Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003apj...582.1190c&link_type=abstract
The Astrophysical Journal, Volume 582, Issue 2, pp. 1190-1205.
Astronomy and Astrophysics
Astronomy
32
Instabilities, Magnetohydrodynamics: Mhd, Sun: Activity, Sun: Magnetic Fields
Scientific paper
Building on Cally's nonlinear model of two-dimensional MHD tachocline instability, we further explore the evolution of a wide variety of toroidal field profiles due to this instability. Cally showed in a recent study that an initially broad toroidal field opens up into a ``clamshell'' pattern because of nonlinear evolution of MHD tachocline instability. Various other toroidal field profiles-single toroidal bands, double bands, and mixed profiles with a band in addition to broad profiles-may also occur in the Sun during various phases of the solar cycle. Detailed study of the evolution of banded profiles shows no occurrence of clamshell instability, but the bands commonly tip relative to the axis of rotation. The higher the latitude location of the band, the more it tips. Extreme tipping results when the band is at 60° latitude or higher-the magnetic ring hangs from the pole on one side of the Sun. For bands of 10° latitude width at sunspot latitudes (<=40°), the band tip is within +/-10° about the mean latitude of the band. This tipping could either enhance or reduce the observed tilt in bipolar active regions. Double bands, or profiles consisting of a band and a broad profile, may exist at certain phases of the solar cycle. We find that double-band systems with two oppositely directed bands separated widely (>15°) in latitude, as well as two close bands of same polarities, do not interact in the same hemisphere-the higher latitude band tips, while the lower latitude band hardly responds. A significant interaction between two individual bands in one hemisphere takes place only when the band separation is <=15° and the bands are oppositely directed, which is a nonsolar case. In this case, the band system either tips or forms the clamshell pattern depending on the dominant mode symmetry. We also show that a mixed profile with oppositely directed narrow fields close to the equator in addition to the broad fields evolve in such a way as to oppose the reconnection of the broad fields across the equator, and thus inhibiting the clamshell formation, at least at certain phases of the solar cycle. Finally, we note that the tipping and clamshell instabilities strongly inhibit differential rotation.
Cally Paul S.
Dikpati Mausumi
Gilman Peter A.
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