Hemispheric Helicity Asymmetry in Active Regions for Solar Cycle 21-23

Details Hemispheric Helicity Asymmetry in Active Regions for Solar Cycle 21-23 Hemispheric Helicity Asymmetry in Active Regions for Solar Cycle 21-23

Jan 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002mwoc.conf..147h&link_type=abstract

Multi-Wavelength Observations of Coronal Structure and Dynamics -- Yohkoh 10th Anniversary Meeting. Proceedings of the conferenc

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Magnetic helicity observed at the surface carries information on the invisible, sub-surface processes such as internal rotation and the behavior of magnetic flux tubes in the convection zone. It has been recognized that magnetic helicity shows a hemispheric rule; the northern (southern) hemisphere tends to show negative (positive) helicity. It is also known that this rule does not change with solar cycle. The determination of the helicity, requiring vector magnetograms, is sensitive to various noises and instrumental effects, and is therefore not an easy task. Considering the importance of magnetic helicity, here we present the results based on our data set and compare them with results from other observatories. We analyzed vector magnetograms in the period of 1992-2000 obtained with the Solar Flare Telescope at the National Astronomical Observatory of Japan. The current helicity was determined by two methods for 180 active regions. The first method calculates the electric currents over active regions by a direct differentiation and then evaluates the average helicity, alpha = frac sum (abla times vec {B})zcdot {m sign} (Bz) + Bz. The second method is the fitting of the linear force-free field vec{B}m cal (alpha) to the observed transverse field vec{B}m obs and finds the best-fit alpha which minimizes frac sum [vec{B}m cal(alpha) - vec{B}m obs]2 sum Bm obs2. The data points with transverse fields larger than 150 G (noise level) and with longitudinal fields weaker than 500 G were used. The latter condition was to minimize the effect of Faraday rotation that may change the azimuth of the transverse field. We plotted the helicity against solar latitude and calculated a linear fit to the data. The slopes of the fit obtained from the first and the second methods are (-1.08 ± 0.51) times 10-10 mm-1 m deg-1 and (-3.30 ± 1.14) times 10-10 mm-1 m deg}-1, respectively. Our results agree with the previous studies and confirm the hemispheric rule.

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