The "Main Sequence" of Explosive Solar Active Regions: Discovery and Interpretation

Details The "Main Sequence" of Explosive Solar Active Regions: Discovery and Interpretation The "Main Sequence" of Explosive Solar Active Regions: Discovery and Interpretation

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsp24a..07f&link_type=abstract

American Geophysical Union, Spring Meeting 2008, abstract #SP24A-07

Physics

7509 Corona, 7513 Coronal Mass Ejections (2101), 7519 Flares, 7524 Magnetic Fields

Scientific paper

From ~ 2000 MDI magnetograms of 44 evolving active regions within 30 heliocentric degrees of disk center, we measured active-region magnetic size and total nonpotentiality. Besides displaying the upper limit on active- region size above which the sun rarely produces active regions and the lower limit on active-region size below which a magnetic flux concentration is not an active region, we discovered that active-region total nonpotentiality has an upper bound that increases with active-region magnetic size. For a given size, an active region can have only so much total nonpotentiality. We show that this limit amounts to an upper bound on a particular measure of an active region's nonpotentiality per unit flux, that is, an upper bound on a flux-normalized measure of an active region's nonpotentiality. This limit plausibly represents an upper bound on the overall degree of twist in an active region's magnetic field. If so, an active region's magnetic twist can increase to this limit but go no further. After being near the limit for a while the active region can loose nonpotentiality and retreat from the limit. Albeit entirely different physics, this evolution is analogous to how stars evolve to the main sequence, stay there a while and then evolve away from it. Unlike the stellar evolution path, an active region can evolve to its limit multiple times. We present evidence that what is enforcing this upper limit on flux-normalized nonpotentiality is that as an active region's magnetic field becomes more twisted, it more rapidly releases energy in the form of flares and CMEs. When an active region's energy-burn-down rate by flares and CMEs equals the rate of buildup of its nonpotential energy, it can get no more nonpotential. The upper limit on flux- normalized nonpotentiality is determined by the burn-down rate dependence on the flux-normalized nonpotentiality and an upper limit on how rapidly an active region's nonpotentiality can buildup. This work is funded by the NASA LWS TR&T Program, by the NSF SHINE Program, by the AFOSR MURI Program, and by the NASA Technical Excellence Initiative.

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