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Magnetic flux emergence in the Sun  

Vasilis Archontis   Submitted: 2008-12-09 07:14

Space weather research is closely connected with the study of the solar magnetic activity. In the past years, many solar missions (e.g., YOHKOH, SOHO, TRACE, 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, STEREO) will study the interaction between the emerging magnetic field and the pre-existing 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, 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 towards 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.

Authors: Archontis, V
Projects: None

Publication Status: Journal of Geophysical Research (published)
Last Modified: 2008-12-09 19:00
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Eruption of magnetic flux ropes during flux emergence.  

Vasilis Archontis   Submitted: 2008-11-07 06:03

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 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. On the contrary, if the expanding tube is allowed to reconnect with a preexisting coronal field, the flux rope experiences a full eruption with a rise profile which is in qualitative agreement with erupting filaments and Coronal Mass Ejections.

Authors: Archontis, V , Torok, T.
Projects: None

Publication Status: A&A (accepted)
Last Modified: 2008-11-07 11:22
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On the structure and evolution of complexity in sigmoids: a flux emergence model  

Vasilis Archontis   Submitted: 2008-10-05 02:54

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 {it J-like} bundles or an overal 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 afore-mentioned complexity in sigmods is build 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 make the system buoyant in the middle and it adopts an Omega-shape as it rises towards the outer atmosphere. During the evolution of the system, expanding fieldlines 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 bald-patch seperatrix surfaces has a {it J-like} shape. Each one of the Js consist of reconnected fieldlines with different shapes and different relative orientation. The further dynamical evolution of the emerging flux tube results in the occurence 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 fieldlines along these layers form an overall S-shaped structure. The reconnection process continues to occur leading to 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 lq post-flare q loops underneath the current structure.

Authors: Archontis, V., Hood, A., Savcheva, A., Golub, L, Deluca, E.
Projects: None

Publication Status: ApJ (accepted)
Last Modified: 2008-10-06 09:40
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Emergence and interaction of twisted flux tubes in the Sun  

Vasilis Archontis   Submitted: 2007-02-02 03:29

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.

Authors: Archontis, V. , Hood, A.W and Brady, C.
Projects: None

Publication Status: A&A (accepted)
Last Modified: 2007-02-02 09:02
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Abstracts by Author
Magnetic flux emergence in the Sun
Eruption of magnetic flux ropes during flux emergence.
On the structure and evolution of complexity in sigmoids: a flux emergence model
Emergence and interaction of twisted flux tubes in the Sun

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