Author name code: archontis ADS astronomy entries on 2022-09-14 author:"Archontis, Vasilis" ------------------------------------------------------------------------ Title: Emergence of magnetic flux in a partially ionized solar atmosphere Authors: Chouliaras, Georgios; Archontis, Vasilis; Syntelis, Petros Bibcode: 2022cosp...44.2552C Altcode: We perform 3-D MHD simulations for a magnetic flux emergence experiment to investigate whether the inclusion of ionization-recombination in the equation of state affects the magnetic flux ascend to the solar atmosphere. We modify the single-fluid MHD equations to include the presence of neutrals on our environment configuration. We used the Lare3D code to numerically solve this 3D time-dependent compressible, resistive MHD equations in cartesian coordinates. Two different models have been considered, the FIP which assumes a fully ionized plasma and the PIP which assumes partially ionized plasma. We performed two different runs with the same beta plasma to be able to observe the differences in the behaviour of the ascend of each flux tube. The axis of the PIP flux tube reaches lower altitudes in the convection zone compared to the axis of the FIP due to the inclusion of the ionization-recombination to the equation of state. The presence of neutrals in the high convection zone affects the structure of the rising magnetic field on the small scale. This different ascended magnetic field on the photosphere has an impact on the evolution of the bipolar region leading to polarities with more spherical and compact shapes than the FIP model and shorter polarity inversion line. Overall our magnetic flux emergence experiment revealed further the role of partial ionization to the magnetic flux emergence into the dynamic solar atmosphere. Title: Decoding the Pre-Eruptive Magnetic Field Configurations of Coronal Mass Ejections Authors: Patsourakos, S.; Vourlidas, A.; Török, T.; Kliem, B.; Antiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou, G.; Georgoulis, M. K.; Green, L. M.; Leake, J. E.; Moore, R.; Nindos, A.; Syntelis, P.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J. Bibcode: 2020SSRv..216..131P Altcode: 2020arXiv201010186P A clear understanding of the nature of the pre-eruptive magnetic field configurations of Coronal Mass Ejections (CMEs) is required for understanding and eventually predicting solar eruptions. Only two, but seemingly disparate, magnetic configurations are considered viable; namely, sheared magnetic arcades (SMA) and magnetic flux ropes (MFR). They can form via three physical mechanisms (flux emergence, flux cancellation, helicity condensation). Whether the CME culprit is an SMA or an MFR, however, has been strongly debated for thirty years. We formed an International Space Science Institute (ISSI) team to address and resolve this issue and report the outcome here. We review the status of the field across modeling and observations, identify the open and closed issues, compile lists of SMA and MFR observables to be tested against observations and outline research activities to close the gaps in our current understanding. We propose that the combination of multi-viewpoint multi-thermal coronal observations and multi-height vector magnetic field measurements is the optimal approach for resolving the issue conclusively. We demonstrate the approach using MHD simulations and synthetic coronal images. Title: Eruptions and flaring activity in emerging quadrupolar regions Authors: Syntelis, P.; Lee, E. J.; Fairbairn, C. W.; Archontis, V.; Hood, A. W. Bibcode: 2019A&A...630A.134S Altcode: 2019arXiv190901446S Context. Solar observations suggest that some of the most dynamic active regions are associated with complex photospheric magnetic configurations such as quadrupolar regions, and especially those that have a δ-spot configuration and a strong polarity inversion line (PIL).
Aims: We study the formation and eruption of magnetic flux ropes in quadrupolar regions.
Methods: We performed 3D magnetohydrodynamics simulations of the partial emergence of a highly twisted flux tube from the solar interior into a non-magnetised stratified atmosphere. We introduced a density deficit at two places along the length of the subphotospheric flux tube to emerge as two Ω-shaped loops, forming a quadrupolar region.
Results: At the photosphere, the emerging flux forms two initially separated bipoles, which later come in contact, forming a δ-spot central region. Above the two bipoles, two magnetic lobes expand and interact through a series of current sheets at the interface between them. Two recurrent confined eruptions are produced. In both cases, the reconnection between sheared low-lying field lines forms a flux rope. The reconnection between the two lobes higher in the atmosphere forms field lines that retract down and push against the flux rope, creating a current sheet between them. It also forms field lines that create a third magnetic lobe between the two emerged lobes, that later acts as a strapping field. The flux rope eruptions are triggered when the reconnection between the flux ropes and the field above the ropes becomes efficient enough to remove the tension of the overlying field. These reconnection events occur internally in the quadrupolar system, as the atmosphere is non-magnetised. The flux rope of the first, weaker, eruption almost fully reconnects with the overlying field. The flux rope of the second, more energetic, eruption is confined by the overlying strapping field. During the second eruption, the flux rope is enhanced in size, flux, and twist, similar to confined-flare-to-flux-rope observations. Proxies of the emission reveal the two erupting filaments channels. A flare arcade is only formed in the second eruption owing to the longer lasting and more efficient reconnection at the current sheet below the flux rope. Title: Particle Acceleration and Heating in Regions of Magnetic Flux Emergence Authors: Isliker, H.; Archontis, V.; Vlahos, L. Bibcode: 2019ApJ...882...57I Altcode: 2019arXiv190704296I The interaction between emerging and pre-existing magnetic fields in the solar atmosphere can trigger several dynamic phenomena, such as eruptions and jets. A key element during this interaction is the formation of large-scale current sheets, and eventually their fragmentation that leads to the creation of a strongly turbulent environment. In this paper, we study the kinetic aspects of the interaction (reconnection) between emerging and ambient magnetic fields. We show that the statistical properties of the spontaneously fragmented and fractal electric fields are responsible for the efficient heating and acceleration of charged particles, which form a power-law tail at high energies on sub-second timescales. A fraction of the energized particles escapes from the acceleration volume, with a super-hot component with a temperature close to 150 MK, and with a power-law high-energy tail with an index between -2 and -3. We estimate the transport coefficients in energy space from the dynamics of the charged particles inside the fragmented and fractal electric fields, and the solution of a fractional transport equation, as appropriate for a strongly turbulent plasma, agrees with the test-particle simulations. We also show that the acceleration mechanism is not related to Fermi acceleration, and the Fokker-Planck equation is inconsistent and not adequate as a transport model. Finally, we address the problem of correlations between spatial transport and transport in energy space. Our results confirm the observations reported for high-energy particles (hard X-rays, type III bursts, and solar energetic particles) during the emission of solar jets. Title: Introduction to the physics of solar eruptions and their space weather impact Authors: Archontis, Vasilis; Vlahos, Loukas Bibcode: 2019RSPTA.37790152A Altcode: 2019arXiv190508361A The physical processes, which drive powerful solar eruptions, play an important role in our understanding of the Sun-Earth connection. In this Special Issue, we firstly discuss how magnetic fields emerge from the solar interior to the solar surface, to build up active regions, which commonly host large-scale coronal disturbances, such as coronal mass ejections (CMEs). Then, we discuss the physical processes associated with the driving and triggering of these eruptions, the propagation of the large-scale magnetic disturbances through interplanetary space and the interaction of CMEs with Earth's magnetic field. The acceleration mechanisms for the solar energetic particles related to explosive phenomena (e.g. flares and/or CMEs) in the solar corona are also discussed. The main aim of this Issue, therefore, is to encapsulate the present state-of-the-art in research related to the genesis of solar eruptions and their space-weather implications.

This article is part of the theme issue `Solar eruptions and their space weather impact'. Title: The emergence of magnetic flux and its role on the onset of solar dynamic events Authors: Archontis, V.; Syntelis, P. Bibcode: 2019RSPTA.37780387A Altcode: 2019arXiv190406274A A plethora of solar dynamic events, such as the formation of active regions, the emission of jets and the occurrence of eruptions is often associated with the emergence of magnetic flux from the interior of the Sun to the surface and above. Here, we present a short review on the onset, driving and/or triggering of such events by magnetic flux emergence. We briefly describe some key observational examples, theoretical aspects and numerical simulations, towards revealing the mechanisms that govern solar dynamics and activity related to flux emergence. We show that the combination of important physical processes like shearing and reconnection of magnetic fieldlines in emerging flux regions or at their vicinity can power some of the most dynamic phenomena in the Sun on various temporal and spatial scales. Based on previous and recent observational and numerical studies, we highlight that, in most cases, none of these processes alone can drive and also trigger explosive phenomena releasing considerable amount of energy towards the outer solar atmosphere and space, such as flares, jets and large-scale eruptions (e.g. coronal mass ejections). In addition, one has to take into account the physical properties of the emerging field (e.g. strength, amount of flux, relative orientation to neighbouring and pre-existing magnetic fields, etc.) in order to better understand the exact role of magnetic flux emergence on the onset of solar dynamic events.

This article is part of the theme issue `Solar eruptions and their space weather impact'. Title: Ellerman bombs and UV bursts: transient events in chromospheric current sheets Authors: Hansteen, V.; Ortiz, A.; Archontis, V.; Carlsson, M.; Pereira, T. M. D.; Bjørgen, J. P. Bibcode: 2019A&A...626A..33H Altcode: 2019arXiv190411524H Context. Ellerman bombs (EBs), observed in the photospheric wings of the Hα line, and UV bursts, observed in the transition region Si IV line, are both brightenings related to flux emergence regions and specifically to magnetic flux of opposite polarity that meet in the photosphere. These two reconnection-related phenomena, nominally formed far apart, occasionally occur in the same location and at the same time, thus challenging our understanding of reconnection and heating of the lower solar atmosphere.
Aims: We consider the formation of an active region, including long fibrils and hot and dense coronal plasma. The emergence of a untwisted magnetic flux sheet, injected 2.5 Mm below the photosphere, is studied as it pierces the photosphere and interacts with the preexisting ambient field. Specifically, we aim to study whether EBs and UV bursts are generated as a result of such flux emergence and examine their physical relationship.
Methods: The Bifrost radiative magnetohydrodynamics code was used to model flux emerging into a model atmosphere that contained a fairly strong ambient field, constraining the emerging field to a limited volume wherein multiple reconnection events occur as the field breaks through the photosphere and expands into the outer atmosphere. Synthetic spectra of the different reconnection events were computed using the 1.5D RH code and the fully 3D MULTI3D code.
Results: The formation of UV bursts and EBs at intensities and with line profiles that are highly reminiscent of observed spectra are understood to be a result of the reconnection of emerging flux with itself in a long-lasting current sheet that extends over several scale heights through the chromosphere. Synthetic spectra in the Hα and Si IV 139.376 nm lines both show characteristics that are typical of the observations. These synthetic diagnostics suggest that there are no compelling reasons to assume that UV bursts occur in the photosphere. Instead, EBs and UV bursts are occasionally formed at opposite ends of a long current sheet that resides in an extended bubble of cool gas.

The movie associated to Fig. 3 is available at https://www.aanda.org Title: Sheared Magnetic Arcades and the Pre-eruptive Magnetic Configuration of Coronal Mass Ejections: Diagnostics, Challenges and Future Observables Authors: Patsourakos, Spiros; Vourlidas, A.; Anthiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou, G.; Georgoulis, M. K.; Green, L. M.; Kliem, B.; Leake, J.; Moore, R. L.; Nindos, A.; Syntelis, P.; Torok, T.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J. Bibcode: 2019shin.confE.194P Altcode: Our thinking about the pre-eruptive magnetic configuration of Coronal Mass Ejections has been effectively dichotomized into two opposing and often fiercely contested views: namely, sheared magnetic arcades and magnetic flux ropes. Finding a solution to this issue will have important implications for our understanding of CME initiation. We first discuss the very value of embarking into the arcade vs. flux rope dilemma and illustrate the corresponding challenges and difficulties to address it. Next, we are compiling several observational diagnostics of pre-eruptive sheared magnetic arcades stemming from theory/modeling, discuss their merits, and highlight potential ambiguities that could arise in their interpretation. We finally conclude with a discussion of possible new observables, in the frame of upcoming or proposed instrumentation, that could help to circumvent the issues we are currently facing. Title: Recurrent CME-like Eruptions in Emerging Flux Regions. II. Scaling of Energy and Collision of Successive Eruptions Authors: Syntelis, P.; Archontis, V.; Tsinganos, K. Bibcode: 2019ApJ...876...61S Altcode: 2019arXiv190403923S We present results of three-dimensional MHD simulations of recurrent eruptions in emerging flux regions. The initial numerical setup is the same as that in the work by Syntelis et al. Here, we perform a parametric study on the magnetic field strength (B 0) of the emerging field. The kinetic energy of the produced ejective eruptions in the emerging flux region ranges from 1026 to 1028 erg, reaching up to the energies of small coronal mass ejections. The kinetic and magnetic energies of the eruptions scale linearly in a logarithmic plot. We find that the eruptions are triggered earlier for higher B 0 and that B 0 is not directly correlated to the frequency of occurrence of the eruptions. Using large numerical domains, we show the initial stage of the partial merging of two colliding erupting fields. The partial merging occurs partly by the reconnection between the field lines of the following and the leading eruption at the interface between them. We also find that tether-cutting reconnection of the field lines of the leading eruption underneath the following eruption magnetically links the two eruptions. Shocks develop inside the leading eruption during the collision. Title: Successful and Failed Flux Tube Emergence in the Solar Interior Authors: Syntelis, P.; Archontis, V.; Hood, A. Bibcode: 2019ApJ...874...15S Altcode: 2019arXiv190207969S We report on our 3D magnetohydrodynamic simulations of cylindrical weakly twisted flux tubes emerging from 18 Mm below the photosphere. We perform a parametric study by varying the initial magnetic field strength (B 0), radius (R), twist (α), and length of the emerging part of the flux tube (λ) to investigate how these parameters affect the transfer of the magnetic field from the convection zone to the photosphere. We show that the efficiency of emergence at the photosphere (i.e., how strong the photospheric field will be in comparison to B 0) depends not only on B 0, but also on the morphology of the emerging field and on the twist. We show that parameters such as B 0 and magnetic flux alone cannot determine whether a flux tube will emerge to the solar surface. For instance, high-B 0 (weak-B 0) fields may fail (succeed) to emerge at the photosphere, depending on their geometrical properties. We also show that the photospheric magnetic field strength can vary greatly for flux tubes with the same B 0 but different geometric properties. Moreover, in some cases we have found scaling laws, whereby the magnetic field strength scales with the local density as B ∝ ρ κ , where κ ≈ 1 deeper in the convection zone and κ < 1 close to the photosphere. The transition between the two values occurs approximately when the local pressure scale (H p ) becomes comparable to the diameter of the flux tube (H p ≈ 2R). We derive forms to explain how and when these scaling laws appear and compare them with the numerical simulations. Title: Recurrent CME-like Eruptions in Emerging Flux Regions. I. On the Mechanism of Eruptions Authors: Syntelis, P.; Archontis, V.; Tsinganos, K. Bibcode: 2017ApJ...850...95S Altcode: 2017arXiv171110249S We report on three-dimensional (3D) magnetohydrodynamic (MHD) simulations of recurrent eruptions in emerging flux regions. We find that reconnection of sheared field lines, along the polarity inversion line of an emerging bipolar region, leads to the formation of a new magnetic structure, which adopts the shape of a magnetic flux rope (FR) during its rising motion. Initially, the FR undergoes a slow-rise phase and, eventually, it experiences a fast-rise phase and ejective eruption toward the outer solar atmosphere. In total, four eruptions occur during the evolution of the system. For the first eruption, our analysis indicates that the torus instability initiates the eruption and that tether-cutting reconnection of the field lines, which envelop the FR, triggers the rapid acceleration of the eruptive field. For the following eruptions, we conjecture that it is the interplay between tether-cutting reconnection and torus instability that causes the onset of the various phases. We show the 3D shape of the erupting fields, focusing more on how magnetic field lines reconnect during the eruptions. We find that when the envelope field lines reconnect mainly with themselves, hot and dense plasma is transferred closer to the core of the erupting FR. The same area appears to be cooler and less dense when the envelope field lines reconnect with neighboring sheared field lines. The plasma density and temperature distribution, together with the rising speeds, energies, and size of the erupting fields, indicate that they may account for small-scale (mini) coronal mass ejections. Title: Bombs and Flares at the Surface and Lower Atmosphere of the Sun Authors: Hansteen, V. H.; Archontis, V.; Pereira, T. M. D.; Carlsson, M.; Rouppe van der Voort, L.; Leenaarts, J. Bibcode: 2017ApJ...839...22H Altcode: 2017arXiv170402872H A spectacular manifestation of solar activity is the appearance of transient brightenings in the far wings of the Hα line, known as Ellerman bombs (EBs). Recent observations obtained by the Interface Region Imaging Spectrograph have revealed another type of plasma “bombs” (UV bursts) with high temperatures of perhaps up to 8 × 104 K within the cooler lower solar atmosphere. Realistic numerical modeling showing such events is needed to explain their nature. Here, we report on 3D radiative magnetohydrodynamic simulations of magnetic flux emergence in the solar atmosphere. We find that ubiquitous reconnection between emerging bipolar magnetic fields can trigger EBs in the photosphere, UV bursts in the mid/low chromosphere and small (nano-/micro-) flares (106 K) in the upper chromosphere. These results provide new insights into the emergence and build up of the coronal magnetic field and the dynamics and heating of the solar surface and lower atmosphere. 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: Ellerman Bombs and IRIS Bombs; In the photosphere and above Authors: Hansteen, Viggo; Archontis, V. Bibcode: 2016usc..confE.118H Altcode: The lower Solar atmosphere, consisting of the photosphere and chromosphere, can occasionally show violent activity more often associated with the magnetically dominated outer layers of the Sun; the upper chromosphere, transition region and corona. However, in regions of strong flux emergence, where Solar active regions are being formed, one can see evidence of photospheric reconnection as the field struggles to emerge through the non-buoyant photosphere and expand into the atmosphere above. Ellerman bombs, short lived, brightness enhancements in the outer wings of strong optical lines are thought to be a result of such reconnection. Observations made with the NASA's Interface Region Imaging Spectrograph, showed similar 'UV bursts' in lines usually associated with the outer Solar atmosphere, while at the same time clearly being situated below much cooler gas. We here present a numerical model of flux emergence in which both Ellerman bombs and perhaps IRIS bombs (UV bursts) are naturally and copiously produced. Title: Emergence of non-twisted magnetic fields in the Sun: Jets and atmospheric response Authors: Syntelis, P.; Archontis, V.; Gontikakis, C.; Tsinganos, K. Bibcode: 2015A&A...584A..10S Altcode: 2015arXiv150902850S
Aims: We study the emergence of a non-twisted flux tube from the solar interior into the solar atmosphere. We investigate whether the length of the buoyant part of the flux tube (i.e. λ) affects the emergence of the field and the dynamics of the evolving magnetic flux system.
Methods: We perform three-dimensional (3D), time-dependent, resistive, compressible magnetohydrodynamic (MHD) simulations using the Lare3D code.
Results: We find that there are considerable differences in the dynamics of the emergence of a magnetic flux tube when λ is varied. In the solar interior, for larger values of λ, the rising magnetic field emerges faster and expands more due to its lower magnetic tension. As a result, its field strength decreases and its emergence above the photosphere occurs later than in the smaller λ case. However, in both cases, the emerging field at the photosphere becomes unstable in two places, forming two magnetic bipoles that interact dynamically during the evolution of the system. Most of the dynamic phenomena occur at the current layer, which is formed at the interface between the interacting bipoles. We find the formation and ejection of plasmoids, the onset of successive jets from the interface, and the impulsive heating of the plasma in the solar atmosphere. We discuss the triggering mechanism of the jets and the atmospheric response to the emergence of magnetic flux in the two cases. Title: Sunspot rotation. I. A consequence of flux emergence Authors: Sturrock, Z.; Hood, A. W.; Archontis, V.; McNeill, C. M. Bibcode: 2015A&A...582A..76S Altcode: 2015arXiv150802437S Context. Solar eruptions and high flare activity often accompany the rapid rotation of sunspots. The study of sunspot rotation and the mechanisms driving this motion are therefore key to our understanding of how the solar atmosphere attains the conditions necessary for large energy release.
Aims: We aim to demonstrate and investigate the rotation of sunspots in a 3D numerical experiment of the emergence of a magnetic flux tube as it rises through the solar interior and emerges into the atmosphere. Furthermore, we seek to show that the sub-photospheric twist stored in the interior is injected into the solar atmosphere by means of a definitive rotation of the sunspots.
Methods: A numerical experiment is performed to solve the 3D resistive magnetohydrodynamic equations using a Lagrangian-Remap code. We track the emergence of a toroidal flux tube as it rises through the solar interior and emerges into the atmosphere investigating various quantities related to both the magnetic field and plasma.
Results: Through detailed analysis of the numerical experiment, we find clear evidence that the photospheric footprints or sunspots of the flux tube undergo a rotation. Significant vertical vortical motions are found to develop within the two polarity sources after the field emerges. These rotational motions are found to leave the interior portion of the field untwisted and twist up the atmospheric portion of the field. This is shown by our analysis of the relative magnetic helicity as a significant portion of the interior helicity is transported to the atmosphere. In addition, there is a substantial transport of magnetic energy to the atmosphere. Rotation angles are also calculated by tracing selected fieldlines; the fieldlines threading through the sunspot are found to rotate through angles of up to 353° over the course of the experiment. We explain the rotation by an unbalanced torque produced by the magnetic tension force, rather than an apparent effect.

The movies associated to Figs. 3, 5, and 11 are available in electronic form at http://www.aanda.org Title: Magnetic Flux Emergence Along the Solar Cycle Authors: Schmieder, B.; Archontis, V.; Pariat, E. Bibcode: 2015sac..book..227S Altcode: No abstract at ADS Title: Helical Blowout Jets in the Sun: Untwisting and Propagation of Waves Authors: Lee, E. J.; Archontis, V.; Hood, A. W. Bibcode: 2015ApJ...798L..10L Altcode: 2014arXiv1412.4853L We report on a numerical experiment of the recurrent onset of helical "blowout" jets in an emerging flux region. We find that these jets are running with velocities of ~100-250 km s-1 and they transfer a vast amount of heavy plasma into the outer solar atmosphere. During their emission, they undergo an untwisting motion as a result of reconnection between the twisted emerging and the non-twisted pre-existing magnetic field in the solar atmosphere. For the first time in the context of blowout jets, we provide direct evidence that their untwisting motion is associated with the propagation of torsional Alfvén waves in the corona. Title: Validation and Benchmarking of a Practical Free Magnetic Energy and Relative Magnetic Helicity Budget Calculation in Solar Magnetic Structures Authors: Moraitis, K.; Tziotziou, K.; Georgoulis, M. K.; Archontis, V. Bibcode: 2014SoPh..289.4453M Altcode: 2014arXiv1406.5381M; 2014SoPh..tmp..122M In earlier works we introduced and tested a nonlinear force-free (NLFF) method designed to self-consistently calculate the coronal free magnetic energy and the relative magnetic helicity budgets of observed solar magnetic structures. In principle, the method requires only a single, photospheric or low-chromospheric, vector magnetogram of a quiet-Sun patch or an active region and performs calculations without three-dimensional magnetic and velocity-field information. In this work we strictly validate this method using three-dimensional coronal magnetic fields. Benchmarking employs both synthetic, three-dimensional magnetohydrodynamic simulations and nonlinear force-free field extrapolations of the active-region solar corona. Our time-efficient NLFF method provides budgets that differ from those of more demanding semi-analytical methods by a factor of approximately three, at most. This difference is expected to come from the physical concept and the construction of the method. Temporal correlations show more discrepancies that are, however, soundly improved for more complex, massive active regions, reaching correlation coefficients on the order of, or exceeding, 0.9. In conclusion, we argue that our NLFF method can be reliably used for a routine and fast calculation of the free magnetic energy and relative magnetic helicity budgets in targeted parts of the solar magnetized corona. As explained in this article and in previous works, this is an asset that can lead to valuable insight into the physics and triggering of solar eruptions. Title: Solar Flaring Activity and Coronal Heating. Authors: Archontis, V.; Hansteen, V. H. Bibcode: 2014AGUFMSH53D..02A Altcode: We report on the formation of small solar flares produced by patchy magnetic reconnection between interacting magnetic loops. Three-dimensional (3D) magnetohydrodynamic (MHD) numerical experiments were performed, where a uniform magnetic flux sheet was injected into a fully developed convective layer. The gradual emergence of the field into the solar atmosphere results in a network of magnetic loops, which interact dynamically forming current layers at their interfaces. The formation and ejection of plasmoids out of the current layers leads to patchy reconnection and the spontaneous formation of several small (size ≈1-2 Mm) flares. We find that these flares are short-lived (30 s-3 minutes) bursts of energy in the range O(1025-1027) erg, which is basically the nanoflare-microflare range. Their persistent formation and co-operative action and evolution leads to recurrent emission of fast EUV/X-ray jets and considerable plasma heating in the active corona. Title: Magnetic Flux Emergence Along the Solar Cycle Authors: Schmieder, B.; Archontis, V.; Pariat, E. Bibcode: 2014SSRv..186..227S Altcode: 2014SSRv..tmp...47S Flux emergence plays an important role along the solar cycle. Magnetic flux emergence builds sunspot groups and solar activity. The sunspot groups contribute to the large scale behaviour of the magnetic field over the 11 year cycle and the reversal of the North and South magnetic polarity every 22 years. The leading polarity of sunspot groups is opposite in the North and South hemispheres and reverses for each new solar cycle. However the hemispheric rule shows the conservation of sign of the magnetic helicity with positive and negative magnetic helicity in the South and North hemispheres, respectively. MHD models of emerging flux have been developed over the past twenty years but have not yet succeeded to reproduce solar observations. The emergence of flux occurs through plasma layers of very high gradients of pressure and changing of modes from a large β to a low β plasma (<1). With the new armada of high spatial and temporal resolution instruments on the ground and in space, emergence of magnetic flux is observed in tremendous detail and followed during their transit through the upper atmosphere. Signatures of flux emergence in the corona depend on the pre-existing magnetic configuration and on the strength of the emerging flux. We review in this paper new and established models as well as the recent observations. Title: Validation of the magnetic energy vs. helicity scaling in solar magnetic structures Authors: Tziotziou, K.; Moraitis, K.; Georgoulis, M. K.; Archontis, V. Bibcode: 2014A&A...570L...1T Altcode: 2014arXiv1409.8117T
Aims: We assess the validity of the free magnetic energy - relative magnetic helicity diagram for solar magnetic structures.
Methods: We used two different methods of calculating the free magnetic energy and the relative magnetic helicity budgets: a classical, volume-calculation nonlinear force-free (NLFF) method applied to finite coronal magnetic structures and a surface-calculation NLFF derivation that relies on a single photospheric or chromospheric vector magnetogram. Both methods were applied to two different data sets, namely synthetic active-region cases obtained by three-dimensional magneto-hydrodynamic (MHD) simulations and observed active-region cases, which include both eruptive and noneruptive magnetic structures.
Results: The derived energy-helicity diagram shows a consistent monotonic scaling between relative helicity and free energy with a scaling index 0.84 ± 0.05 for both data sets and calculation methods. It also confirms the segregation between noneruptive and eruptive active regions and the existence of thresholds in both free energy and relative helicity for active regions to enter eruptive territory.
Conclusions: We consider the previously reported energy-helicity diagram of solar magnetic structures as adequately validated and envision a significant role of the uncovered scaling in future studies of solar magnetism. Title: Clusters of Small Eruptive Flares Produced by Magnetic Reconnection in the Sun Authors: Archontis, V.; Hansteen, V. Bibcode: 2014ApJ...788L...2A Altcode: 2014arXiv1405.6420A We report on the formation of small solar flares produced by patchy magnetic reconnection between interacting magnetic loops. A three-dimensional (3D) magnetohydrodynamic (MHD) numerical experiment was performed, where a uniform magnetic flux sheet was injected into a fully developed convective layer. The gradual emergence of the field into the solar atmosphere results in a network of magnetic loops, which interact dynamically forming current layers at their interfaces. The formation and ejection of plasmoids out of the current layers leads to patchy reconnection and the spontaneous formation of several small (size ≈1-2 Mm) flares. We find that these flares are short-lived (30 s-3 minutes) bursts of energy in the range O(1025-1027) erg, which is basically the nanoflare-microflare range. Their persistent formation and co-operative action and evolution leads to recurrent emission of fast EUV/X-ray jets and considerable plasma heating in the active corona. Title: Distribution of electric currents in source regions of solar eruptions Authors: Torok, Tibor; Leake, James E.; Titov, Viacheslav; Archontis, Vasilis; Mikic, Zoran; Linton, Mark; Dalmasse, Kevin; Aulanier, Guillaume; Kliem, Bernhard Bibcode: 2014AAS...22431202T Altcode: There has been a long-lasting debate on the question of whether or not electric currents in the source regions of solar eruptions are neutralized. That is, whether or not the direct coronal currents connecting the photospheric polarities in such regions are surrounded by return currents of equal amount and opposite direction. In order to address this question, we consider several mechanisms of source region formation (flux emergence, photospheric shearing/twisting flows, and flux cancellation) and quantify the evolution of the electric currents, using 3D MHD simulations. For the experiments conducted so far, we find a clear dominance of the direct currents over the return currents in all cases in which the models produce significant magnetic shear along the source region's polarity inversion line. This suggests that pre-eruptive magnetic configurations in strongly sheared active regions and filament channels carry substantial net currents. We discuss the implications of this result for the modeling of solar eruptions. Title: Recurrent Explosive Eruptions and the "Sigmoid-to-arcade" Transformation in the Sun Driven by Dynamical Magnetic Flux Emergence Authors: Archontis, V.; Hood, A. W.; Tsinganos, K. Bibcode: 2014ApJ...786L..21A Altcode: 2014arXiv1405.6955A We report on three-dimensional MHD simulations of recurrent mini coronal mass ejection (CME)-like eruptions in a small active region (AR), which is formed by the dynamical emergence of a twisted (not kink unstable) flux tube from the solar interior. The eruptions develop as a result of the repeated formation and expulsion of new flux ropes due to continuous emergence and reconnection of sheared field lines along the polarity inversion line of the AR. The acceleration of the eruptions is triggered by tether-cutting reconnection at the current sheet underneath the erupting field. We find that each explosive eruption is followed by reformation of a sigmoidal structure and a subsequent "sigmoid-to-flare arcade" transformation in the AR. These results might have implications for recurrent CMEs and eruptive sigmoids/flares observations and theoretical studies. Title: Distribution of Electric Currents in Solar Active Regions Authors: Török, T.; Leake, J. E.; Titov, V. S.; Archontis, V.; Mikić, Z.; Linton, M. G.; Dalmasse, K.; Aulanier, G.; Kliem, B. Bibcode: 2014ApJ...782L..10T Altcode: 2014arXiv1401.2931T There has been a long-standing debate on the question of whether or not electric currents in solar active regions are neutralized. That is, whether or not the main (or direct) coronal currents connecting the active region polarities are surrounded by shielding (or return) currents of equal total value and opposite direction. Both theory and observations are not yet fully conclusive regarding this question, and numerical simulations have, surprisingly, barely been used to address it. Here we quantify the evolution of electric currents during the formation of a bipolar active region by considering a three-dimensional magnetohydrodynamic simulation of the emergence of a sub-photospheric, current-neutralized magnetic flux rope into the solar atmosphere. We find that a strong deviation from current neutralization develops simultaneously with the onset of significant flux emergence into the corona, accompanied by the development of substantial magnetic shear along the active region's polarity inversion line. After the region has formed and flux emergence has ceased, the strong magnetic fields in the region's center are connected solely by direct currents, and the total direct current is several times larger than the total return current. These results suggest that active regions, the main sources of coronal mass ejections and flares, are born with substantial net currents, in agreement with recent observations. Furthermore, they support eruption models that employ pre-eruption magnetic fields containing such currents. Title: Realistic 3D simulations of a small flare resulting from flux emergence Authors: Hansteen, Viggo; Archontis, Vasilis Bibcode: 2014cosp...40E1151H Altcode: We have performed three-dimensional (3d) magnetohydrodynamic simulations of magnetic flux emergence in a model that spans the convection zone and into the outer solar atmosphere with the Bifrost code. This is a ``realistic'' model, in the sense that the parameters and physical effects that control the atmosphere can be used to produce diagnostics that can be directly compared with observations. The emerging flux leads to the formation of several current sheets as it rises into the modeled corona. Multiple plasmoids are ejected from the current sheets. Reconnection occurs impulsively, producing heating and fast outflows near or in the current sheet, arranged in a manner reminiscent of the CSHKP flare model. This includes a cusp like arcade and a flux rope in the lower atmospere underneath the current sheet. We discuss the evolution of the model and several synthetic observables. Title: Particle acceleration in regions of magnetic flux emergence: a statistical approach using test-particle- and MHD-simulations Authors: Vlahos, Loukas; Archontis, Vasilis; Isliker, Heinz Bibcode: 2014cosp...40E3539V Altcode: We consider 3D nonlinear MHD simulations of an emerging flux tube, from the convection zone into the corona, focusing on the coronal part of the simulations. We first analyze the statistical nature and spatial structure of the electric field, calculating histograms and making use of iso-contour visualizations. Then test-particle simulations are performed for electrons, in order to study heating and acceleration phenomena, as well as to determine HXR emission. This study is done by comparatively exploring quiet, turbulent explosive, and mildly explosive phases of the MHD simulations. Also, the importance of collisional and relativistic effects is assessed, and the role of the integration time is investigated. Particular aim of this project is to verify the quasi- linear assumptions made in standard transport models, and to identify possible transport effects that cannot be captured with the latter. In order to determine the relation of our results to Fermi acceleration and Fokker-Planck modeling, we determine the standard transport coefficients. After all, we find that the electric field of the MHD simulations must be downscaled in order to prevent an un-physically high degree of acceleration, and the value chosen for the scale factor strongly affects the results. In different MHD time-instances we find heating to take place, and acceleration that depends on the level of MHD turbulence. Also, acceleration appears to be a transient phenomenon, there is a kind of saturation effect, and the parallel dynamics clearly dominate the energetics. The HXR spectra are not yet really compatible with observations, we have though to further explore the scaling of the electric field and the integration times used. Title: Free magnetic energy and relative magnetic helicity diagnostics for the quality of NLFF field extrapolations Authors: Moraitis, Kostas; Archontis, Vasilis; Tziotziou, Konstantinos; Georgoulis, Manolis K. Bibcode: 2014cosp...40E2169M Altcode: We calculate the instantaneous free magnetic energy and relative magnetic helicity of solar active regions using two independent approaches: a) a non-linear force-free (NLFF) method that requires only a single photospheric vector magnetogram, and b) well known semi-analytical formulas that require the full three-dimensional (3D) magnetic field structure. The 3D field is obtained either from MHD simulations, or from observed magnetograms via respective NLFF field extrapolations. We find qualitative agreement between the two methods and, quantitatively, a discrepancy not exceeding a factor of 4. The comparison of the two methods reveals, as a byproduct, two independent tests for the quality of a given force-free field extrapolation. We find that not all extrapolations manage to achieve the force-free condition in a valid, divergence-free, magnetic configuration. This research has been co-financed by the European Union (European Social Fund - ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: Thales. Investing in knowledge society through the European Social Fund. Title: Free magnetic energy and relative magnetic helicity in active and quiet solar regions and their role in solar dynamics Authors: Tziotziou, Konstantinos; Archontis, Vasilis; Tsiropoula, Georgia; Georgoulis, Manolis K.; Moraitis, Kostas; Kontogiannis, Ioannis Bibcode: 2014cosp...40E3428T Altcode: We present a novel non-linear force-free method for the calculation of the instantaneous free magnetic energy and relative magnetic helicity budgets of a solar region from a single photospheric/chromospheric vector magnetogram. Our objective is to study the role of these quantities both in solar eruptions and in quiet-Sun dynamics. The validity of the method is tested using both observations and synthetic magnetohydrodynamical (MHD) models. The method is applied for the derivation of the energy-helicity (EH) diagram of solar active regions (ARs) from a sample of 162 vector magnetograms corresponding to 42 different ARs, suggesting the existence of 4×10(31) erg and 2×10(42) Mx(2) thresholds in free energy and relative helicity, respectively, for ARs to enter eruptive territory. Furthermore, the dynamical evolution of both quantities in eruptive NOAA AR 11158, using a high-cadence 5-day time series of vector magnetograms, suggests the formation of increasingly helical pre-eruption structures and a causal relation between flares and Coronal Mass Ejections (CMEs). The method is also used to derive helicity and energy budgets in quiet Sun regions and construct the respective EH diagram. Our results highlight the importance of both energy and helicity in AR evolution and quiet-Sun dynamics and instigate further research on the underlying physics with three-dimensional MHD models. This work is supported by EU's Seventh Framework Programme via a Marie Curie Fellowship. Title: The Emergence of Weakly Twisted Magnetic Fields in the Sun Authors: Archontis, V.; Hood, A. W.; Tsinganos, K. Bibcode: 2013ApJ...778...42A Altcode: We have studied the emergence of a weakly twisted magnetic flux tube from the upper convection zone into the solar atmosphere. It is found that the rising magnetized plasma does not undergo the classical, single Ω-shaped loop emergence, but it becomes unstable in two places, forming two magnetic lobes that are anchored in small-scale bipolar structures at the photosphere, between the two main flux concentrations. The two magnetic lobes rise and expand into the corona, forming an overall undulating magnetic flux system. The dynamical interaction of the lobes results in the triggering of high-speed and hot jets and the formation of successive cool and hot loops that coexist in the emerging flux region. Although the initial emerging field is weakly twisted, a highly twisted magnetic flux rope is formed at the low atmosphere, due to shearing and reconnection. The new flux rope (hereafter post-emergence flux rope) does not erupt. It remains confined by the overlying field. Although there is no ejective eruption of the post-emergence rope, it is found that a considerable amount of axial and azimuthal flux is transferred into the solar atmosphere during the emergence of the magnetic field. Title: Magnetic helicity and free energy in solar active regions Authors: Moraitis, K.; Georgoulis, M.; Tziotziou, K.; Archontis, V. Bibcode: 2013hell.confS..21M Altcode: We study the evolution of the non-potential free magnetic energy and relative magnetic helicity budgets in solar active regions (ARs). For this we use a time-series of a three-dimensional, synthetic AR produced by magnetohydrodynamical (MHD) simulations. As a first step, we calculate the potential magnetic field that has the same normal components with the MHD field along all boundaries of the AR, by solving Laplace's equation. The free magnetic energy of the AR is then easily derived. From the two fields, MHD and potential one, we calculate the corresponding vector potentials with a recently proposed integration method. The knowledge of both fields and their respective vector potentials throughout the AR, allows us to estimate the relative magnetic helicity budget of the AR. Following this procedure for each snapshot of the AR, we reconstruct the evolution of free energy and helicity in the AR. Our method reproduces, for a synthetic AR, the energy/helicity relations known to hold in real active regions. Title: SDO Observations of Solar Jets Authors: Moschou, S. P.; Tsinganos, K.; Vourlidas, A.; Archontis, V. Bibcode: 2013SoPh..284..427M Altcode: 2012SoPh..tmp..310M We present an analysis of high cadence observations of solar jets observed in the Extreme Ultraviolet (EUV), at 304 Å, with the Atmospheric Imaging Assembly instrument aboard the Solar Dynamics Observatory (SDO). The jets in our sample lie very close to the solar limb to minimize projection effects. Two of the events show clear helical patterns during ejection. We also find that some of the jets are recurrent and that most of them cannot overcome solar gravity. Title: Erratum: "A Numerical Model of Standard to Blowout Jets" (2013, ApJL, 769, L21) Authors: Archontis, V.; Hood, A. W. Bibcode: 2013ApJ...770L..41A Altcode: No abstract at ADS Title: A Numerical Model of Standard to Blowout Jets Authors: Archontis, V.; Hood, A. W. Bibcode: 2013ApJ...769L..21A Altcode: We report on three-dimensional (3D) MHD simulations of the formation of jets produced during the emergence and eruption of solar magnetic fields. The interaction between an emerging and an ambient magnetic field in the solar atmosphere leads to (external) reconnection and the formation of "standard" jets with an inverse Y-shaped configuration. Eventually, low-atmosphere (internal) reconnection of sheared fieldlines in the emerging flux region produces an erupting magnetic flux rope and a reconnection jet underneath it. The erupting plasma blows out the ambient field and, moreover, it unwinds as it is ejected into the outer solar atmosphere. The fast emission of the cool material that erupts together with the hot outflows due to external/internal reconnection form a wider "blowout" jet. We show the transition from "standard" to "blowout" jets and report on their 3D structure. The physical plasma properties of the jets are consistent with observational studies. Title: Modelling magnetic flux emergence in the solar convection zone Authors: Bushby, P. J.; Archontis, V. Bibcode: 2012A&A...545A.107B Altcode: 2012arXiv1208.1667B Context. Bipolar magnetic regions are formed when loops of magnetic flux emerge at the solar photosphere. Magnetic buoyancy plays a crucial role in this flux emergence process, particularly at larger scales. However it is not yet clear to what extent the local convective motions influence the evolution of rising loops of magnetic flux.
Aims: Our aim is to investigate the flux emergence process in a simulation of granular convection. In particular we aim to determine the circumstances under which magnetic buoyancy enhances the flux emergence rate (which is otherwise driven solely by the convective upflows).
Methods: We used three-dimensional numerical simulations, solving the equations of compressible magnetohydrodynamics in a horizontally-periodic Cartesian domain. A horizontal magnetic flux tube was inserted into fully developed hydrodynamic convection. We systematically varied the initial field strength, the tube thickness, the initial entropy distribution along the tube axis and the magnetic Reynolds number.
Results: Focusing upon the low magnetic Prandtl number regime (Pm < 1) at moderate magnetic Reynolds number, we find that the flux tube is always susceptible to convective disruption to some extent. However, stronger flux tubes tend to maintain their structure more effectively than weaker ones. Magnetic buoyancy does enhance the flux emergence rates in the strongest initial field cases, and this enhancement becomes more pronounced when we increase the width of the flux tube. This is also the case at higher magnetic Reynolds numbers, although the flux emergence rates are generally lower in these less dissipative simulations because the convective disruption of the flux tube is much more effective in these cases. These simulations seem to be relatively insensitive to the precise choice of initial conditions: for a given flow, the evolution of the flux tube is determined primarily by the initial magnetic field distribution and the magnetic Reynolds number. Title: Magnetic flux emergence and associated dynamic phenomena in the Sun Authors: Archontis, V. Bibcode: 2012RSPTA.370.3088A Altcode: No abstract at ADS Title: The Creation of Outflowing Plasma in the Corona at Emerging Flux Regions: Comparing Observations and Simulations Authors: Harra, L. K.; Archontis, V.; Pedram, E.; Hood, A. W.; Shelton, D. L.; van Driel-Gesztelyi, L. Bibcode: 2012SoPh..278...47H Altcode: In this paper we analyse the flux emergence that occurred in the following polarity area of an active region on 1 - 2 December 2006. Observations have revealed the existence of fast outflows at the edge of the emerging flux region. We have performed 3-D numerical simulations to study the mechanisms responsible for these flows. The results indicate that these outflows are reconnection jets or pressure-driven outflows, depending on the relative orientation of the magnetic fields in contact (i.e. the emerging flux and the active region's field which is favourable for reconnection on the west side and nearly parallel with the pre-existing field on the east side of the emerging flux). In the observations, the flows are larger on the west side until late in the flux emergence, when the reverse is true. The simulations show that the flows are faster on the west side, but do not show the east flows increasing with time. There is an asymmetry in the expansion of the emerging flux region, which is also seen in the observations. The west side of the emerging flux region expands faster into the corona than the other side. In the simulations, efficient magnetic reconnection occurs on the west side, with new loops being created containing strong downflows that are clearly seen in the observations. On the other side, the simulations show strong compression as the dominant mechanism for the generation of flows. There is evidence of these flows in the observations, but the flows are stronger than the simulations predict at the later stages. There could be additional small-angle reconnection that adds to the flows from the compression, as well as reconnection occurring in larger loops that lie across the whole active region. Title: 3D MHD Flux Emergence Experiments: Idealised Models and Coronal Interactions Authors: Hood, A. W.; Archontis, V.; MacTaggart, D. Bibcode: 2012SoPh..278....3H Altcode: 2011arXiv1103.3685H This paper reviews some of the many 3D numerical experiments of the emergence of magnetic fields from the solar interior and the subsequent interaction with the pre-existing coronal magnetic field. The models described here are idealised, in the sense that the internal energy equation only involves the adiabatic, Ohmic and viscous shock heating terms. However, provided the main aim is to investigate the dynamical evolution, this is adequate. Many interesting observational phenomena are explained by these models in a self-consistent manner. Title: Magnetic flux emergence: a precursor of solar plasma expulsion Authors: Archontis, V.; Hood, A. W. Bibcode: 2012A&A...537A..62A Altcode:
Aims: We model the emergence of magnetized plasma from the top of the convection zone to the lower corona. We investigate the eruption of coronal flux ropes above emerging flux regions.
Methods: We performed three-dimensional numerical experiments in which the time-dependent and resistive equations of MHD are solved self-consistently, using the Lare3D code.
Results: A subphotospheric magnetic flux tube rises from the convectively unstable layer into the solar surface, followed by the formation and eruption of a new flux rope into the corona. Firstly, we examined the case where the corona is field-free. The expansion of the emerging field forms an envelope sheath that surrounds the newly formed flux rope. The erupting ropes are confined by the envelope field. The eruptions are driven by the gradient of the gas pressure and the tension of fieldlines that reconnect within the emerging flux region. The amount of the initial twist of the emerging field and the dense plasma that is lifted up, determine the height-time profile of the erupting ropes. Secondly, we examined the case of emergence into a pre-existing magnetic field in the upper solar atmosphere. A variety of different ambient field configurations was used in the experiments. External reconnection between the emerging and the pre-existing field may result in the removal of sufficient flux from the interacting fields and the full ejection of the flux ropes.
Conclusions: The results indicate that the relative contact angle of the interacting flux systems and their field strengths are crucial parameters that ultimately affect the evolution of the eruption of the rope into the higher solar atmosphere. One important result is that for any contact angle that favors reconnection, ejective eruptions occur earlier when the ambient field is relatively strong. In many cases, the erupting plasma adopts an S-like configuration. The sigmoidal structure accelerates during the fast eruption of the rope. The acceleration is enhanced by the external and internal reconnection of fieldlines during the rising motion of the rope. A key result is that in the reconnection-favored cases, the flux ropes experience ejective eruptions when the envelope flux is reduced (owing to removal by external reconnection) below that of the erupting flux rope. If the envelope flux stays higher than the erupting flux, the magnetic flux rope remains confined by the envelope field. Title: Evolution of Electric Currents during Active Region Formation Authors: Torok, T.; Archontis, V.; Titov, V. S. Bibcode: 2011AGUFMSH33C..08T Altcode: In previous work it has been shown that the emergence of twisted magnetic flux tubes into the corona can lead to the formation of both stable and eruptive coronal flux ropes, either by the rigid emergence of the tube or by shear flows and reconnection occurring within its expanding upper part. Such an intrusion of new magnetic flux into the corona naturally produces return currents that flow in the opposite direction of the flux rope current. It has been argued that such return currents significantly change the local force balance -- thus could prevent the flux rope from eruption -- and that therefore coronal flux rope models that employ a non-neutralized flux rope current are not suitable to model filament eruptions or coronal mass ejections. Recently, however, Georgoulis et al. have shown from observations that strong non-neutralized currents can exist close to the polarity inversion lines of active regions, particularly in regions that produce eruptions. This raises the question on the physical origin of such non-neutralized currents. In this talk, we will present results from our investigation of the evolution of photospheric and coronal electric currents in the course of the formation of active regions and coronal flux ropes, using the flux emergence simulations by Archontis et al., and we will discuss the implications of our results for coronal eruptions. Title: Flux Emergence and Associated Dynamic Events in the Sun Authors: Archontis, V. Bibcode: 2010ASPC..424....3A Altcode: One of the most important processes, responsible for many dynamical phenomena observed in the Sun, is the emergence of magnetic flux from the solar interior in active regions and the modification of the coronal magnetic field in response to the emergence. In fact, magnetic flux emergence might be responsible for the appearance of small-scale events (e.g., compact flares, plasmoids, active-region-associated X-ray brightenings) and large-scale events (e.g., X-class flares and CMEs). However, it is clear that the question of how exactly the magnetic fields rise through the convection zone of the Sun and emerge through the photosphere and chromosphere into the corona has still not been solved. Studying the process of flux emergence is an important step towards the understanding of the dynamic coupling between the solar interior and the outer solar atmosphere. This paper provides a brief review of some numerical models, which have been used to study the process of magnetic flux emergence into the outer solar atmosphere and the dynamics of associated explosive events. Title: Study of a Solar Active Region Jet Authors: Gontikakis, C.; Archontis, V.; Tsinganos, K. Bibcode: 2010ASPC..424...19G Altcode: We present the observations of an active region jet originating from the east side of NOAA 8531 on May 15 1999. The observations include a series of TRACE 171 Å filtergrams, and simultaneous observations from SUMER in Ne VIII, 770 Å, C IV 1548 Å, as well as MDI magnetograms. The observations were compared with the results of a 3D MHD numerical simulation of magnetic flux emergence and its subsequent reconnection with preexisting magnetic flux. The numerical simulation reproduces the observed 100 km/s outflow at the right temperature range (0.6-1×106 Kelvin). Moreover, the observations seem to suggest that the jet plasma falls back on the solar surface along an active region loop, in agreement with our model. Title: Flux emergence and coronal eruption Authors: Archontis, V.; Hood, A. W. Bibcode: 2010A&A...514A..56A Altcode: 2010arXiv1003.2333A
Aims: Our aim is to study the photospheric flux distribution of a twisted flux tube that emerges from the solar interior. We also report on the eruption of a new flux rope when the emerging tube rises into a pre-existing magnetic field in the corona.
Methods: To study the evolution, we use 3D numerical simulations by solving the time-dependent and resistive MHD equations. We qualitatively compare our numerical results with MDI magnetograms of emerging flux at the solar surface.
Results: We find that the photospheric magnetic flux distribution consists of two regions of opposite polarities and elongated magnetic tails on the two sides of the polarity inversion line (PIL), depending on the azimuthal nature of the emerging field lines and the initial field strength of the rising tube. Their shape is progressively deformed due to plasma motions towards the PIL. Our results are in qualitative agreement with observational studies of magnetic flux emergence in active regions (ARs). Moreover, if the initial twist of the emerging tube is small, the photospheric magnetic field develops an undulating shape and does not possess tails. In all cases, we find that a new flux rope is formed above the original axis of the emerging tube that may erupt into the corona, depending on the strength of the ambient field. Title: Recurrent solar jets in active regions Authors: Archontis, V.; Tsinganos, K.; Gontikakis, C. Bibcode: 2010A&A...512L...2A Altcode: 2010arXiv1003.2349A
Aims: We study the emergence of a toroidal flux tube into the solar atmosphere and its interaction with a pre-existing field of an active region. We investigate the emission of jets as a result of repeated reconnection events between colliding magnetic fields.
Methods: We perform 3D simulations by solving the time-dependent, resistive MHD equations in a highly stratified atmosphere.
Results: A small active region field is constructed by the emergence of a toroidal magnetic flux tube. A current structure is build up and reconnection sets in when new emerging flux comes into contact with the ambient field of the active region. The topology of the magnetic field around the current structure is drastically modified during reconnection. The modification results in a formation of new magnetic systems that eventually collide and reconnect. We find that reconnection jets are taking place in successive recurrent phases in directions perpendicular to each other, while in each phase they release magnetic energy and hot plasma into the solar atmosphere. After a series of recurrent appearance of jets, the system approaches an equilibrium where the efficiency of the reconnection is substantially reduced. We deduce that the emergence of new magnetic flux introduces a perturbation to the active region field, which in turn causes reconnection between neighboring magnetic fields and the release of the trapped energy in the form of jet-like emissions. This is the first time that self-consistent recurrency of jets in active regions is shown in a three-dimensional experiment of magnetic flux emergence. Title: Formation of Ellerman bombs due to 3D flux emergence Authors: Archontis, V.; Hood, A. W. Bibcode: 2009A&A...508.1469A Altcode: Aims. We investigate the emergence of a “sea-serpent” magnetic field into the outer solar atmosphere and the connection between undulating fieldlines and formation of Ellerman bombs.
Methods: We perform 3D numerical experiments solving the time-dependent and resistive MHD equations.
Results: A sub-photospheric magnetic flux sheet develops undulations due to the Parker instability. It rises from the convectively unstable sub-photospheric layer and emerges into the highly stratified atmosphere through successive reconnection events along the undulating system. Brightenings with the characteristics of Ellerman bombs are produced due to reconnection, which occurs during the emergence of the field. At an advanced stage of the evolution of the system, the resistive emergence leads to the formation of long, arch-like magnetic fields that expand into the corona. The enhancement of the magnetic field at the low atmosphere and episodes of emergence of new magnetic flux are also discussed.
Title: Observations and 3D MHD simulations of a solar active region jet Authors: Gontikakis, C.; Archontis, V.; Tsinganos, K. Bibcode: 2009A&A...506L..45G Altcode: Aims: We study an active region jet originating from NOAA 8531 on May 15 1999. We perform 3D MHD numerical simulations of magnetic flux emergence and its subsequent reconnection with preexisting magnetic flux. Then, we compare the physical properties of the observed jet with the reconnecting outflow produced in the numerical model.
Methods: We report observations of this jet using a series of TRACE 171 Å filtergrams, simultaneous observations from SUMER in Ne viii 770Å and C iv 1548 Å as well as MDI magnetograms. In the numerical simulation, the full compressible and resistive MHD equations are solved, including viscous and Ohmic heating.
Results: A high-velocity upflow (≃100 km s-1) is observed after the emergence of new magnetic flux at the edge of the active region. The jet is recorded over a range of temperatures between 105 K and 1.5 × 106 K. In our numerical experiments, we find that the jet is the result of magnetic reconnection between newly emerging flux and the preexisting magnetic field of the active region.
Conclusions: The hot and high-velocity bidirectional flows occur as a result of the interaction between oppositely directed magnetic fields. Observations and numerical results are strongly suggestive of effective reconnection process being responsible for producing jets when emerging flux appears in solar active regions. 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: On the Structure and Evolution of Complexity in Sigmoids: A Flux Emergence Model Authors: Archontis, V.; Hood, A. W.; Savcheva, A.; Golub, L.; Deluca, E. Bibcode: 2009ApJ...691.1276A Altcode: Sigmoids are structures with a forward or inverse S-shape, generally observed in the solar corona in soft X-ray emission. It is believed that the appearance of a sigmoid in an active region is an important factor in eruptive activity. The association of sigmoids with dynamic phenomena such as flares and coronal mass ejections (CMEs) make the study of sigmoids important. Recent observations of a coronal sigmoid, obtained with the X-Ray Telescope (XRT) on board Hinode, showed the formation and eruption phase with high spatial resolution. These observations revealed that the topological structure of the sigmoid is complex: it consists of many differently oriented loops that all together form two opposite J-like bundles or an overall S-shaped structure. A series of theoretical and numerical models have been proposed, over the past years, to explain the nature of sigmoids but there is no explanation on how the aforementioned complexity in sigmoids is built up. In this paper, we present a flux emergence model that leads to the formation of a sigmoid, whose structure and evolution of complexity are in good qualitative agreement with the recent observations. For the initial state of the experiment a twisted flux tube is placed below the photosphere. A density deficit along the axis of the tube makes the system buoyant in the middle and it adopts an Ω-shape as it rises toward the outer atmosphere. During the evolution of the system, expanding field lines that touch the photosphere at bald-patches (BPs) form two seperatrix surfaces where dissipation is enhanced and current sheets are formed. Originally, each of the BP seperatrix surfaces has a J-like shape. Each one of the J's consist of reconnected field lines with different shapes and different relative orientation. The further dynamical evolution of the emerging flux tube results in the occurrence of many sites that resemble rotational discontinuities. Thus, additional current layers are formed inside the rising magnetized volume increasing the complexity of the system. The reconnected field lines along these layers form an overall S-shaped structure. The reconnection process continues to occur leading to the formation of another current concentration in the middle of the sigmoid where a flaring episode occurs. This central brightening is accompanied by the eruption of a flux rope from the central area of the sigmoid and the appearance of "post-flare" loops underneath the current structure. Title: Eruption of magnetic flux ropes during flux emergence Authors: Archontis, V.; Török, T. Bibcode: 2008A&A...492L..35A Altcode: 2008arXiv0811.1134A Aims: We investigate the formation of flux ropes in a flux emergence region and their rise into the outer atmosphere of the Sun.
Methods: We perform 3D numerical experiments by solving the time-dependent and resistive MHD equations.
Results: A sub-photospheric twisted flux tube rises from the solar interior and expands into the corona. A flux rope is formed within the expanding field, due to shearing and reconnection of field lines at low atmospheric heights. If the tube emerges into a non-magnetized atmosphere, the flux rope rises, but remains confined inside the expanding magnetized volume. In contrast, if the expanding tube is allowed to reconnect with a pre-existing coronal field, the flux rope experiences a full eruption with a rise profile that is in qualitative agreement with erupting filaments and Coronal Mass Ejections. Title: NLFF Model of a Coronal Sigmoid Authors: Savcheva, A. S.; Archontis, V.; van Ballegooijen, A. Bibcode: 2008AGUSMSP31A..05S Altcode: Between Feb 10 and 12, 2007, the X-ray telescope on Hinode produced some very high-cadence and high- resolution observations of a prominent coronal sigmoid. Here we show our results from computing a NLFF model of the sigmoid and compare it qualitatively to the XRT and TRACE observations. In addition we include some preliminary qualitative and quantitative results from 2.5D flux emergence simulation. We also discuss the future goals of this project. Title: Magnetic flux emergence in the Sun Authors: Archontis, V. Bibcode: 2008JGRA..113.3S04A Altcode: Space weather research is closely connected with the study of the solar magnetic activity. In past years, many solar missions (e.g., YOHKOH, SOHO, TRACE, and RHESSI) have provided outstanding observations, which have been used to improve our understanding of the structure and the dynamical evolution of solar magnetic fields. In addition, the newly launched solar missions (e.g., Hinode and STEREO) will study the interaction between the emerging magnetic field and the preexisting field in the corona (increasing our understanding of the causes of solar variability), and they will also observe the three-dimensional evolution of solar eruptions as they leave the Sun and move into the interplanetary space. One of the most important processes, responsible for many dynamical phenomena observed in the Sun, is the emergence of magnetic flux from the solar interior in active regions and the modification of the coronal magnetic field in response to the emergence. In fact, magnetic flux emergence might be responsible for the appearance of small-scale events (e.g., compact flares, plasmoids, and active-region-associated X-ray brightenings) and large-scale events (e.g., X-class flares and CMEs), which are major drivers of space weather. However, it is clear that the question of how exactly the magnetic fields rise through the convection zone of the Sun and emerge through the photosphere and chromosphere into the corona has still not been solved. It is believed that understanding the process of flux emergence is an important step toward the understanding of the initiation mechanism of eruptive events in the Sun, which is another topic of great debate. This paper provides a brief review of the theory and the numerical models, which have been used to study the process of magnetic flux emergence into the outer atmosphere of the Sun. We underline the similarities and differences between these models, and we compare the basic features of the numerical results with observations. Finally, we review the recent progress and discuss what further developments are required in the models to best describe the essential physics in the process of flux emergence. Title: A Flux Emergence Model for Solar Eruptions Authors: Archontis, V.; Hood, A. W. Bibcode: 2008ApJ...674L.113A Altcode: 2008arXiv0801.1649A We have simulated the three-dimensional (3D) emergence and interaction of two twisted flux tubes, which rise from the interior into the outer atmosphere of the Sun. We present evidence for the multiple formation and eruption of flux ropes inside the emerging flux systems and hot arcade-like structures in between them. Their formation is due to internal reconnection, occurring between oppositely directed, highly stretched, and sheared field lines at photospheric heights. Most of the eruptions escape into the corona, but some are confined and fade away without leaving the low atmosphere. As these flux ropes erupt, new reconnected field lines accumulate around the main axis of the initial magnetic flux systems. We also show the complex 3D field-line geometry and the structure of the multiple current sheets, which form as a result of the reconnection between the emerging flux systems. Title: Magnetic flux emergence in the Sun Authors: Archontis, Vasilis Bibcode: 2008cosp...37..117A Altcode: 2008cosp.meet..117A One of the most important processes, responsible for many dynamical phenomena observed in the Sun, is the emergence of magnetic flux from the solar interior in active regions and the modification of the coronal magnetic field in response to the emergence. In fact, magnetic flux emergence might be responsible for the appearance of small-scale events (e.g., compact flares, plasmoids, active region associated X-ray brightenings) and large-scale events (e.g., X-class flares and CMEs). However, it is clear that the question of how exactly the magnetic fields rise through the convection zone of the Sun and emerge through the photosphere and chromosphere into the corona has still not been solved. It is believed that understanding the process of flux emergence is an important step towards the understanding of the initiation mechanism of eruptive events in the Sun. In this talk, we review the recent progress and discuss what further developments are required in numerical experiments to best describe the essential physics in the process of flux emergence. 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: Nonlinear MHD dynamo operating at equipartition Authors: Archontis, V.; Dorch, S. B. F.; Nordlund, Å. Bibcode: 2007A&A...472..715A Altcode: Context: We present results from non linear MHD dynamo experiments with a three-dimensional steady and smooth flow that drives fast dynamo action in the kinematic regime. In the saturation regime, the system yields strong magnetic fields, which undergo transitions between an energy-equipartition and a turbulent state. The generation and evolution of such strong magnetic fields is relevant for the understanding of dynamo action that occurs in stars and other astrophysical objects.
Aims: We study the mode of operation of this dynamo, in the linear and non-linear saturation regimes. We also consider the effect of varying the magnetic and fluid Reymolds number on the non-linear behaviour of the system.
Methods: We perform three-dimensional non-linear MHD simulations and visualization using a high resolution numerical scheme.
Results: We find that this dynamo has a high growth rate in the linear regime, and that it can saturate at a level significantly higher than intermittent turbulent dynamos, namely at energy equipartition, for high values of the magnetic and fluid Reynolds numbers. The equipartition solution however does not remain time-independent during the simulation but exhibits a much more intricate behaviour than previously thought. There are periods in time where the solution is smooth and close to energy-equipartition and others where it becomes turbulent. Similarities and differences in the way the magnetic field is amplified and sustained for experiments with varying Reynolds numbers are discussed.
Conclusions: Strong magnetic fields, in near equipartition, can be generated also by a non-turbulent dynamo. A striking result is that the saturation state of this dynamo reveals interesting transitions between turbulent and laminar states. Title: Emergence and interaction of twisted flux tubes in the Sun Authors: Archontis, V.; Hood, A. W.; Brady, C. Bibcode: 2007A&A...466..367A Altcode: Aims: We present results from numerical simulations that study the interaction of a pair of twisted, buoyant magnetic flux tubes, which rise from the solar interior into the outer atmosphere of the Sun. The aim of our new model is to reproduce some of the dynamic solar phenomena in a self-consistent manner.
Methods: We perform non-linear simulations in 2.5D numerical experiments by solving the compressible and resistive MHD equations using a Lagrangian remap, shock capturing code (Lare2D). For some aspects of the problem, we consider the evolution of the system using both uniform and locally enhanced resistivity.
Results: The two flux tubes start to rise at the same time but from a different height below the photosphere. The leading (first) tube, which is originally located nearer to the surface, rises and eventually expands above the photosphere forming a magnetized atmosphere for the upcoming system (second tube). Current sheets, high-velocity reconnection jets, plasmoids, loop brightnenings and arcade flare-like structures are formed, for the first time in such numerical experiments, self-consistently by the emergence, expansion and the dynamical interaction between the two emerging flux systems. Title: Ellerman Bombs and Jets Associated with Resistive Flux Emergence Authors: Isobe, H.; Tripathi, D.; Archontis, V. Bibcode: 2007ApJ...657L..53I Altcode: Using two-dimensional (2D) magnetohydrodynamic simulations we study the effects of resistive processes in the dynamics of magnetic flux emergence and its relation to Ellerman bombs and other dynamic phenomena in the Sun. The widely accepted scenario of flux emergence is the formation and expansion of Ω-shaped loops due to the Parker instability. Since the Parker instability has the largest growth rate at finite wavelength λp~10H-20H, where H is the scale height (~200 km in the solar photosphere), a number of magnetic loops may rise from the initial flux sheet if it is sufficiently long. This process is shown in our numerical simulations. The multiple emerging loops expand in the atmosphere and interact with each other, leading to magnetic reconnection. At first reconnection occurs in the lower atmosphere, which allows the sinking part of the flux sheet to emerge above the photosphere. This reconnection also causes local heating that may account for Ellerman bombs. In the later stage, reconnection between the expanding loops occurs at higher levels of the atmosphere and creates high-temperature reconnection jets, and eventually a large (>>λp) coronal loop is formed. Cool and dense plasma structures, which are similar to Hα surges, are also formed. This is not because of magnetic reconnection but due to the compression of the plasma in between the expanding loops. 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: New Results on an Equipartition Dynamo Authors: Dorch, S. B. F.; Archontis, V. Bibcode: 2006IAUJD...8E...4D Altcode: This contribution presents results from numerical computer experiments with a 3-d steady sine flow (with zero mean helicity) that drives fast dynamo action. The mode of operation of this so-called ``no-cosines" dynamo (recently dubbed ``the Archontis dynamo"" by David Galloway) was studied during linear and non-linear saturation regimes. The means were 3-d non-linear MHD simulations and visualization using the high resolution numerical scheme by Nordlund, Galsgaard and others. We have found that the dynamo has a high growth rate in the linear regime, and that it can saturate at a level significantly higher that intermittent turbulent dynamos: Namely very close to energy equipartition for high Reynolds numbers. The equipartition solution however is not turbulent but a laminar solution that acts as an attractor to other modes. Similarities and differences, in the way the magnetic field is amplified and sustained, between experiments with varying Reynolds numbers are illustrated. The conclusion is that strong astrophysical magnetic fields at equipartition are not necessarily generated by turbulent dynamos. 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: 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: 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: 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: A non-helical dynamo - MHD simulations of dynamo action by a non-helical flow Authors: Archontis, V.; Dorch, S. B. F. Bibcode: 2005HiA....13..136A Altcode: We performed numerical 3-d MHD simulations to study whether or not the presence of helicity is a necessary ingredient for fast dynamo action. A steady 3-d flow with no mean helicity is used and is turned out that apart from the high growth rate in the linear regime the dynamo saturates at a level significantly higher that the intermittent turbulent dynamos. It becomes clear from this example that the precense of a mean helicity is not at all a requirement for dynamo action but it is rather the stretching ability of the flow that amplifies the magnetic energy in an exponential manner. 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: On the Saturation of Astrophysical Dynamos: Numerical Experiments with the No-Cosines Flow Authors: Dorch, S. B. F.; Archontis, V. Bibcode: 2004SoPh..224..171D Altcode: 2004astro.ph..9193D; 2005SoPh..224..171D In the context of astrophysical dynamos we illustrate that the no-cosines flow, with zero mean helicity, can drive fast dynamo action and we study the dynamo's mode of operation during both the linear and non-linear saturation regimes. It turns out that in addition to a high growth rate in the linear regime, the dynamo saturates at a level significantly higher than normal turbulent dynamos, namely at exact equipartition when the magnetic Prandtl number Prm∼ 1. Visualization of the magnetic and velocity fields at saturation will help us to understand some of the aspects of the non-linear dynamo problem. Title: Dynamo action in turbulent flows Authors: Archontis, V.; Dorch, S. B. F.; Nordlund, Å. Bibcode: 2003A&A...410..759A Altcode: 2003astro.ph..6069A We present results from numerical simulations of nonlinear MHD dynamo action produced by three-dimensional flows that become turbulent for high values of the fluid Reynolds number. The magnitude of the forcing function driving the flow is allowed to evolve with time in such way as to maintain an approximately constant velocity amplitude (and average kinetic energy) when the flow becomes hydrodynamically unstable. It is found that the saturation level of the dynamo increases with the fluid Reynolds number (at constant magnetic Prandtl number), and that the average growth rate approaches an asymptotic value for high fluid Reynolds number. The generation and destruction of magnetic field is examined during the laminar and turbulent phase of the flow and it is found that in the neighborhood of strong magnetic flux ``cigars" Joule dissipation is balanced by the work done against the Lorentz force, while the steady increase of magnetic energy occurs mainly through work done in the weak part of the magnetic field. Title: Helicity and Dynamo Action Authors: Archontis, Vasilis D.; Dorch, Bertil F. Bibcode: 2003IAUJD...3E..10A Altcode: We performed numerical 3-d MHD simulations to study whether or not the presence of helicity is a necessary ingredient for fast dynamo action. A steady 3-d flow with no mean helicity is used and is turned out that apart from the high growth rate in the linear regime the dynamo saturates at a level significantly higher that the intermittent turbulent dynamos. It becomes clear from this example that the precense of a mean helicity is not at all a requirement for dynamo action but it is rather the stretching ability of the flow that amplifies the magnetic energy in an exponential manner. Title: Numerical simulations of kinematic dynamo action Authors: Archontis, V.; Dorch, S. B. F.; Nordlund, Å. Bibcode: 2003A&A...397..393A Altcode: 2002astro.ph..4208A Numerical simulations of kinematic dynamo action in steady and three-dimensional ABC flows are presented with special focus on the difference in growth rates between cases with single and multiple periods of the prescribed velocity field. It is found that the difference in growth rate (apart from a trivial factor stemming from a scaling of the rate of strain with the wavenumber of the velocity field) is due to differences in the recycling of the weakest part of the magnetic field. The single wavelength classical ABC-flow experiments impose stronger symmetry requirements, which results in a suppression of the growth rate. The experiments with larger wave number achieve growth rates that are more compatible with the turn-over time scale by breaking the symmetry of the resulting dynamo-generated magnetic field. Differences in topology in cases with and without stagnation points in the imposed velocity field are also investigated, and it is found that the cigar-like structures that develop in the classical A=B=C dynamos are replaced by ribbon structures in cases where the flow is without stagnation points. Title: Dynamo action in turbulent flows Authors: Archontis, V.; Nordlund, Å. Bibcode: 2002ESASP.505...95A Altcode: 2002solm.conf...95A; 2002IAUCo.188...95A We present results from numerical simulations of kinematic and nonlinear MHD dynamo action produced by turbulent flows. Traditionally, turbulence was thought to be essential to dynamo action. There is new evidence that indicates that laminar and turbulent dynamos are surprisingly similar, with growth rates similar to large scale turn-over time in both cases. An analysis of the Lorentz work and Joule dissipation shows that dynamo occurs primarily in regions where the field is weak by bending and stretching the magnetic field lines. Title: Linear, non-linear and turbulent dynamos Authors: Archontis, Vasilis Bibcode: 2000PhDT.......179A Altcode: No abstract at ADS Title: Linear, Non-Linear and Turbulent Dynamos Authors: Archontis, V. Bibcode: 2000PhDT..........A Altcode: The properties of magnetic structures in linear, non-linear and turbulent dynamos are studied by applying numerical 3-D magneto-hydrodynamical simulations to parametrized smooth flows, such as e.g. ABC flows. Title: 3D simulations of twisted magnetic flux ropes Authors: Dorch, S. B. F.; Archontis, V.; Nordlund, Å. Bibcode: 1999A&A...352L..79D Altcode: Several numerical simulations of buoyant 2D and 3D twisted flux ropes have been performed. It is found that the apex region of an anchored 3D flux rope behaves similarly to the simpler case of a 2D horizontal twisted flux tube while the overall structure of such a 3D flux rope developes quite differently. Upon emergence a characteristic S-shape of the magnetic field lines is displayed in agreement with observations in soft X-ray. Title: Numerical Simulations of Dynamos Associated with ABC Flows Authors: Archontis, V.; Dorch, B. Bibcode: 1999ASPC..178....1A Altcode: 1999sdnc.conf....1A No abstract at ADS