AAS 201st Meeting, January, 2003
Session 83. Solar Activity
Poster, Wednesday, January 8, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[83.06U] Calculation of Magnetic Helicity in a Force-Free Field

A. C. Updike (Smith College / National Solar Observatory), A. Pevtsov (National Solar Observatory)

One of the goals of solar physics is to understand the solar dynamo. By studying the nature of magnetic helicity, we are studying the solar magnetic field driven by the dynamo. We used chromospheric magnetographs in the Ca II line (8543 Å) and EIT images in the Fe XII line (195 Å) to determine the degree of twist in the magnetic field. Using this degree of twist and the known magnetic energy for a given active region, we were able to measure the magnetic helicity in this force-free region. Over a period of three years (August 1996 to November 1998), we studied 37 active regions. We found a 73% hemispheric preference in the sign of the helicity - in the northern hemisphere, 70% of the active regions exhibited negative helicity; in the southern hemisphere, 76.5% of the active regions exhibited positive helicity. Our observations agree with earlier studies of the sign of helicity, which used vector magnetograms and studies of quiescent filiments.

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