Abstract
We studied two active regions appeared in 2001 (NOAA 9415 and 9661), both of which have produced X-class flares. The time evolutions of magnetic flux and magnetic helicity were derived based on vector magnetograph observations. These regions satisfied the so-called hemispheric rule of magnetic helicity, namely regions in the northern (southern) hemisphere tend to have negative (positive) helicity. The magnetic helicity integrated over the regions evolved slowly and did not show abrupt changes at the time of the flares, although the distributions of magnetic helicity changed significantly over a few days in the regions. The appearance of localized regions whose helicity was opposite to that expected from the hemispheric rule was seen in both cases.
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