Relationship between decrease of net magnetic flux and solar major events

Using the full disk magnetograms taken by SOHO/MDI, we studied the evolution of magnetic fluxes in the line-of-sight for twelve solar active regions in the 23rd cycle, which produced strong solar activities. It is measured that the fluxes of these regions were appreciably imbalanced when they moved past the central meridian, where reliable measurements were obtained. In addition, it is found that, for seven regions for which the vector magnetic field was well observed at Huairou Solar Observing Station and thereby a vertical electric current can be derived, the vertical electric currents were out of balance. It has been found that the net fluxes of seven active regions which were associated with strong solar storms significantly decreased within 2 or 3 days prior to the major flares associated with the storms, and tended toward minimization before the onsets of the flares, suggesting a possible relationship between significant decrease of the flux imbalance of active regions and instability of large-scale magnetic fields.