Latitude Variation of Magnetic Helicity

The figure shows the observed latitude distribution of the average magnetic helicity density, alpha-average, of 69 solar active regions, based primarily on vector magnetograms made with the Haleakala Stokes Polarimeter at Mees Solar Observatory from 1988 - 1994. The helicity density is measured using the linear force-free field parameter alpha, where curl B = alpha B. Its average value for a single vector magnetogram of a single region is determined by least-squares minimization of the difference between the observed horizontal magnetic fields and those of a computed constant-alpha force-free field. The error base show the estimated standard deviation, sigma, of both alpha-average and latitude, for regions represented by more than one magnetogram. The box and triangle symbols without error bars show alpha-average of active regions represented by only one magnetogram. Boxes and points at the intersections of error bars indicate data from our Stokes Polarimeter during cycle 22; triangles mark values of alpha taken from published papers, cycles 20-21. The short-dashed line shows the least-square best fit of the data by a first-degree Legendre polynomial, and the dashed lines show the 95% confidence band of this fit.

In this data set, the average magnetic helicity show a hemispheric sign difference at the 2 sigma level. 76% of the active regions in the northern hemisphere have negative helicity and 69% in the southern hemisphere, positve. The interpretation of these data in terms of the generation of the magnetic fields in the solar dynamo, presumably near the base of the convetion zone, their transport through the convection zone, and their reconnection in the corona is not yet well understood.

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