Modeling the Magnetic Field Evolution of the December 13 2006 Eruptive Flare Yuhong Fan High Altitude Observatory, National Center for Atmospheric Research FEW 2011
Outline • A set of simulations of CME onset with an idealized configuration: • Consider a pre-existing coronal potential arcade field and impose the emergence of a twisted flux rope at the lower boundary • Critical conditions for the eruption of a coronal flux rope • Formation of current sheet and the role of “tether cutting” reconnections • An observationally guided simulation • Both the pre-existing field and the lower boundary driving conditions are derived to some degree from observations • Qualitatively models the magnetic field evolution associated with the December 13 2006 eruptive flare
MHD simulations of the eruption of coronal flux ropes • Numerically solve the isothermal MHD equations in a spherical domain of the solar corona: • The domain is resolved by a non-uniform grid of 432x192x240 • Initially the corona is a static isothermal atmosphere at 1MK with a pre-existing potential arcade field: the isothermal sound speed as=128km/s, the peak Alfven speed at the foot point of the arcade vA0=1951km/s. • At the lower boundary, we impose (kinematically) the emergence of a twisted torus for t=0 to t=tstp after which the emergence is stopped and the field lines are rigidly anchored subsequently. • A sequence of simulations are carried out where tstp is varied such that a varying amount of the twisted flux of the torus is transported into the corona. Fan (2010)
Formation of current sheet and “tether cutting” reconnections Orange surfaces: iso-surfaces of J/B with the level set at 1/l where l = 10 grid resolution elements.
Hinode observation of the d-region NOAA 10930 and the eruptive flare on 2006-12-13 Min and Chae (2009) Images and movies from http://solar-b.nao.ac.jp/news_e/20061213_flare_e.shtml • The small sunspot of positive polarity rotated counter-clockwise about its center by 240° as measured by Zhang et al. (2007) and 540° as measured by Min and Chae (2009). Liu et al. (2008)
Hinode observation of the d-region NOAA 10930 and the eruptive flare on 2006-12-13 Images and movies from http://solar-b.nao.ac.jp/news_e/20061213_flare_e.shtml
Constructing the initial pre-existing field and the lower boundary driving conditions 20:51:01 UT on Dec. 12, 2006 • A region centered on the d-spot is extracted from the MDI full disk magnetogram • Smoothing of Br with a Gaussian filter • The magnetic flux in a central area enclosing the region of flux emergence is zeroed out • Construct potential field from the lower boundary normal flux distribution as the pre-existing coronal field • On the lower boundary, in the zeroed out area, drive the emergence of an idealized, twisted magnetic torus. initial normal flux distribution final normal flux distribution
We solve the following MHD equations, assuming an ideal polytropic gas with g = 1.1:
Simulation domain: Grid: Initial atmosphere is assumed to be a static polytropic atmosphere with g=1.1. Initial potential magnetic field
t = 3.25 t = 3.55
t = 3.65 Liu et al. (2008)
Summary • The simulated coronal magnetic field resulting from the emergence of an east-west oriented flux rope with its positive emerging flux bordering the southern edge of the dominant pre-existing negative sunspot captures the gross structure of the actual magnetic field evolution associated with the eruptive flare (Fan 2011 ApJ in press). • Improvement of the model: • Much wider simulation domain • Increase spatial decline rate of the ambient potential field faster eruption • Remove the interference of the sidewall boundaries on the trajectory and writhing of the erupting flux rope • Reduce smoothing of the observed lower boundary flux density • More quantitative determination of the lower boundary electric field that results in better matching of the observe flux emergence pattern. Acknowledgements This work is supported in part by NASA LWS TR&T grant NNX09AJ89G to NCAR. The numerical simulations were carried out on the Pleiades supercomputer at the NASA Advanced Supercomputing Division.