  We analyse the temporal evolution of the Differential Emission Measure (DEM)
of solar active regions and explore its usage in solar flare prediction. The
DEM maps are provided by the Gaussian Atmospheric Imaging Assembly (GAIA-DEM)
archive, calculated assuming a Gaussian dependence of the DEM on the
logarithmic temperature. We analyse time-series of sixteen solar active regions
and a statistically significant sample of 9454 point-in-time observations
corresponding to hundreds of regions observed during solar cycle 24. The
time-series analysis shows that the temporal derivatives of the Emission
Measure dEM/dt and the maximum DEM temperature dTmax/dt frequently exhibit high
positive values a few hours before M- and X-class flares, indicating that
flaring regions become brighter and hotter as the flare onset approaches. From
the point-in-time observations we compute the conditional probabilities of
flare occurrences using the distributions of positive values of the dEM/dt, and
dTmax/dt and compare them with corresponding flaring probabilities of the total
unsigned magnetic flux, a conventionally used, standard flare predictor. For
C-class flares, conditional probabilities have lower or similar values with the
ones derived for the unsigned magnetic flux, for 24 and 12 hours forecast
windows. For M- and X-class flares, these probabilities are higher than those
of the unsigned flux for higher parameter values. Shorter forecast windows
improve the conditional probabilities of dEM/dt, and dTmax/dt in comparison to
those of the unsigned magnetic flux. We conclude that flare forerunner events
such as preflare heating or small flare activity prior to major flares reflect
on the temporal evolution of EM and Tmax. Of these two, the temporal derivative
of the EM could conceivably be used as a credible precursor, or short-term
predictor, of an imminent flare.
