Statistical analysis of solar H alpha flares

A statistical analysis of a large data set of H alpha flares comprising alm ost 100000 single events that occurred during the period January 1975 to De cember 1999 is presented. We analyzed the flares evolution steps, i.e. dura tion, rise times, decay times and event asymmetries. Moreover, these parame ters characterizing the temporal behavior of flares, as well as the spatial distribution on the solar disk, i.e. N-S and E-W asymmetries, are analyzed in terms of their dependency on the solar cycle. The main results are: 1) The duration, rise and decay times increase with increasing importance clas s. The increase is more pronounced for the decay times than for the rise ti mes. The same relation is valid with regard to the brightness classes but i n a weaker manner. 2) The event asymmetry indices, which characterize the p roportion of the decay to the rise time of an event, are predominantly posi tive (approximate to 90%). For about 50% of the events the decay time is ev en more than 4 times as long as the rise time. 3) The event asymmetries inc rease with the importance class. 4) The are duration and decay times vary i n phase with the solar cycle; the rise times do not. 5) The event asymmetri es do not reveal a distinct correlation with the solar cycle. However, they drop during times of solar minima, which can be explained by the shorter d ecay times found during minimum activity. 6) There exists a significant N-S asymmetry over longer periods, and the dominance of one hemisphere over th e other can persist for more than one cycle. 7) For certain cycles there ma y be evidence that the N-S asymmetry evolves with the solar cycle, but in g eneral this is not the case. 8) There exists a slight but significant E-W a symmetry with a prolonged eastern excess.