronal loops during which the loop system was subjected to an altitude decrease of ~20 Mm for an extended span of ~30 min. 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 of the flaring region which together with X-ray spectra indicate strong localized heating in the source region before the filament activation and associated M-class flare. With the onset of the impulsive phase of the M6.2 flare, we detect HXR and MW sources that exhibit intricate temporal and spatial evolution in relation with the fast rise of the prominence. Following the flare maximum, the filament eruption slowed down and subsequently confined within the large overlying active region loops; the event did not lead to a coronal mass ejection (CME). 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 observations reveal high plasma temperature (~30 MK) and substantial non-thermal characteristics with electron spectral index (~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 suggest that the energy of accelerated particles efficiently converted to hot flare plasma implying an effective validation of the Neupert effect.

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arXiv2014

Impulsive energy release and non-thermal emission in a confined M4.0 flare triggered by rapidly evolving magnetic structures

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 Solar Dynamics Observatory, Reuven Ramaty High Energy Solar Spectroscopic Imager, and Nobeyama Radioheliograph 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 ($δ$) $\sim$3. Another non-thermal peak, observed 32 s later, was more pronounced in the MW flux than the HXR profiles. Dual peaked structure in the MW and HXR light curves suggest a two-step magnetic reconnection process. Extreme ultraviolet (EUV) images exhibit a sequential evolution of the inner and outer core regions of magnetic loop system while the overlying loop configuration remained unaltered. Combined observations in HXR, (E)UV, and H$α$ provide support for flare models involving interaction of coronal loops. The magnetograms obtained from Helioseismic and Magnetic Imager (HMI) reveal the emergence of magnetic flux which started $\sim$5 hr before the flare. However, the more crucial changes in the photospheric magnetic flux occurred about 1 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.

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arXiv2013

RHESSI and TRACE observations of multiple flare activity in AR 10656 and associated filament eruption

We present RHESSI and TRACE observations of multiple flare activity that occurred in the active region NOAA 10656 over the period of two hours on 2004 August 18. Out of four successive flares, there were three events of class C while the final event was a major X1.8 solar eruptive flare. The events during the pre-eruption phase, i.e., before the X1.8 flare, are characterized by localized episodes of energy release occurring in the vicinity of an active region filament which produced 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 ~12 km/s. The filament eruption is accompanied with an X1.8 flare during which multiple 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 with very high plasma temperatures of ~31 MK and 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 a current sheet formed below the erupting filament. 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 a crucial role in destabilizing the filament by a tether-cutting process leading to large-scale eruption and X-class flare.

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arXiv2012

Signatures of magnetic reconnection in solar eruptive flares: A multi-wavelength perspective

In this article, we review some key aspects of a multi-wavelength flare which have essentially contributed to form a standard flare model based on the magnetic reconnection. The emphasis is given on the recent observations taken by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on the X-ray emission originating from different regions of the coronal loops. We also briefly summarize those observations which do not seem to accommodate within the canonical flare picture and discuss the challenges for future investigations.

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arXiv2011

Pre-flare activity and magnetic reconnection during the evolutionary stages of energy release in a solar eruptive flare

In this paper, we present a multi-wavelength analysis of an eruptive white-light M3.2 flare which occurred in active region NOAA 10486 on November 1, 2003. Excellent set of high resolution observations made by RHESSI and TRACE provide clear evidence of significant pre-flare activities for ~9 minutes in the form of an initiation phase observed at EUV/UV wavelengths followed by the 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 ~30 keV and coincided with the beginning of flux emergence at the flaring location along with early signatures of the eruption. From the RHESSI observations, we conclude 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, intense 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 100 keV and significant non-thermal characteristics. The present study indicates a causal relation between the activities in the preflare and main flare. We also conclude that pre-flare activities, occurred in the form of subtle magnetic reorganization along with localized magnetic reconnection, played a crucial role in destabilizing the active region filament leading to solar eruptive flare and associated large-scale phenomena.

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arXiv2009

Periodicities in sunspot activity during solar cycle 23

The data of sunspot numbers, sunspot areas and solar flare index during cycle 23 are analyzed to investigate the intermediate-term periodicities. Power spectral analysis has been performed separately for the data of the whole disk, northern and southern hemispheres of the Sun. Several significant midrange periodicities ($\sim$175, 133, 113, 104, 84, 63 days) are detected in sunspot activity. Most of the periodicities in sunspot numbers generally agree with those of sunspot areas during the solar cycle 23. The study reveals that the periodic variations in the northern and southern hemispheres of the Sun show a kind of asymmetrical behavior. Periodicities of $\sim$175 days and $\sim$133 days are highly significant in the sunspot data of northern hemisphere showing consistency with the findings of Lean (1990) during solar cycles 12-21. On the other hand, southern hemisphere shows a strong periodicity of about 85 days in terms of sunspot activity. The analysis of solar flare index data of the same time interval does not show any significant peak. The different periodic behavior of sunspot and flare activity can be understood in the light of hypothesis proposed by Ballester et al. (2002), which suggests that during cycle 23, the periodic emergence of magnetic flux partly takes place away from developed sunspot groups and hence may not necessarily increase the magnetic complexity of sunspot groups that leads to the generation of flares.

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arXiv2009

Magnetic Reconnection During the Two-Phase Evolution of a Solar Eruptive Flare

We present a detailed multi-wavelength analysis and interpretation of the evolution of an M7.6 flare on October 24, 2003. The X-ray observations of the flare taken from the 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 $\sim$11 minutes. Such a long duration of the descending LT source motion is reported for the first time. The EUV loops, located below the X-ray LT source, also undergo contraction with similar speed ($\sim$15 km s$^{-1}$) in this interval. During the second phase the two distinct hard X-ray footpoints (FP) sources are observed which correlate well with UV and H$α$ flare ribbons. The X-ray LT source now exhibits upward motion. 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 ($\varepsilon \gtrsim$25 keV) follow hard power laws during the second phase ($γ= 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.

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arXiv2007

North-South Asymmetry of Solar Activity during Cycle 23

In this paper, we have made a statistical analysis of solar H$α$ flares that occurred during the period 1996 to 2005 to investigate their spatial distribution with respect to northern and southern hemispheres of the Sun. The analysis includes a total of 21608 single events. The study shows a significant N$-$S asymmetry which is persistent with the evolution of the solar cycle. The flare activity favors the northern hemisphere in general during the rising and maximum phase of the solar cycle (i.e., in 1997, 1999, and 2000), while the declining phase (i.e., from 2001 to 2005) shows a southern dominance. Further, the monthly N$-$S asymmetry index for flares, sunspot numbers and sunspot areas suggests similar variations for these phenomena with the progress of solar cycle. We also find that in terms of asymmetric behavior of solar flares, cycle 23 seems to act quite differently from cycle 22 but comparably to cycle 21.

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arXiv2007

Multi-wavelength signatures of magnetic reconnection of a flare associated coronal mass ejection

The evolution of an X2.7 solar flare, that occurred in a complex $βγδ$ magnetic configuration region on 2003 November 3 is discussed utilizing a multi-wavelength data set. The very first signature of pre-flare coronal activity is observed in radio wavelengths as type III burst that occurred several minutes prior to the flare signature in H$α$. This type III is followed by the appearance of a looptop source in hard X-ray (HXR) images obtained from RHESSI. During the main phase of the event, H$α$ images observed from the solar tower telescope at ARIES, Nainital, reveal well-defined footpoint (FP) and looptop (LT) sources. As the flare evolves, the LT source moves upward and the separation between the two FP sources increases. The co-alignment of H$α$ with HXR images shows spatial correlation between H$α$ and HXR footpoints, while the rising LT source in HXR is always located above the LT source seen in H$α$. The evolution of LT and FP sources is consistent with the reconnection models of solar flares. The EUV images at 195 Å taken by SOHO/EIT reveal intense emission on the disk at the flaring region during the impulsive phase. Further, slow drifting type IV bursts, observed at low coronal heights at two time intervals along the flare period, indicate rising plasmoids or loop systems. The intense type II radio burst at time in between these type IV bursts, but at a relatively larger height indicates the onset of CME and its associated coronal shock wave. The study supports the standard CSHKP model of flares, which is consistent with nearly all eruptive flare models. More important, the results also contain evidence for breakout reconnection before the flare phase.

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arXiv2006

Intermediate-term periodicities in soft X-ray flare index during solar cycles 21, 22 and 23

We have analyzed the intermediate term periodicities in soft X-ray flare index ($FI_{SXR}$) during solar cycles 21, 22 and 23. Power spectral analysis of daily $FI_{SXR}$ reveals a significant period of 161 days in cycle 21 which is absent during cycle 22 and 23. We have found that in cycle 22 periodicities of 74 and 83 days are in operation. A 123 day periodicity has been found to be statistically significant during the part of the current solar cycle 23. The existence of these periodicities has been discussed in the light of earlier results.

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