Slowly but surely towards the huge amount of energy I

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Nugget
Number: 162
1st Author: Urszula Bak-Stęślicka
2nd Author: Tomasz Mrozek
Published: 31 October 2011
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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 hard X-ray (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 the 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−3s−1 ,

• the heating rate decreases very slowly with time after reaching its maximum value.

A detailed analysis of SLDEs in the hard X-ray range requires high spectral resolution, much better than the resolution provided by the Yohkoh/HXT. This requirement was fulfilled by RHESSI, which 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. The parameters thus obtained (e.g., temperature and emission measure) can be used for an energy-balance analysis.


References

[1] "Hard X-rays from 'Slow LDEs'" [2] "Investigation of X-Ray Flares with Long Rising Phases"

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