Magnetic helicity is now regarded as an important physical quantity in understanding solar magnetic activities. We have studied the injection of magnetic helicity through the photosphere of four active regions during the long periods of their lifetimes. The rate of helicity injection was determined as a function of time in each active region by applying the local correlation tracking (LCT) method to full-disk, 96 minute cadence magnetograms taken by SOHO MDI. As a result, we have found that most helicity was intensively injected while active regions emerged and grew, suggesting that magnetic flux emergence may be the major process of helicity injection. In all the active regions studied, the rate of helicity injection during flux emergence was initially low, increased and stayed high for a while, and then became low again, while magnetic flux steadily increased at a more or less constant rate all the time. Flux cancellation, as a minor process, resulted in some loss of coronal magnetic helicity, or inverse helicity injection. The contribution of differential rotation to helicity injection was found to be insignificant in these active regions. It was also found that the magnetic helicity budget of an active region strongly depends on its average magnetic flux.
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