In this article, we identify the magnetic reconnections at the hyperbolic flux tube (HFT), aided by slipping reconnection at quasi-separatrix layers (QSLs), which are pivotal to the occurrence of a confined M2.1 class flare in NOAA active region 12268. The magnetic field topology before the flare onset is obtained through a non-force-free field extrapolation scheme that accommodates a non-zero Lorentz force. A key aspect is the presence of an HFT in the computational domain above the flaring region, along with two QSLs at the lower boundary. To simulate the dynamics of the active region, we conduct a data-constrained magnetohydrodynamic (MHD) simulation initiated by the extrapolated field. The dynamics captured in the simulation document the formation of a current sheet within the HFT configuration, leading to magnetic reconnection at the HFT. Additionally, we observe the slipping motion of the footpoints of the magnetic field lines in the QSLs at the bottom boundary, which indicates the occurrence of slipping reconnection in the QSLs. Importantly, the magnetic reconnection at the HFT is suggested to be the primary driver in the development of the intricate flare brightenings and the flare ribbons.

We investigate the magnetic topology and trigger mechanism of the X1.5-class flare hosted by NOAA Active region 13006 on 10 May 2022, combining multi-wavelength observations with a Non-Force-Free-Field (NFFF) extrapolation. Observations show a circular brightening and a remote brightening that evolve into corresponding circular and remote flare ribbons during the impulsive phase. GONG H α data reveal a pre-existing arc-shaped filament within the flaring region that stays stable throughout the flare and subsequently shows signs of eruption shortly afterward. The NFFF extrapolation of the active region depicts the presence of a three dimensional (3D) null exhibiting a fan-spine structure, a filament located beneath the fan dome within the flaring kernel. The extrapolated field accurately reproduces the observed coronal morphology and demonstrates a close spatial relationship between the fan foot-points and the circular ribbon. The analysis also reveals a circular quasi-separatrix layer (QSL) surrounding the fan foot-points, while enhanced |𝐉|/|𝐁| near the null point indicates favorable conditions for current-sheet formation and reconnection. Measurements of the photospheric magnetic flux further indicate flux cancellation beneath the fan structure during the flare, implying a gradual buildup of magnetic free energy. The flux evolution implies that the sheared arcade beneath the fan may progressively transform into a flux rope, eventually leading to eruption. We propose that the flare is triggered by slipping magnetic reconnection within the circular QSL, leading to the initial circular brightening, followed by reconnection at the 3D null that amplifies the circular ribbon and produces the remote ribbon.

Abstract We investigate the onset of an M3.4-class solar flare that occurred on 8 May 2024 in the merged active region NOAA AR 13668/13664. Multiwavelength observations from the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) are used to examine the chromospheric flare ribbons, coronal loop dynamics, and photospheric magnetic flux evolution. The event exhibits a complex three-ribbon configuration consisting of two parallel J-shaped ribbons located near the flaring polarity inversion line and a spatially separated third ribbon. To investigate the underlying magnetic configuration, we reconstruct the three-dimensional coronal magnetic field using non–force-free field (NFFF) extrapolations based on HMI vector magnetograms. The extrapolated field reveals a hierarchical magnetic topology composed of a low-lying sheared arcade, multiple hyperbolic flux tubes (HFTs), and an overlying three-dimensional magnetic null point. The observed elongation and apparent motion of the flare ribbons suggest slipping magnetic reconnection along quasi-separatrix layers associated with the HFT system, with an estimated slipping speed of approximately 4.6 − 77.5 km s−1. These results indicate that the flare is driven by multiple magnetic configurations, including a sheared arcade, HFTs, and an overlying null point, which together produce hierarchical reconnection processes and explain the formation of multiple flare ribbons.

Understanding the initiation mechanisms of major solar flares remains a central problem in solar physics, particularly when multiple complex magnetic topologies such as null points and flux ropes are involved. This study aims to investigate the magnetic configuration that led to the X1.3-class flare on 2022 March 30 in NOAA Active Region (AR) 12975 and to understand the role of the magnetic topology in triggering the flare. For this purpose, we employ a non-force-free-field (NFFF) extrapolation technique on vector magnetogram data from Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamic Observatory (SDO) to reconstruct a temporal sequence of the three-dimensional (3D) coronal magnetic-field configurations during the flare. In the flaring region, we identify key magnetic features such as a 3D magnetic null, sheared arcades, a magnetic flux rope (MFR), and quasi-separatrix layers (QSLs). The sequence of the extrapolation shows the development of a magnetic-flux rope from an initially sheared arcade structure. The development is suggested to be facilitated by the magnetic reconnections in the magnetic field lines of the sheared arcade. In addition, reconnection at the 3D null is also found to occur. The footpoints of the null point coincide with observed pre-flare brightenings and the some part of the flare ribbons, indicating null-point reconnection as a key flare trigger. Furthermore, there is also indication of the onset of the slipping reconnection at the footpoints of the null which may further contribute to the ribbon brightenings. Overall, based on the extrapolation sequence, a plausible scenario can be proposed in which the reconnection between sheared arcades may lead to the formation and subsequent rise of the magnetic flux rope, which becomes unstable by the removal of the overlying flux through 3D null-point reconnection leading to the flaring event.

We study the magnetic reconnection during a flare by investigating flare ribbon dynamics using observations and data-constrained magnetohydrodynamics (MHD) simulation. In particular, we estimate the reconnection flux and the reconnection flux rates using flare ribbons of an M1.1 flare hosted by the active region 12184 utilizing the technique developed by Qiu et al. (2002). The reconnection flux and corresponding flux rates are found to be 10^20 Mx and 10^18 Mx s−1 respectively. To understand the flare onset and the origin of flare ribbons, we perform an MHD simulation initiated by the non-force-free-field extrapolation. Importantly, the extrapolated configuration identifies a three-dimensional (3D) magnetic neutral point and a flux rope in the flaring region, which is crucial to the flaring activity. The reconnection initiates at the null point and, subsequently the flux rope rises and appears to reconnect there, which is favorable for the eruption of the filament. The surrounding field lines also seem to take part in the null point reconnection. In later stage, a current sheet is formed below the null point ensuing a secondary reconnection near an X-type topology, further contributing to the energy release process in the flare. We trace the footpoint evolution of the field lines lying over the flare ribbons and find a significant similarity between the observed flare ribbons and the evolution of footpoints computed from the MHD simulation. We estimated induced electric field during the flare and found it to be ≈ 0.52 V cm−1, a slight less value, as per many past literatures. Additional findings are the enhancement of vertical current density near the flaring ribbons, a signature of successive reconnections near the null point. Overall, the present work contributes to the understanding of the ribbon formation in a flaring process and the involved magnetic reconnection.

The Sun is the only star that harbours a planet known to host life. Our home, the Earth, and other solar system planets reside within the heliosphere – the sphere of influence of the Sun. Within this domain, the Sun’s radiation, energetic particles, plasma wind, magnetic fluxes, and dynamic events, such as flares and coronal mass ejections influence planetary environments. While the Sun provides the basis for life, it also produces severe space weather that is hazardous to humanity’s space-based technologies. Long-term solar variations also influence planetary evolution and habitability. Dynamic solar variability originates in magnetohydrodynamic processes in its interior and atmosphere that provide a window to the plasma universe. Therefore, exploring the origin, impact, and overarching astrophysical implications of the Sun’s activity is of fundamental importance to humanity. In this vision for solar and heliospheric physics – contributing to the vision document of the Astronomical Society of India – we provide a brief synopsis of the current status of the field, focus on outstanding challenges that are expected to drive the field over the next decade or so, and based on an assessment of the expertise available within India, we provide specific recommendations that the Indian community is well poised to address.

In this paper, we present a magnetohydrodynamics simulation of NOAA active region 11166 to understand the origin of a confined X-class flare that peaked at 23:23 UT on 2011 March 9. The simulation is initiated with a magnetic field extrapolated from the corresponding photospheric magnetogram, using a non-force-free-field extrapolation technique. Importantly, the initial magnetic configuration identifies three-dimensional (3D) magnetic nulls and quasi-separatrix layers (QSLs), which nearly agree with the bright structures appeared in multi-wavelength observations. The Lorentz force associated with the extrapolated field self-consistently generates the dynamics that leads to the magnetic reconnections at the 3D nulls and the QSLs. These reconnections are found to contribute to the pre-flare activities and, ultimately, lead to the development of the flare ribbons. Notably, the anchored spine of the 3D null and the complete absence of flux rope in the flaring region are congruent with the confined nature of the flare. Furthermore, the simulation also suggests the role of reconnections at the 3D null with an open spine in the onset of a jet away from the flaring site.

Abstract. We investigate variations in the position of the magnetopause in response to the interplanetary magnetic field (IMF), and different phases of magnetospheric substorms. The average location of magnetopause is examined using magnetic field observations from multiple satellites (THEMIS, RBSP, and MMS), and the Shue model utilizing OMNI solar wind data for a period of five years from 2016–2020. We estimate average position of the magnetopause using Shue model through superposed epoch analysis of standoff distance and tail flaring angle at different substorm timings (onset, peak and end) and from in-situ measurements through 2D equatorial maps of average Δ BZ under IMF |Bz|> 0 conditions. Our findings reveal the occurrence of substorms during both northward and southward IMF orientations and highlight an earthward movement of the magnetopause during substorm onset and peak, followed by a relaxation during the substorm end time, for both northward and southward IMF orientations. Notably, the magnetopause undergoes significant compression and reaches its closest point to the Earth during instances of strong southward IMF (BZ < -5), particularly during the substorm peak. The empirical model provides accurate estimation of the magnetopause location during periods of both strong northward and southward IMF |Bz|>5, as the model curve traverses a distinct location (Δ BZ = 0) representing the magnetopause shown in the 2D average map of Δ BZ.

Three-dimensional (3D) magnetic nulls are abundant in the solar atmosphere, as has been firmly established through contemporary observations. They are established to be important magnetic structures in, for example, jets and circular ribbon flares. Although simulations and extrapolations support this, the mechanisms behind 3D null generation remain an open question. Recent magnetohydrodynamic simulations demonstrated magnetic reconnections to be responsible for both generating and annihilating 3D nulls. However, these simulations began with initial magnetic fields already supporting preexisting nulls, raising the question of whether magnetic reconnection can create nulls in fields initially devoid of them. Previously, this question was briefly explored in a simulation with an initial chaotic magnetic field. However, the study failed to precisely identify locations, topological degrees, and natures (spiral or radial) of nulls, and it approximated magnetic reconnection without tracking the magnetic field lines in time. In this paper, these findings are revisited in light of recent advancements and tools used to locate and trace nulls, along with the tracing of field lines, through which the concept of generation/annihilation of 3D nulls from chaotic fields is established in a precise manner.

Context. Erupting magnetic flux ropes (MFRs) are believed to play a crucial role in producing solar flares. However, the formation of erupting MFRs in complex coronal magnetic configurations and the role of their subsequent evolution in the flaring events are not fully understood.Aims. We perform a magnetohydrodynamic (MHD) simulation of active region NOAA 12241 to understand the formation of a rising magnetic flux rope during the onset of an M6.9 flare on 2014 December 18 around 21:41 UT (SOL2014-12- 18T21:41M6.9), which was followed by the appearance of parallel flare ribbons.Methods. The MHD simulation was initialised with an extrapolated non-force-free magnetic field generated from the photospheric vector magnetogram of the active region taken a few minutes before the flare.Results. The initial magnetic field topology displays a pre-existing sheared arcade enveloping the polarity inversion line. The simulated dynamics exhibit the movement of the oppositely directed legs of the sheared arcade field lines towards each other due to the converging Lorentz force, resulting in the onset of tether-cutting magnetic reconnection that produces an underlying flare arcade and flare ribbons. Concurrently, a magnetic flux rope above the flare arcade develops inside the sheared arcade and shows a rising motion. The flux rope is found to be formed in a torus-unstable region, thereby explaining its eruptive nature. Interestingly, the location and rise of the rope are in good agreement with the corresponding observations seen in extreme-ultraviolet channels of the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO). Furthermore, the foot points of the simulation's flare arcade match well with the location of the observed parallel ribbons of the flare.Conclusions. The presented simulation supports the development of the MFR by the tether-cutting magnetic reconnection inside the sheared coronal arcade during flare onset. The MFR is then found to extend along the polarity inversion line (PIL) through slip-running reconnection. The MFR's eruptive nature is ascribed both to its formation in the torus-unstable region and also to the runaway tether-cutting reconnection.

We present a novel Hall magnetohydrodynamics (HMHD) numerical simulation of a three-dimensional (3D) magnetic flux rope (MFR)-generated by magnetic reconnections from an initial 3D bipolar sheared field. Magnetic reconnections during the HMHD evolution are compared with the MHD. In both simulations, the MFRs generate as a consequence of the magnetic reconnection at null points which has not been realized in contemporary simulations. Interestingly, the evolution is faster and more intricate in the HMHD simulation. Repetitive development of the twisted magnetic field lines (MFLs) in the vicinity of 3D nulls (reconnection site) is unique to the HMHD evolution of the MFR. The dynamical evolution of magnetic field lines around the reconnection site being affected by the Hall forcing correspondingly affects the large-scale structures.

&lt;p&gt;Magnetospheric substorm is recognized as an important mechanism for transferring and dissipating solar wind energy to the ionosphere and near-Earth regions. A substorm is generally thought to consist of three phases: growth phase, expansion phase, and the recovery phase, and the total duration of a substorm is about 2&amp;#8211;4 hour. In this work, we present a statistical study of the magnetotail state during different phases of substorms, recovery phase in particular, for a period of 5 years from 2016-2020 using multi-spacecraft and ground magnetic measurements. For best spatial and temporal coverage of the inner magnetosphere and magnetotail, we use THEMIS, RBSP, MMS mission observations complemented by the SuperMAG database of measurements from ground-based magnetometers. To examine the duration of substorm expansion and recovery phases in the ionosphere, inner magnetosphere and magnetotail, we first find the substorm peak and end times from a list of substorm onsets available on the SuperMAG website. Substorm peak corresponds to the peak intensity of the westward electrojet provided by the SML (SuperMAG AL) index. For the current analysis period, we obtain a few thousand events when there are at least two spacecraft in the tail, which provides good statistics. To determine the time scales of expansion and recovery phases in the inner magnetosphere and magnetotail, we divide the observations into different bins based on X and Y position of the spacecraft. Keeping focus at the center of the tail, i. e., -5 &lt; Y &lt; 8 RE, the bins are chosen to be -4 to -7 RE, -7 to -10 RE, -10 to -15 RE, and -15 to -25 RE. A superposed epoch analysis is performed on the IGRF field subtracted ($ \Delta Bz =Bz_{Measured}- Bz_{IGRF}$) $Bz$ component of observed magnetic field for complete period of analysis. To find the time scale for recovery phase, we center the superposed epoch around the peak time. Our results show that the timescale of the field recovery is &amp;#160;more than an hour near the geostationary orbit (-4 to -7 RE), 30 min to less than an hour in the range -7 to -10 RE and even shorter as we go beyond -10 RE. The results presented in this work will help understand the spatial and temporal evolution of substorms in the magnetotail, and will significantly improve our understanding of space physics.&lt;/p&gt;

Magnetic flux ropes (MFRs) play an important role in high-energetic events like solar flares and coronal mass ejections in the solar atmosphere. Importantly, solar observations suggest an association of some flaring events with quadrupolar magnetic configurations. However, the formation and subsequent evolution of MFRs in such magnetic configurations still need to be fully understood. In this paper, we present idealized magnetohydrodynamics (MHD) simulations of MFR formation in a quadrupolar magnetic configuration. A suitable initial magnetic field having a quadrupolar configuration is constructed by modifying a three-dimensional linear force-free magnetic field. The initial magnetic field contains neutral lines, which consist of X-type null points. The simulated dynamics initially demonstrate the oppositely directed magnetic field lines located across the polarity inversion lines (PILs) moving towards each other, resulting in magnetic reconnections. Due to these reconnections, four highly twisted MFRs form over the PILs. With time, the foot points of the MFRs move towards the X-type neutral lines and reconnect, generating complex magnetic structures around the neutral lines, thus making the MFR topology more complex in the quadrupolar configuration than those formed in bipolar loop systems. Further evolution reveals the non-uniform rise of the MFRs. Importantly, the simulations indicate that the pre-existing X-type null points in magnetic configurations can be crucial to the evolution of the MFRs and may lead to the observed brightenings during the onset of some flaring events in the quadrupolar configurations.

Magnetohydrodynamics simulation of active region NOAA 11515 is performed to examine the initiation of the M5.6 flaring event that starts around 10:43 UT on 2 July 2012. The simulation is conducted using an extrapolated non-force-free magnetic field generated from the photospheric vector magnetogram of the active region as the initial magnetic field. The magnetic field shows the presence of a three-dimensional (3D) magnetic null with the corresponding dome overlying a filament and a low-lying magnetic flux rope, observed in 304 Å and 131 Å respectively. The simulated dynamics, triggered by the initial Lorentz force, lead to the bifurcations of the flux rope, which is similar to the observed bifurcation in the 131 Å brightenings. Additionally, the rope exhibits a rise and reconnects at the 3D null. These reconnections convert field lines of the rope into the anchored outer spine of the 3D null—explaining the occurrence of a nearby confined C-class flare. Further, the results show that the field lines of the flux rope reach the vicinity of the filament and become non-parallel to the field lines of the filament. This initiates the reconnections between the rope and the field lines of the filament—activating the filament for the eruption. This interesting interaction of the flux rope and filament seems to contribute to the onset of the M-class flare.

Three-dimensional (3D) magnetohydrodynamic simulations are carried out to explore magnetic reconnections in the presence of 3D magnetic nulls and quasi-separatrix layers (QSLs). The initial magnetic fields are created by superposing uniform vertical magnetic fields of two different magnitudes on a linear force-free field. The interior of the numerical box contains two 3D nulls with separatrix domes separated by a quasi-separator (or hyperbolic flux tube) with QSLs. In the first simulation, the uniform vertical field is so large that the nulls are located at low heights and the domes are separate. Initially unbalanced Lorentz forces drive rotational flows that form strong electric currents and strong torsional fan reconnection at the 3D nulls and weak QSL reconnection at the hyperbolic flux tube. Flipping or slipping of field lines is observed in both cases. In the second simulation, with a weaker vertical field and larger domes, the separatrix surfaces meet at the central quasi-separator and their rotation drives stronger QSL reconnection than before.

We present a three-dimensional magnetohydrodynamic simulation of the NOAA active region 12017 to explore the onset of an X-class flare on 2014 March 29 at 17:48 UT. The simulation is initiated with a magnetic field constructed by non-force-free-field extrapolation of the photospheric magnetic field. Importantly, the initial field contains a three-dimensional magnetic null, a pair of magnetic flux ropes, and a set of sheared arcades overlying the flux ropes at the flare location. A set of magnetic field lines are also identified which connect the flaring location to a distant region affected by the flare. The simulated evolution documents the magnetic reconnections at the null, which can potentially explain the observed chromospheric brightenings during the flare. The sheared arcades also appear to rise and participate in reconnections at the null. Notably, under the favorable initial Lorentz force, the flux ropes lose their twist due to reconnection. The terminal geometry of the ropes and the arcades are similar to the observed post-flare loops. Interestingly, the connecting field lines appear to take part in reconnections at the null, which may lead to the brightenings at the distant region. Further, the free energy released during the evolution which turns out to be ≈6.8×1031 ergs matches well with the observation.

Three-dimensional magnetic nulls relate to magnetic topology, and are propitious for triggering solar coronal transients. Although abundant in nature, their generation is not established. This paper reports magnetohydrodynamic simulations indicating the nulls to be dissipative self-organized structures. Categorically, the results of two case studies are presented. First, a potential null located at the origin of a Cartesian coordinate system is subjected to a sinusoidal flow. The null is seen to bifurcate while conserving the net topological degree. Using the corresponding deformed magnetic field as an initial condition, the magnetofluid is subsequently evolved by dissipating its magnetic and kinetic energies through magnetic reconnection and viscous dissipation. In effect, a current-carrying null develops in the process. Second, another simulation is initiated with a modified Arnold–Beltrami–Childress (ABC) magnetic field which exerts a Lorentz force on the magnetofluid and has no nulls within the computational volume. Astoundingly, allowed the magnetofluid to relax, nulls having mixed topological degrees are generated. The modified ABC field being chaotic, the spontaneous appearance of nulls establishes emergence of ordered magnetic structures from chaos—a trait of self-organized structures—explaining their ubiquity in naturally existing plasmas.

The magnetohydrodynamics of active region NOAA 11283 is simulated using an initial non-force-free magnetic field extrapolated from its photospheric vector magnetogram. We focus on the magnetic reconnections at a magnetic null point that participated in the X2.1 flare on 2011 September 6 around 22:21 UT (SOL2011-09-06T22:21X2.1) followed by the appearance of circular flare ribbons and coronal dimmings. The initial magnetic field from extrapolation displays a three-dimensional (3D) null topology overlying a sheared arcade. Prior to the flare, magnetic loops rise due to the initial Lorentz force, and reconnect at the 3D null, leading to expansion and loss of confined plasma that produce the observed pre-flare coronal dimmings. Further, the simulated dynamics documents the transfer of twist from the arcade to the overlying loops through reconnections, developing a flux rope. The nonparallel field lines comprising the rope and lower-lying arcades form an X-type geometry. Importantly, the simultaneous reconnections at the 3D null and the X-type geometry can explain the observed circular and parallel flare ribbons. Reconnections at the 3D null transform closed inner spine field lines into open field lines of the outer spine. The footpoints of these open field lines correspond to a ring-shaped coronal dimming region, tracing the dome. Further, the flux rope bifurcates because of these reconnections, which also results in the generation of open magnetic field lines. The plasma loss along the open field lines can potentially explain the observed coronal dimming.