  A long-standing problem is to predict the future flare productivity of an
active region (AR) when it is on the stage of early emergence. The aim of this
study is to probe two parameters of the photospheric magnetic field, both
derived during the emergence phase of an AR, and to compare them with the flare
productivity of the well-developed AR. The parameters are: (i) the index of the
magnetic power spectrum (the slope of the spectrum) on the stage of emergence,
and (ii) the flux emergence rate. Analysis of 243 emerging ARs showed that the
magnetic power index increases from values typical for quiet-Sun regions to the
values typical for mature ARs within a day while the emergence proceeds for
several days; frequently, after the increase, the value of the power index
undulates around some mean value with fluctuations being several time less than
the growth of the power index during the emergence onset. For a subset of 34
flare-productive ARs we found no correlation between the power spectrum index
at the stage of emergence and flare index derived from the entire interval of
the AR's presence on the disc. At the same time, the flux emergence rate
correlates well with the flare index (the Pearson's correlation coefficient is
0.74). We conclude that high flux emergence rate is a necessary condition for
an AR to produce strong flares in the future, and so, the flux emergence rate
can be used to probe the future flare productivity of the AR.
