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Solar Flares with an Exponential Growth of the Emission Measure in the Impulsive Phase Derived from X-ray Observations

The light curves of solar flares in the impulsive phase are complex in general, indicating that multiple physical processes are involved in. With the GOES (Geostationary Operational Environmental Satellite) observations, we find that there are a subset of flares, whose impulsive phases are dominated by a period of exponential growth of the emission measure. The flares occurred from January 1999 to December 2002 are analyzed, and the results from the observations made with both GOES 8 and GEOS 10 satellites are compared to estimate the instrumental uncertainties. Their mean temperatures during this exponential growth phase have a normal distribution. Most flares within the 1σ range of this temperature distribution belong to the GOES class B or C, with the peak fluxes at the GOES low-energy channel following a log-normal distribution. The growth rate and duration of the exponential growth phase also follow a lognormal distribution, in which the duration is distributed in the range from half a minute to about half an hour. As expected, the growth time is correlated with the decay time of the soft X-ray flux. We also find that the growth rate of the emission measure is strongly anti-correlated with the duration of the exponential growth phase, and the mean temperature increases slightly with the increase of the growth rate. The implications of these results on the study of energy release in solar flares are discussed in the end.