Detalles de publicación
PP 05031
The 3D interaction between emerging magnetic flux and a large scale coronal field: reconnection, current sheets and jets
(1) Instituto de Astrofísica de Canarias, Spain, (2) Department of Astrophysics, Universidad de La Laguna, Spain, (3) Niels Bohr Institute, Copenhagen, Denmark, (4) School of Mathematics and Statistics, University of St.Andrews, UK,
Using MHD numerical experiments in three dimensions, we study the emergence of a bipolar magnetic region from the solar interior into a model corona endowed with a horizontal magnetic field mimicking a large-scale coronal field. The process starts with a twisted magnetic flux tube below the photosphere that has enough buoyancy for it to break through the photosphere. An arch-shaped concentrated current sheet is formed at the interface between the rising magnetized plasma and the ambient coronal field. Active reconnection takes place all along the current sheet which, following the intrinsic three-dimensionality of the field configuration across the sheet, does not conform to standard two-dimensional scenarios. As a result of the reconnection, the corona and the photosphere become magnetically connected, a process in agreement with observational evidence obtained through satellite missions in past years.
We first show the structure and evolution of the current sheet. Given our initial configuration, the abrupt change of the magnetic field across the current sheet starts as a comparatively simple tangential discontinuity. However, in the intermediate and advanced stages, it becomes a rotational discontinuity with no vanishing magnetic field. The paper then focuses on the reconnection process in the later stages of emergence. We find clear indications that individual reconnection events in this 3D environment at this stage are not one-off events, but, rather, 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 suffer multiple processes of reconnection at different points in the corona, thus creating {it photospheric pockets} for the coronal field. We calculate global measures for the amount of subphotospheric flux that becomes linked to the corona along the experiment and find that most of the original subphotospheric flux becomes connected to the coronal field lines. This is in part due to the fact that the initial tube is twisted, so that most of its field lines can reach coronal heights during the experiment. 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 2D models but the geometry of the jets is clearly three-dimensional: the jets are generated all along the rims of the arch-shaped current concentration, and, thus, appear like curved sheets. Temperatures and velocities of the jets obtained in the experiment are comparable to those measured in solar X-Ray jets by the SXT detector on board the Yohkoh satellite.
We first show the structure and evolution of the current sheet. Given our initial configuration, the abrupt change of the magnetic field across the current sheet starts as a comparatively simple tangential discontinuity. However, in the intermediate and advanced stages, it becomes a rotational discontinuity with no vanishing magnetic field. The paper then focuses on the reconnection process in the later stages of emergence. We find clear indications that individual reconnection events in this 3D environment at this stage are not one-off events, but, rather, 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 suffer multiple processes of reconnection at different points in the corona, thus creating {it photospheric pockets} for the coronal field. We calculate global measures for the amount of subphotospheric flux that becomes linked to the corona along the experiment and find that most of the original subphotospheric flux becomes connected to the coronal field lines. This is in part due to the fact that the initial tube is twisted, so that most of its field lines can reach coronal heights during the experiment. 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 2D models but the geometry of the jets is clearly three-dimensional: the jets are generated all along the rims of the arch-shaped current concentration, and, thus, appear like curved sheets. Temperatures and velocities of the jets obtained in the experiment are comparable to those measured in solar X-Ray jets by the SXT detector on board the Yohkoh satellite.