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Introduction

Solar flares may have complicated structures and time developments, but since Skylab days we have distinguished "impulsive" and "gradual" classes. A Long Duration Event (LDE) is a flare characterized by a slow decrease of the soft X-ray (SXR) emission. The decay phase of an LDE may last more than one day. Some of LDEs have also the unusually long rising phase that may last more than 30 minutes. This group is called slow long-duration events (SLDEs). During the rising phase of an SLDE there is no typical impulsive phase [1]. Instead of short pulses we observe a gradual increase of HXR emission and/or a smooth, broad (several minutes long) bursts of HXR emission (see Figure 1). RHESSI's first X-class flare was just such an event.

Figure 1: RHESSI light curves (6-12 keV, 25-50 keV) for two LDE flares: Left: a long (50 min) rise phase and Right: a short (15 min) rise phase. Note that SOL2002-04-21 (left) was RHESSI's first X-class flare observation.

Chronologically the first studies of SLDEs were based on Yohkoh/SXT images [2]. These studies provided us with basic observational characteristics of SLDEs:

• most SLDEs occur in high or mid-high structures, • loop-top sources are characterized by low temperature (T<10 MK), low density (N ≈ 1010 cm− 3 and large size (r > 7 × 108 cm), • the heating rate of plasma is small, below 1 erg cm− 3 s− 1 , • the heating rate decreases very slowly with time after reaching its maximum value.

A detailed analysis of SLDEs in the HXR range requires high spectral resolution, much better than the resolution provided by the Yohkoh/HXT. The requirement was fulfilled by RHESSI data. RHESSI allows us to investigate spatially resolved HXR emission of SLDEs with 1 keV energy resolution. This energy resolution enables the estimation of physical parameters in SLDEs through imaging spectroscopy. Parameters obtained like temperature and emission measure can be used for an energy balance analysis.

References

[1] "Hard X-rays from 'Slow LDEs'"