Slowly but surely towards the huge amount of energy I
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Our study (ref. [3]) dealt with six SLDEs, with the following results: | Our study (ref. [3]) dealt with six SLDEs, with the following results: | ||
- | + | <li><b>The character of the loop-top emission:</b> we find a thermal component to be dominant dominant, however for four of the six events we observed significant non-thermal emission | |
- | + | <li> <b>The heating rate of the plasma:</b> the values obtained are not high, usually below 10 erg cm<sup>3</sup>s<sup>-1</sup>, | |
but larger than those obtained from the earlier SXT observations. | but larger than those obtained from the earlier SXT observations. | ||
This difference is caused by the fact that the SXT had less sensitivity to higher-temperature plasma (> 10 MK). | This difference is caused by the fact that the SXT had less sensitivity to higher-temperature plasma (> 10 MK). | ||
- | + | <li><b>The time evolution of the heating rate:</b> the long duration of the rise phase of an SLDE is consistent with a very slow decrease of heating rate during this phase. In most cases the characteristic time τ is larger than 1000 s. | |
- | + | <li> <b>The total thermal energy released during the rise phase:</b> the low limit of the total energy released during the rising phase was calculated under the following assumptions: (a) the heating rate is constant through the entire rise phase and equal to its minimum measured value, (b) loop-top volume was calculated from mean area of the source, A, as seen in images, and assuming a sphere, and assumed to be constant during the whole rise phase. For the analyzed events we obtained at least about 10<sup>31</sup> erg as the total thermal energy released during the rise phase. | |
== Conclusions == | == Conclusions == |
Revision as of 20:00, 25 October 2011
Nugget | |
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Number: | 162 |
1st Author: | Urszula Bak-Stęślicka |
2nd Author: | Tomasz Mrozek, Sylwester Kołomański |
Published: | 31 October 2011 |
Next Nugget: | Slowly II |
Previous Nugget: | [1] |
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Contents |
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.
Chronologically the first studies of SLDEs were based on Yohkoh/SXT images [2]. These studies provided us with the basic observational characteristics of SLDEs:
Data Analysis
For our analysis we selected six limb or near-the-limb SLDEs, with rise phases lasted between 25 – 150 min. RHESSI light curves, GOES/SXI with the RHESSI contours, and the RHESSI spectrum (for the flare with the longest rising phase) are shown in Figure 2. The images were reconstructed with the PIXON algorithm. We used time intervals of 20 – 40 s and narrow energy intervals of 1 – 2 keV. In our analysis we were concentrated on coronal sources, so-called loop-top sources only. For each observed such source we estimated size (area projected on image) and altitude above the photosphere. In the next step we performed a spectroscopic analysis, using the standard RHESSI software, to obtain the physical parameters. These parameters were used as an input data for calculation of the heating rate. It was assumed that a change of thermal energy of a loop-top source is due to expansion, radiation, conduction (cooling processes) and heating:
Observed change of thermal energy = adiabatic expansion – conductive cooling – radiative cooling + heating rate
Knowing the change of LTS thermal energy and values of the three cooling processes we can calculate how efficiently the LTS was heated. As an additional parameter describing temporal evolution of the heating rate we calculated the characteristic time τ of the decrease of heating rate (EH) after it reaches its maximum value:
Results
Our study (ref. [3]) dealt with six SLDEs, with the following results:
Conclusions
Do the "slow LDEs" represent different physics, not found in ordinary flares? Despite the low value of the heating rate, during the whole rise phase of SLDE the total released energy is huge. In our examples it is around 1031 – 1032 erg. This value is larger, by at least an order of magnitude, than the total energy released during the rise phase of a short-rise flares of comparable GOES magnitude. In some cases no detectable HXR (nor, then, intense particle acceleration) may have occurred. It remains to be seen whether this signifies different physical processes.
References
[1] "Hard X-rays from 'Slow LDEs'"
[2] "Investigation of X-Ray Flares with Long Rising Phases"
[3] "Energy Release During Slow Long-Duration Flares Observed by RHESSI"
RHESSI Nugget Date | 31 October 2011 + |
RHESSI Nugget First Author | Urszula Bak-Stęślicka + |
RHESSI Nugget Index | 162 + |
RHESSI Nugget Second Author | Tomasz Mrozek, Sylwester Kołomański + |