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P. Vemareddy

Publications and source records attributed to P. Vemareddy.

At least 19 recordsLinked to original sources
arXiv2026

Formation and Eruption of Filament Channel in Solar Active Region 12975: Insights from Observations and Simulations of Magnetic Field Evolution

We studied the magnetic field evolution of active region (AR) 12975 using a time-dependent magnetofrictional (TMF) model. This AR produced two consecutive CMEs associated with M-class flares on March 28, 2022. The AR exhibited a simple bipolar configuration, with new bipolar flux emerging from March 27. These emerging flux regions evolved through shear motions, forming a filament-channel that ultimately erupted on March 28 at 12:00 UT. The simulation, initialized at 12:00 UT on March 26, is driven by electric fields derived from a time-series of photospheric vector-magnetograms. It reproduces the observed coronal evolution, including the gradual development of a sigmoidal, twisted flux rope (FR) over approximately 50 hours. The modeled temporal evolution of magnetic energy and helicity within the computational domain is consistent with the observed injection of both quantities. Furthermore, the ratio of current-carrying to total relative helicity reaches 0.23 at the time of observed eruption, however the torus-unstable regime is attained when the helicity ratio reaches 0.32, approximately 7 hr after the observed eruption. Notably, the FR forms adjacent to pre-existing magnetic fields, and a substantial portion of the coronal structure does not belong to the FR system. Consequently, the derived helicity thresholds vary and deviate from the proposed value of 0.29. While reproducing filament formation with high morphological accuracy, this study underscores the quantitative challenges involved in modeling and evaluating the eruptive behavior of different ARs.

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arXiv2026

Understanding the formation and eruption of sigmoidal structure through data-driven modeling of magnetic evolution in solar active region 13500

We investigate the magnetic origin of the coronal mass ejection (CME) that occurred on November 28, 2023, at 19:50UT from active region (AR) 13500 located near the solar disk-center. The eruption was associated with an S-shaped sigmoidal structure formed by the inner AR polarities along a sheared polarity inversion line, while the outer polarities evolved through proper motions. During November 26-28, the AR exhibited a decrease in net magnetic flux while progressively injecting magnetic helicity and energy into the corona toward the eruption onset, highlighting the key role of helicity-injection in triggering eruptions. To simulate this magnetic evolution, we employed a data-driven magnetofrictional (MF) simulation starting 2.8 days prior to the eruption. The energy input for the model was constrained using the observed energy injection through an ad-hoc parameter. The initial potential-field configuration gradually evolved into a sheared-arcade and eventually developed into a twisted flux rope (FR) over the observed time-scale. Proxy emission maps based on electric currents show remarkable morphological agreement between the simulated and observed sigmoidal structure. The average FR-core twist increasingly builds-up leading the FR to initiate slow-rise motion of FR top from 50Mm until its eruption onset at 80Mm. Importantly, the ratio of current-carrying to total relative-helicity increased from 0.13 at FR formation to 0.30 at eruption, when the FR core entered the torus-unstable regime, suggesting an association between torus-instability and a threshold helicity ratio. These results demonstrate that data-driven MF simulations can successfully reproduce the evolving coronal magnetic configuration and may provide a robust tool for assessing the eruptive potential of ARs, particularly the helicity ratio.

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arXiv2025

An intense geomagnetic storm originated from stealth Coronal Mass Ejection: remote and $in situ$ observations by near radially aligned spacecraft

We investigate the solar origin and heliospheric evolution of an intense geomagnetic storm that occurred on March 23-24, 2023. Despite multiple candidate CMEs observed between March 19-21, a weak CME detected on March 19 at 18:00 UT was identified as the cause, originating from the eruption of a longitudinal-filament channel near center of the sun. The channel underwent a smooth transition to eruption phase without detectable low coronal signatures. Wide-angle heliospheric imaging revealed asymmetric expansion and acceleration by solar wind drag, achieving an average CME velocity of $\approx$640 km/s. The radial evolution of the interplanetary coronal mass ejection (ICME) was analyzed by three spacecraft in close radial alignment. Arrival times and propagation speeds were consistent across spacecraft, with a 21 hour delay between STEREO-A and WIND attributed to solar rotation and longitudinal separation. The ICME exhibits magnetic cloud (MC) signatures characterized by right-handed helicity, enhanced density at all three spacecraft. The MC underwent expansion (radial-size increases from 0.08AU at SolO to 0.18AU at STEREO-A), decrease in magnetic field strength with distance; $B_{av}\propto R_H^{-1.97}$ (SolO-STA) and $B_{av}\propto R_H^{-1.53}$ (SolO-WIND). The MC axis is inclined with the ecliptic at $-69^o$ at SolO, $-25^o$ at STA and $-34^o$ at WIND, indicating rotation during heliospheric transit. Importantly, the storm's main phase leads to a peak intensity ($SYM-H=-169$nT) occurring at 24/02:40UT followed by a second peak ($SYM-H=-170$nT) at 24/05:20UT due to density enhancement towards MC's tail. The study emphasizes the significant geoeffectiveness of weak, stealth CMEs with southward Bz and density enhancements.

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arXiv2025

Visible Emission Line Coronagraph (VELC) onboard ADITYA-L1

Aditya-L1, India's first dedicated mission to study the Sun and its atmosphere from the Sun-Earth Lagrangian L1 location was successfully launched on 2023 September 2. It carries seven payloads. The Visible Emission Line Coronagraph (VELC) is a major payload on Aditya-L1. VELC is designed to carry out imaging and spectroscopic observations (the latter in three emission lines of the corona), simultaneously. Images of the solar corona in the continuum at 5000 A, with a field of view (FoV) from 1.05 Ro to 3 Ro can be obtained at variable intervals depending on the data volume that can be downloaded. Spectroscopic observations of the solar corona in three emission lines, namely 5303 A Fexiv, 7892 A Fexi, and 10747 A Fexiii are possible simultaneously, with different exposure times and cadence. Four slits, each of width 50 um, separated by 3.75 mm help to simultaneously obtain spectra at four positions in the solar corona at all the aforementioned lines. A Linear Scan Mechanism (LSM) makes it possible to scan the solar corona up to +/-1.5 Ro. The instrument has the facility to carry out spectropolarimetric observations at 10747 A also in the FoV range 1.05 - 1.5 Ro. Various components of the instrument were tested interferometrically on the optical bench before installation. The individual components were aligned and performance of the payload was checked in the laboratory using a laser source and tungsten lamp. Wavelength calibration of the instrument was verified using Sun as a light source. All the detectors were calibrated for different parameters such as dark current and its variation with exposure time. Here, we discuss the various features of the VELC, alignment, calibration, performance, possible observations, initial data analysis and results of initial tests conducted in-orbit.

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arXiv2024

Magnetic evolution of active regions: formation and eruption of magnetic flux ropes

Magnetic flux ropes (FRs) are twisted structures appearing on the sun, predominantly in the magnetically concentrated regions. These structures appear as coronal features known as filaments or prominences in H$\alpha$ observations, and as sigmoids in X-ray, EUV observations. Using the continuous vector magnetic field observations from \textit{Helioseismic and Magnetic Imager} onboard \textit{Solar Dynamics Observatory}, we study the evolution of the magnetic fields in the active regions (ARs) to understand the conditions of twisted flux formation. While ARs emerge and evolve further, flux motions such as shearing and rotation are efficient mechanisms to form twisted flux ropes. Magnetic helicity quantifies the twisted magnetic fields and helicity injection through photosphere leads to its accumulation in the corona. Therefore, coronal helicity accumulation leads to twisted FR formation and its eruption. The magnetic helicity injection is seen to evolve distinctly in the regions of flux rope formation and eruption. The ARs that are associated with eruptive activity are observed with helicity injection predominantly with one sign over a period of a few days. The ARs that inject helicity with a changing sign are unlikely to form twisted FRs because coronal helicity during the period of one sign of injected helicity gets cancelled by the opposite sign of injection in the later period. As a result, the coronal field reconfigures from shared to potential structure. For a given AR, the upper limit of helicity that could cause a CME eruption is not yet understood, which can be the subject of future studies of ARs. Magnetic reconnection plays a crucial role in both the initiation and driving of FR eruptions after their formation. Data-driven simulations of the AR evolution provide more insights on the flux rope formation and its onset of eruption.

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arXiv2024

Simulating the formation and eruption of flux rope by magneto-friction model driven by time-dependent electric fields

Aiming to capture the formation and eruption of flux ropes (FRs) in the source active regions (ARs), we simulate the coronal magnetic field evolution of the AR 11429 employing the time-dependent magneto-friction model (TMF). The initial field is driven by electric fields that are derived from time-sequence photospheric vector magnetic field observations by invoking ad-hoc assumptions. The simulated magnetic structure evolves from potential to twisted fields over the course of two days, followed by rise motion in the later evolution, depicting the formation of FR and its slow eruption later. The magnetic configuration resembles an inverse S-sigmoidal structure, composed of a potential field enveloping the inverse J-shaped fields that are shared past one another and a low lying twisted field along the major PIL. To compare with observations, proxy emission maps based on averaged current density along the field lines are generated from the simulated field. These emission maps exhibit a remarkable one-to-one correspondence with the spatial characteristics in coronal EUV images, especially the filament-trace supported by the twisted magnetic field in the south-west subregion. Further, the topological analysis of the simulated field reveals the co-spatial flare ribbons with the quasi-separatrix layers, which is consistent with the standard flare models; therefore, the extent of the twist and orientation of the erupting FR is indicated to be the real scenario in this case. The TMF model simulates the coronal field evolution, correctly capturing the formation of the FR in the observed time scale and the twisted field generated from these simulations serve as the initial condition for the full MHD simulations.

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arXiv2023

Data-driven simulations of magnetic field evolution in Active Region 11429: Magneto-friction method using PENCIL code

Coronal magnetic fields evolve quasi statically over long time scales and dynamically over short time scales. As of now there exists no regular measurements of coronal magnetic fields, and therefore generating the coronal magnetic field evolution using the observations of the magnetic field at the photosphere is of fundamental requirement to understand the origin of the transient phenomena from the solar active regions. Using the magnetofriction (MF) approach, we aim to simulate the coronal field evolution in the solar active region 11429. The MF method is implemented in open source \PC along with a driver module to drive the initial field with different boundary conditions prescribed from observed vector magnetic fields at the photosphere. In order to work with vector potential and the observations, we prescribe three types of bottom boundary drivers with varying free-magnetic energy. The MF simulation reproduces the magnetic structure, which better matches to the sigmoidal morphology exhibited by AIA images at the pre-eruptive time. We found that the already sheared field further driven by the sheared magnetic field, will maintain and further build the highly sheared coronal magnetic configuration, as seen in AR 11429. Data-driven MF simulation is a viable tool to generate the coronal magnetic field evolution, capturing the formation of the twisted flux rope and its eruption.

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arXiv2023

Filament eruption from active region 13283 leading to fast halo-CME and intense geomagnetic storm on 23 April 2023

Using multi-instrument and multi-wavelength observations, we studied a CME eruption that led to intense geomagnetic storm on 23 April 2023. The eruption occurred on April 21 in solar active region 13283 near the disk-center. The AR was in its decay stage, with fragmented polarities and a pre-existing long filament channel, a few days before the eruption. The study of magnetic field evolution suggests that the flux-rope (filament) has been built up by monotonous helicity accumulation over several days, and further converging and canceling fluxes lead to helicity injection change, resulting in the unstable nature of the magnetic flux-rope (MFR) and its further eruption. Importantly, the CME morphology revealed that the MFR apex underwent a rotation of upto 56\degree~in clockwise-direction owing to its positive helicity. The CME decelerates in the LASCO-FOV and has a plane-of-sky velocity of 1226 km/s at 20\,R$_\odot$. In the Heliospheric Imager FOV, the CME lateral expansion is tracked more than the earthward motion. This implies that the arrival time estimation is difficult to assess. The in-situ arrival of ICME shock was at 07:30 UT on April 23, and a geomagnetic storm commenced at 08:30\,UT. The flux rope fitting to the in-situ magnetic field observations reveals that the MC flux rope orientation is consistent with its near Sun orientation, which has a strong negative Bz-component. The analysis of this study indicates that the near-Sun rotation of the filament during its eruption to the CME is the key to the negative Bz-component and consequently the intense geomagnetic storm.

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arXiv2023

Data processing of Visible Emission Line Coronagraph Onboard ADITYA L1

ADITYA-L1 is India's first dedicated mission to observe the sun and its atmosphere from a halo orbit around L1 point. Visible emission line coronagraph (VELC) is the prime payload on board at Aditya-L1 to observe the sun's corona. VELC is designed as an internally occulted reflective coronagraph to meet the observational requirements of wide wavelength band and close to the solar limb (1.05 Ro). Images of the solar corona in continuum and spectra in three emission lines 5303{\AA} [Fe xiv], 7892{\AA} [Fe xi] and 10747 [Fe xiii] obtained with high cadence to be analyzed using software algorithms automatically. A reasonable part of observations will be made in synoptic mode, those, need to be analyzed and results made available for public use. The procedure involves the calibration of instrument and detectors, converting the images into fits format, correcting the images and spectra for the instrumental effects, align the images etc. Then, develop image processing algorithms to detect the occurrence of energetic events using continuum images. Also derive physical parameters, such as temperature and velocity structure of solar corona using emission line observations. Here, we describe the calibration of detectors and the development of software algorithms to detect the occurrence of CMEs and analyze the spectroscopic data.

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arXiv2022

Nature of helicity injection in non-erupting solar active regions

Using time-sequence vector magnetic field and coronal observations from \textit{Solar Dynamics Observatory}, we report the observations of the magnetic field evolution and coronal activity in four emerging active regions (ARs). The ARs emerge with leading polarity being the same as for the majority of ARs in a hemisphere of solar cycle 24. After emergence, the magnetic polarities separate each other without building a sheared polarity inversion line. In all four ARs, the magnetic fields are driven by foot point motions such that the sign of the helicity injection ($dH/dt$) in the first half of the evolution is changed to the opposite sign in the later part of the observation time. This successive injection of opposite helicity is also consistent with the sign of mean force-free twist parameter ($\alpha_{av}$). Further, the EUV light curves of the ARs in 94\AA~and GOES X-ray flux reveal flaring activity below C-class magnitude. Importantly, the white-light coronagraph images in conjunction with the AR images in AIA 94 \AA~delineate the absence of associated CMEs with the studied ARs. These observations imply that the ARs with successive injection of opposite sign magnetic helicity are not favorable to twisted flux rope formation with excess coronal helicity, and therefore are unable to launch CMEs, according to recent reports. This study provides the characteristics of helicity flux evolution in the ARs referring to the conservative property of magnetic helicity and more such studies would help to quantify the eruptive capability of a given AR.

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arXiv2022

Eruption of EUV Hot-Channel near Solar Limb and Associated Moving Type-IV Radio Burst

Using the observations from Solar Dynamics Observatory, we study an eruption of a hot-channel flux rope (FR) near the solar-limb on February 9, 2015. The pre-eruptive structure is visible mainly in EUV 131 $\mathring{\mathrm{A}}$ images with two highly-sheared loop structures. They undergo slow rise motion and then reconnect to form an eruptive hot-channel as in the tether-cutting reconnection model. The J-shaped flare-ribbons trace the footpoint of the FR which is identified as the hot-channel. Initially, the hot channel is observed to rise slowly at 40 km s$^{-1}$, followed by an exponential rise from 22:55 UT at a coronal height of 87$\pm$2 Mm. Following the onset of the eruption at 23:00 UT, the flare-reconnection adds to the acceleration process of the CME within 3 R$_\odot$. Later on, the CME continues to accelerate at 8 m s$^{-2}$ during its propagation period. Further, the eruption launched type-II followed by III, IVm radio bursts. The start and end times of type-IVm correspond to the CME core height of 1.5 and 6.1 R$_\odot$, respectively. Also the spectral index is negative suggesting the non-thermal electrons trapped in the closed loop structure. Accompanied with type-IVm, this event is unique in the sense that the flare ribbons are very clearly observed along with the erupting hot channel, which strongly supports that the hooked-part of J-shaped flare ribbons outlines the boundary of the erupting FR.

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arXiv2022

Magnetic imprints of eruptive and non-eruptive Solar flares as observed by Solar Dynamics Observatory

The abrupt and permanent changes of photospheric magnetic field in the localized regions of active regions during solar flares called magnetic imprints (MIs), have been observed for the past nearly three decades. The well known "coronal implosion" model is assumed to explain such flare associated changes but the complete physical understanding is still missing and debatable. In this study, we made a systematic analysis of flare-related changes of photospheric magnetic field during 21 flares (14 eruptive and 7 non-eruptive) using the high-cadence (\texttt{135s}) vector-magnetogram data obtained from Helioseismic and Magnetic Imager. The MI regions for eruptive flares are found to be strongly localised, whereas the majority of non-eruptive events ($>70~\%$) have scattered imprint regions. To quantify the strength of the MIs, we derived the integrated change of horizontal field and total change of Lorentz force over an area. These quantities correlate well with the flare strength, irrespective of whether flares being eruptive or not, short or long duration. Further, the free-energy (FE), determined from virial-theorem estimates, exhibits statistically significant downward trend which starts around the flare time is observed in majority of flares. The change of FE during flares do not depend on eruptivity but have a strong positive correlation ($\approx 0.8$) with the Lorentz force change, indicating that the part of FE released would penetrate into the photosphere. While these results strongly favor the idea of significant feedback from corona on the photospheric magnetic field, the characteristics of MIs are quite indistinguishable for flares being eruptive or not.

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arXiv2021

Magnetic Structure in Successively Erupting Active Regions: Comparison of Flare-Ribbons with Quasi-Separatrix Layers

This paper studies the magnetic topology of successively erupting active regions (ARs) 11429 and 12371. Employing vector magnetic field observations from Helioseismic and Magnetic Imager, the pre-eruptive magnetic structure is reconstructed by a model of non-linear force-free field (NLFFF). For all the five CMEs from these ARs, the pre-eruptive magnetic structure identifies an inverse-S sigmoid consistent with the coronal plasma tracers in EUV observations. In all the eruption cases, the quasi-separatrix layers (QSLs) of Large Q values are continuously enclosing core field bipolar regions in which inverse-S shaped flare ribbons are observed. These QSLs essentially represent the large connectivity gradients between the domains of twisted core flux within the inner bipolar region and the surrounding potential like arcade. It is consistent with the observed field structure largely with the sheared arcade. The QSL maps in the chromosphere are compared with the flare-ribbons observed at the peak time of the flares. The flare ribbons are largely inverse-S shape morphology with their continuity of visibility is missing in the observations. For the CMEs in the AR 12371, the QSLs outline the flare ribbons as a combination of two inverse J-shape sections with their straight parts being separated. These QSLs are typical with the weakly twisted flux rope. Similarly, for the CMEs in the AR 11429, the QSLs are co-spatial with the flare ribbons both in the middle of the PIL and in the hook sections. In the frame work of standard model of eruptions, the observed flare ribbons are the characteristic of the pre-eruptive magnetic structure being sigmoid which is reproduced by {\bf the} NLFFF model with a weakly twisted flux rope at the core.

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arXiv2021

Successive Injection of Opposite Magnetic Helicity: Evidence for Active Regions without Coronal Mass Ejections

Magnetic helicity (MH) is a measure of twist and shear of magnetic field. MH is injected in the active region (AR) corona through photospheric footpoint motions causing twisted and sheared magnetic fields. From the conservation property of the helicity, it was conjectured that an already twisted flux rope (FR) with continuous injection of MH inevitably erupts to remove the excess accumulated coronal helicity. Therefore, understanding the nature and evolution of the photospheric helicity flux transfer is crucial to reveal the intensity of the flare/CME activity. Using the time-sequence vector-magnetograms of \textit{Helioseismic Magnetic Imager}, we study the evolution of MH injection in emerging AR 12257. The photospheric flux motions in this AR inject positive helicity in the first 2.5 days followed by negative helicity later. This successive injection of opposite helicity is consistent with the sign of mean force-free twist parameter ($\alpha_{av}$), orientation of magnetic-tongues. Also, the extrapolated AR magnetic structure exhibits transformation of global-shear without a twisted FR in the core of the AR. No CMEs are launched from this AR but C-class flaring activity is observed predominantly in the second half of the evolution period. The ARs with sign reversal of the MH injection are not favorable to twisted FR formation with excess coronal helicity and therefore are important to identify CME-less ARs readily. A possible scenario in these ARs is that when one sign of helicity flux is replaced by opposite sign, the magnetic field of different connectivity with opposite shear undergoes reconnection at different scales giving rise to both intermittent flares and enhanced coronal heating.

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arXiv2019

Finding the critical decay index in solar prominence eruptions

The background field is assumed to play prime role in the erupting structures like prominences. In the flux rope models, the critical decay index ($n_c$) is a measure of the rate at which background field intensity decreases with height over the flux rope or erupting structure. In the real observations, the critical height of the background field is unknown, so a typical value of $n_{c}=1.5$ is adopted from the numerical studies. In this study, we determined the $n_c$ of 10 prominence eruptions (PEs). The prominence height in 3D is derived from two-perspective observations of \textit{Solar Dynamics Observatory} and \textit{Solar TErrestrial RElations Observatory}. Synoptic maps of photospheric radial magnetic field are used to construct the background field in the corona. During the eruption, the height-time curve of the sample events exhibits the slow and fast-rise phases and is fitted with the linear-cum-exponential model. From this model, the onset height of fast-rise motion is determined and is considered as the critical height for the onset of the torus-instability because the erupting structure is allowed to expand exponentially provided there is no strapping background field. Corresponding to the critical height, the $n_c$ values of our sample events are varied to be in the range of 0.8-1.3. Additionally, the kinematic analysis suggests that the acceleration of PEs associated with flares are significantly enhanced compared to flare-less PEs. We found that the flare magnetic reconnection is the dominant contributor than the torus-instability to the acceleration process during the fast-rise phase of flare-associated PEs in low corona ($<1.3R_{\odot}$).

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arXiv2019

Degree of electric current neutralization and the activity in solar Active Regions

Using time-sequence vector magnetic field observation from \textit{Helioseismic and Magnetic Imager}, we examined the connection of non-neutralized currents and the observed activity in 20 solar active-regions (ARs). The net current in a given magnetic polarity is algebraic sum of direct-current (DC) and return-current (RC) and the ratio $|DC/RC|$ is a measure of degree of net-current-neutralization (NCN). In the emerging ARs, the non-neutrality of these currents builds with the onset of flux emergence, following the relaxation to neutrality during the separation motion of bipolar regions. Accordingly, some emerging ARs are source regions of CMEs occurring at the time of higher level non-neutrality. ARs in the post-emergence phase can be CME productive provided they have interacting bipolar regions with converging and shearing motions. In these cases, the net current evolves with higher level ($>1.3$) of non-neutrality. Differently, the $|DC/RC|$ in flaring and quiet ARs vary near unity. In all the AR samples, the $|DC/RC|$ is higher for chiral current density than that for vertical current density. Owing to the fact that the non-neutralized currents arise in the vicinity of sheared polarity-inversion-lines (SPILs), the profiles of the total length of SPIL segments and the degree of NCN follow each other with a positive correlation. We find that the SPIL is localized as small segments in flaring-ARs whereas it is long continuous in CME-producing ARs. These observations demonstrate the dividing line between the CMEs and flares with the difference being in global or local nature of magnetic shear in the AR that reflected in non-neutralized currents.

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arXiv2019

Formation and eruption of sigmoidal structure from a weak field region of NOAA 11942

Using observations from Solar Dynamics Observatory, we studied an interesting example of a sigmoid formation and eruption from small-scale flux canceling regions of active region (AR) 11942. Analysis of HMI and AIA observations infer that initially the AR is compact and bipolar in nature, evolved to sheared configuration consisting of inverse J-shaped loops hosting a filament channel over a couple of days. By tracking the photospheric magnetic features, shearing and converging motions are observed to play a prime role in the development of S-shaped loops and further flux cancellation leads to tether-cutting reconnection of J-loops. This phase is co-temporal with the filament rise motion followed by sigmoid eruption at 21:32 UT on January 6. The flux rope rises in phases of slow (v$_{avg}$ = 26 km~s$^{-1}$) and fast (a$_{avg}$= 55 ms$^{-2}$) rise motion categorizing the CME as slow with an associated weak C1.0 class X-ray flare. The flare ribbon separation velocity peaks at around peak time of the flare at which maximum reconnection rate (2.14 Vcm$^{-1}$) occurs. Further, the EUV light-curves of 131, 171Å~have delayed peaks of 130 minutes compared to 94Å~and is explained by differential emission measure. Our analysis suggests that the energy release is proceeded in a much long time duration, manifesting the onset of filament rise and eventual eruption driven by converging and canceling flux in the photosphere. Unlike strong eruption events, the observed slow CME and weak flare are indications of slow runway tether-cutting reconnection where most of the sheared arcade is relaxed during the extended post phase of the eruption.

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arXiv2019

Very fast helicity injection leading to critically stable state and large eruptive activity in solar active region NOAA 12673

Using the photospheric magnetic and coronal observations of Solar Dynamics Observatory, we studied the build-up and eruption of coronal non-potential magnetic structure in emerging active region (AR) 12673. The velocity field derived from tracked vector-magnetograms indicates persistent shear and converging motions of flux regions about the polarity inversion line (PIL). A major helicity injection occurs during rapid flux emergence consistent with the very fast flux emergence phase. While this helicity flux builds-up the sigmoid by September 4, the helicity injection by the continued shear and converging motions in the later evolution contributes to sigmoid sustenance and its core field twist as a manifestation of the flux rope which erupts after exceeding critical value of twist. Moreover, the total length of sheared PIL segments correlates with the non-neutralized current and maintains a higher value in both the polarity regions as a signature of eruptive capability of the AR according to the flux rope models. The modelled magnetic field qualitatively reproduces the sigmoidal structure capturing major features like twisted core flux as flux rope, and hook-shaped parts connecting at the middle of the PIL. Study of quasi-separatrix-layers reveals that the sheared arcade, enclosing the flux rope, is stressed to a critically stable state and its coronal height becomes doubled from September 4-6. While demonstrating the fast injection of helicity per unit flux as the crucial factor for severe space-weather events, this study explains the formation of the flux rope and recurrent eruptive nature of the AR by the critically stable state of sheared arcade early on September 6.

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