Author name code: moreno-insertis ADS astronomy entries on 2022-09-14 author:"Moreno-Insertis, Fernando" ------------------------------------------------------------------------ Title: A 2D Model for Coronal Bright Points: Association with Spicules, UV Bursts, Surges, and EUV Coronal Jets Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F. Bibcode: 2022ApJ...935L..21N Altcode: 2022arXiv220804308N Coronal bright points (CBPs) are ubiquitous structures in the solar atmosphere composed of hot small-scale loops observed in extreme-ultraviolet (EUV) or X-rays in the quiet Sun and coronal holes. They are key elements to understanding the heating of the corona; nonetheless, basic questions regarding their heating mechanisms, the chromosphere underneath, or the effects of flux emergence in these structures remain open. We have used the Bifrost code to carry out a 2D experiment in which a coronal-hole magnetic null-point configuration evolves perturbed by realistic granulation. To compare with observations, synthetic SDO/AIA, Solar Orbiter EUI-HRI, and IRIS images have been computed. The experiment shows the self-consistent creation of a CBP through the action of stochastic granular motions alone, mediated by magnetic reconnection in the corona. The reconnection is intermittent and oscillatory, and it leads to coronal and transition-region temperature loops that are identifiable in our EUV/UV observables. During the CBP lifetime, convergence and cancellation at the surface of its underlying opposite polarities takes place. The chromosphere below the CBP shows a number of peculiar features concerning its density and the spicules in it. The final stage of the CBP is eruptive: Magnetic flux emergence at the granular scale disrupts the CBP topology, leading to different ejections, such as UV bursts, surges, and EUV coronal jets. Apart from explaining observed CBP features, our results pave the way for further studies combining simulations and coordinated observations in different atmospheric layers. Title: Ambipolar diffusion: Self-similar solutions and MHD code testing. Cylindrical symmetry Authors: Moreno-Insertis, F.; Nóbrega-Siverio, D.; Priest, E. R.; Hood, A. W. Bibcode: 2022A&A...662A..42M Altcode: 2022arXiv220306272M Context. Ambipolar diffusion is a process occurring in partially ionised astrophysical systems that imparts a complicated mathematical and physical nature to Ohm's law. The numerical codes that solve the magnetohydrodynamic (MHD) equations have to be able to deal with the singularities that are naturally created in the system by the ambipolar diffusion term.
Aims: The global aim is to calculate a set of theoretical self-similar solutions to the nonlinear diffusion equation with cylindrical symmetry that can be used as tests for MHD codes which include the ambipolar diffusion term.
Methods: First, following the general methods developed in the applied mathematics literature, we obtained the theoretical solutions as eigenfunctions of a nonlinear ordinary differential equation. Phase-plane techniques were used to integrate through the singularities at the locations of the nulls, which correspond to infinitely sharp current sheets. In the second half of the paper, we consider the use of these solutions as tests for MHD codes. To that end, we used the Bifrost code, thereby testing the capabilities of these solutions as tests as well as (inversely) the accuracy of Bifrost's recently developed ambipolar diffusion module.
Results: The obtained solutions are shown to constitute a demanding, but nonetheless viable, test for MHD codes that incorporate ambipolar diffusion. Detailed tabulated runs of the solutions have been made available at a public repository. The Bifrost code is able to reproduce the theoretical solutions with sufficient accuracy up to very advanced diffusive times. Using the code, we also explored the asymptotic properties of our theoretical solutions in time when initially perturbed with either small or finite perturbations.
Conclusions: The functions obtained in this paper are relevant as physical solutions and also as tests for general MHD codes. They provide a more stringent and general test than the simple Zeldovich-Kompaneets-Barenblatt-Pattle solution.

Movies associated to Figs. 4 and 7 are available at https://www.aanda.org Title: Large-amplitude Prominence Oscillations following Impact by a Coronal Jet Authors: Luna, Manuel; Moreno-Insertis, Fernando Bibcode: 2021ApJ...912...75L Altcode: 2021arXiv210302661L Observational evidence shows that coronal jets can hit prominences and set them in motion. The impact leads to large-amplitude oscillations (LAOs) in the prominence. In this paper, we attempt to understand this process via 2.5D MHD numerical experiments. In our model, the jets are generated in a sheared magnetic arcade above a parasitic bipolar region located in one of the footpoints of the filament channel (FC) supporting the prominence. The shear is imposed at velocities not far above the observed photospheric values; this leads to a multiple reconnection process, as obtained in previous jet models. Both a fast Alfvénic perturbation and a slower supersonic front preceding a plasma jet are issued from the reconnection site; in the later phase, a more violent (eruptive) jet is produced. The perturbation and jets run along the FC; they are partially reflected at the prominence, and partially transmitted through it. This results in a pattern of counter-streaming flows along the FC, and oscillations in the prominence. The oscillations are LAOs (i.e., with amplitudes above 10 km s-1) in some areas of the prominence, both in the longitudinal and transverse directions. In some field lines, the impact is so strong that the prominence mass is brought out of the dip and down to the chromosphere along the FC. Two cases are studied, with respect to arcades at different heights above the parasitic bipolar region, leading to different heights for the region of the prominence perturbed by the jets. The obtained oscillation amplitudes and periods are in general agreement with the observations. Title: The chromospheric component of coronal bright points. Coronal and chromospheric responses to magnetic-flux emergence Authors: Madjarska, Maria S.; Chae, Jongchul; Moreno-Insertis, Fernando; Hou, Zhenyong; Nóbrega-Siverio, Daniel; Kwak, Hannah; Galsgaard, Klaus; Cho, Kyuhyoun Bibcode: 2021A&A...646A.107M Altcode: 2020arXiv201209426M Context. We investigate the chromospheric counterpart of small-scale coronal loops constituting a coronal bright point (CBP) and its response to a photospheric magnetic-flux increase accompanied by co-temporal CBP heating.
Aims: The aim of this study is to simultaneously investigate the chromospheric and coronal layers associated with a CBP, and in so doing, provide further understanding on the heating of plasmas confined in small-scale loops.
Methods: We used co-observations from the Atmospheric Imaging Assembly and Helioseismic Magnetic Imager on board the Solar Dynamics Observatory, together with data from the Fast Imaging Solar Spectrograph taken in the Hα and Ca II 8542.1 Å lines. We also employed both linear force-free and potential field extrapolation models to investigate the magnetic topology of the CBP loops and the overlying corona, respectively. We used a new multi-layer spectral inversion technique to derive the temporal variations of the temperature of the Hα loops (HLs).
Results: We find that the counterpart of the CBP, as seen at chromospheric temperatures, is composed of a bundle of dark elongated features named in this work Hα loops, which constitute an integral part of the CBP loop magnetic structure. An increase in the photospheric magnetic flux due to flux emergence is accompanied by a rise of the coronal emission of the CBP loops, that is a heating episode. We also observe enhanced chromospheric activity associated with the occurrence of new HLs and mottles. While the coronal emission and magnetic flux increases appear to be co-temporal, the response of the Hα counterpart of the CBP occurs with a small delay of less than 3 min. A sharp temperature increase is found in one of the HLs and in one of the CBP footpoints estimated at 46% and 55% with respect to the pre-event values, also starting with a delay of less than 3 min following the coronal heating episode. The low-lying CBP loop structure remains non-potential for the entire observing period. The magnetic topological analysis of the overlying corona reveals the presence of a coronal null point at the beginning and towards the end of the heating episode.
Conclusions: The delay in the response of the chromospheric counterpart of the CBP suggests that the heating may have occurred at coronal heights.

Movies are available at https://www.aanda.org Title: Case study of multi-temperature coronal jets for emerging flux MHD models Authors: Joshi, Reetika; Chandra, Ramesh; Schmieder, Brigitte; Moreno-Insertis, Fernando; Aulanier, Guillaume; Nóbrega-Siverio, Daniel; Devi, Pooja Bibcode: 2020A&A...639A..22J Altcode: 2020arXiv200506064J Context. Hot coronal jets are a basic observed feature of the solar atmosphere whose physical origin is still actively debated.
Aims: We study six recurrent jets that occurred in active region NOAA 12644 on April 4, 2017. They are observed in all the hot filters of AIA as well as cool surges in IRIS slit-jaw high spatial and temporal resolution images.
Methods: The AIA filters allow us to study the temperature and the emission measure of the jets using the filter ratio method. We studied the pre-jet phases by analysing the intensity oscillations at the base of the jets with the wavelet technique.
Results: A fine co-alignment of the AIA and IRIS data shows that the jets are initiated at the top of a canopy-like double-chambered structure with cool emission on one and hot emission on the other side. The hot jets are collimated in the hot temperature filters, have high velocities (around 250 km s-1) and are accompanied by cool surges and ejected kernels that both move at about 45 km s-1. In the pre-phase of the jets, we find quasi-periodic intensity oscillations at their base that are in phase with small ejections; they have a period of between 2 and 6 min, and are reminiscent of acoustic or magnetohydrodynamic waves.
Conclusions: This series of jets and surges provides a good case study for testing the 2D and 3D magnetohydrodynamic emerging flux models. The double-chambered structure that is found in the observations corresponds to the regions with cold and hot loops that are in the models below the current sheet that contains the reconnection site. The cool surge with kernels is comparable with the cool ejection and plasmoids that naturally appears in the models.

Movies are available at https://www.aanda.org Title: Ambipolar diffusion in the Bifrost code Authors: Nóbrega-Siverio, D.; Martínez-Sykora, J.; Moreno-Insertis, F.; Carlsson, M. Bibcode: 2020A&A...638A..79N Altcode: 2020arXiv200411927N Context. Ambipolar diffusion is a physical mechanism related to the drift between charged and neutral particles in a partially ionized plasma that is key to many different astrophysical systems. However, understanding its effects is challenging due to basic uncertainties concerning relevant microphysical aspects and the strong constraints it imposes on the numerical modeling.
Aims: Our aim is to introduce a numerical tool that allows us to address complex problems involving ambipolar diffusion in which, additionally, departures from ionization equilibrium are important or high resolution is needed. The primary application of this tool is for solar atmosphere calculations, but the methods and results presented here may also have a potential impact on other astrophysical systems.
Methods: We have developed a new module for the stellar atmosphere Bifrost code that improves its computational capabilities of the ambipolar diffusion term in the generalized Ohm's law. This module includes, among other things, collision terms adequate to processes in the coolest regions in the solar chromosphere. As the main feature of the module, we have implemented the super time stepping (STS) technique, which allows an important acceleration of the calculations. We have also introduced hyperdiffusion terms to guarantee the stability of the code.
Results: We show that to have an accurate value for the ambipolar diffusion coefficient in the solar atmosphere it is necessary to include as atomic elements in the equation of state not only hydrogen and helium, but also the main electron donors like sodium, silicon, and potassium. In addition, we establish a range of criteria to set up an automatic selection of the free parameters of the STS method that guarantees the best performance, optimizing the stability and speed for the ambipolar diffusion calculations. We validate the STS implementation by comparison with a self-similar analytical solution. Title: Quasi Periodic Oscillations in the Pre Phases of Recurrent Jets Highlighting Plasmoids in Current Sheet Authors: Joshi, Reetika; Chandra, Ramesh; Schmieder, Brigitte; Aulanier, Guillaume; Devi, Pooja; Moreno-Insertis, Fernando; Nóbrega-Siverio, Daniel Bibcode: 2020EGUGA..2222351J Altcode: Solar jets observed at the limb are important to determine the location of reconnection sites in the corona. In this study, we investigate six recurrent hot and cool jets occurring in the active region NOAA 12644 as it is crossing the west limb on April 04, 2017. These jets are observed in all the UV/EUV filters of SDO/AIA and in cooler temperature formation lines in IRIS slit jaw images. The jets are initiated at the top of a double chamber vault with cool loops on one side and hot loops on the other side. The existence of such double chamber vaults suggests the presence of emerging flux with cool loops, the hot loops being the reconnected loops similarly as in the models of Moreno-Insertiset al. 2008, 2013 and Nóbrega-Siverio et al. 2016. In the preliminary phase of the main jets, quasi periodic intensity oscillations accompanied by smaller jets are detected in the bright current sheet between the vault and the preexisting magnetic field. Individual kernels and plasmoids are ejected in open field lines along the jets. Plasmoids may launch torsional Alfven waves and the kernels would be the result of the untwist of the plasmoids in open magnetic field as proposed in the model of Wyper et al. 2016. Title: Nonequilibrium ionization and ambipolar diffusion in solar magnetic flux emergence processes Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F.; Martínez-Sykora, J.; Carlsson, M.; Szydlarski, M. Bibcode: 2020A&A...633A..66N Altcode: 2019arXiv191201015N Context. Magnetic flux emergence from the solar interior has been shown to be a key mechanism for unleashing a wide variety of phenomena. However, there are still open questions concerning the rise of the magnetized plasma through the atmosphere, mainly in the chromosphere, where the plasma departs from local thermodynamic equilibrium (LTE) and is partially ionized.
Aims: We aim to investigate the impact of the nonequilibrium (NEQ) ionization and recombination and molecule formation of hydrogen, as well as ambipolar diffusion, on the dynamics and thermodynamics of the flux emergence process.
Methods: Using the radiation-magnetohydrodynamic Bifrost code, we performed 2.5D numerical experiments of magnetic flux emergence from the convection zone up to the corona. The experiments include the NEQ ionization and recombination of atomic hydrogen, the NEQ formation and dissociation of H2 molecules, and the ambipolar diffusion term of the generalized Ohm's law.
Results: Our experiments show that the LTE assumption substantially underestimates the ionization fraction in most of the emerged region, leading to an artificial increase in the ambipolar diffusion and, therefore, in the heating and temperatures as compared to those found when taking the NEQ effects on the hydrogen ion population into account. We see that LTE also overestimates the number density of H2 molecules within the emerged region, thus mistakenly magnifying the exothermic contribution of the H2 molecule formation to the thermal energy during the flux emergence process. We find that the ambipolar diffusion does not significantly affect the amount of total unsigned emerged magnetic flux, but it is important in the shocks that cross the emerged region, heating the plasma on characteristic times ranging from 0.1 to 100 s. We also briefly discuss the importance of including elements heavier than hydrogen in the equation of state so as not to overestimate the role of ambipolar diffusion in the atmosphere.

Movies associated to Figs. 2-5, 8, 9, and A.1 are available at https://www.aanda.org Title: Self-similar Approach for Rotating Magnetohydrodynamic Solar and Astrophysical Structures Authors: Luna, M.; Priest, E.; Moreno-Insertis, F. Bibcode: 2018ApJ...863..147L Altcode: 2018arXiv180702473L Rotating magnetic structures are common in astrophysics, from vortex tubes and tornadoes in the Sun all the way to jets in different astrophysical systems. The physics of these objects often combine inertial, magnetic, gas pressure, and gravitational terms. Also, they often show approximate symmetries that help simplify the otherwise rather intractable equations governing their morphology and evolution. Here we propose a general formulation of the equations assuming axisymmetry and a self-similar form for all variables: in spherical coordinates (r, θ, ϕ), the magnetic field and plasma velocity are taken to be of the form {\boldsymbol{B}}={\boldsymbol{f}}(θ )/{r}n and {\boldsymbol{v}}={\boldsymbol{g}}(θ )/{r}m, with corresponding expressions for the scalar variables like pressure and density. Solutions are obtained for potential, force-free, and non-force-free magnetic configurations. Potential field solutions can be found for all values of n. Nonpotential force-free solutions possess an azimuthal component B ϕ and exist only for n ≥ 2 the resulting structures are twisted and have closed field lines but are not collimated around the system axis. In the non-force-free case, including gas pressure, the magnetic field lines acquire an additional curvature to compensate for an outward pointing pressure gradient force. We have also considered a pure rotation situation with no gravity, in the zero-β limit: the solution has cylindrical geometry and twisted magnetic field lines. The latter solutions can be helpful in producing a collimated magnetic field structure; but they exist only when n < 0 and m < 0: for applications they must be matched to an external system at a finite distance from the origin. Title: Small-scale Magnetic Flux Emergence in the Quiet Sun Authors: Moreno-Insertis, F.; Martinez-Sykora, J.; Hansteen, V. H.; Muñoz, D. Bibcode: 2018ApJ...859L..26M Altcode: 2018arXiv180600489M Small bipolar magnetic features are observed to appear in the interior of individual granules in the quiet Sun, signaling the emergence of tiny magnetic loops from the solar interior. We study the origin of those features as part of the magnetoconvection process in the top layers of the convection zone. Two quiet-Sun magnetoconvection models, calculated with the radiation-magnetohydrodynamic (MHD) Bifrost code and with domain stretching from the top layers of the convection zone to the corona, are analyzed. Using 3D visualization as well as a posteriori spectral synthesis of Stokes parameters, we detect the repeated emergence of small magnetic elements in the interior of granules, as in the observations. Additionally, we identify the formation of organized horizontal magnetic sheets covering whole granules. Our approach is twofold, calculating statistical properties of the system, like joint probability density functions (JPDFs), and pursuing individual events via visualization tools. We conclude that the small magnetic loops surfacing within individual granules in the observations may originate from sites at or near the downflows in the granular and mesogranular levels, probably in the first 1 or 1.5 Mm below the surface. We also document the creation of granule-covering magnetic sheet-like structures through the sideways expansion of a small subphotospheric magnetic concentration picked up and pulled out of the interior by a nascent granule. The sheet-like structures that we found in the models may match the recent observations of Centeno et al. Title: On the Importance of the Nonequilibrium Ionization of Si IV and O IV and the Line of Sight in Solar Surges Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F.; Martínez-Sykora, J. Bibcode: 2018ApJ...858....8N Altcode: 2018arXiv180310251N Surges are ubiquitous cool ejections in the solar atmosphere that often appear associated with transient phenomena like UV bursts or coronal jets. Recent observations from the Interface Region Imaging Spectrograph show that surges, although traditionally related to chromospheric lines, can exhibit enhanced emission in Si IV with brighter spectral profiles than for the average transition region (TR). In this paper, we explain why surges are natural sites to show enhanced emissivity in TR lines. We performed 2.5D radiative-MHD numerical experiments using the Bifrost code including the nonequilibrium (NEQ) ionization of silicon and oxygen. A surge is obtained as a by-product of magnetic flux emergence; the TR enveloping the emerged domain is strongly affected by NEQ effects: assuming statistical equilibrium would produce an absence of Si IV and O IV ions in most of the region. Studying the properties of the surge plasma emitting in the Si IV λ1402.77 and O IV λ1401.16 lines, we find that (a) the timescales for the optically thin losses and heat conduction are very short, leading to departures from statistical equilibrium, and (b) the surge emits in Si IV more and has an emissivity ratio of Si IV to O IV larger than a standard TR. Using synthetic spectra, we conclude the importance of line-of-sight effects: given the involved geometry of the surge, the line of sight can cut the emitting layer at small angles and/or cross it multiple times, causing prominent, spatially intermittent brightenings in both Si IV and O IV. Title: Surges and Si IV Bursts in the Solar Atmosphere: Understanding IRIS and SST Observations through RMHD Experiments Authors: Nóbrega-Siverio, D.; Martínez-Sykora, J.; Moreno-Insertis, F.; Rouppe van der Voort, L. Bibcode: 2017ApJ...850..153N Altcode: 2017arXiv171008928N Surges often appear as a result of the emergence of magnetized plasma from the solar interior. Traditionally, they are observed in chromospheric lines such as Hα 6563 \mathringA and Ca II 8542 \mathringA . However, whether there is a response to the surge appearance and evolution in the Si IV lines or, in fact, in many other transition region lines has not been studied. In this paper, we analyze a simultaneous episode of an Hα surge and a Si IV burst that occurred on 2016 September 03 in active region AR 12585. To that end, we use coordinated observations from the Interface Region Imaging Spectrograph and the Swedish 1-m Solar Telescope. For the first time, we report emission of Si IV within the surge, finding profiles that are brighter and broader than the average. Furthermore, the brightest Si IV patches within the domain of the surge are located mainly near its footpoints. To understand the relation between the surges and the emission in transition region lines like Si IV, we have carried out 2.5D radiative MHD (RMHD) experiments of magnetic flux emergence episodes using the Bifrost code and including the nonequilibrium ionization of silicon. Through spectral synthesis, we explain several features of the observations. We show that the presence of Si IV emission patches within the surge, their location near the surge footpoints and various observed spectral features are a natural consequence of the emergence of magnetized plasma from the interior to the atmosphere and the ensuing reconnection processes. Title: Magnetic topological analysis of coronal bright points Authors: Galsgaard, K.; Madjarska, M. S.; Moreno-Insertis, F.; Huang, Z.; Wiegelmann, T. Bibcode: 2017A&A...606A..46G Altcode: 2017arXiv170704174G Context. We report on the first of a series of studies on coronal bright points which investigate the physical mechanism that generates these phenomena.
Aims: The aim of this paper is to understand the magnetic-field structure that hosts the bright points.
Methods: We use longitudinal magnetograms taken by the Solar Optical Telescope with the Narrowband Filter Imager. For a single case, magnetograms from the Helioseismic and Magnetic Imager were added to the analysis. The longitudinal magnetic field component is used to derive the potential magnetic fields of the large regions around the bright points. A magneto-static field extrapolation method is tested to verify the accuracy of the potential field modelling. The three dimensional magnetic fields are investigated for the presence of magnetic null points and their influence on the local magnetic domain.
Results: In nine out of ten cases the bright point resides in areas where the coronal magnetic field contains an opposite polarity intrusion defining a magnetic null point above it. We find that X-ray bright points reside, in these nine cases, in a limited part of the projected fan-dome area, either fully inside the dome or expanding over a limited area below which typically a dominant flux concentration resides. The tenth bright point is located in a bipolar loop system without an overlying null point.
Conclusions: All bright points in coronal holes and two out of three bright points in quiet Sun regions are seen to reside in regions containing a magnetic null point. An as yet unidentified process(es) generates the brigh points in specific regions of the fan-dome structure.

The movies are available at http://www.aanda.org Title: Solar Coronal Jets: Observations, Theory, and Modeling Authors: Raouafi, N. E.; Patsourakos, S.; Pariat, E.; Young, P. R.; Sterling, A. C.; Savcheva, A.; Shimojo, M.; Moreno-Insertis, F.; DeVore, C. R.; Archontis, V.; Török, T.; Mason, H.; Curdt, W.; Meyer, K.; Dalmasse, K.; Matsui, Y. Bibcode: 2016SSRv..201....1R Altcode: 2016arXiv160702108R; 2016SSRv..tmp...31R Coronal jets represent important manifestations of ubiquitous solar transients, which may be the source of significant mass and energy input to the upper solar atmosphere and the solar wind. While the energy involved in a jet-like event is smaller than that of "nominal" solar flares and coronal mass ejections (CMEs), jets share many common properties with these phenomena, in particular, the explosive magnetically driven dynamics. Studies of jets could, therefore, provide critical insight for understanding the larger, more complex drivers of the solar activity. On the other side of the size-spectrum, the study of jets could also supply important clues on the physics of transients close or at the limit of the current spatial resolution such as spicules. Furthermore, jet phenomena may hint to basic process for heating the corona and accelerating the solar wind; consequently their study gives us the opportunity to attack a broad range of solar-heliospheric problems. Title: Mini-CME eruptions in a flux emergence event in a coronal hole environment Authors: Galsgaard, K.; Moreno-Insertis, F. Bibcode: 2016usc..confE..64G Altcode: Small scale jets are observed to take place at the interface between the open magnetic field in coronal holes and bipolar magnetic field concentrations. A fraction of these shows an eruptive behavior, where a combination of cold dense and hot light plasma has been observed to propagate out along the jet region, combining traditional jets with what looks like the eruption of mini-CMEs. Here we discuss a simple model scenario for the explosive energy release process that leads to a mixture of hot and cold plasma being accelerated upwards simultaneously. The model explains both the typical steady state inverted-Y jet and the subsequent mini-CME eruptions found in blowout jets. The numerical experiment consists of a buoyant unstable flux rope that emerges into an overlying slanted coronal field, thereby creating a bipolar magnetic field distribution in the photosphere with coronal loops linking the polarities. Reconnection between the emerged and preexisting magnetic systems including the launching of a classical inverted-Y jet. The experiment shows that this simple model provides for a very complicated dynamical behavior in its late phases. Five independent mini-CME eruptions follow the initial near steady-state jet phase. The first one is a direct consequence of the reconnection of the emerged magnetic flux, is mediated by the formation of a strongly sheared arcade followed by a tether-cutting reconnection process, and leads to the eruption of a twisted flux rope. The final four explosive eruptions, instead, are preceded by the formation of a twisted torus-like flux rope near the strong magnetic concentrations at the photosphere. As the tube center starts emerging an internal current sheet is formed below it. This sheet experiences a tether cutting process that provides the important upwards kick of the newly formed mini-CME structure. As the fast rising cold and dense tube interacts with the overlying magnetic field, it reconnects at different spatial locations, either through a null region or through a local strong shear region without nulls. The restructuring of the magnetic field lines generate magneto-acoustic waves that transport twist and cold plasma out along the less stressed parts of the newly reconnected field lines. The emphasis of the talk will be on the physical forces responsible for the initial flux tube rising and the effects and reasons for the following destruction of the mini-CMEs. Title: The Cool Surge Following Flux Emergence in a Radiation-MHD Experiment Authors: Nóbrega-Siverio; D.; Moreno-Insertis, F.; Martínez-Sykora, J. Bibcode: 2016usc..confE..68N Altcode: Cool and dense ejections, typically Hα surges, often appear alongside EUV or X-ray coronal jets as a result of the emergence of magnetized plasma from the solar interior. Idealized numerical experiments explain those ejections as being indirectly associated with the magnetic reconnection taking place between the emerging and preexisting systems. However, those experiments miss basic elements that can importantly affect the surge phenomenon. In this paper we study the cool surges using a realistic treatment of the radiation transfer and material plasma properties. To that end, the Bifrost code is used, which has advanced modules for the equation of state of the plasma, photospheric and chromospheric radiation transfer, heat conduction, and optically thin radiative cooling. We carry out a 2.5D experiment of the emergence of magnetized plasma through (meso) granular convection cells and the low atmosphere to the corona. Through detailed Lagrange tracing we study the formation and evolution of the cool ejection and, in particular, the role of the entropy sources; this allows us to discern families of evolutionary patterns for the plasma elements. In the launch phase, many elements suffer accelerations well in excess of gravity; when nearing the apex of their individual trajectories, instead, the plasma elements follow quasi-parabolic trajectories with accelerations close to the solar gravity . We show how the formation of the cool ejection is mediated by a wedge-like structure composed of two shocks, one of which leads to the detachment of the surge from the original emerged plasma dome. Title: The Cool Surge Following Flux Emergence in a Radiation-MHD Experiment Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F.; Martínez-Sykora, J. Bibcode: 2016ApJ...822...18N Altcode: 2016arXiv160104074N Cool and dense ejections, typically Hα surges, often appear alongside EUV or X-ray coronal jets as a result of the emergence of magnetized plasma from the solar interior. Idealized numerical experiments explain those ejections as being indirectly associated with the magnetic reconnection taking place between the emerging and preexisting systems. However, those experiments miss basic elements that can importantly affect the surge phenomenon. In this paper we study the cool surges using a realistic treatment of the radiation transfer and material plasma properties. To that end, the Bifrost code is used, which has advanced modules for the equation of state of the plasma, photospheric and chromospheric radiation transfer, heat conduction, and optically thin radiative cooling. We carry out a 2.5D experiment of the emergence of magnetized plasma through (meso) granular convection cells and the low atmosphere to the corona. Through detailed Lagrange tracing we study the formation and evolution of the cool ejection and, in particular, the role of the entropy sources; this allows us to discern families of evolutionary patterns for the plasma elements. In the launch phase, many elements suffer accelerations well in excess of gravity; when nearing the apex of their individual trajectories, instead, the plasma elements follow quasi-parabolic trajectories with accelerations close to {g}⊙ . We show how the formation of the cool ejection is mediated by a wedge-like structure composed of two shocks, one of which leads to the detachment of the surge from the original emerged plasma dome. Title: Commission 12: Solar Radiation and Structure Authors: Cauzzi, Gianna; Shchukina, Nataliya; Kosovichev, Alexander; Bianda, Michele; Brandenburg, Axel; Chou, Dean-Yi; Dasso, Sergio; Ding, Ming-De; Jefferies, Stuart; Krivova, Natalie; Kuznetsov, Vladimir D.; Moreno-Insertis, Fernando Bibcode: 2016IAUTA..29..278C Altcode: Commission 12 of the International Astronomical Union encompasses investigations of the internal structure and dynamics of the Sun, the quiet solar atmosphere, solar radiation and its variability, and the nature of relatively stable magnetic structures like sunspots, faculae and the magnetic network. The Commission sees participation of over 300 scientists worldwide. Title: Are Tornado-Like Magnetic Structures Able to Support Solar Prominence Plasma? Authors: Ogunjo, S. T.; Luna Bennasar, M.; Moreno-Insertis, F.; Priest, E. R. Bibcode: 2015AGUFMSH53B2483O Altcode: Recent high-resolution and high-cadence observations have surprisingly suggested that prominence barbs exhibit apparent rotating motions suggestive of a tornado-like structure. Additional evidence has been provided by Doppler measurements. The observations reveal opposite velocities for both hot and cool plasma on the two sides of a prominence barb. This motion is persistent for several hours and has been interpreted in terms of rotational motion of prominence feet. Several authors suggest that such barb motions are rotating helical structures around a vertical axis similar to tornadoes on Earth. One of the difficulties of such a proposal is how to support cool prominence plasma in almost-vertical structures against gravity. In this work we model analytically a tornado-like structure and try to determine possible mechanisms to support the prominence plasma. We have found that the Lorentz force can indeed support the barb plasma provided the magnetic structure is sufficiently twisted and/or significant poloidal flows are present. Title: Multi-parametric Study of Rising 3D Buoyant Flux Tubes in an Adiabatic Stratification Using AMR Authors: Martínez-Sykora, Juan; Moreno-Insertis, Fernando; Cheung, Mark C. M. Bibcode: 2015ApJ...814....2M Altcode: 2015arXiv150701506M We study the buoyant rise of magnetic flux tubes embedded in an adiabatic stratification using two-and three-dimensional, magnetohydrodynamic simulations. We analyze the dependence of the tube evolution on the field line twist and on the curvature of the tube axis in different diffusion regimes. To be able to achieve a comparatively high spatial resolution we use the FLASH code, which has a built-in Adaptive Mesh Refinement (AMR) capability. Our 3D experiments reach Reynolds numbers that permit a reasonable comparison of the results with those of previous 2D simulations. When the experiments are run without AMR, hence with a comparatively large diffusivity, the amount of longitudinal magnetic flux retained inside the tube increases with the curvature of the tube axis. However, when a low-diffusion regime is reached by using the AMR algorithms, the magnetic twist is able to prevent the splitting of the magnetic loop into vortex tubes and the loop curvature does not play any significant role. We detect the generation of vorticity in the main body of the tube of opposite sign on the opposite sides of the apex. This is a consequence of the inhomogeneity of the azimuthal component of the field on the flux surfaces. The lift force associated with this global vorticity makes the flanks of the tube move away from their initial vertical plane in an antisymmetric fashion. The trajectories have an oscillatory motion superimposed, due to the shedding of vortex rolls to the wake, which creates a Von Karman street. Title: Are Tornado-like Magnetic Structures Able to Support Solar Prominence Plasma? Authors: Luna, M.; Moreno-Insertis, F.; Priest, E. Bibcode: 2015ApJ...808L..23L Altcode: 2015arXiv150701455L Recent high-resolution and high-cadence observations have surprisingly suggested that prominence barbs exhibit apparent rotating motions suggestive of a tornado-like structure. Additional evidence has been provided by Doppler measurements. The observations reveal opposite velocities for both hot and cool plasma on the two sides of a prominence barb. This motion is persistent for several hours and has been interpreted in terms of rotational motion of prominence feet. Several authors suggest that such barb motions are rotating helical structures around a vertical axis similar to tornadoes on Earth. One of the difficulties of such a proposal is how to support cool prominence plasma in almost-vertical structures against gravity. In this work we model analytically a tornado-like structure and try to determine possible mechanisms to support the prominence plasma. We have found that the Lorentz force can indeed support the barb plasma provided the magnetic structure is sufficiently twisted and/or significant poloidal flows are present. Title: Continuum Intensity and [O I] Spectral Line Profiles in Solar 3D Photospheric Models: The Effect of Magnetic Fields Authors: Fabbian, D.; Moreno-Insertis, F. Bibcode: 2015ApJ...802...96F Altcode: 2015arXiv150106916F The importance of magnetic fields in three-dimensional (3D) magnetoconvection models of the Sun’s photosphere is investigated in terms of their influence on the continuum intensity at different viewing inclination angles and on the intensity profile of two [O i] spectral lines. We use the RH numerical radiative transfer code to perform a posteriori spectral synthesis on the same time series of magnetoconvection models used in our publications on the effect of magnetic fields on abundance determination. We obtain a good match of the synthetic disk-center continuum intensity to the absolute continuum values from the Fourier Transform Spectrometer (FTS) observational spectrum; the match of the center-to-limb variation synthetic data to observations is also good, thanks, in part, to the 3D radiation transfer capabilities of the RH code. The different levels of magnetic flux in the numerical time series do not modify the quality of the match. Concerning the targeted [O i] spectral lines, we find, instead, that magnetic fields lead to nonnegligible changes in the synthetic spectrum, with larger average magnetic flux causing both of the lines to become noticeably weaker. The photospheric oxygen abundance that one would derive if instead using nonmagnetic numerical models would thus be lower by a few to several centidex. The inclusion of magnetic fields is confirmed to be important for improving the current modeling of the Sun, here in particular in terms of spectral line formation and of deriving consistent chemical abundances. These results may shed further light on the still controversial issue regarding the precise value of the solar oxygen abundance. Title: The power spectrum of solar convection flows from high-resolution observations and 3D simulations Authors: Yelles Chaouche, L.; Moreno-Insertis, F.; Bonet, J. A. Bibcode: 2014A&A...563A..93Y Altcode: 2014arXiv1402.2293Y Context. Understanding solar surface magnetoconvection requires the study of the Fourier spectra of the velocity fields. Nowadays, observations are available that resolve very small spatial scales, well into the subgranular range, almost reaching the scales routinely resolved in numerical magnetoconvection simulations. Comparison of numerical and observational data at present can provide an assessment of the validity of the observational proxies.
Aims: Our aims are: (1) to obtain Fourier spectra for the photospheric velocity fields using the spectropolarimetric observations with the highest spatial resolution so far (~120 km), thus reaching for the first time spatial scales well into the subgranular range; (2) to calculate corresponding Fourier spectra from realistic 3D numerical simulations of magnetoconvection and carry out a proper comparison with their observational counterparts considering the residual instrumental degradation in the observational data; and (3) to test the observational proxies on the basis of the numerical data alone, by comparing the actual velocity field in the simulations with synthetic observations obtained from the numerical boxes.
Methods: (a) For the observations, data from the SUNRISE/IMaX spectropolarimeter are used. (b) For the simulations, we use four series of runs obtained with the STAGGER code for different average signed vertical magnetic field values (0, 50, 100, and 200 G). Spectral line profiles are synthesized from the numerical boxes for the same line observed by IMaX (Fe I 5250.2 Å) and degraded to match the performance of the IMaX instrument. Proxies for the velocity field are obtained via Dopplergrams (vertical component) and local correlation tracking (LCT, for the horizontal component). Fourier power spectra are calculated and a comparison between the synthetic and observational data sets carried out. (c) For the internal comparison of the numerical data, velocity values on constant optical depth surfaces are used instead of on horizontal planes.
Results: A very good match between observational and simulated Fourier power spectra is obtained for the vertical velocity data for scales between 200 km and 6 Mm. Instead, a clear vertical shift is obtained when the synthetic observations are not degraded to emulate the degradation in the IMaX data. The match for the horizontal velocity data is much less impressive because of the inaccuracies of the LCT procedure. Concerning the internal comparison of the direct velocity values of the numerical boxes with those from the synthetic observations, a high correlation (0.96) is obtained for the vertical component when using the velocity values on the log τ500 = -1 surface in the box. The corresponding Fourier spectra are near each other. A lower maximum correlation (0.5) is reached (at log τ500 = 0) for the horizontal velocities as a result of the coarseness of the LCT procedure. Correspondingly, the Fourier spectra for the LCT-determined velocities is well below that from the actual velocity components.
Conclusions: As measured by the Fourier spectra, realistic numerical simulations of surface magnetoconvection provide a very good match to the observational proxies for the photospheric velocity fields at least on scales from several Mm down to around 200 km. Taking into account the spatial and spectral instrumental blurring is essential for the comparison between simulations and observations. Dopplergrams are an excellent proxy for the vertical velocities on constant-τ isosurfaces, while LCT is a much less reliable method of determining the horizontal velocities. Title: Twisting solar coronal jet launched at the boundary of an active region Authors: Schmieder, B.; Guo, Y.; Moreno-Insertis, F.; Aulanier, G.; Yelles Chaouche, L.; Nishizuka, N.; Harra, L. K.; Thalmann, J. K.; Vargas Dominguez, S.; Liu, Y. Bibcode: 2013A&A...559A...1S Altcode: 2013arXiv1309.6514S
Aims: A broad jet was observed in a weak magnetic field area at the edge of active region NOAA 11106 that also produced other nearby recurring and narrow jets. The peculiar shape and magnetic environment of the broad jet raised the question of whether it was created by the same physical processes of previously studied jets with reconnection occurring high in the corona.
Methods: We carried out a multi-wavelength analysis using the EUV images from the Atmospheric Imaging Assembly (AIA) and magnetic fields from the Helioseismic and Magnetic Imager (HMI) both on-board the Solar Dynamics Observatory, which we coupled to a high-resolution, nonlinear force-free field extrapolation. Local correlation tracking was used to identify the photospheric motions that triggered the jet, and time-slices were extracted along and across the jet to unveil its complex nature. A topological analysis of the extrapolated field was performed and was related to the observed features.
Results: The jet consisted of many different threads that expanded in around 10 minutes to about 100 Mm in length, with the bright features in later threads moving faster than in the early ones, reaching a maximum speed of about 200 km s-1. Time-slice analysis revealed a striped pattern of dark and bright strands propagating along the jet, along with apparent damped oscillations across the jet. This is suggestive of a (un)twisting motion in the jet, possibly an Alfvén wave. Bald patches in field lines, low-altitude flux ropes, diverging flow patterns, and a null point were identified at the basis of the jet.
Conclusions: Unlike classical λ or Eiffel-tower-shaped jets that appear to be caused by reconnection in current sheets containing null points, reconnection in regions containing bald patches seems to be crucial in triggering the present jet. There is no observational evidence that the flux ropes detected in the topological analysis were actually being ejected themselves, as occurs in the violent phase of blowout jets; instead, the jet itself may have gained the twist of the flux rope(s) through reconnection. This event may represent a class of jets different from the classical quiescent or blowout jets, but to reach that conclusion, more observational and theoretical work is necessary. Title: Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations (Corrigendum) Authors: Beck, C.; Fabbian, D.; Moreno-Insertis, F.; Puschmann, K. G.; Rezaei, R. Bibcode: 2013A&A...559C...1B Altcode: No abstract at ADS Title: Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations Authors: Beck, C.; Fabbian, D.; Moreno-Insertis, F.; Puschmann, K. G.; Rezaei, R. Bibcode: 2013A&A...557A.109B Altcode: 2013arXiv1306.6093B Context. Numerical three-dimensional (3D) radiative (magneto-)hydrodynamical [(M)HD] simulations of solar convection are nowadays used to understand the physical properties of the solar photosphere and convective envelope, and, in particular, to determine the Sun's photospheric chemical abundances. To validate this approach, it is important to check that no excessive thermodynamic fluctuations arise as a consequence of the partially incomplete treatment of radiative transfer causing radiative damping that is too modest.
Aims: We investigate the realism of the thermodynamics in recent state-of-the-art 3D convection simulations of the solar atmosphere carried out with the Stagger code.
Methods: We compared the characteristic properties of several Fe i lines (557.6 nm, 630 nm, 1565 nm) and one Si i line at 1082.7 nm in solar disc-centre observations of different spatial resolution with spectra synthesized from 3D convection simulations. The observations were taken with ground-based (Echelle spectrograph, Göttingen Fabry-Pérot Interferometer (GFPI), POlarimetric LIttrow Spectrograph, Tenerife Infrared Polarimeter, all at the Vacuum Tower Telescope on Tenerife) and space-based instruments (Hinode/Spectropolarimeter). We degraded the synthetic spectra to the spatial resolution of the observations, based on the distribution of the continuum intensity Ic. We estimated the spectral degradation to be applied to the simulation results by comparing atlas spectra with averaged observed spectra. In addition to deriving a set of line parameters directly from the intensity profiles, we used the SIR (Stokes Inversion based on Response functions) code to invert the spectra.
Results: The spatial degradation kernels yield a similar generic spatial stray-light contamination of about 30% for all instruments. The spectral stray light inside the different spectrometers is found to be between 2% and 20%. Most of the line parameters from the observational data are matched by the degraded HD simulation spectra. The inversions predict a macroturbulent velocity vmac below 10 m s-1 for the HD simulation spectra at full spatial resolution, whereas they yield vmac ≲ 1000 m s-1 at a spatial resolution of 0.″3. The temperature fluctuations in the inversion of the degraded HD simulation spectra do not exceed those from the observational data (of the order of 100-200 K rms for -2 ⪉ log τ500 nm ⪉ -0.5). The comparison of line parameters in spatially averaged profiles with the averaged values of line parameters in spatially resolved profiles indicates a significant change in (average) line properties on a spatial scale between 0.″13 and 0.″3.
Conclusions: Up to a spatial resolution of 0.″3 (GFPI spectra), we find no indications of excessive thermodynamic fluctuations in the 3D HD simulation. To definitely confirm that simulations without spatial degradation contain fully realistic thermodynamic fluctuations requires observations at even higher spatial resolution (i.e. <0.″13).

Appendices A and B are available in electronic form at http://www.aanda.org Title: Plasma Jets and Eruptions in Solar Coronal Holes: A Three-dimensional Flux Emergence Experiment Authors: Moreno-Insertis, F.; Galsgaard, K. Bibcode: 2013ApJ...771...20M Altcode: 2013arXiv1305.2201M A three-dimensional (3D) numerical experiment of the launching of a hot and fast coronal jet followed by several violent eruptions is analyzed in detail. These events are initiated through the emergence of a magnetic flux rope from the solar interior into a coronal hole. We explore the evolution of the emerging magnetically dominated plasma dome surmounted by a current sheet and the ensuing pattern of reconnection. A hot and fast coronal jet with inverted-Y shape is produced that shows properties comparable to those frequently observed with EUV and X-ray detectors. We analyze its 3D shape, its inhomogeneous internal structure, and its rise and decay phases, lasting for some 15-20 minutes each. Particular attention is devoted to the field line connectivities and the reconnection pattern. We also study the cool and high-density volume that appears to encircle the emerged dome. The decay of the jet is followed by a violent phase with a total of five eruptions. The first of them seems to follow the general pattern of tether-cutting reconnection in a sheared arcade, although modified by the field topology created by the preceding reconnection evolution. The two following eruptions take place near and above the strong-field concentrations at the surface. They show a twisted, Ω-loop-like rope expanding in height, with twist being turned into writhe, thus hinting at a kink instability (perhaps combined with a torus instability) as the cause of the eruption. The succession of a main jet ejection and a number of violent eruptions that resemble mini-CMEs and their physical properties suggest that this experiment may provide a model for the blowout jets recently proposed in the literature. Title: MHD simulations of flux emergence in an open field region: Jet formation and explosive events. Authors: Galsgaard, Klaus; Moreno-Insertis, Fernando Bibcode: 2013enss.confE..32G Altcode: The launch of Hinode in 2006 was the start of a new interest in the jet phenomena in open field regions. Since then observations by the Hinode, SDO and Stereo satellites have shown that one characteristic jet type dominates, namely the so-called Eiffeltower or inverted-Y jet. The names arise from the jet's appearance in X-ray, where they are seen having two small "legs" below a long monolith structure representing the jet. This structure is interpreted as the result of the interaction between a newly emerged bipolar field into an unipolar magnetic field region. This picture naturally leads to magnetic reconnection between the two flux regions, where two the high velocity outflows from the diffusion region forms both the long jet structure and the underlying loop structure. To investigate this scenario in detail, we have preformed new MHD experiments of the emergence of a magnetic dipole region into an uniform open field region. The new experiments represent a significant extension of both the domain size, the duration of the experiment and the details of the analysis compared to the one presented in Moreno-Insertis et al. 2008. We find the initial jet phase to last on the order of 10 minutes, showing a smoothly evolving structure which, for a part of the evolution, closely resemblance the inverted-y structure. A number of characteristic structures arises around the footpoint region of the loop that may be compared with observations. Towards the end of this "steady state" inverted-y jet phase, the amount of flux in the emerged bipolar region is being exhausted by the reconnection process and the dynamical evolution enters a new phase. In this phase we find five explosive eruption from different parts of the remaining structure. These eruptions arises from only three main areas of the emerged flux region, implying that the same physical region can host repeated instabilities in the magnetic field. Title: Solar Fe abundance and magnetic fields. Towards a consistent reference metallicity Authors: Fabbian, D.; Moreno-Insertis, F.; Khomenko, E.; Nordlund, Å. Bibcode: 2012A&A...548A..35F Altcode: 2012arXiv1209.2771F
Aims: We investigate the impact on Fe abundance determination of including magnetic flux in series of 3D radiation-magnetohydrodynamics (MHD) simulations of solar convection, which we used to synthesize spectral intensity profiles corresponding to disc centre.
Methods: A differential approach is used to quantify the changes in theoretical equivalent width of a set of 28 iron spectral lines spanning a wide range in wavelength, excitation potential, oscillator strength, Landé factor, and formation height. The lines were computed in local thermodynamic equilibrium (LTE) using the spectral synthesis code LILIA. We used input magnetoconvection snapshots covering 50 min of solar evolution and belonging to series having an average vertical magnetic flux density of ⟨ Bvert ⟩ = 0,50,100, and 200 G. For the relevant calculations we used the Copenhagen Stagger code.
Results: The presence of magnetic fields causes both a direct (Zeeman-broadening) effect on spectral lines with non-zero Landé factor and an indirect effect on temperature-sensitive lines via a change in the photospheric T - τ stratification. The corresponding correction in the estimated atomic abundance ranges from a few hundredths of a dex up to |Δlog ɛ(Fe)⊙| ~ 0.15 dex, depending on the spectral line and on the amount of average magnetic flux within the range of values we considered. The Zeeman-broadening effect gains relatively more importance in the IR. The largest modification to previous solar abundance determinations based on visible spectral lines is instead due to the indirect effect, i.e., the line-weakening caused by a warmer stratification as seen on an optical depth scale. Our results indicate that the average solar iron abundance obtained when using magnetoconvection models can be ~ 0.03-0.11 dex higher than when using the simpler hydrodynamics (HD) convection approach.
Conclusions: We demonstrate that accounting for magnetic flux is important in state-of-the-art solar photospheric abundance determinations based on 3D convection simulations. Title: The emergence of magnetized plasma from the solar interior into the atmosphere Authors: Moreno-Insertis, Fernando Bibcode: 2012cosp...39.1275M Altcode: 2012cosp.meet.1275M Magnetized plasma is continually emerging from the solar interior into the atmosphere on a very large range of space- and timescales, extending from, e.g., the tiny bipolar magnetic elements observed to emerge within granules, all the way up to the largest active regions. Magnetic flux emergence is one of the fundamental processes that shape the solar atmosphere. However, it poses an enormous challenge for detailed theoretical understanding given that, among other things, it typically affects all atmospheric layers from the photosphere to the corona, that it requires the simultaneous solution of the plasma physics and radiation transfer problems, and that proper check with the observations requires the use of data from detectors in the visible/IR, the EUV and X-Rays. In this lecture, a number recent results concerning the modeling of flux emergence events in three dimensions are reviewed. Recent 3D numerical experiments are able to follow the emergence of small to intermediate bipolar regions from the topmost thousands of km below the surface into the low atmosphere and the corona. The most complete models are now carried out using massively-parallel radiation-MHD codes and can cope with selected NLTE aspects of the radiation transfer problem; other models sacrifice the radiation transfer side of the problem and concentrate onto the magnetodynamical aspects (including the topology/connectivity changes following reconnection, eruption episodes, etc). With the breathtaking pace of advance of supercomputing installations (now already beyond the Petaflop level and heading toward the Exaflop mark) and with the impressive number of ongoing or planned space missions and new-generation ground-based telescopes, we can expect substantial advances in this field in the coming years. Title: Sources and Removal of Magnetic Flux in the Solar Atmosphere Authors: Moreno-Insertis, F. Bibcode: 2012ASPC..455...91M Altcode: Recent advances in the observation and numerical modeling of magnetic flux emergence on small-scales are reviewed. The high-resolution limit of solar photospheric observations has reached scales of order 0".2, or 100-200 km, in recent years. Observations with that resolution show individual flux tubes emerging within single granules in the quiet Sun as small bipolar features of flux as low as 1016 Mx. Also, high-resolution observations of emerging ephemeral active regions have been carried out simultaneously at heights from the photosphere to the corona using different instruments in space and on the ground, and providing views of the emergence process with unprecedented detail. This paper starts with a brief review of some of the highest-resolution flux emergence observations. On the theory side, there is an increasing number of realistic numerical simulations of flux emergence that solve the equations of magnetohydrodynamics and radiation transfer. Various groups have studied different aspects of the radiation-MHD modeling of flux emergence, but their simulations in part cover the same processes. In this paper, a number of conclusions of the models are discussed with special focus on the comparison between the results of the different groups. The removal of magnetic fields from the surface is a less explored field than the inverse process, both observationally and theoretically. Yet, there is a good number of observations of flux disappearance from the photosphere and other atmospheric layers, typically in the form of cancellation of colliding flux elements of opposite polarity. On the simulation side, various numerical experiments of emerging flux regions find clear instances of flux cancellation and removal in the runs. In those cases, reconnection of field lines of opposite polarity is taking place and leads to phenomena akin to those reported in some of the observations. In this review a number of recent results from theory and observation are discussed which help understand the removal of flux from the solar atmosphere. Title: Magnetic flux emergence into the atmosphere: 3D numerical models. Authors: Moreno-Insertis, Fernando Bibcode: 2012decs.confE.114M Altcode: The emergence of magnetic flux from the solar interior is one of the fundamental processes that shape the solar photosphere, chromosphere and corona. Taking place on a bewildering range of space- and timescales, it has defied detailed understanding for a long time, among other things due to insufficient observational and computing/modeling power. With the current golden age of solar space missions and with the advent of Petaflop massively parallel computing, the situation is quickly improving. Recent 3D numerical experiments are able to follow the emergence of small to intermediate bipolar regions from the topmost thousands of km below the surface into the low atmosphere and the corona. Some of those models include the simultaneous solution of the MHD and radiation transfer problems, with the divergence of the radiation flux then being used as entropy source in the plasma physics problem. In all cases, post-facto spectral synthesis based on the computed data permits comparison with observational data in the visible/IR, EUV and X-ray ranges. The interaction between theory and observation is thus reaching an excellent level and it must be strengthened for the benefit of future solar physics research. In this lecture, a review of recent modeling efforts of flux emergence processes will be provided. Although with a theoretical bias, the lecture will also provide results concerning the comparison with observations. A number of shortcomings of the current modeling capabilities will be discussed. Title: The Three-dimensional Structure of an Active Region Filament as Extrapolated from Photospheric and Chromospheric Observations Authors: Yelles Chaouche, L.; Kuckein, C.; Martínez Pillet, V.; Moreno-Insertis, F. Bibcode: 2012ApJ...748...23Y Altcode: 2012arXiv1201.2456Y The three-dimensional structure of an active region filament is studied using nonlinear force-free field extrapolations based on simultaneous observations at a photospheric and a chromospheric height. To that end, we used the Si I 10827 Å line and the He I 10830 Å triplet obtained with the Tenerife Infrared Polarimeter at the Vacuum Tower Telescope (Tenerife). The two extrapolations have been carried out independently from each other and their respective spatial domains overlap in a considerable height range. This opens up new possibilities for diagnostics in addition to the usual ones obtained through a single extrapolation from, typically, a photospheric layer. Among those possibilities, this method allows the determination of an average formation height of the He I 10830 Å signal of ≈2 Mm above the surface of the Sun. It allows, as well, a cross-check of the obtained three-dimensional magnetic structures to verify a possible deviation from the force-free condition, especially at the photosphere. The extrapolations yield a filament formed by a twisted flux rope whose axis is located at about 1.4 Mm above the solar surface. The twisted field lines make slightly more than one turn along the filament within our field of view, which results in 0.055 turns Mm-1. The convex part of the field lines (as seen from the solar surface) constitutes dips where the plasma can naturally be supported. The obtained three-dimensional magnetic structure of the filament depends on the choice of the observed horizontal magnetic field as determined from the 180° solution of the azimuth. We derive a method to check for the correctness of the selected 180° ambiguity solution. Title: On the Origin of the Type II Spicules: Dynamic Three-dimensional MHD Simulations Authors: Martínez-Sykora, Juan; Hansteen, Viggo; Moreno-Insertis, Fernando Bibcode: 2011ApJ...736....9M Altcode: 2010arXiv1011.4703M Recent high temporal and spatial resolution observations of the chromosphere have forced the definition of a new type of spicule, "type II's," that are characterized by rising rapidly, having short lives, and by fading away at the end of their lifetimes. Here, we report on features found in realistic three-dimensional simulations of the outer solar atmosphere that resemble the observed type II spicules. These features evolve naturally from the simulations as a consequence of the magnetohydrodynamical evolution of the model atmosphere. The simulations span from the upper layer of the convection zone to the lower corona and include the emergence of a horizontal magnetic flux. The state-of-art Oslo Staggered Code is used to solve the full MHD equations with non-gray and non-LTE radiative transfer and thermal conduction along the magnetic field lines. We describe in detail the physics involved in a process which we consider a possible candidate for the driver mechanism that produces type II spicules. The modeled spicule is composed of material rapidly ejected from the chromosphere that rises into the corona while being heated. Its source lies in a region with large field gradients and intense electric currents, which lead to a strong Lorentz force that squeezes the chromospheric material, resulting in a vertical pressure gradient that propels the spicule along the magnetic field, as well as Joule heating, which heats the jet material, forcing it to fade. Title: Mesogranulation and the Solar Surface Magnetic Field Distribution Authors: Yelles Chaouche, L.; Moreno-Insertis, F.; Martínez Pillet, V.; Wiegelmann, T.; Bonet, J. A.; Knölker, M.; Bellot Rubio, L. R.; del Toro Iniesta, J. C.; Barthol, P.; Gandorfer, A.; Schmidt, W.; Solanki, S. K. Bibcode: 2011ApJ...727L..30Y Altcode: 2010arXiv1012.4481Y The relation of the solar surface magnetic field with mesogranular cells is studied using high spatial (≈100 km) and temporal (≈30 s) resolution data obtained with the IMaX instrument on board SUNRISE. First, mesogranular cells are identified using Lagrange tracers (corks) based on horizontal velocity fields obtained through local correlation tracking. After ≈20 minutes of integration, the tracers delineate a sharp mesogranular network with lanes of width below about 280 km. The preferential location of magnetic elements in mesogranular cells is tested quantitatively. Roughly 85% of pixels with magnetic field higher than 100 G are located in the near neighborhood of mesogranular lanes. Magnetic flux is therefore concentrated in mesogranular lanes rather than intergranular ones. Second, magnetic field extrapolations are performed to obtain field lines anchored in the observed flux elements. This analysis, therefore, is independent of the horizontal flows determined in the first part. A probability density function (PDF) is calculated for the distribution of distances between the footpoints of individual magnetic field lines. The PDF has an exponential shape at scales between 1 and 10 Mm, with a constant characteristic decay distance, indicating the absence of preferred convection scales in the mesogranular range. Our results support the view that mesogranulation is not an intrinsic convective scale (in the sense that it is not a primary energy-injection scale of solar convection), but also give quantitative confirmation that, nevertheless, the magnetic elements are preferentially found along mesogranular lanes. Title: Solar Abundance Corrections Derived Through Three-dimensional Magnetoconvection Simulations Authors: Fabbian, D.; Khomenko, E.; Moreno-Insertis, F.; Nordlund, Å. Bibcode: 2010ApJ...724.1536F Altcode: 2010arXiv1006.0231F We explore the effect of the magnetic field when using realistic three-dimensional convection experiments to determine solar element abundances. By carrying out magnetoconvection simulations with a radiation-hydro code (the Copenhagen stagger code) and through a posteriori spectral synthesis of three Fe I lines, we obtain evidence that moderate amounts of mean magnetic flux cause a noticeable change in the derived equivalent widths compared with those for a non-magnetic case. The corresponding Fe abundance correction for a mean flux density of 200 G reaches up to ~0.1 dex in magnitude. These results are based on space- and time-averaged line profiles over a time span of 2.5 solar hours in the statistically stationary regime of the convection. The main factors causing the change in equivalent widths, namely the Zeeman broadening and the modification of the temperature stratification, act in different amounts and, for the iron lines considered here, in opposite directions; yet, the resulting |Δlog epsilonsun(Fe)| coincides within a factor of 2 in all of them, even though the sign of the total abundance correction is different for the visible and infrared lines. We conclude that magnetic effects should be taken into account when discussing precise values of the solar and stellar abundances and that an extended study is warranted. Title: Galaxies at High Redshift Authors: Pérez-Fournon, I.; Balcells, M.; Moreno-Insertis, F.; Sánchez, F. Bibcode: 2010gahr.book.....P Altcode: Participants; Group photograph; Preface; Acknowledgements; 1. Galaxy formation and evolution: recent progress R. Ellis; 2. Galaxies at high redshift M. Dickinson; 3. High-redshift galaxies: the far-infrared and sub-millimeter view A. Franceschini; 4. Quasar absorption lines J. Bechtold; 5. Stellar population synthesis models at low and high redshift G. Bruzual A.; 6. Elliptical galaxies K. C. Freeman; 7. Disk galaxies K. C. Freeman; 8. Dark matter in disk galaxies K. C. Freeman. Title: Comparison Of Observations And Advanced Numerical Simulations Of Type II Spicules Authors: Martinez-Sykora, Juan; De Pontieu, B.; Hansteen, V.; Moreno-Insertis, F. Bibcode: 2010AAS...21640306M Altcode: 2010BAAS...41..878M We have performed realistic 3D radiation MHD simulations of the solar atmosphere. These simulations show jet-like features that are similar to the type II spicules discovered with Hinode's Solar Optical Telescope. These type II spicules have been associated with so-called rapid blueshifted events (RBE's) on the solar disk, and with significant blueward asymmetries in transition region and coronal lines at the footpoints of coronal loops (discovered with Hinode's EIS). These observational results and their ubiquity suggest they may play a significant role in providing the corona with hot plasma. We will present a detailed comparison of the properties of the simulated jets, with those of type II spicules (observed with Hinode) and RBE's (with ground-based instruments). We will present analysis of a wide variety of synthetic emission lines from the simulations covering temperatures from 10,000 K to several million K, and compare their intensities, velocities, line widths and asymmetry with those of the observed phenomena. We will also show how the formation mechanism of these jets (reconnection at tangential discontinuities) complicates efforts to establish a firm link between observations of magnetic fields and of chromospheric flows, and suggests that magnetic field observations at chromospheric heights may be crucial to establish from observations how these jets are formed. Title: X-Ray Jets in Coronal Holes: Numerical Simulation and Hinode Observations Authors: Moreno-Insertis, F.; Galsgaard, K.; Ugarte-Urra, I. Bibcode: 2009ASPC..415...51M Altcode: We report on our recent 3D numerical models of the launching of hot, high-speed jets in a coronal hole following the emergence of magnetized plasma from the solar interior. As part of the same research, we have also analyzed Hinode (EIS and XRT) and Soho-MDI observational data of an actual process of flux emergence followed by jet launching in a coronal hole. From the observations, we reconstruct the magnetic topology at the emergence site and calculate velocity and further physical properties of the observed event. The 3D model was calculated for realistic conditions in a coronal hole, including, in particular, a low-density (108 particles cm-3), high Alfven speed plasma prior to the emergence. After emergence, a ribbon-like current sheet is created at the site of collision of the emerging and preexisting magnetic systems. Field line reconnection ensues, which leads to the ejection of the X-Ray jet. We analyze the global magnetic topology, and the temperature, velocity and current distribution in the 3D experiment. The numerical results provide a good match to the observed features of the coronal hole jets. This is meant regarding both our own observational results as well as the ranges and average values of the statistical study by Savcheva et al. (2007). Title: Magnetic flux emergence into the solar photosphere and chromosphere Authors: Tortosa-Andreu, A.; Moreno-Insertis, F. Bibcode: 2009A&A...507..949T Altcode: Aims: We model the emergence of magnetized plasma across granular convection cells and the low atmosphere, including layers up to the mid-chromosphere.
Methods: Three-dimensional numerical experiments are carried out in which the equations of MHD and radiative transfer are solved self-consistently. We use the MURaM code, which assumes local thermodynamic equilibrium between plasma and radiation.
Results: In the photosphere, we find good agreement between our simulation predictions and observational results obtained with the Hinode satellite for the velocity and magnetic fields. We also confirm earlier simulation results by other authors. Our experiments reveal a natural mechanism of formation of twisted magnetic flux tubes that results from the retraction of photospheric horizontal fields at new intergranular lanes in decaying granules. In the chromosphere, we present evidence for the non-radiative heating of the emerging magnetized plasma due to the passage of shocks and/or ohmic dissipation. We study the formation of high-temperature points in the magnetic domain. We detect two types of points, classified according to whether they have a photospheric counterpart or otherwise. We also find evidence of those two types in Hinode observations. Using Lagrangian tracing of a large statistical sample of fluid particles, we detect and study episodes of convective collapse of magnetic elements returning to the photosphere. On the other hand, we study the maximum heights reached by all tracers, magnetized or otherwise. Only a small fraction (1.3%) of the magnetic elements reach the mid-chromosphere (z>750 km), while virtually no unmagnetized elements in the sample rise above the level of the reverse granulation (a few 100 km above the photosphere). We find that the rise into the chromosphere occurs in the form of successive jumps with intermediate stops rather than in a smooth continuous fashion and propose a tentative explanation of this behavior. Finally, also using Lagrange tracing, we document the creation of high-temperature points in the chromosphere via rising shock fronts. Title: The emergence of toroidal flux tubes from beneath the solar photosphere Authors: Hood, A. W.; Archontis, V.; Galsgaard, K.; Moreno-Insertis, F. Bibcode: 2009A&A...503..999H Altcode: Context: Models of flux emergence frequently use a twisted cylindrical loop as the initial starting configuration and ignore the coupling between the radiation field and plasma. In these models, the axis of the original tube never emerges through the photosphere. Without the axis emerging, it is very difficult to form a realistic sunspot.
Aims: The aim is to use a toroidal flux loop, placed beneath the solar photosphere and study whether the axis of the system emerges fully into the atmosphere. The toroidal curvature means that the plasma can drain more effectively than in a straight cylindrical tube.
Methods: Three-dimensional magnetohydrodynamic numerical simulations of an emerging magnetic flux tube are presented for an initial toroidal loop model. The simulations use a Lagrangian-Remap code that is particularly suited to dealing with shocks and strong current sheets.
Results: The evolution of the toroidal loop is followed and the characteristics of the emergence process are compared with the traditional cylindrical loops. The flux sources seen at the photosphere are more circular, and there are less shearing motions in the upper photosphere. When the initial magnetic field strength is relatively weak the evolution of the system is similar to the cylindrical loop case, with the formation of a new flux rope above the photosphere. A striking result is that for large values of field strength the axial field of the toroidal loop emerges fully into the corona. This is reported for the first time in experiments of flux emergence in a highly stratified atmosphere that do not solve self-consistently the radiation transfer problem. In addition, the new flux rope forms below the original axis of the toroidal tube when the field strength is sufficiently strong. Title: X-ray jets and magnetic flux emergence in the Sun Authors: Moreno-Insertis, Fernando Bibcode: 2009IAUS..259..201M Altcode: Magnetized plasma is emerging continually from the solar interior into the atmosphere. Magnetic flux emergence events and their consequences in the solar atmosphere are being observed with high space, time and spectral resolution by a large number of space missions in operation at present (e.g. SOHO, Hinode, Stereo, Rhessi). The collision of an emerging and a preexisting magnetic flux system in the solar atmosphere leads to the formation of current sheets and to field line reconnection. Reconnection under solar coronal conditions is an energetic event; for the field strengths, densities and speeds involved in the collision of emerging flux systems, the reconnection outflows lead to launching of high-speed (hundreds of km/s), high-temperature (107 K) plasma jets. Such jets are being observed with the X-Ray and EUV detectors of ongoing satellite missions. On the other hand, the spectacular increase in computational power in recent years permits to carry out three-dimensional numerical experiments of the time evolution of flux emerging systems and the launching of jets with a remarkable degree of detail.

In this review, observation and modeling of the solar X-Ray jets are discussed. A two-decade long computational effort to model the magnetic flux emergence events by different teams has led to numerical experiments which explain, even quantitatively, many of the observed features of the X-ray jets. The review points out that, although alternative mechanisms must be considered, flux emergence is a prime candidate to explain the launching of the solar jets. Title: Magnetic Flux Emergence and Jet Formation in Coronal Holes Authors: Galsgaard, K.; Moreno-Insertis, F. Bibcode: 2008ESPM...12.3.27G Altcode: Recent observations of coronal holes with Hinode show with unprecedented detail the launching of fast and hot jets. Many of these jets are found to coincide with the emergence of new magnetic flux, and it is generally assumed that the jets are initiated by magnetic reconnection between the new emerging flux and the existing open magnetic field. Further to this a comparison of a larger sample of jets show that about 70% of these are followed by the formation of plumes within minutes to an hour.

How do we understand these events from a physical point of view? To investigate this we have carried out numerical 3D MHD experiment modeling the emergence of magnetic flux from the upper convection zone into an open magnetic flux region resembling a coronal hole. The emergence process drives the formation of a strong and highly localised current sheet. Time-dependent reconnection in the current sheet gives rise to a high-velocity jet that eventually flows along the previously open coronal field lines. Initially the jet has transition region temperature, but as time progresses it eventually exceeds the coronal temperature in the model. Investigating the development of the structure of the magnetic field, it is found that it changes in a very characteristic way, leading to a horizontal drift of the jet. The experiment also shows how the reconnection speed influences the dynamical properties of both the jet parameters and the evolution of the underlying magnetic structure. Towards the end of the experiment the jet speed decreases and leaves a large funnel-like region above the emerging flux domain with an enhanced temperature and density distribution. Title: Magnetic Flux Emergence in the Solar Photosphere Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F. Bibcode: 2008ASPC..384..181C Altcode: 2008csss...14..181C The most prominent magnetic structures on the surface of the Sun are bipolar active regions. These magnetic complexes are comprised of a hierarchy of magnetic structures of different sizes, the largest of which are sunspots. Observations indicate that the appearance of active regions on the solar surface result from the emergence of bundles of magnetic flux from the underlying convection zone. We study the emergence process by means of 3D radiation MHD simulations. In the simulations, an initially buoyant magnetic flux tube is introduced into the near-surface layers of the convection zone. Subject to the buoyancy force, the flux tube rises towards the photosphere. Our simulations highlight the importance of magneto-convection on the evolution of the magnetic flux tube. The external convective flow field has an important influence on the emergence morphology of the emerging magnetic field. Depending on the initial properties of the magnetic flux tube (e.g. field strength, twist, entropy etc.), flux emergence may lead to a disturbance of the local granulation pattern. The observational signatures associated with emerging magnetic flux in our simulations are in qualitative and quantitative agreement with observational studies of emerging flux regions on the Sun. Title: Jets in Coronal Holes: Hinode Observations and Three-dimensional Computer Modeling Authors: Moreno-Insertis, F.; Galsgaard, K.; Ugarte-Urra, I. Bibcode: 2008ApJ...673L.211M Altcode: 2007arXiv0712.1059M Recent observations of coronal hole areas with the XRT and EIS instruments on board the Hinode satellite have shown with unprecedented detail the launching of fast, hot jets away from the solar surface. In some cases these events coincide with episodes of flux emergence from beneath the photosphere. In this Letter we show results of a three-dimensional numerical experiment of flux emergence from the solar interior into a coronal hole and compare them with simultaneous XRT and EIS observations of a jet-launching event that accompanied the appearance of a bipolar region in MDI magnetograms. The magnetic skeleton and topology that result in the experiment bear a strong resemblance to linear force-free extrapolations of the SOHO/MDI magnetograms. A thin current sheet is formed at the boundary of the emerging plasma. A jet is launched upward along the open reconnected field lines with values of temperature, density, and velocity in agreement with the XRT and EIS observations. Below the jet, a split-vault structure results with two chambers: a shrinking one containing the emerged field loops and a growing one with loops produced by the reconnection. The ongoing reconnection leads to a horizontal drift of the vault-and-jet structure. The timescales, velocities, and other plasma properties in the experiment are consistent with recent statistical studies of this type of event made with Hinode data. Title: Photospheric Magnetic Flux Emergence: A comparative study between Hinode/SOT Observations and MHD simulations Authors: Cheung, M. C.; Schüssler, M.; Moreno-Insertis, F.; Tarbell, T. D. Bibcode: 2007AGUFMSH53A1073C Altcode: With high angular resolution, high temporal cadence and a stable point spread function, the Solar Optical Telescope (SOT) onboard the Hinode satellite is the ideal instrument for the study of magnetic flux emergence and its manifestations on the solar surface. In this presentation, we focus on the development of ephemeral regions and small active regions. In many instances, SOT has been able to capture the entire emergence process from beginning to end: i.e. from the initial stages of flux appearance in granule interiors, through the intermediate stages of G-band bright point formation, and finally to the coalescence of small vertical flux elements to form pores. To investigate the physics of the flux emergence process, we performed 3D numerical MHD simulations with the MURaM code. The models are able to reproduce, and help us explain, various observational signatures of magnetic flux emergence. Title: Three-dimensional numerical experiments of flux emergence into the corona Authors: Moreno-Insertis, F. Bibcode: 2007ASPC..369..335M Altcode: Understanding the complicated non-linear thermal and magnetodynamic processes observed to continuously reshape the solar atmosphere requires the use of three-dimensional computer simulations and numerical experimentation. In this paper, a number of results from the recent generation of 3D numerical models of magnetic flux emergence from the topmost layers of the solar interior to the corona are reviewed. Four main areas are covered: current sheet formation, plasmoid generation and ejection, 3D reconnection and jet launching. Multidimensional computer simulation of MHD and radiative processes in the solar atmosphere is particularly important at present given the high-quality data expected from Solar B and other satellites in the coming years. Title: The Effect of the Relative Orientation between the Coronal Field and New Emerging Flux. I. Global Properties Authors: Galsgaard, K.; Archontis, V.; Moreno-Insertis, F.; Hood, A. W. Bibcode: 2007ApJ...666..516G Altcode: 2007arXiv0705.1097G The emergence of magnetic flux from the convection zone into the corona is an important process for the dynamical evolution of the coronal magnetic field. In this paper we extend our previous numerical investigations, by looking at the process of flux interaction as an initially twisted flux tube emerges into a plane-parallel, coronal magnetic field. Significant differences are found in the dynamical appearance and evolution of the emergence process depending on the relative orientation between the rising flux system and any preexisting coronal field. When the flux systems are nearly antiparallel, the experiments show substantial reconnection and demonstrate clear signatures of a high-temperature plasma located in the high-velocity outflow regions extending from the reconnection region. However, the cases that have a more parallel orientation of the flux systems show very limited reconnection and none of the associated features. Despite the very different amount of reconnection between the two flux systems, it is found that the emerging flux that is still connected to the original tube reaches the same height as a function of time. As a compensation for the loss of tube flux, a clear difference is found in the extent of the emerging loop in the direction perpendicular to the main axis of the initial flux tube. Increasing amounts of magnetic reconnection decrease the volume, which confines the remaining tube flux. Title: Magnetic Flux Emergence In Granular Convection: Radiative MHD Simulations And Hinode SOT Observations Authors: Cheung, Mark; Schüssler, M.; Moreno-Insertis, F.; Tarbell, T.; SOT Team Bibcode: 2007AAS...210.9425C Altcode: 2007BAAS...39..221C We model the emergence of buoyant magnetic flux from the convection zone into the photosphere by means of 3D radiative MHD simulations using the MURaM code. In a series of simulations, we study how an initially buoyant magnetic flux tube rises in the presence of granular convection. The simulations take into account the effects of radiative energy exchange, ionization effects in the equation of state and compressibility. An emphasis of this talk is the comparison of observational diagnostics from the simulations with recent observations from Hinode SOT. Title: Magnetic flux emergence in granular convection: radiative MHD simulations and observational signatures Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F. Bibcode: 2007A&A...467..703C Altcode: 2007astro.ph..2666C 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 1019 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.

Movies are only available in electronic form at http://www.aanda.org Title: The origin of the reversed granulation in the solar photosphere Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F. Bibcode: 2007A&A...461.1163C Altcode: 2006astro.ph.12464C Aims:We study the structure and reveal the physical nature of the reversed granulation pattern in the solar photosphere by means of 3-dimensional radiative hydrodynamics simulations.
Methods: We used the MURaM code to obtain a realistic model of the near-surface layers of the convection zone and the photosphere.
Results: The pattern of horizontal temperature fluctuations at the base of the photosphere consists of relatively hot granular cells bounded by the cooler intergranular downflow network. With increasing height in the photosphere, the amplitude of the temperature fluctuations diminishes. At a height of z=130-140 km in the photosphere, the pattern of horizontal temperature fluctuations reverses so that granular regions become relatively cool compared to the intergranular network. Detailed analysis of the trajectories of fluid elements through the photosphere reveal that the motion of the fluid is non-adiabatic, owing to strong radiative cooling when approaching the surface of optical depth unity followed by reheating by the radiation field from below. The temperature structure of the photosphere results from the competition between expansion of rising fluid elements and radiative heating. The former acts to lower the temperature of the fluid whereas the latter acts to increase it towards the radiative equilibrium temperature with a net entropy gain. After the fluid overturns and descends towards the convection zone, radiative energy loss again decreases the entropy of the fluid. Radiative heating and cooling of fluid elements that penetrate into the photosphere and overturn do not occur in equal amounts. The imbalance in the cumulative heating and cooling of these fluid elements is responsible for the reversal of temperature fluctuations with respect to height in the photosphere. Title: Flux Emergence at the Photosphere Authors: Cheung, M. C. M.; Schüssler, M.; Moreno-Insertis, F. Bibcode: 2006ASPC..354...97C Altcode: To model the emergence of magnetic fields at the photosphere, we carried out 3D magneto-hydrodynamics (MHD) simulations using the MURaM code. 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. In the simulations, an initially buoyant magnetic flux tube is embedded in the upper layers of the convection zone. We find that the interaction between the 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 inclusion of radiative transfer allows us to directly compare the simulation results with real observations of emerging flux. Title: 3D simulations identifying the effects of varying the twist and field strength of an emerging flux tube Authors: Murray, M. J.; Hood, A. W.; Moreno-Insertis, F.; Galsgaard, K.; Archontis, V. Bibcode: 2006A&A...460..909M Altcode: Aims.We investigate the effects of varying the magnetic field strength and the twist of a flux tube as it rises through the solar interior and emerges into the atmosphere.
Methods: .Using a 3D numerical MHD code, we consider a simple stratified model, comprising of one solar interior layer and three overlying atmospheric layers. We set a horizontal, twisted flux tube in the lowest layer. The specific balance of forces chosen results in the tube being fully buoyant and the temperature is decreased in the ends of the tube to encourage the formation of an Ω-shape along the tube's length. We vary the magnetic field strength and twist independently of each other so as to give clear results of the individual effects of each parameter.
Results: .We find a self-similar evolution in the rise and emergence of the flux tube when the magnetic field strength of the tube is modified. During the rise through the solar interior, the height of the crest and axis, the velocity of the crest and axis, and the decrease in the magnetic field strength of the axis of the tube are directly dependent upon the initial magnetic field strength given to the tube. No such self-similarity is evident when the twist of the flux tube is changed, due to the complex interaction of the tension force on the rise of the tube. For low magnetic field strength and twist values, we find that the tube cannot fully emerge into the atmosphere once it reaches the top of the interior since the buoyancy instability criterion cannot be fulfilled. For those tubes that do advance into the atmosphere, when the magnetic field strength has been modified, we find further self-similar behaviour in the amount of tube flux transported into the atmosphere. For the tubes that do emerge, the variation in the twist results in the buoyancy instability, and subsequent emergence, occurring at different locations along the tube's length.
Title: Flux Emergence from the Solar Interior to the Atmosphere: the Passage through the Photosphere Authors: Moreno-Insertis, F. Bibcode: 2006ASPC..354..183M Altcode: The atmosphere of the Sun is being stirred and reshaped continuously through the emergence of magnetized plasma from the convection zone. Magnetic elements coming from the solar interior produce structures in the photosphere, chromosphere and corona in a bewildering range of length and time scales, from the smallest magnetic tubes to the largest active regions. Although still at an early stage, the computer simulation of these intrinsically three-dimensional processes can already provide important physical insight and explain some of the observational results obtained with ground-based detectors and in space. This review summarizes the challenge posed by flux emergence processes to numerical simulation, discusses why the magnetic plasma can rise through the photosphere in spite of the latter's subadiabatic stratification and presents a number of results on active region formation at the photosphere obtained in recent years. Title: Three-dimensional Plasmoid Evolution in the Solar Atmosphere Authors: Archontis, V.; Galsgaard, K.; Moreno-Insertis, F.; Hood, A. W. Bibcode: 2006ApJ...645L.161A Altcode: We present clear evidence of the formation of three-dimensional (3D) plasmoids in the current sheet between two magnetic flux systems in a 3D numerical experiment of flux emergence into the solar atmosphere and study their properties and time evolution. Plasmoids are most likely the result of resistive tearing mode instabilities. They adopt the shape of a solenoid contained within the current sheet: the solenoid is tightly wound when the field in the two flux systems is close to antiparallel. The plasmoids are expelled to the sides of the sheet as a result of a reconnection imbalance between the two x-lines on their sides. We show the complex, 3D field line geometry in various plasmoids: individual plasmoid field lines have external linkages to the flux system on either side of the current sheet; we also find field lines that go through a few plasmoids in succession, probably indicating that the field line has resulted from multiple reconnection events. Title: Moving magnetic tubes: fragmentation, vortex streets and the limit of the approximation of thin flux tubes Authors: Cheung, M. C. M.; Moreno-Insertis, F.; Schüssler, M. Bibcode: 2006A&A...451..303C Altcode: Aims.We study the buoyant rise of magnetic flux tubes in a stratified layer over a range of Reynolds numbers (25 ⪉ Re ⪉ 2600) by means of numerical simulations. Special emphasis is placed on studying the fragmentation of the rising tube, its trailing wake and the formation of a vortex street in the high-Reynolds number regime. Furthermore, we evaluate the relevance of the thin flux tube approximation with regard to describing the evolution of magnetic flux tubes in the simulations.
Methods: .We used the FLASH code, which has an adaptive mesh refinement (AMR) algorithm, thus allowing the simulations to be carried out at high Reynolds numbers.
Results: .The evolution of the magnetic flux tube and its wake depends on the Reynolds number. At Re up to a few hundred, the wake consists of two counter-rotating vortex rolls. At higher Re, the vortex rolls break up and the shedding of flux into the wake occurs in a more intermittent fashion. The amount of flux retained by the central portion of the tube increases with the field line twist (in agreement with previous literature) and with Re. The time evolution of the twist is compatible with a homologous expansion of the tube. The motion of the central portion of the tube in the simulations is very well described by the thin flux tube model whenever the effects of flux loss or vortex forces can be neglected. If the flux tube has an initial net vorticity, it undergoes asymmetric vortex shedding. In this case, the lift force accelerates the tube in such a way that an oscillatory horizontal motion is super-imposed on the vertical rise of the tube, which leaves behind a vortex street. This last result is in accordance with previous simulations reported in the literature, which were carried out at lower Reynolds number.
Title: Flux emergence and interaction with a coronal field: 3D MHD simulations Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood, A. W. Bibcode: 2006IAUS..233...53A Altcode: The dynamic process of magnetic flux emergence from the solar interior to the outer atmosphere may well be related with eruptive phenomena and intense events of the Solar activity. However, the physics of the emergence is not still well understood. Thus, we have performed 3D MHD simulations to study the rising motion of a twisted flux tube from the convection zone of the Sun and its interaction with a preexisting coronal magnetic field. The results show that the reconnection process depends criticaly on the initial relative orientation between the two magnetic flux systems into contact. On the other hand, the overal process of emergence depends mostly on the dynamics of the sub-photospheric plasma. Title: Magnetic flux emergence into the atmosphere Authors: Moreno-Insertis, F. Bibcode: 2006IAUS..233...33M Altcode: Understanding the mechanism that causes the emergence of magnetic flux from the solar interior to the atmosphere, the drastic changes in the properties of the matter and magnetic fields along the rise and the interplay of dynamic and resistive phenomena that shape the emerged regions is one of the major open tasks in solar physics. Important advances are being made both in the theoretical modelling and in the observation of the emergence events. This review concentrates on recent advances through 3D numerical experiments carried out with massively parallel MHD and radiative transfer codes. Title: The Three-dimensional Interaction between Emerging Magnetic Flux and a Large-Scale Coronal Field: Reconnection, Current Sheets, and Jets Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood, A. W. Bibcode: 2005ApJ...635.1299A Altcode: Using MHD numerical experiments in three dimensions, we study the emergence of a bipolar magnetic region from the solar interior into a model corona containing a large-scale, horizontal magnetic field. An arch-shaped concentrated current sheet is formed at the interface between the rising magnetized plasma and the ambient coronal field. Three-dimensional reconnection takes place along the current sheet, so that the corona and the photosphere become magnetically connected, a process repeatedly observed in recent satellite missions. We show the structure and evolution of the current sheet and how it changes in time from a simple tangential discontinuity to a rotational discontinuity with no null surface. We find clear indications that individual reconnection events in this three-dimensional environment in the advanced stage are not one-off events, but instead take place in a continuous fashion, with each field line changing connectivity during a finite time interval. We also show that many individual field lines of the rising tube undergo multiple processes of reconnection at different points in the corona, thus creating photospheric pockets for the coronal field. We calculate global measures for the amount of subphotospheric flux that becomes linked to the corona during the experiment and find that most of the original subphotospheric flux becomes connected to coronal field lines. The ejection of plasma from the reconnection site gives rise to high-speed and high-temperature jets. The acceleration mechanism for those jets is akin to that found in previous two-dimensional models, but the geometry of the jets bears a clear three-dimensional imprint, having a curved-sheet appearance with a sharp interface to the overlying coronal magnetic field system. Temperatures and velocities of the jets in the simulations are commensurate with those measured in soft X-rays by the Yohkoh satellite. Title: D Magneto-Convection and Flux Emergence in the Photosphere Authors: Cheung, M.; Schüssler, M.; Moreno-Insertis, F. Bibcode: 2005ESASP.596E..54C Altcode: 2005ccmf.confE..54C No abstract at ADS Title: Magnetic Flux Emergence and its Interaction with AN Existing Coronal Field Authors: Galsgaard, K.; Moreno-Insertis, F.; Archontis, V.; Hood, A. Bibcode: 2005ESASP.596E..27G Altcode: 2005ccmf.confE..27G No abstract at ADS Title: Magnetic Flux Emergence and its Interaction with AN Existing Coronal Field Authors: Galsgaard, K.; Moreno-Insertis, F.; Archontis, V.; Hood, A. Bibcode: 2005ESASP.596E..55G Altcode: 2005ccmf.confE..55G No abstract at ADS Title: A Three-dimensional Study of Reconnection, Current Sheets, and Jets Resulting from Magnetic Flux Emergence in the Sun Authors: Galsgaard, K.; Moreno-Insertis, F.; Archontis, V.; Hood, A. Bibcode: 2005ApJ...618L.153G Altcode: 2004astro.ph.10057G We present the results of a set of three-dimensional numerical simulations of magnetic flux emergence from below the photosphere and into the corona. The corona includes a uniform and horizontal magnetic field as a model for a preexisting large-scale coronal magnetic system. Cases with different relative orientations of the upcoming and coronal fields are studied. Upon contact, a concentrated current sheet with the shape of an arch is formed at the interface that marks the positions of maximum jump in the field vector between the two systems. Relative angles above 90° yield abundant magnetic reconnection and plasma heating. The reconnection is seen to be intrinsically three-dimensional in nature and to be accompanied by marked local heating. It generates collimated high-speed outflows only a short distance from the reconnection site, and these propagate along the ambient magnetic field lines as jets. As a result of the reconnection, magnetic field lines from the magnetized plasma below the surface end up connecting to coronal field lines, thus causing a profound change in the connectivity of the magnetic regions in the corona. The experiments presented here yield a number of features repeatedly observed with the TRACE and Yohkoh satellites, such as the establishment of connectivity between emergent and preexisting active regions, local heating, and high-velocity outflows. Title: Flux Emergence from the Solar Interior Into a Uniformly Magnetized Corona Authors: Moreno-Insertis, F.; Galsgaard, K.; Archontis, V.; Hood, A. Bibcode: 2004ESASP.575..216M Altcode: 2004soho...15..216M No abstract at ADS Title: 3D MHD Simulations on Magnetic Flux Emergence Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood, A. Bibcode: 2004ESASP.575..342A Altcode: 2004soho...15..342A No abstract at ADS Title: Emergence of magnetic flux from the convection zone into the corona Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood, A.; O'Shea, E. Bibcode: 2004A&A...426.1047A Altcode: Numerical experiments of the emergence of magnetic flux from the uppermost layers of the solar interior to the photosphere and its further eruption into the low atmosphere and corona are carried out. We use idealized models for the initial stratification and magnetic field distribution below the photosphere similar to those used for multidimensional flux emergence experiments in the literature. The energy equation is adiabatic except for the inclusion of ohmic and viscous dissipation terms, which, however, become important only at interfaces and reconnection sites. Three-dimensional experiments for the eruption of magnetic flux both into an unmagnetized corona and into a corona with a preexisting ambient horizontal field are presented. The shocks preceding the rising plasma present the classical structure of nonlinear Lamb waves. The expansion of the matter when rising into the atmosphere takes place preferentially in the horizontal directions: a flattened (or oval) low plasma-β ball ensues, in which the field lines describe loops in the corona with increasing inclination away from the vertical as one goes toward the sides of the structure. Magnetograms and velocity field distributions on horizontal planes are presented simultaneously for the solar interior and various levels in the atmosphere. Since the background pressure and density drop over many orders of magnitude with increasing height, the adiabatic expansion of the rising plasma yields very low temperatures. To avoid this, the entropy of the rising fluid elements should be increased to the high values of the original atmosphere via heating mechanisms not included in the present numerical experiments. The eruption of magnetic flux into a corona with a preexisting magnetic field pointing in the horizontal direction yields a clear case of essentially three-dimensional reconnection when the upcoming and ambient field systems come into contact. The coronal ambient field is chosen at time t=0 perpendicular to the direction of the tube axis and thus, given the twist of the magnetic tube, almost anti-parallel to the field lines at the upper boundary of the rising plasma ball. A thin, dome-shaped current layer is formed at the interface between the two flux systems, in which ohmic dissipation and heating are taking place. The reconnection proceeds by merging successive layers on both sides of the reconnection site; however, this occurs not only at the cusp of the interface, but, also, gradually along its sides in the direction transverse to the ambient magnetic field. The topology of the magnetic field in the atmosphere is thereby modified: the reconnected field lines typically are part of the flanks of the tube below the photosphere but then join the ambient field system in the corona and reach the boundaries of the domain as horizontal field lines. Title: The Emergence of Magnetic Field into Stellar Atmospheres Authors: Moreno-Insertis, F. Bibcode: 2004Ap&SS.292..587M Altcode: The magnetic activity in the atmosphere of the Sun, possibly of most other magnetically active late-type stars, is a consequence of the emergence of magnetic flux from the stellar interior. Research in this field has to consider the rise of magnetized plasma from the depths of the convective envelope through the stellar surface and out across the corona. In the present review, a number of key questions, difficulties and results are discussed. Title: Galaxies at High Redshift Authors: Pérez-Fournon, Ismael; Balcells, Marc; Moreno-Insertis, Fernando; Sánchez, Francisco Bibcode: 2003ghr..conf.....P Altcode: This book covers normal galaxies, distant galaxies, studies based on far-infrared diagnostics, quasar absorption lines, and the properties of nearby galaxies. The timely volume provides an essential reference for astronomers working in the field of high-redshift galaxies. It includes the lectures delivered at the XI Canary Islands Winter School of Astrophysics, and reviews scientific results as well as main questions in the field. Title: Galaxies at High Redshift Authors: Pérez-Fournon, I.; Balcells, M.; Moreno-Insertis, F.; Sánchez, F. Bibcode: 2003gahr.book.....P Altcode: Participants; Group photograph; Preface; Acknowledgements; 1. Galaxy formation and evolution: recent progress R. Ellis; 2. Galaxies at high redshift M. Dickinson; 3. High-redshift galaxies: the far-infrared and sub-millimeter view A. Franceschini; 4. Quasar absorption lines J. Bechtold; 5. Stellar population synthesis models at low and high redshift G. Bruzual A.; 6. Elliptical galaxies K. C. Freeman; 7. Disk galaxies K. C. Freeman; 8. Dark matter in disk galaxies K. C. Freeman. Title: Preface (Galaxies at high redshift) Authors: Pérez-Fournon, Ismael; Balcells, Marc; Moreno-Insertis, Fernando; Sánchez, Francisco Bibcode: 2003ghr..confD..13P Altcode: No abstract at ADS Title: Thermal properties of magnetic flux tubes. I. Solution of the diffusion problem Authors: Moreno-Insertis, F.; Schüssler, M.; Glampedakis, K. Bibcode: 2002A&A...388.1022M Altcode: The heat flow and temperature structure within and surrounding a magnetic flux tube stored in mechanical equilibrium in a stellar convection zone are considered. The stationary thermal equilibrium state is determined through the analytical solution of a two-dimensional heat diffusion problem for an infinitely long cylinder with different thermal conductivities inside and outside the cylinder, both spatially variable. In the exterior of the cylinder, convective heat transport is approximated in terms of a linear diffusive process, while in its interior convection is assumed to be suppressed and only the much smaller radiative conductivity remains. The results show that, under the conditions prevailing near the bottom of the solar convection zone and in the limit of small cylinder radius, the temperature disturbance (thermal shadow) in the exterior of the insulating cylinder is almost negligible due to the large effiency of convective energy transport. The spatial dependence of the conductivities and the curvature of the external temperature profile lead to a temperature excess in the interior with respect to the undisturbed temperature profile far away from the cylinder. We show that, within the framework of the thin magnetic flux tube approximation, this temperature excess is due to a heating term equal to the negative divergence of the undisturbed radiative heat flow, as suggested earlier by Fan & Fisher (\cite{Fan:Fisher:1996}). These results are independent of the treatment of the convective transport in the exterior as long as the stratification is almost adiabatic. The consequences for the storage of magnetic flux in the solar convection zone, brought about by the enhanced buoyancy and caused by the heating effect, are discussed. Title: Thermal Instability in a Cooling and Expanding Medium Including Self-Gravity and Conduction Authors: Gomez-Pelaez, A. J.; Moreno-Insertis, F. Bibcode: 2002ApJ...569..766G Altcode: 2002astro.ph..1150G A systematic study of the linear thermal stability of a medium subject to cooling, self-gravity, and thermal conduction is carried out for the case when the unperturbed state is subject to global cooling and expansion. A general, recursive WKB solution for the perturbation problem is obtained that can be applied to a large variety of situations in which there is a separation of timescales for different physical processes. Solutions are explicitly given and discussed for the case in which sound propagation and/or self-gravity are the fastest processes, with cooling, expansion, and thermal conduction operating on slower timescales. A brief discussion is also added for the solutions in the cases in which cooling or conduction operate on the fastest timescale. The general WKB solution obtained in this paper permits solving the problem of the effect of thermal conduction and self-gravity on the thermal stability of a globally cooling and expanding medium. As a result of the analysis, the critical wavelength (often called the ``Field length'') above which cooling makes the perturbations unstable against the action of thermal conduction is generalized to the case of an unperturbed background with net cooling. As an astrophysical application, the ``generalized Field length'' is calculated for a hot (104-108 K), optically thin medium (as pertains, for instance, for the hot interstellar medium of supernova remnants or superbubbles) using a realistic cooling function and including a weak magnetic field. The stability domains are compared with the predictions made on the basis of models for which the background is in thermal equilibrium. The instability domain of the sound waves in particular is seen to be much larger in the case with net global cooling. Title: Astrophysical spectropolarimetry Authors: Trujillo-Bueno, J.; Moreno-Insertis, F.; Sánchez, F. Bibcode: 2002apsp.conf.....T Altcode: The polarization of light is the key to obtaining a wealth of essential information that lies encoded in the electromagnetic radiation from cosmic objects. Spectropolarimetry and imaging polarimetry provide powerful diagnostics of the physical conditions in astrophysical plasmas, which cannot be obtained via conventional spectroscopy. Whilst its application to other fields of astrophysics is still at an early stage of development, spectropolarimetry is being used with great success in solar physics. The book contains the lectures delivered at the XII Canary Islands Winter School of Astrophysics. Written by eight prestigious astrophysics researchers, it covers the physics of polarization, polarized radiation diagnostics of solar magnetic fields, stellar magnetic fields, polarization insights for active galactic nuclei, compact objects and accretion disks, astronomical masers and their polarization, infrared-submillimeter spectropolarimetry, and instrumentation for astrophysical spectropolarimetry. This timely volume will provide graduate students and researchers with an unprecedented introduction to the field of astrophysical spectropolarimetry. Title: Preface (Astrophysical spectropolarimetry) Authors: Trujillo-Bueno, Javier; Moreno-Insertis, Fernando Bibcode: 2002apsp.confD..11T Altcode: No abstract at ADS Title: Astrophysical Spectropolarimetry Authors: Trujillo-Bueno, Javier; Moreno-Insertis, Fernando; Sanchez Martinez, Francisco Bibcode: 2002assp.book.....T Altcode: This book contains the lectures delivered at the XII Canary Islands Winter School of Astrophysics on Astrophysical Spectropolarimetry. It highlights how recent developments in theoretical astrophysics and astronomical instrumentation are leading an ever-growing number of astrophysicists to appreciate the enormous diagnostic potential offered by spectropolarimetry. Title: The Zigzag Path of Buoyant Magnetic Tubes and the Generation of Vorticity along Their Periphery Authors: Emonet, T.; Moreno-Insertis, F.; Rast, M. P. Bibcode: 2001ApJ...549.1212E Altcode: We study the generation of vorticity in the magnetic boundary layer of buoyant magnetic tubes and its consequences for the trajectory of magnetic structures rising in the solar convection zone. When the Reynolds number is well above 1, the wake trailing the tube sheds vortex rolls, producing a von Kármán vortex street, similar to the case of flows around rigid cylinders. The shedding of a vortex roll causes an imbalance of vorticity in the tube. The ensuing vortex force excites a transverse oscillation of the flux tube as a whole so that it follows a zigzag upward path instead of rising along a straight vertical line. In this paper, the physics of vorticity generation in the boundary layer is discussed and scaling laws for the relevant terms are presented. We then solve the two-dimensional magnetohydrodynamic equations numerically, measure the vorticity production, and show the formation of a vortex street and the consequent sinusoidal path of the magnetic flux tube. For high values of the plasma beta, the trajectory of the tubes is found to be independent of β but varying with the Reynolds number. The Strouhal number, which measures the frequency of vortex shedding, shows in our rising tubes only a weak dependence with the Reynolds numbers, a result also obtained in the rigid-tube laboratory experiments. In fact, the actual values measured in the latter are also close to those of our numerical calculations. As the Reynolds numbers are increased, the amplitude of the lift force grows and the trajectory becomes increasingly complicated. It is shown how a simple analytical equation (which includes buoyancy, drag, and vortex forces) can satisfactorily reproduce the computed trajectories. The different regimes of rise can be best understood in terms of a dimensionless parameter, χ, which measures the importance of the vortex force as compared with the buoyancy and drag forces. For χ2<<1, the rise is drag dominated and the trajectory is mainly vertical with a small lateral oscillation superposed. When χ becomes larger than 1, there is a transition toward a drag-free regime and epicycles are added to the trajectory. Title: Magnetic Fields in Cool Stars (CD-ROM Directory: contribs/moreno) Authors: Moreno Insertis, F.; Saar, S. H.; Solanki, S. K. Bibcode: 2001ASPC..223..435M Altcode: 2001csss...11..435M No abstract at ADS Title: Storage of a Strong Magnetic Field Below the Solar Convection Zone (CD-ROM Directory: contribs/rempel) Authors: Rempel, M.; Schüssler, M.; Moreno-Insertis, F.; Tóth, G. Bibcode: 2001ASPC..223..738R Altcode: 2001csss...11..738R No abstract at ADS Title: Solar Interior: Emerging Magnetic Flux Tubes Authors: Moreno-Insertis, F. Bibcode: 2000eaa..bookE2521M Altcode: The magnetic field observed in the photosphere and at higher levels of the solar atmosphere is generated in the convective envelope that extends along the first 200 000 km below the solar surface (see DYNAMOS: SOLAR AND STELLAR and SOLAR INTERIOR). To properly understand the solar magnetic cycle, it is necessary to explain the processes whereby the magnetic field, once generated, emerges across ... Title: The Dynamics of Buoyant Magnetic Ropes and the Generation of Vorticity in their Periphery Authors: Emonet, T.; Moreno-Insertis, F.; Rast, M. P. Bibcode: 2000SPD....31.0133E Altcode: 2000BAAS...32..807E When the Reynolds number is not small, the wake trailing a buoyant magnetic flux tube sheds vortex rolls therefore producing a Von Karman vortex street and an imbalance of vorticity in the tube which results in a transverse oscillation of the tube as a whole. The actual path followed by the magnetic structure is therefore directly affected by the amount of vorticity being produced in its boundary. Analytical expressions for the magnetic generation and viscous dissipation of vorticity in the boundary layer of buoyant magnetic flux tubes are obtained. Corresponding scaling laws are deduced and checked using a full compressible 2D MHD code. Interestingly, the observed trajectories can be satisfactorily reproduced by a simple analytical equation (which includes buoyancy, drag and vortex forces). I will conclude with some comparisons with classical results from the hydrodynamical literature (Strouhal number), and some comments about the rise time of buoyant magnetic structures through the solar convection zone. Title: Effects of Non-Uniform Thermal Conduction on Solar Convection-Zone Flux Tubes Authors: Lenz, D. D.; Moreno-Insertis, F. Bibcode: 2000SPD....31.0137L Altcode: 2000BAAS...32..807L The dynamo model of solar magnetic field generation assumes that magnetic flux is retained at the dynamo site for times of the order of the solar-cycle period. However, flux tubes in the solar convection zone are expected to be buoyant, rising to the surface on timescales much shorter than the solar cycle. Since the initial 1955 paper by Parker on this puzzle, there have been numerous investigations into the detailed physics of buoyant flux tubes, but no definitive conclusions have yet been reached. We investigate the role of thermal conduction in flux tube dynamics using MHD simulations. We expect that the thermal conductivity can vary with conditions inside the tube relative to those in the ambient fluid and that such variation in turn affects the tube's energetics and evolution. Preliminary results suggest that suppressed thermal conductivity inside the tube can significantly affect tube morphology and evolution, depending on the characteristics of the tube and its surroundings. We discuss possible implications of our results for solar surface magnetic fields. D.D.L. acknowledges support from an NSF-NATO Postdoctoral Fellowship in Science and Engineering. We thank Thierry Emonet for helpful discussions. We are grateful to Mark Rast for the use of his MHD code. The numerical simulations were performed on the Cray T3E at CIEMAT in Madrid, Spain. Title: Distribution of magnetic flux on the solar surface and low-degree p-modes Authors: Moreno-Insertis, F.; Solanki, S. K. Bibcode: 2000MNRAS.313..411M Altcode: The frequencies of solar p-modes are known to change over the solar cycle. There is also recent evidence that the relation between frequency shift of low-degree modes and magnetic flux or other activity indicators differs between the rising and falling phases of the solar cycle, leading to a hysteresis in such diagrams. We consider the influence of the changing large-scale surface distribution of the magnetic flux on low-degree (l<=3) p-mode frequencies. To that end, we use time-dependent models of the magnetic flux distribution and study the ensuing frequency shifts of modes with different order and degree as a function of time. The resulting curves are periodic functions (in simple cases just sine curves) shifted in time by different amounts for the different modes. We show how this may easily lead to hysteresis cycles comparable to those observed. Our models suggest that high-latitude fields are necessary to produce a significant difference in hysteresis between odd- and even-degree modes. Only magnetic field distributions within a small parameter range are consistent with the observations by Jiménez-Reyes et al. Observations of p-mode frequency shifts are therefore capable of providing an additional diagnostic of the magnetic field near the solar poles. The magnetic distribution that is consistent with the p-mode observations also appears reasonable compared with direct measurements of the magnetic field. Title: Storage of toroidal magnetic field below the solar convection zone Authors: Rempel, M.; Schüssler, M.; Moreno-Insertis, F. Bibcode: 1999AGAb...15R..74R Altcode: 1999AGM....15..J15R Simulations of erupting flux tubes in the thin flux tube approximation show that essential properties of sunspots can only be explained if the initial field strength of the flux tube at the base of the convection zone is about 10 T. Such strong magnetic field can only be stored below the solar convection zone in a subadiabatic stratification. We consider mechanical equilibria in form of magnetic flux tubes and magnetic sheets and discuss the influence of radiative and convective energy transport on these configurations. In the case of magnetic flux tubes, radiative inflow of heat leads to enhanced buoyancy which causes the flux tube to move upwards and leave the storage region. In the case of magnetic sheets, the compensation of the poleward directed magnetic tension force requires a deviation of the temperature from the hydrostatic background stratification. Convective energy transport disturbs the equilibrium and leads to thermal circulations. Title: Emerging Flux Tubes in the Solar Convection Zone. II. The Influence of Initial Conditions Authors: Caligari, P.; Schüssler, M.; Moreno-Insertis, F. Bibcode: 1998ApJ...502..481C Altcode: Numerical simulations of rising magnetic flux tubes in the solar convection zone have contributed significantly to our understanding of the basic properties of sunspot groups. They have provided an important clue to the operation of the solar dynamo by predicting strong (super-equipartition) magnetic fields near the bottom of the convection zone. We have investigated to what extent the simulation results (obtained on the basis of the thin flux tube approximation) depend on the assumptions made about the initial state of a magnetic flux tube at the start of the simulation. Two initial conditions used in the literature have been considered in detail: mechanical equilibrium (MEQ) and temperature balance (TBL). It turns out that the requirement of super-equipartition field strength is a robust feature of the simulations, largely independent of the choice of initial conditions: emergence of active regions at low latitudes and the correct dependence of their tilt angle (with respect to the east-west direction) as a function of heliographic latitude require an initial magnetic field strength on the order of 105 G. Other properties of rising flux tubes, such as the asymmetries of shape and field strength between the leading and following wings (with respect to the direction of rotation) of a rising loop, or the anchoring of part of the flux tube in the overshoot region, depend on the initial condition. Observed asymmetries in the magnetic flux distribution and of proper motions in emerging active regions favor MEQ over TBL as the proper initial condition. MEQ should also be preferred for other theoretical reasons: it allows for fewer free parameters, it requires no fine tuning for the tube geometry and background stratification in the overshoot region, and it can be easily made compatible with an encompassing model of the generation, storage, and eruption of the magnetic flux. We have also studied whether an external upflow (convective updraft) can trigger the eruption of an otherwise stably stored flux tube in the overshoot region. We find that a significant deformation and destabilization of a flux tube with equipartition field strength requires coherent upflow velocities of 20-50 m s-1 in the overshoot layer, which is an order of magnitude larger than current estimates for such velocities. Title: A crossroads for European solar and heliospheric physics. Recent achievements and future mission possibilities. Proceedings. Authors: Priest, E. R.; Moreno-Insertis, F.; Harris, R. A. Bibcode: 1998cesh.conf.....P Altcode: The following topics were dealt with: the solar interior, photosphere, solar corona, the solar wind, SOHO spacecraft mission results, the Ulysses mission, the Yohkoh mission, scientific objectives and future missions, the RAMSES proposal, INTERHELIOS, SOLARNET, the solar STEREO mission, HIREX, Solar-B mission, SOLAR LITE, UV coronagraphs. Title: The Physics of Twisted Magnetic Tubes Rising in a Stratified Medium: Two-dimensional Results Authors: Emonet, T.; Moreno-Insertis, F. Bibcode: 1998ApJ...492..804E Altcode: 1997astro.ph.11043E The physics of a twisted magnetic flux tube rising in a stratified medium is studied using a numerical magnetohydrodynamic (MHD) code. The problem considered is fully compressible (has no Boussinesq approximation), includes ohmic resistivity, and is two-dimensional, i.e., there is no variation of the variables in the direction of the tube axis. We study a high-plasma β-case with a small ratio of radius to external pressure scale height. The results obtained will therefore be of relevance to understanding the transport of magnetic flux across the solar convection zone.

We confirm that a sufficient twist of the field lines around the tube axis can suppress the conversion of the tube into two vortex rolls. For a tube with a relative density deficit on the order of 1/β (the classical Parker buoyancy) and a radius smaller than the pressure scale height (R2<<H2p), the minimum amount of twist necessary corresponds to an average pitch angle on the order of sin-1 [(R/Hp)1/2]. The evolution of a tube with this degree of twist is studied in detail, including the initial transient phase, the internal torsional oscillations, and the asymptotic, quasi-stationary phase. During the initial phase, the outermost, weakly magnetized layers of the tube are torn off its main body and endowed with vorticity. They yield a trailing magnetized wake with two vortex rolls. The fraction of the total magnetic flux that is brought to the wake is a function of the initial degree of twist. In the weakly twisted case, most of the initial tube is turned into vortex rolls. With a strong initial twist, the tube rises with only a small deformation and no substantial loss of magnetic flux. The formation of the wake and the loss of flux from the main body of the tube are basically complete after the initial transient phase.

A sharp interface between the tube interior and the external flows is formed at the tube front and sides; this area has the characteristic features of a magnetic boundary layer. Its structure is determined as an equilibrium between ohmic diffusion and field advection through the external flows. It is the site of vorticity generation via the magnetic field during the whole tube evolution.

From the hydrodynamical point of view, this problem constitutes an intermediate case between the rise of air bubbles in water and the motion of a rigid cylinder in an external medium. As with bubbles, the tube is deformable and the outcome of the experiment (the shape of the rising object and the wake) depends on the value of the Weber number. Several structural features obtained in the present simulation are also observed in rising air bubbles, such as a central tail, and a skirt enveloping the wake. As in rigid cylinders, the boundary layer satisfies a no-slip condition (provided for in the tube by the magnetic field), and secondary rolls are formed at the lateral edges of the moving object. Title: Turbulent Erosion of Magnetic Flux Tubes Authors: Petrovay, K.; Moreno-Insertis, F. Bibcode: 1997ApJ...485..398P Altcode: 1997astro.ph..3152P Results from a numerical and analytical investigation of the solution of a nonlinear axisymmetric diffusion equation for the magnetic field are presented for the case when the nonlinear dependence of the diffusivity ν(B) on the magnetic field satisfies basic physical requirements. We find that for sufficiently strong nonlinearity (i.e., for sufficiently strong reduction of ν inside the tube) a current sheet is spontaneously formed around the tube within one diffusion timescale. This sheet propagates inward with a velocity inversely proportional to the ratio of the field strength just inside the current sheet to the equipartition field strength B0/Be, so the lifetime of a tube with constant internal flux density is increased approximately by a factor not exceeding B0/Be, even for infinitely effective inhibition of turbulence inside the tube. Among the applications of these results, we point out that toroidal flux tubes in the solar convective zone are subject to significant flux loss owing to turbulent erosion on a timescale of ~1 month and that turbulent erosion may be responsible for the formation of a current sheet around a sunspot. It is further proposed that, despite the simplifying assumptions involved, our solutions correctly reflect the essential features of the sunspot decay process. Title: Emergence of magnetic flux in the convection zone. The limits of the thin flux tube approximation. Authors: Moreno-Insertis, F. Bibcode: 1997smf..conf....3M Altcode: The thin flux tube approximation cannot describe some important phenomena occurring during the journey of the tubes across the convection zone. This review summarizes five of those processes. One of them is related to the dramatic off-axis expansion suffered by the top of some magnetic tubes of moderate field strength in the middle of the convection zone. Another one concerns the expansion that all rising tubes experience in the final few 10000 km below the photosphere. The thin flux tube approximation does not account for the development of vorticity and twist in the magnetic tubes (or only under very restrictive circumstances). However, vorticity and twist are fundamental ingredients that have to be considered at various stages of the rise. All this is explained on the basis of the recent results of two-dimensional MHD simulations of the initial stages of the rise. In the final section, the validity of the local approach for a rectilinear and non-rotating tube as well as its failure in more general cases are explained. Title: The Rise of Magnetic Flux Tubes across a Stratified Medium: Effects of the Twist Authors: Emonet, T.; Moreno-Insertis, F. Bibcode: 1997ASPC..118...71E Altcode: 1997fasp.conf...71E The results of a 2D numerical simulation of the rise of twisted magnetic flux tubes are sketched. The theoretical criterion for the minimum twist necessary to prevent the conversion of the tube into a pair of strong vortices is shown to be correct. The transition from a low-twist to a high-twist regime is exemplified. There is a sharp transition between the tube interior and the outside medium. A well-developed wake is formed, which however contains only a fraction of the original magnetic flux of the tube. Title: Emergence of magnetThe limits of the thin flux tube approximation Authors: Moreno-Insertis, F. Bibcode: 1997smf..proc....3M Altcode: The studies of the emergence of magnetic flux across the convection zone show how the individual magnetic tubes rise toward the surface at the same time remaining anchored at the interface between the convection zone and the radiative interior. A number of recent numerical simulations, in particular, reproduce several observed features of the resulting active regions (e.g., tilt angles, preceding-following asymmetries, etc). These simulations are based on the thin flux tube approximation, which simplifies the magnetic ropes as a one-dimensional continuum. The thin flux tube approximation has provided valuable insights concerning the rise of the magnetic tubes. Yet, it cannot describe some important phenomena occurring during the journey of the tubes across the convection zone. This review summarizes five of those processes. One of them is related to the dramatic off-axis expansion suffered by the top of some magnetic tubes of moderate field strength in the middle of the convection zone (a phenomenon dubbed explosion) as a consequence of the adiabatic character of their evolution. Another one concerns the expansion that all rising tubes experience in the final few $10,000$ km below the photosphere. Further, the thin flux tube approximation does not account for the development of vorticity and twist in the magnetic tubes (or only under very restrictive circumstances). However, vorticity and twist are fundamental ingredients that have to be considered at various stages of the rise. Without twist, the buoyant tubes tend to split and yield vortex filament pairs which separate horizontally instead of rising. The transverse field component of a twisted tube helps maintain the unity of the tube, but it yields an evolutionary pattern far more complicated than as described by the thin flux tube approximation. All this is explained on the basis of the recent results of two-dimensional MHD simulations of the initial stages of the rise. Finally, the back reaction of the external medium to the advance of the magnetic region is difficult to treat in the thin tube approximation. The simple local prescription commonly used is inconsistent in that it is based on the assumption of a potential (i.e., essentially non-local) flow around the tube. As a result, the expression for the enhanced inertia it yields may violate the condition of global momentum conservation. In the final section of the paper, the validity of the local approach for a rectilinear and non-rotating tube as well as its failure in more general cases are explained. Title: Multidimensional simulation of the rise of magnetic flux tubes Authors: Moreno-Insertis, F. Bibcode: 1997ASPC..118...45M Altcode: 1997fasp.conf...45M Recent results obtained through the 2D or 3D numerical simulation of the rise of magnetic flux tubes in the solar convection zone are discussed. The one--dimensional approximation known as the slender flux tube approximation permits calculation of the time evolution of a rising magnetic tube through most of the phases before it erupts at the surface. However, the 1D model disregards the off-axis structure of the magnetic tube, which turns out to play a non-negligible role in the tube evolution from the very beginning of its rise. This is discussed on the basis of a few examples: if the tube is untwisted, the hydrodynamical forces of the surrounding flow may easily convert it into a vortex tube pair whose components, asymptotically, stop to rise. If the tube is sufficiently twisted, then the development of vorticity is prevented in most of the tube interior, and the tube rises in a way reminiscent of air bubbles in a liquid. This suggests that the magnetic flux may be transported to the photosphere by means of twisted magnetic tubes. The physical conditions under which the magnetic flux is stored may be decisive for this issue: if the magnetic tubes are stored in mechanical equilibrium, then the minimum degree of twist required to prevent the generation of vorticity in the tube may be reduced. This review centers on the evolution of the magnetic flux in a tube-like geometry. However, recent 2D and 3D simulations of the time evolution of a Rayleigh-Taylor unstable magnetic slab are briefly discussed. Title: Origin of the Proper Motions of Emerging Bipolar Magnetic Regions Authors: Caligari, P.; Schussler, M.; Moreno-Insertis, F. Bibcode: 1997ASPC..118...76C Altcode: 1997fasp.conf...76C We have performed numerical simulations of the rise of magnetic flux tubes through the convection zone. We find that the observed proper motions of pores and sunspots in young active regions can be understood as a consequence of the Coriolis force: conservation of angular momentum leads to a retardation of the rising flux loop with respect to those parts of the flux tube that remain anchored in the overshoot layer below the convection zone proper. The result is an asymmetric shape with the following flank of the loop being more vertical than the leading part. When emerging at the solar surface, the asymmetric shape of the tube leads to proper motions which are in qualitative agreement with the observations. By studying the dependence of the asymmetry on the initial state of the flux tube we find that the observed proper motions favor a mechanical equilibrium of the magnetic field in the overshoot layer. We also find that small active regions (emerging from flux tubes with little magnetic flux) are less asymmetric and should show weaker proper motions than large bipolar regions. This prediction can be put to an observational test. Title: Emergence of magnetic flux from the solar interior Authors: Moreno-Insertis, F. Bibcode: 1997MmSAI..68..429M Altcode: No abstract at ADS Title: The Rise of Twisted Magnetic Tubes in a Stratified Medium Authors: Moreno-Insertis, F.; Emonet, T. Bibcode: 1996ApJ...472L..53M Altcode: First results from a two-dimensional numerical study of the buoyant rise of twisted magnetic flux tubes in the solar convection zone are presented. We show in detail the process by which the transverse component of the field can suppress the splitting of the rising tube into two vortex filaments. For the suppression to be effective, the pitch angle of the twisted field lines has to be above a threshold given by the condition that the magnetic equivalent of the Weber number (see § 2.2) be below 1. The shape obtained for the tube and wake is strongly reminiscent of laboratory experiments with air bubbles rising in liquids. The magnetized region outside an equipartition boundary is peeled away from the tube: two sidelobes are formed, which lag behind the tube and contain only a fraction of the initial magnetic flux. This is similar to the formation of a skirt in the fluid dynamical case. The velocities of rise predicted by the thin flux tube approximation are compared with those obtained here. Title: Enhanced inertia of thin magnetic flux tubes. Authors: Moreno-Insertis, F.; Schuessler, M.; Ferriz-Mas, A. Bibcode: 1996A&A...312..317M Altcode: Accelerated bodies immersed in a fluid experience enhanced inertia due to the associated co-acceleration of a certain volume of fluid in their environment. We discuss the concept of enhanced inertia in the framework of the approximation of thin flux tubes, which is widely used to describe the dynamics of concentrated magnetic structures in astrophysical objects. Previous attempts to incorporate this effect have used a local approach, in which the reaction force of the external medium on a given tube mass element solely depends on the relative acceleration of tube and environment at that element. We show that those previous formulations are inconsistent (either on physical or geometrical grounds). We present here an alternative derivation of the enhanced inertia term by geometrical means, still within a local treatment of the problem but avoiding the pitfalls of previous works. Our formulation, on the other hand, reveals a basic problem: all local approaches are bound to give incorrect answers for the reaction force in as far as they disregard the variation of the external flow in the direction parallel to the flux tube: in doing so, they generally fail to provide for global momentum conservation. An exact solution and detailed analysis for an instance of this failure is given. The discussion of this paper may be of use also in the hydrodynamical framework of vortex tube dynamics. Title: Equilibrium of Twisted Horizontal Magnetic Flux Tubes Authors: Emonet, T.; Moreno-Insertis, F. Bibcode: 1996ApJ...458..783E Altcode: The equilibrium of non-force-free twisted horizontal magnetic flux tubes is studied including gravity and an arbitrary pressure perturbation on the tube boundary. To solve this free-boundary problem, we use general nonorthogonal flux coordinates and consider the two-dimensional case in which there is no variation of the physical quantities along the tube axis. For the applications in the convection zone and corona, we consider the case of weak external stratification by assuming that the radius of the tube is smaller than the external pressure scale height. This allows us to introduce a perturbation scheme which is much less restrictive than the customary slender flux-tube approximation. In particular, it has the advantage of not imposing any limitation on the strength of the azimuthal field as compared to the longitudinal field. Within this scheme, one retains to zero order all the functional degrees of freedom of a general axisymmetric magnetostatic equilibrium; the geometry of the perturbed azimuthal field lines is then obtained from the equilibrium equations as a consequence of the zero-order density (or rather buoyancy) distribution in the tube and of the circular wavenumber of the external pressure perturbation. We show that, as a result of the presence of gravity, the field lines are no longer concentric, although they continue being circular. The resulting changes in magnetic pressure and tension of the azimuthal field exactly counteract the differences in buoyancy in the tube cross section. On the other hand, external pressure fluctuations of circular wavenumber higher than one can only be countered by bending the azimuthal field lines. In general terms, the present scheme allows one to study in detail the mutual dependence of the (differential) buoyancy in the tube, the intensity and field line geometry of the azimuthal magnetic field, and the gas pressure and longitudinal magnetic field distributions.

The main application of the equations and results of this paper is to study the transverse structure of magnetic flux rings embedded in a stratified medium with a flow around the tube that causes pressure fluctuations on its surface. This includes tubes in the deep convection zone, e.g., in its subadiabatic lower part, or those kept in place by a meridional flow. It also applies to flux rings moving in a quasi-static regime in which the drag force of the relative motion with respect to the external medium exactly compensates the total buoyancy of the tube. In this way, this study can complement the numerical simulations of the rise of magnetized tubes and bubbles toward the surface. Title: Asymmetries in solar active regions and flux emergence models Authors: Cauzzi, G.; Moreno-Insertis, F.; van Driel-Gesztelyi, L. Bibcode: 1996ASPC..109..121C Altcode: 1996csss....9..121C No abstract at ADS Title: MHS-Equilibrium of Twisted Magnetic Tubes Authors: Emonet, T.; Moreno-Insertis, F. Bibcode: 1996ApL&C..34....9E Altcode: No abstract at ADS Title: ``Explosion'' and Intensification of Magnetic Flux Tubes Authors: Moreno-Insertis, F.; Caligari, P.; Schuessler, M. Bibcode: 1995ApJ...452..894M Altcode: A magnetic flux tube anchored at the bottom of the solar convection zone and rising toward the surface as a result of an undulatory instability can be affected by a sudden catastrophic expansion and weakening of the magnetic field at its apex if the original field strength is below a few times 104 G. Such an "explosion" occurs if the flux tube evolves close enough to (adiabatic) hydrostatic equilibrium along the magnetic field lines in a super-adiabatically stratified environment. This condition is satisfied if the diameter of the tube is small enough for the drag force to dominate the dynamical evolution. For example, rising flux tubes with equipartition field (104 G) at their basis explode in the middle of the convection zone if their magnetic flux is below ≌1021 Mx.

Apart from preventing flux tubes with equipartition field from reaching the surface, the explosion process may have other consequences for the evolution of magnetic fields in the convection zone: (a) it provides a source of weak field to be acted upon by the convective flows in the course of a turbulent dynamo process; (b) upflow of matter into the inflated top of a loop intensifies the magnetic field in the submerged part of the flux tube at the bottom of the convection zone. This might constitute a mechanism to produce the strong azimuthal fields in the overshoot region suggested by recent studies of the undular instability of magnetic flux tubes. Title: Emerging Flux Tubes in the Solar Convection Zone. I. Asymmetry, Tilt, and Emergence Latitude Authors: Caligari, P.; Moreno-Insertis, F.; Schussler, M. Bibcode: 1995ApJ...441..886C Altcode: The process of emergence of magnetic flux from the depths of the convection zone to the surface is presented in the framework of self-consistent model for the storage of field in the lower overshoot regions and as the mechanism responsible for some of the regularities observed in active regions. We have performed numerical simulations of the emergence of flux tubes in the solar convection zone including the effects of spherical geometry and rotation. The magnetic flux tubes can be stored in mechanical equilibrium in the overshoot region, which is the natural equilibrium of the flux rings in a subadiabatic layer. An undular instability leads to the formation of loops once a critical magnetic field strength of the order of 105 G is exceeded. In the nonlinear phase of their unstable evolution, the tubes move across the convection zone on a very fast time-scale, typically about one month. The geometry and dynamics of the flux tubes studied in these simulations permit prediction of some of the observed properties of the active regions. First, the wings of the tube show a marked asymmetry of inclination and velocity, which is compatible with the observed asymmetric proper motions of sunspots and with the position of the neutral line in emerging active regions. Second, upon emergence the flux tubes show a tilt angle with respect to the equator which fits reasonably well with the observed values. Third, the flux tubes rise roughly within a cone of radial directions in the Sun so that no outbreak at high latitudes takes place. The calculations lend further support to the possibility of superequipartition field strengths in the overshoot region. The implications of the present results for the dynamo mechanism are discussed and hints for observational work are also given. Title: Active Region Asymmetry as a Result of the Rise of Magnetic Flux Tubes Authors: Moreno-Insertis, F.; Caligari, P.; Schuessler, M. Bibcode: 1994SoPh..153..449M Altcode: The magnetic flux tubes that rise across the convection zone to produce active regions are shown to develop a difference in inclination between their preceding and follower sides. This asymmetry is such that the follower wing is more vertical (i.e., closer to the radial direction) than the preceding side. An asymmetry of this kind can be obtained as a natural consequence of the conservation of angular momentum along the rise. This process may explain a number of the observed asymmetries in morphology and behavior of the preceding and follower parts of the active regions. We present results of numerical simulations showing this effect and discuss possible observational consequences. Title: Forces on Magnetic Flux Tubes Moving in Inhomogeneous Flows Authors: Moreno-Insertis, F.; Ferriz-Mas, A.; Schussler, M. Bibcode: 1994ApJ...422..652M Altcode: The back-reaction of an incompressible two-dimensional flow of constant strain to the accelerated motion of a straight cylinder is calculated in the general case that the flow is inhomogeneous, nonstationary, and with nonvanishing (although constant) vorticity. The resulting enhanced inertia of the cylinder is basically given by its relative acceleration with respect to the background flow. Further force terms that appear because of the nonstationary and inhomogeneity of the background flow are the following: a force identical to that experienced by the mass elements of the unperturbed flow at the position of the axis of the cylinder; the customary lift force because of the circulation around the body; finally, a force term that appears only with nonvanishing relative speed between the body and the background flow and which is related to the energy which has to be imparted by the body to the surrounding fluid in order to adapt the perturbation to the new local velocity. The results have application to the dynamics of magnetic flux tubes in the convection zone and atmosphere of the Sun as well as to other astrophysical problems. Title: Dynamics of erupting magnetic flux tubes Authors: Moreno-Insertis, F.; Schüssler, M.; Caligari, P. Bibcode: 1994ASIC..433..407M Altcode: 1994ssm..work..407M The eruption of magnetic flux tubes from the overshoot layer due to instability and the dynamics of their subsequent rise through the solar convection zone are followed by numerical simulation. Special emphasis is put on the possibility of explaining observed regularities of the active regions at the surface (tilt angles, latitude of emergence, asymmetry between preceding and following parts, etc). Instability sets in with non-axisymmetric (undular) modes at azimuthal wavenumbers $m=1$ and $m=2$ if the field strength exceeds values of the order of $10^5$ G. At the same time, such strong initial fields are required to reproduce the observable properties of sunspots and active regions. Consequently, a consistent picture of storage, instability and eruption of solar magnetic fields emerges. Title: Instability and eruption of magnetic flux tubes Authors: Caligari, P.; Ferriz-Mas, A.; Moreno-Insertis, F.; Schüssler, M. Bibcode: 1994smf..conf..139C Altcode: No abstract at ADS Title: The magnetic field in the convection zone as a link between the active regions on the surface and the field in the solar interior Authors: Moreno-Insertis, F. Bibcode: 1994smf..conf..117M Altcode: Recent developments in the theory of the rise of magnetic flux from the bottom of the convection zone to the surface to produce active regions are reviewed. These include the explanation for the observed tilt angle of the main magnetic axis of the active region as well as for some of the asymmetries between the preceding and follower polarities. It is stressed how this kind of studies may provide a way to gain insight about the magnetic field in the dynamo layer at the same time explaining some of the regularities observed in the active regions at the surface. Title: Instability and eruption of magnetic flux tubes in the solar convection zone. Authors: Schussler, M.; Caligari, P.; Ferriz-Mas, A.; Moreno-Insertis, F. Bibcode: 1994A&A...281L..69S Altcode: We present a consistent model of storage, instability and dynamical eruption of magnetic flux tubes in the solar convection zone and underlying overshoot region. Using a convection zone model with self-consistent overshoot layer, we calculate equilibrium configurations of magnetic flux tubes and determine their linear stability properties, taking into consideration the effects of stratification and rotation. Instability of flux tubes stored in the overshoot layer with growth times below one year requires field strengths of the order of 105 G; in many cases, the dominant mode has an azimuthal wave number of m = 2. Numerical simulations are used to follow the nonlinear evolution of such unstable flux tubes and their rise through the convection zone, from which they emerge to form active regions. The results are in accordance with the following two requirements, based on observational facts: (a) the upward motion of the tubes is not significantly deflected by the Coriolis force so that they can emerge at low latitudes, (b) their inclination with respect to the East-West direction (tilt angle) as a function of the latitude of emergence is consistent with observations of acitve regions. Title: The distribution of sunspot decay rates. Authors: Martinez Pillet, V.; Moreno-Insertis, F.; Vazquez, M. Bibcode: 1993A&A...274..521M Altcode: The distribution of sunspot decay rates is studied using the Greenwich Photoheliographic Results (GPR) for a total of approximately hundred years between 1874 and 1976.

The decay rates are seen to be lognormally distributed. The discrepancies between the decay rates given in the past by different authors are shown to originate as a consequence of this asymmetric distribution. It is pointed out that the extended tails shown by the lognormal distributions are associated to spots decaying much faster than suggested by Bumba's (1963) work. A cycle by cycle analysis of the lognormal distributions associated with each sunspot group type and for single spots is presented. The differences between the nine solar cycles involved are studied.

As a remarkable property of the decay process, we show that it happens at a nearly constant total to umbral area ratio. This property holds for decaying spots which are still large enough to show a penumbra.

We have studied the suitability of a decay law with the instantaneous decay rate proportional to the length of the spot boundary. This law predicts a parabolic decay pattern with some specific characteristics. No definite conclusion in favour of this law is reached, but it is suggested that a linear decay is as weakly supported by the GPR data as a peripheral one. On the other hand, weak non-linearities are seen in the decay of isolated spots with a clear tendency to produce a convex pattern in the area vs. time diagram. The implication is that sunspot decay is braked as time proceeds. Title: Storage of Magnetic Flux in the Overshoot Region Authors: Moreno-Insertis, F.; Schussler, M.; Ferriz-Mas, A. Bibcode: 1993IAUS..157...41M Altcode: No abstract at ADS Title: The Distribution of Sunspot Decay Rates Authors: Martinez Pillet, V.; Moreno-Insertis, F.; Vazquez, M. Bibcode: 1993ASPC...46...67M Altcode: 1993mvfs.conf...67M; 1993IAUCo.141...67M No abstract at ADS Title: The Motion of Magnetic Flux Tubes in the Convection Zone and the Subsurface Origin of Active Regions Authors: Moreno-Insertis, F. Bibcode: 1993sto..work..385M Altcode: The rise of kink-unstable magnetic flux tubes through the solar convection zone is discussed with a view to explaining the subsurface origin of active regions. In spite of the advances and insights obtained by analytical and numerical means in the past decade(s), there are still important uncertainties as to the viability of a mechanism of this kind to explain the origin of sunspots and active regions. In this paper, the main features of the rise of kink unstable tubes and some open questions regarding this process are presented. Title: Shock wave propagation in a magnetic flux tube Authors: Ferriz-Mas, A.; Moreno-Insertis, F. Bibcode: 1992PhFlA...4.2700F Altcode: 1992PhFl....4.2700F The propagation of a shock wave in a magnetic flux tube is studied within the framework of the Brinkley-Kirkwood theory adapted to a radiating gas. Simplified thermodynamic paths along which the compressed plasma returns to its initial state are considered. It is assumed that the undisturbed medium is uniform and that the flux tube is optically thin. The shock waves investigated, which are described with the aid of the thin flux-tube approximation, are essentially slow magnetohydrodynamic shocks modified by the constraint of lateral pressure balance between the flux tube and the surrounding field-free fluid; the confining external pressure must be balanced by the internal gas plus magnetic pressures. Exact analytical solutions giving the evolution of the shock wave are obtained for the case of weak shocks. Title: Storage of magnetic flux tubes in a convective overshoot region Authors: Moreno-Insertis, F.; Schuessler, M.; Ferriz-Mas, A. Bibcode: 1992A&A...264..686M Altcode: Consideration is given to the suppression of the radial and polar escape of magnetic flux in the form of toroidal flux tubes (flux rings) from low latitudes in the overshoot region below the solar convection zone through the combined action of the subadiabatic ambient stratification and the rotationally induced forces. It is shown that a flux ring which is initially in thermal equilibrium with its environment and rotates with the ambient angular velocity moves radially and latitudinally towards an equilibrium configuration of lower internal temperature and larger internal rotation rate with respect to the surrounding nonmagnetic gas. Flux rings perform superposed buoyancy and inertial oscillations around their equilibrium positions. From a study of the frequencies and amplitudes of these oscillations, it is concluded that flux rings with B of less than about 100,000 G can be kept within the overshoot region if the superadiabaticity is sufficiently negative, i.e., less than about -0.00004. Title: Instability of magnetic flux tubes in the solar convection zone. Authors: Caligari, P.; Moreno-Insertis, F.; Schüssler, M. Bibcode: 1992AGAb....7..152C Altcode: No abstract at ADS Title: The Motion of Magnetic Flux Tubes in the Convection Zone and the Subsurface Origin of Active Regions Authors: Moreno-Insertis, F. Bibcode: 1992ASIC..375..385M Altcode: 1992sto..work..385M The rise of kink-unstable magnetic flux tubes through the solar convection zone is discussed with a view to explaining the subsurface origin of active regions. In spite of the advances and insights obtained by analytical and numerical means in the past decade(s), there are still important uncertainties as to the viability of a mechanism of this kind to explain the origin of sunspots and active regions. In this paper, the main features of the rise of kink unstable tubes and some open questions regarding this process are presented. Title: Damping of Shocks in Magnetic Flux Tubes Authors: Ferriz Mas, A.; Moreno Insertis, F. Bibcode: 1991mcch.conf..417F Altcode: No abstract at ADS Title: Decay rates of sunspot groups from 1874 to 1976 Authors: Martinez Pillet, V.; Moreno-Insertis, F.; Vazquez, M. Bibcode: 1990Ap&SS.170....3M Altcode: The global behaviour and fine structure of the distribution of sunspot decay rates from activity cycle 13 to 20 are presented. It is shown that the distribution of this parameter is lognormal. Statistically significantly lower values of decay rates are found in cycles 13, 14, and 18 for isolated spots. The complex groups had no appreciable changes. Title: Stability of Sunspots to Convective Motions. I. Adiabatic Instability Authors: Moreno-Insertis, F.; Spruit, H. C. Bibcode: 1989ApJ...342.1158M Altcode: For determining the adiabatic stability of a uniform vertical field in an arbitrary stratification it is sufficient to consider the limit of infinitesimal horizontal wavelength. It is shown how the behavior of the instability can be estimated qualitatively from the dependence of the equipartition field strength on depth. Modes are calculated numerically for analytic stratification models and for a detailed sunspot stratification, including the effects of partial ionization. It is concluded that for the observed field strengths of umbrae the stratification is indeed unstable, with a growth time of about 18 minutes. The unstable eigenfunctions have a maximum at about 2300 km below the surface of the umbra and are about 3900 km deep. Deeper layers may also be unstable depending on unknown details of the stratification. A connection between fluting instability and convective instability is noted. Title: A statistical study of the decay phase of sunspot groups from 1874 to 1939 Authors: Moreno-Insertis, F.; Vazquez, M. Bibcode: 1988A&A...205..289M Altcode: A statistical study of the decay of sunspots and sunspot groups is carried out using data contained in the Greenwich Photoheliographic Results. The period studied spans more than six decades, namely from 1874 to 1939. The adequacy of exponential, quadratic and linear decay laws to the area decay data is considered. The dependence, or otherwise, of the mean area decay rate for each single spot, D, on its maximum area, A0, is studied; no relationship of the form D(A0) can be ascertained. In fact, when plotted on a D vs. A0 diagram, the couples (A0, D) cluster in two pencils issuing from the origin of coordinates. These two clusters can be seen to correspond with the families of the recurrent and non-recurrent spot groups. The adequacy of a quadratic law to the decay data is considered. A most probable parabolic decay law is obtained; this law corresponds to the case in which the decay rate is proportional to the spot perimeter. Finally, the variation of the decay rates in successive activity cycles is also discussed. Title: An analytical study of shock waves in thin magnetic flux tubes Authors: Ferriz-Mas, A.; Moreno-Insertis, F. Bibcode: 1987A&A...179..268F Altcode: The jump conditions across a shock front in a thin magnetic flux tube are studied by purely analytical means. Some properties of magnetohydrodynamic shock waves in extended media are shown also to hold in the more complicated case of thin magnetic flux tubes. It is shown that flux tube shock waves are always compressive, thus being accompanied by a weakening of the magnetic field strength and increase of the tube radius. Some consequences of this are examined, such as the sub- or supercritical character of the flow velocity with respect to the Alfvén, sound and "tube" speeds. The range of variation of the ratios of the different variables across the shock front is determined along with the equivalent of the Hugoniot curve and further properties of the shocks in magnetic flux tubes. The analogies and differences with HD and MHD shocks in extended media are pointed out. Finally, some implications of the shock wave structure for the solar atmosphere are briefly discussed. Title: The Subsurface Structure of Sunspots and the Origin of Solar Active Regions Authors: Moreno-Insertis, F. Bibcode: 1987rfsm.conf..167M Altcode: Several topics concerning the subsurface structure and evolution of sunspots are discussed. These include the morphology and stability of the magnetized plasma below a sunspot as well as the heat blockage in it. Current ideas on the origin and physical processes preceding the formation of an active region are reviewed. Title: The subsurface structure of sunspots and the origin of solar active regions Authors: Moreno-Insertis, F. Bibcode: 1987fsmf.conf..167M Altcode: Several topics concerning the subsurface structure and evolution of sunspots are discussed. These include the morphology and stability of the magnetized plasma below a sunspot as well as the heat blockage in it. In the second part of this paper, current ideas on the origin and physical processes preceding the formation of an active region are reviewed. Title: Decay Rates of Sunspot Groups from 1874 to 1939 Authors: Moreno-Insertis, F.; Vázquez, M. Bibcode: 1987rfsm.conf..196M Altcode: Various possible decay laws (exponential, linear, parabolic) are tried to fit the observational material. The dependence of the decay rates on the spot's maximum area as well as their variation along the activity cycle are studied. Title: Nonlinear time-evolution of kink-unstable magnetic flux tubes in the convective zone of the sun Authors: Moreno-Insertis, F. Bibcode: 1986A&A...166..291M Altcode: The nonlinear development of the kink instability in a horizontal magnetic flux tube with a field strength exceeding the equipartition value lying initially in the deep convective zone is calculated. A Lagrangian numerical code is developed to obtain solutions for the system of equations dictating the evolution of a thin magnetic flux tube in a stratified medium. The relevance of this calculation to the understanding of active region formation and solar activity cycle is considered. Title: Non linear time-evolution of kink-unstable magnetic flux tubes in theconvection zone of the Sun. Authors: Moreno-Insertis, F. Bibcode: 1986MPARp.226.....M Altcode: No abstract at ADS Title: Observations of the Birth and Fine Structure of Sunspot Penumbrae Authors: Collados, M.; Garcia de La Rosa, J. I.; Moreno-Insertis, F.; Vazquez, M. Bibcode: 1985LNP...233..133C Altcode: 1985hrsp.proc..133C High resolution white-light pictures of sunspot penumbrae are presented. These include pictures showing details of their filamentary structure and some instances of the birth of a penumbra. The observations are discussed in the framework of current penumbra theories. Title: Nonlinear time-evolution of kink-unstable magnetic flux tubes in the convection zone of the Sun. Authors: Moreno-Insertis, F. Bibcode: 1984ESASP.220...81M Altcode: 1984ESPM....4...81M The nonlinear development of the kink instability of a magnetic flux tube initially lying in mechanical equilibrium in the deep convection zone is presented. A numerical code was developed; for the convection zone stratification the model by Spruit (1977) was used. Title: Rise times of horizontal magnetic flux tubes in the convection zone of the sun Authors: Moreno-Insertis, F. Bibcode: 1983A&A...122..241M Altcode: Solutions of the equations describing the motion of a straight horizontal magnetic flux tube in the convection zone of the sun including inertial term and radiative exchange with the surroundings are presented. Cases where the tube is initially both in thermal or in mechanical equilibrium with the surroundings are considered. It is shown that the superadiabatic stratification of the convection zone affects greatly the time scale for the rise of tubes with weak field strength (less than the equipartition value), reducing it by a large factor compared with tubes in permanent thermal equilibrium with the surroundings. The various possible regimes for the rise starting from a state of initial mechanical equilibrium are discussed. It is shown that all tubes with magnetic fluxes approximately equal to or greater than 10 to the 20th Mx will cross the convection zone in time scales of less than two years.