Emergence of a Twisted Ω -loop Into the Solar Atmosphere

Details Emergence of a Twisted Ω -loop Into the Solar Atmosphere Emergence of a Twisted Ω -loop Into the Solar Atmosphere

May 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sp51b09f&link_type=abstract

American Geophysical Union, Spring Meeting 2001, abstract #SP51B-09

Astronomy and Astrophysics

Astrophysics

7500 Solar Physics, Astrophysics, And Astronomy, 7524 Magnetic Fields, 7529 Photosphere

Scientific paper

We report the results of a 3D MHD simulation of the dynamic emergence of a twisted Ω -loop from the top layer of the convection zone (where the plasma β is high) into the solar atmosphere and the corona. Compared to previous 2D simulations of the emergence of an infinitely long horizontal flux tube by Magara (2001), our calculations include the 3D effect of bending the flux tube so that only a segment of the Ω -loop intersects the photosphere. Similar to the 2D results, we find that as the tube segment at the apex of the Ω -loop enters the photosphere boundary, the tube first decelerates because the flux tube encounters the stable stratification of the isothermal atmosphere. Subsequently, the magnetic flux entering the photosphere boundary develops a magnetic buoyancy instability (or the Parker instability) which causes the upper part of the apex tube segment to rapidly expand into the atmosphere. The growth of the Parker instability is characterized by an exponential increase with time of the rise velocity of the front boundary of the flux tube expanding into the atmosphere, and by downflows of plasma along the emerged field lines. At the beginning of the emergence, the emerged field lines are nearly north-south oriented. Subsequently, a shear motion develops where the field-line footpoints on the two sides of the polarity inversion line are sheared in the east-west direction, with the footpoints of the leading (following) polarity moving westward (eastward). We find that with time an intensification of vertical magnetic field of the leading and following polarities takes place respectively at the west and east corners of the emerged magnetic region on the photosphere, in agreement with the Hale polarity rule. The line connecting the two major intensification spots of the two polarities is tilted slightly away from the east-west direction in the sense that is anti-Joy's law if the emerging tube has left-handed twist in the northern hemisphere. We find that our simulation of the emergence of a left-hand-twisted Ω -loop can explain the major observed features in a newly emerged active region studied by Strous et al. (1996,1999), including the orientation of the arch-filament system, the distribution and horizontal motion of flux on the photosphere, and the location of formation of sunspot pores.

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