Solar and Magnetospheric MHD Theory Group - University of St Andrews

REVEALING THE NATURE OF SOLAR SIGMOIDS

Archontis, V. (1), Hood, A. (1), Savcheva, A. (2), Golub, L. (2) and Deluca, Ed.(2)

(1) Mathematical Institute, St Andrews University, UK (2) Harvard-Smithsonian Center for Astrophysics, USA

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'Sigmoids' are structures with a forward or inverse S-shape, observed in the solar corona in soft X-ray emission. It is believed that the appearance of a sigmoid in an active region is a very important factor in eruptive activity. In fact, most of the sigmoids (~80%) are associated with dynamic phenomena such as FLARES and CORONAL MASS EJECTIONS (CMEs) and this relationship makes the study of sigmoids unique.

Recent observations of a coronal sigmoid, obtained with the X-Ray Telescope (XRT) on board Hinode mission, showed (for the first time) its 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. They also showed that, at the end of its life, the sigmoid produces a 'flare' and twisted magnetic fields erupt to the outer solar atmosphere.

A series of theoretical and numerical models have been proposed, over the past years, to explain the nature of sigmoids but there was no explanation on how such complex 'sigmoids' form, erupt and fade away.

We have build up a numerical model that, for the first time in solar physics, can reproduce and explain the nature of all the afore-mentioned stages of a sigmoid's life. In this model, we show that sigmoids are consist of many, thin and twisted layers ( or 'ribbons') of strong electric current. Reconnection of magnetic field lines in these current layers leads to strong heating along the sigmoid and the formation of powerful flares. Another result of this dynamical evolution of the sigmoid is the formation and eruption twisted magnetic fields and high energetic particles into the outer interplanetary space.

The results of our numerical experiments and the recent, high-resolution observations on the formation and evolution of coronal sigmoids appear in the Astrophysical Journal, Volume 691, Issue 2, pp. 1276-1291 (2009). Comparing the new observations with the results of our numerical model we explain how complex sigmoids are formed and evolve in the Sun and how they are connected to dramatic, explosive events of the solar corona. This is a copy of the paper.

Figure 1. The left panel shows one snapshot from the numerical experiments and the right panel shows the 'corresponding' snapshot from the observations (as recorded by Hinode mission). The two figures show the 'internal complex structure' of the solar sigmoid, which has been observed with very high resolution and it is reproduced succesfully by our numerical experiments. As you can see, 'sigmoids' consist of a netwrok of thin 'ribbons' where the electric current is strong (the isosurfaces in the left panel show electric current) and thus, the plasma is heated to 1-2 million Kelvin degrees temperature. As you can see, there is a very good agreement between the observations and the numerical experiments.

Credit: Hinode is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA and STFC (UK) as international partners. It is operated by these agencies in co-operation with ESA and the NSC (Norway). AS, LG and ED were supported by NASA contract NNM07AB07C.

Figure 2. This figure shows the time evolution and final eruption of the sigmoid. It consists of three columns (time is running from top to bottom). Columns 1 and 2 show results from numerical experiments. The yellow isosurfaces are surfaces of electric current (left panels). Column 2 (middle panels) shows temperature. Column 3 shows 'temperature' (intensity) as it is recorded by the observations (Hinode mission). Notice that the agreement on the shape of the sigmoid, internal structure and thermal distribution along the sigmoid, between numerical experiments and observations is very good and fairly balanced. Notice, that even the 'flaring' episode (flashing) at the middle of the sigmoid at the down-right snapshot from observations is reproduced exceptionally well by our numerical experiments (down-middle).