Emergence Of Magnetic Flux Tubes Research Articles

Bayesian approach for modeling global magnetic parameters for the solar active region

Context. Active regions (ARs) appear in the solar atmosphere as a consequence of the emergence of magnetic flux tubes. The presence of elongated magnetic polarities in line-of-sight (LOS) magnetograms indicates the existence of twist in the flux tubes that form them. These polarity elongations, referred to as magnetic tongues, bias the measurement of AR characteristics obtained during their emergence phase (e.g., their tilt angle and magnetic flux). In particular, obtaining a good estimation of the tilt angle evolution plays a key role in constraining flux-transport dynamo models. Aims. In this work, we aim to estimate the intrinsic properties of the twisted flux tubes, or flux ropes, that form ARs by quantitatively comparing observed LOS magnetograms with synthetic ones derived from a toroidal magnetic flux tube model. Methods. We developed a Bayesian inference method to obtain the statistical distributions of the inferred model parameters. As an example, we applied the method to NOAA AR 10268. Next, we tested the results using a synthetic-AR generator to quantify the effect of small-scale perturbations over the inferred parameter distributions. Results. We conclude that this method can significantly remove the effects of magnetic tongues on the derived AR global characteristics, providing a better understanding of the intrinsic properties of the emerging flux rope. Conclusions. These results provide a framework for future analyses of the physical properties of emerging ARs using Bayesian statistics.

Solar Dynamical Processes I

The article gives a concise overview of solar dynamical processes and their impacts on the space weather. This article is based on the observational and theoretical developments made during last few decades. The article begins with a brief discussion of the Sun and the solar interior, from the core to the solar corona. We discuss the solar magnetic field and provide some basic understanding of the solar dynamo model. The solar dynamical processes, the transient as well as the gradual, are the manifestations of the Sun’s magnetic field. Magnetic reconnection, as well as submergence and emergence of magnetic flux tubes, plays an important role in the solar activities. This article tries to cover a range of dynamical processes, including sunspots, solar prominences and bright points. We also discussed various models of the dynamical processes along with their properties and effect on other activities occurring on the Sun.

Magnetic activity of interacting binaries

AbstractInteracting binaries provide unique parameter regimes, both rapid rotation and tidal distortion, in which to test stellar dynamo theories and study the resulting magnetic activity. Close binaries such as cataclysmic variables (CVs) have been found to differentially rotate, and so can provide testbeds for tidal dissipation efficiency in stellar convective envelopes, with implications for both CV and planet-star evolution. Furthermore, CVs show evidence of preferential emergence of magnetic flux tubes towards the companion star, as well as large, long-lived prominences that form preferentially within the binary geometry. Moreover, RS CVn binaries also show clear magnetic interactions between the two components in the form of coronal X-ray emission. Here, we review several examples of magnetic interactions in different types of close binaries.

Velocity Characteristics of Rotating Sunspots

A statistical study is carried out to investigate the detailed relationship between rotating sunspots and the emergence of magnetic flux tubes. This paper presents the velocity characteristics of 132 sunspots in 95 solar active regions. The rotational characteristics of the sunspots are calculated from successive SOHO/MDI magnetograms by applying the Differential Affine Velocity Estimator (DAVE) technique (Schuck, 2006, Astrophys. J.646, 1358). Among 82 sunspots in active regions exhibiting strong flux emergence, 63 showed rotation with rotational angular velocity larger than 0.4° h−1. Among 50 sunspots in active regions without well-defined flux emergence, 14 showed rotation, and the rotation velocities tend to be slower, compared to those in emerging regions. In addition, we investigated 11 rotating sunspot groups in which both polarities show evidence for co-temporary rotation. In seven of these cases the two polarities co-rotate, while the other four are found to be counter-rotating. Plausible reasons for the observed characteristics of the rotating sunspots are discussed.

Dynamo action and magnetic buoyancy in convection simulations with vertical shear

A hypothesis for sunspot formation is the buoyant emergence of magnetic flux tubes created by the strong radial shear at the tachocline. In this scenario, the magnetic field has to exceed a threshold value before it becomes buoyant and emerges through the whole convection zone. We follow the evolution of a random seed magnetic field with the aim of study under what conditions it is possible to excite the dynamo instability and whether the dynamo generated magnetic field becomes buoyantly unstable and emerges to the surface as expected in the flux-tube context. We perform numerical simulations of compressible turbulent convection that include a vertical shear layer. Like the solar tachocline, the shear is located at the interface between convective and stable layers. We find that shear and convection are able to amplify the initial magnetic field and form large-scale elongated magnetic structures. The magnetic field strength depends on several parameters such as the shear amplitude, the thickness and location of the shear layer, and the magnetic Reynolds number ($\Rm$). Whenever the toroidal magnetic field reaches amplitudes greater a threshold value which is close to the equipartition value, it becomes buoyant and rises into the convection zone where it expands and forms mushroom shape structures. Some events of emergence, i.e. those with the largest amplitudes of the initial field, are able to reach the very uppermost layers of the domain. These episodes are able to modify the convective pattern forming either broader convection cells or convective eddies elongated in the direction of the field. However, in none of these events the field preserves its initial structure.

Twist and writhe of δ-island active regions

AbstractWe study the magnetic helicity properties of a set of peculiar active regions (ARs) including δ-islands and other high-tilt bipolar configurations. These ARs are usually identified as the most active in terms of flare and CME production. Due to their observed structure, they have been associated with the emergence of magnetic flux tubes that develop a kink instability. Our main goal is to determine the chirality of the twist and writhe components of the AR magnetic helicity in order to set constrains on the possible mechanisms producing the flux tube deformations. We determine the magnetic twist comparing observations of the AR coronal structure with force-free models of the magnetic field. We infer the flux-tube writhe from the rotation of the main magnetic bipole during the observed evolution. From the relation between the obtained twist and writhe signs we conclude that the development of the kink instability cannot be the single mechanism producing deformed flux-tubes.

Magnetic flux emergence in granular convection: radiative MHD simulations and observational signatures

Aims. We study the emergence of magnetic flux from the near-surface layers of the solar convection zone into the photosphere. Methods. To model magnetic flux emergence, we carried out a set of numerical radiative magnetohydrodynamics simulations. Our simulations take into account the effects of compressibility, energy exchange via radiative transfer, and partial ionization in the equation of state. All these physical ingredients are essential for a proper treatment of the problem. Furthermore, the inclusion of radiative transfer allows us to directly compare the simulation results with actual observations of emerging flux. Results. We find that the interaction between the magnetic flux tube and the external flow field has an important influence on the emergent morphology of the magnetic field. Depending on the initial properties of the flux tube (e.g. field strength, twist, entropy etc.), the emergence process can also modify the local granulation pattern. The emergence of magnetic flux tubes with a flux of 10 19 Mx disturbs the granulation and leads to the transient appearance of a dark lane, which is coincident with upflowing material. These results are consistent with observed properties of emerging magnetic flux.