We present a three-dimensional numerical magnetohydrodynamic simulation designed to model the emergence of a magnetic flux rope passing from below the photosphere into the corona. For the initial state, we prescribe a plane-parallel atmosphere that comprises a polytropic convection zone, photosphere, transition region, and corona. Embedded in this system is an isolated horizontal magnetic flux rope located 10 photospheric pressure scale heights below the photosphere. The flux rope is uniformly twisted, with the plasma temperature inside the rope reduced to compensate for the magnetic pressure. Density is reduced in the middle of the rope, so that this section buoyantly rises. The early evolution proceeds with the middle of the rope rising to the photosphere and expanding into the corona. Just as it seems the system might approach equilibrium, the upper part of the flux rope begins to separate from the lower, mass-laden part. The separation occurs through stretching of the field, which forms a current sheet, where reconnection severs the field lines to form a new system of closed flux. This flux then erupts into the corona. Essential to the eruption process are shearing motions driven by the Lorentz force, which naturally occur as the rope expands in the pressure-stratified atmosphere. The shearing motions transport axial flux and energy to the expanding portion of the magnetic field, driving the eruption. Subject headingg s: MHD -Sun: magnetic fields -Sun: photosphere

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Active regions on the solar surface are generally thought to originate from a strong toroidal magnetic field generated by a deep seated solar dynamo mechanism operating at the base of the solar convection zone. Thus the magnetic fields need to traverse the entire convection zone before they reach the photosphere to form the observed solar active regions. Understanding this process of active region flux emergence is therefore a crucial component for the study of the solar cycle dynamo. This article reviews studies with regard to the formation and rise of active region scale magnetic flux tubes in the solar convection zone and their emergence into the solar atmosphere as active regions. The date given as then uniquely identifies the version of the article you are referring to. Article RevisionsLiving Reviews supports two different ways to keep its articles up-to-date:Fast-track revision A fast-track revision provides the author with the opportunity to add short notices of current research results, trends and developments, or important publications to the article. A fast-track revision is refereed by the responsible subject editor. If an article has undergone a fast-track revision, a summary of changes will be listed here.Major update A major update will include substantial changes and additions and is subject to full external refereeing. It is published with a new publication number.

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We report the results of a magneto-hydrodynamic (MHD) simulation of a convective dynamo in a model solar convective envelope driven by the solar radiative diffusive heat flux. The convective dynamo produces a large-scale mean magnetic field that exhibits irregular cyclic behavior with oscillation time scales ranging from about 5 to 15 years and undergoes irregular polarity reversals. The mean axisymmetric toroidal magnetic field is of opposite signs in the two hemispheres and is concentrated at the bottom of the convection zone. The presence of the magnetic fields is found to play an important role in the self-consistent maintenance of a solar-like differential rotation in the convective dynamo model. Without the magnetic fields, the convective flows drive a differential rotation with a faster rotating polar region. In the midst of magneto-convection, we found emergence of strong super-equipartition flux bundles at the surface, exhibiting properties that are similar to emerging solar active regions.

We present a 3D simulation of the dynamic emergence of a twisted magnetic flux tube from the top layer of the solar convection zone into the solar atmosphere and corona. It is found that after a brief initial stage of flux emergence during which the two polarities of the bipolar region become separated and the tubes intersecting the photosphere become vertical, significant rotational motion sets in within each polarity. The rotational motions of the two polarities are found to twist up the inner field lines of the emerged fields such that they change their orientation into an inverse configuration (i.e. pointing from the negative polarity to the positive polarity over the neutral line). As a result, a flux rope with sigmoid-shaped, dipped core fields form in the corona, and the center of the flux rope rises in the corona with increasing velocity as the twisting of the flux rope footpoints continues. The rotational motion in the two polarities is a result of propagation of non-linear torsional Alfvén waves along the flux tube, which transports significant twist from the tube's interior portion towards its expanded coronal portion. This is a basic process whereby twisted flux ropes are developed in the corona with increasing twist and magnetic energy, leading up to solar eruptions.

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