Slowly but surely towards the huge amount of energy I
From RHESSI Wiki
|1st Author:||Urszula Bak-Stęślicka|
|2nd Author:||Tomasz Mrozek, Sylwester Kołomański|
|Published:||31 October 2011|
|Next Nugget:||Slowly II|
Solar flares may have complicated structures and time developments, but since Skylab days we have distinguished between "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 a day. Some LDEs may also have an unusually long rising phase that may lasts more than 30 minutes. We refer to this type of LDE as a slow long-duration events (SLDEs). During the rising phase of an SLDE there is no typical impulsive phase . 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 . These studies provided us with the basic observational characteristics of SLDEs:
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:
Our study (ref. ) dealt with six SLDEs, with the following results:
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.