Active chromospheric fibril singularity. Coordinated observations from Solar Orbiter, SST, and IRIS

The coronal blowout jet, extreme ultraviolet (EUV) wave and coronal mass e-
jection (CME) are common phenomena in the solar atmosphere. In this paper, we report the
occurrence of an M-shaped CME event associated with a blowout jet and an EUV wave using
high-resolution, multi-angle and multi-wavelength observations taken from Solar Dynamics
Observatory,andSolarTErrestrialRElationsObservatory.Interestingly,andforthefirsttime,
it is found that two bubble-like CMEs and a jet-like CME were simultaneously triggered by
the same eruptive event. Our observational analyses and findings indicate the following: (1)
the eruption of a blowout jet led to a large-scale EUV wave; (2) the eruption of the EUV
wave swept a small filament (prominence) and a long filament; (3) eventually the EUV wave
split-up into two parts, leading to the two bubble-like CMEs, while the blowout jet induced
a jet-like CME. The combined events appear to form an M-shape like structure CME, that
we sketch throughout a proposed cartoon tentatively explaining the observed complex con-
figuration. Based on observational diagnosis, we argue that the jet, the EUV wave and the
multi-CME are highly interlinked. A suggested eruption-model, from the solar atmosphere
to the space, is outlined and discussed, providing a possibly new way to probe the relation-
ship between the solar eruptions and the surrounding space. The investigation of such rare phenomenon can be a key point for better understanding of the physical associated triggering
mechanisms and energy transport in the solar atmosphere, crucial for MHD simulations and
modeling.

Emerging Flux Regions (EFR’s) on the Sun observed in the Meudon–Mitaka Collaborative Solar Observation Program (1979) are discussed. The rule found earlier (Martres et al. 1973) for the growth (or decay) of the N polarity or the decay (or growth) of the S polarity magnetic field in relation to the photospheric clockwise (or counter-clockwise) vortex motion is reconfirmed for the EFR in the present solar cycle. The rule is emphasized as an indication of active generation of an electric current system by a photospheric motion. The chromospheric manifestations of EFR’s seen in Hα as arch filament systems (AFS’s) and the enhanced brightness are examined with respect to the photospheric magnetic field structure derived from the white light and magnetic observations. An isolated EFR is associated with an AFS which is well coaligned with the photospheric transverse field. The typical EFR configuration may be distorted if the EFR appears in the region of the pre-existing strong field. The observational characteristics of such EFR’s can also be interpreted in terms of the emergence of a nearly potential field associated with the dissipation of an excess magnetic energy. An EFR born in an older EFR shows that the flux loop is low-lying and tends to relax into a potential field configuration.

Coronal jets are collimated plasma eruptions that are ubiquitous in the solar atmosphere. Believed to be triggered by magnetic reconnection, these jets can contribute to various phenomena, including coronal heating and particle acceleration. Coronal jets are a contemporary area of research, with their onset mechanism meriting further attention. Importantly, a subclass of jets, the blowout jets, is particularly interesting because of their broad spire, suggesting substantial 3D reconnection between open and closed field lines involving 3D null points. Consequently, here we explore the onset of a blowout jet associated with Active Region SPoCA 29093 detected by the Spatial Possibilistic Clustering Algorithm (SPoCA). This AR produced a C1.1-class flare on 2022 November 10, and we investigate it using a data-constrained magnetohydrodynamic simulation initiated with a non-force-free field extrapolation of the photospheric magnetic field. Key elements of the extrapolated field lines are the presence of a 3D null and a magnetic flux rope (MFR) colocated with the jet activity region, the evolution of which is further traced in the simulation. The simulation suggests that magnetic reconnection is responsible for the evolution of the MFR, leading to a near-simultaneous onset of the flare and jet as observed by the Atmospheric Imaging Assembly/Solar Dynamics Observatory. In particular, the simulation shows spontaneous creation and annihilation of 3D null pairs via magnetic reconnection near the jet region. Such spontaneous null pair generation, in principle, can trigger or contribute to coronal jets, opening up a new avenue for further research.

Magnetohydrodynamic simulation is carried out for the NOAA AR 12615 in the time span t ∈ {05:48, 06:18} UT on 2016 December 5; covering events of a blowout jet and a C1.2 flare. The events are selected based on the small interval between their occurrences, which provides us with an opportunity to explore two energetically different events of magnetic reconnection (MR) within the run time of a single magnetohydrodynamic (MHD) simulation. The simulation is initiated with magnetic field extrapolated from the vector magnetogram provided by the Helioseismic Magnetic Imager/Solar Dynamics Observatory, using a non-force-free approximation. The extrapolated Lorentz force is found to decay at a rate faster than the volume current density, making the higher corona to be effectively force-free while the Lorentz force affects only the lower corona and the photosphere—a desirable feature that agrees with the contemporary understanding. For the simulation, the plasma is idealized to be incompressible, thermally homogeneous and having perfect electrical conductivity. The results affirm MRs near a set of two three-dimensional (3D) magnetic nulls to be responsible for initiating the jet. Moreover, a flux rope located near the nulls contributes to the jet by changing its magnetic field lines from an anchored to an open topology. The scenario agrees with the standard mini-filament breakout model for blowout jets and provides its first demonstration from a 3D data-constrained MHD simulation where the computational output is reconciled with magnetogram(s) only once. The generation of flare ribbons is attributed to reconnections at a 3D null and a quasi-separatrix layer (QSL), highlighting the importance of topological complexity in flare initiation.

Recurrent and homologous jets were observed to the west edge of active region NOAA 11513 at the boundary of a coronal hole. We find two kinds of cancellations between opposite polarity magnetic fluxes, inducing the generation of recurrent jets. First, a satellite spot continuously collides with a pre-existing opposite polarity magnetic field and causes recurrent solar jets. Second, moving magnetic features, which emerge near the sunspot penumbra, pass through the ambient plasma and eventually collide with the opposite polarity magnetic field. Among these recurrent jets, a blowout jet that occurred around 21:10 UT is investigated. The rotation of the pre-existing magnetic field and the shear motion of the satellite spot accumulate magnetic energy, which creates the possibility for the jet to experience blowout right from the standard.

An improved single-cell photonic band-gap (PBG) structure with an inner row of elliptical rods (PBG-E) was tested with high power at a 60 Hz repetition rate at X-band (11.424 GHz), achieving a gradient of $128\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at a breakdown probability of $3.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ per pulse per meter at a pulse length of 150 ns. The tested standing-wave structure was a single high-gradient cell with an inner row of elliptical rods and an outer row of round rods; the elliptical rods reduce the peak surface magnetic field by 20% and reduce the temperature rise of the rods during the pulse by several tens of degrees, while maintaining good damping and suppression of high order modes. When compared with a single-cell standing-wave undamped disk-loaded waveguide structure with the same iris geometry under test at the same conditions, the PBG-E structure yielded the same breakdown rate within measurement error. The PBG-E structure showed a greatly reduced breakdown rate compared with earlier tests of a PBG structure with round rods, presumably due to the reduced magnetic fields at the elliptical rods vs the fields at the round rods, as well as use of an improved testing methodology. A post-testing autopsy of the PBG-E structure showed some damage on the surfaces exposed to the highest surface magnetic and electric fields. Despite these changes in surface appearance, no significant change in the breakdown rate was observed in testing. These results demonstrate that PBG structures, when designed with reduced surface magnetic fields and operated to avoid extremely high pulsed heating, can operate at breakdown probabilities comparable to undamped disk-loaded waveguide structures and are thus viable for high-gradient accelerator applications.

We have succeeded in obtaining magnetized star models that have extremely strong magnetic fields in the interior of the star. In our formulation, arbitrary functions of the magnetic flux function appear in the expression for the current density. By appropriately choosing the functional form for one of the arbitrary functions that corresponds to the distribution of the toroidal current density, we have obtained configurations with magnetic field distributions that are highly localized within the central part and near the magnetic axis region. The absolute values of the central magnetic fields are stronger than those of the surface region by two orders of magnitude. By applying our results to magnetars, the internal magnetic poloidal fields could be 10^17 G, although the surface magnetic fields are about 10^15 G in the case of magnetars. For white dwarfs, the internal magnetic poloidal fields could be 10^12 G, when the surface magnetic fields are 10^9-10^10 G .

Context. Solar jets are collimated plasma ejections along magnetic field lines observed in hot (extreme-ultraviolet (EUV) jets) and cool (chromospheric surges) temperature diagnostics. Their trigger mechanisms and the relationship between hot and cool jets are still not completely understood. Aims. We aim to investigate the generation of a sequence of active-region solar jets and their evolution from the photospheric to the coronal heights using multithermal observations from ground-based and space-borne instruments. Methods. Using the synergy of high-spatial-resolution and high-temporal-resolution observations by the Swedish 1-m Solar Telescope (SST), along with the Solar Dynamics Observatory (SDO), we analyzed a sequence of solar jets originating in a mixed-polarity region between the leading and following sunspots of an active region. We investigated the kinematics of these jets using the spectra from the SST observations. We used a non-force-free field (NFFF) extrapolation technique to derive the magnetic field topology of the active region. Results. A mixed-polarity region is formed over a long period (24 hours) with persistent magnetic flux emergence. This region has been observed as an arch filament system (AFS) in chromospheric SST observations. In this region, negative polarities surrounded by positive polarities create a fan surface with a null point at a height of 6 Mm detected in the NFFF extrapolation. SST observations in the Hβ spectral line reveal a large flux rope over the AFS moving from north to south, causing successive EUV and cool jets to move in the east–west direction and later towards the south along the long open loops. Conclusions. The high-resolution SST observations (0″.038 per pixel) resolve the dark area observed at the jet base and reveal the existence of an AFS with an extended cool jet, which may be the result of a peeling-like mechanism of the AFS. Based on the combined analysis of SST and AIA observations along with extrapolated magnetic topology, it is suggested that the magnetic reconnection site may move southward by approximately 20 Mm until it reaches a region where the open magnetic field lines are oriented north–south.

Nonlinear interactions in coronal heating

The dynamics of magnetic fields in closed regions of solar and stellar coronae are investigated with a reduced magnetohydrodynamic (MHD) model in the framework of Parker scenario for coronal heating. A novel analysis of reduced MHD equilibria shows that their magnetic fields have an asymmetric structure in the axial direction with variation length-scale $z_\ell \sim \ell B_0/b$, where $B_0$ is the intensity of the strong axial guide field, $b$ that of the orthogonal magnetic field component, and $\ell$ the scale of $\mathbf{b}$. Equilibria are then quasi-invariant along the axial direction for variation scales larger than approximatively the loop length $z_\ell \gtrsim L_z$, and increasingly more asymmetric for smaller variation scales $z_\ell \lesssim L_z$. The $critical$ $length$ $z_\ell \sim L_z$ corresponds to the magnetic field intensity threshold $b \sim \ell B_0/L_z$. Magnetic fields stressed by photospheric motions cannot develop strong axial asymmetries. Therefore fields with intensities below such threshold evolve quasi-statically, readjusting to a nearby equilibrium, without developing nonlinear dynamics nor dissipating energy. But stronger fields cannot access their corresponding asymmetric equilibria, hence they are out-of-equilibrium and develop nonlinear dynamics. The subsequent formation of current sheets and energy dissipation is $necessary$ for the magnetic field to relax to equilibrium, since dynamically accessible equilibria have variation scales larger than the loop length $z_\ell \gtrsim L_z$, with intensities smaller than the threshold $b \lesssim \ell B_0/L_z$. The dynamical implications for magnetic fields of interest to solar and stellar coronae are investigated numerically and the impact on coronal physics discussed.

Magnetic field observations made utilizing a \(\frac{1}{7}\) A birefringent filter for the FeI line at 5324 A resolve magnetic field features comparable in scale to the Hα fine structure. Without exception, low lying filaments in the center of the disk coincide with boundaries of polarity change in the line of sight component of the magnetic field. Small filaments can be identified on a scale comparable to the Hα fibril structures. The arch filament systems are associated with areas of mixed polarity. ‘Satellite spots’ may sometimes be identified as small localized enhancements of magnetic field of both polarities around the perimeter of a sunspot.

We discuss a model for radio pulsars and magnetars, which explain different observational properties of these objects. Recently developed model of the partially screened polar gap for radio pulsars assumes that the magnetic field at the stellar surface is very strong and differs essentially from the pure dipolar structure. This assumption is well supported by recent calculations of the cohesive energy for the neutron star surface, which show that the accelerating gap above polar cap can form provided that the local surface magnetic field exceeds 1014 G. In addition, X‐ray observations of radio pulsars make it clear that the hot spot surface is much smaller than that of the canonical one obtained from the dipolar geometry. The only natural explanation of this observational feature is that the hot spot is defined by the crust anchored magnetic field anomalies, dominating the surface magnetic field in radio pulsars. The open field lines of the dipolar field reconnect with the actual surface magnetic field lines forming a much smaller polar cap. The flux conservation law implies that the magnetic field at the surface is few times 1014 G, with the curvature radius of field lines near the polar cap is much smaller than that of dipolar field lines. The high curvature of the field lines, which is an essential condition for creation of dense electron‐positron plasma necessary for the radio emission generation, depends on the ratio of the dipolar and crust anchored fields. In the case of the magnetars this ratio is near unity, because dipolar field estimated by the pulsar spin‐down rate is the same order as the actual surface magnetic field. Therefore, although magnetars have large enough surface magnetic field to create the inner gap accelerator, the curvature radius of field lines is too large to magnetic pair creation in quantities large enough for generation of coherent radio emission. Thus, magnetars should be radio quiet as it is for majority of these objects.

Solar energetic events are caused by the release of magnetic energy accumulated in the solar atmosphere. To understand their initiating physical mechanisms, the dynamics of the coronal magnetic fields must be studied. Unfortunately, the dominant mechanisms are still unclear due to a lack of direct measurements. Numerical simulations based on magnetohydrodynamics (MHD) can reproduce the dynamical evolution of solar coronal magnetic field, providing a useful tool to explore flare initiation. Data-driven MHD simulations, in which the time-series observational data of the photospheric magnetic field is used as the simulation boundary condition, can explore different mechanisms. To investigate the accumulation of free magnetic energy through a solar eruption, we simulated the first of several large flares in NOAA active region 11283. We used a data-driven model that was governed by zero-beta MHD, focusing on the free magnetic energy accumulation prior to the M5.3 flare (2011 September 6 at 01:59 UT). We reproduced the flare-associated eruption following the formation of twisted magnetic fields, or a magnetic flux rope (MFR), formed by photospheric motions at its footpoints. We found that the eruption was first triggered by the growth of the torus instability. The erupting MFR caused magnetic reconnections with neighboring magnetic field lines located along the direction of the eruption. Using the simulation results and an axial-radial decay index centered on the MFR, we find a natural explanation for the inclination of the eruption and a possible approach to predict the direction of solar eruptive events.

Based on a topological model for the magnetic field of a solar active region (AR), we suggest a criterion for the existence of magnetic null points on the separators in the corona. With the problem of predicting solar flares in mind, we have revealed a model parameter whose decrease means that the AR evolves toward a major eruptive flare. We analyze the magnetic field evolution for AR 9077 within two days before the Bastille Day flare on July 14, 2000. The coronal conditions are shown to have become more favorable for magnetic reconnection, which led to a 3B/X5.7 eruptive flare.

The origin of ultra-intense magnetic fields on magnetars is a mystery in modern astrophysics. We model the core collapse dynamics of massive progenitor stars with high surface magnetic fields in the theoretical framework of a self-similar general polytropic magnetofluid under the self-gravity with a quasi-spherical symmetry. With the specification of physical parameters such as mass density, temperature, magnetic field and wind mass loss rate on the progenitor stellar surface and the consideration of a rebound shock breaking through the stellar interior and envelope, we find a remnant compact object (i.e. neutron star) left behind at the centre with a radius of $\sim 10^6$ cm and a mass range of $\sim 1-3$ solar masses. Moreover, we find that surface magnetic fields of such kind of compact objects can be $\sim 10^{14}-10^{15}$ G, consistent with those inferred for magnetars which include soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs). The magnetic field enhancement factor critically depends on the self-similar scaling index $n$, which also determines the initial density distribution of the massive progenitor. We propose that magnetized massive stars as magnetar progenitors based on the magnetohydrodynamic evolution of the gravitational core collapse and rebound shock. Our physical mechanism, which does not necessarily require ad hoc dynamo amplification within a fast spinning neutron star, favours the `fossil field' scenario of forming magnetars from the strongly magnetized core collapse inside massive progenitor stars. With a range of surface magnetic field strengths over massive progenitor stars, our scenario allows a continuum of magnetic field strengths from pulsars to magnetars.