Slowly but surely towards the huge amount of energy II
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|publish_date = 28 November 2011 | |publish_date = 28 November 2011 | ||
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- | == A search for extreme == | + | == A search for the extreme == |
- | How much energy does a solar flare need to exist? How much energy must be accumulated in magnetic field before a flare starts? And how fast the accumulated energy | + | How much energy does a solar flare need to exist? How much energy must be accumulated in the magnetic field before a flare starts? And how fast must the accumulated energy be released? Should it be discharged in a short time during the flare initial (impulsive) stage? Or should it leak continuously out throughout the entire flare duration? These are several of the basic questions in flare physics, and the answers will help us to understand solar and stellar flares. |
- | To answer the questions we decided to go to the extreme in our search | + | To answer the questions we decided to go to the extreme in our search: we studied flares that last very long, the so-called ''long-duration events''. |
+ | A long-duration event (LDE) is a solar flare characterized by a slow decrease in X-ray thermal emission (see Figure 1). | ||
+ | This decrease may last from several hours to more than a day. | ||
+ | Studies of LDEs based on past solar space missions ([http://en.wikipedia.org/wiki/Skylab Skylab], [http://en.wikipedia.org/wiki/Yohkoh Yohkoh]) resulted ''inter alia'' in the conclusion that, without a continuous energy input during the whole decay phase, LDEs would decay much faster than observed. | ||
+ | If energy is continuously released during the many hours of the decay, then these LDEs may be extremely energy-hungry. | ||
+ | But which part of an LDE is the best tool to find out how much energy is generated? | ||
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Knowing all these we decided to do the search for how much energy can be produced during flares by studying LTSs of long duration flares. RHESSI satellite was chosen as a source of observational data because of its high sensitivity and high angular and energy resolutions. A sample of LDEs lasting at least 7 hours was selected. | Knowing all these we decided to do the search for how much energy can be produced during flares by studying LTSs of long duration flares. RHESSI satellite was chosen as a source of observational data because of its high sensitivity and high angular and energy resolutions. A sample of LDEs lasting at least 7 hours was selected. | ||
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==How to see a weak source?== | ==How to see a weak source?== |
Revision as of 14:22, 23 November 2011
Nugget | |
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Number: | 164 |
1st Author: | Sylwester Kołomański |
2nd Author: | Tomasz Mrozek |
Published: | 28 November 2011 |
Next Nugget: | Hα and Hard X-rays |
Previous Nugget: | X-ray and H-alpha Flare Impulses |
List all |
Contents |
A search for the extreme
How much energy does a solar flare need to exist? How much energy must be accumulated in the magnetic field before a flare starts? And how fast must the accumulated energy be released? Should it be discharged in a short time during the flare initial (impulsive) stage? Or should it leak continuously out throughout the entire flare duration? These are several of the basic questions in flare physics, and the answers will help us to understand solar and stellar flares.
To answer the questions we decided to go to the extreme in our search: we studied flares that last very long, the so-called long-duration events. A long-duration event (LDE) is a solar flare characterized by a slow decrease in X-ray thermal emission (see Figure 1). This decrease may last from several hours to more than a day. Studies of LDEs based on past solar space missions (Skylab, Yohkoh) resulted inter alia in the conclusion that, without a continuous energy input during the whole decay phase, LDEs would decay much faster than observed. If energy is continuously released during the many hours of the decay, then these LDEs may be extremely energy-hungry. But which part of an LDE is the best tool to find out how much energy is generated?
Coronal sources called loop-top sources (LTSs) are observed in every flare. LTSs are remarkable X-rays features seen close to a flare loop apex. They form before the flare maximum, and in LDEs they may last many hours. LTSs should be located close to the primary energy release site. Thus, they are a very promising tool for analysing energy release during the decay phase. The most demanding case for the energy release is long-lasting X-ray emission from LTSs seen in LDE flares. If a long-lasting X-ray source is thermal, then it must be continuously heated, because the characteristic radiative cooling time of hot (above 10MK) and dense (≈ 1010 cm-3) plasma is ≈ 1 hour. If an X-ray source is non-thermal then there should be a continuous acceleration of particles, to counteract fast thermalization of non-thermal electrons. The timescale of the thermalization in the sense of momentum loss of electrons is about several seconds.
Knowing all these we decided to do the search for how much energy can be produced during flares by studying LTSs of long duration flares. RHESSI satellite was chosen as a source of observational data because of its high sensitivity and high angular and energy resolutions. A sample of LDEs lasting at least 7 hours was selected.
How to see a weak source?
It is a large difficulty to reconstruct a RHESSI image when an emitting source is weak. And LTSs are weak at very late decay phase indeed. Usually RHESSI image reconstruction is performed for detectors nos. 3–6, 8, 9. However, we were not able to reconstruct any reliable image in a case of a very late phase of the LDE decay with the use of this standard set of grids. The weak signal was not the problem since by taking integration times up to several minutes we were able to collect enough counts, i.e. about several thousands. The problem may be solved if we remember that when a source size is comparable to the resolution of a particular grid, then the detector records very weak modulation or even no modulation of the signal (Hurford et al. 2002). The resulting image obtained for this grid is then dominated by noise. This grid added to the set used for reconstruction introduces noise only. In the worst case we may not get convergence of the reconstruction algorithm. Thus, we used only these grids, for which detector recorded significant modulation and reconstructed images showed a definite source in a single detector image.
The images used in further analysis were reconstructed with a PIXON algorithm in very narrow (1 keV) energy intervals. With described method of selection of detectors we were able to obtain good quality images up to 20 keV in the early decay and up to 10 keV in the very late decay of the LDE flares. It enabled us to obtain reliable fits to observed spectra of loot-top sources.
Searching for energy with RHESSI
To find out how effective energy release and heating processes are at the decay phase of LDEs we calculated the energy balance of the observed LTS based on physical parameters estimated through imaging spectroscopy. A change of thermal energy of a loop-top source is due to some processes that cool and heat plasma of the source. Three major cooling processes where included into our calculations: expansion, radiation and conduction. Knowing the change of LTS thermal energy and values of the three cooling processes we can calculate if and how efficiently the LTS was heated. The equation of energy balance can be written as follows:
observed change of thermal energy = adiabatic expansion − conductive cooling − radiative cooling + heating rate
Having the heating rate EH estimated we are able to calculate the total thermal energy deposited in an LTS during the whole decay of a flare. The calculation was done under the following assumptions: (a) duration of a flare was taken from GOES 1-8 Å lightcurve, (b) volume was calculated from the mean area of a source as seen in images, assuming a sphere with a constant volume during the whole decay phase. With the first assumption involved one obtains the lower limit of the total thermal energy released during the decay of LDEs.
Slow but energy hungry
Estimated values of heating rates EH are low, usually about 1 erg cm− 3 s− 1 at a very early stage of the decay and may drop below 102 erg cm-3 s-1 at the end of the decay, But EH is never equal to zero, i.e. LTSs need to be ‘fed‘ with energy continuously for many hours. Even with such a low heating rate an LTS of typical volume of the of 1028 cm3 existing for 10 hours needs in total about 1031 erg of thermal energy to be visible as it is. And 1031 erg is just the thermal energy released to sustain only long-lasting LTS. Energy released in LDEs surely is not deposited only in loop-top sources and only as thermal energy. Thus total energy ‘produced’ by an LDE during the decay must be larger than 1031 erg. This is huge amount of energy, which can be higher than energy released during the impulsive phase of a flare (Jiang et al., 2006). This severe requirement should be taken into account when building models of a solar flare. But a flare is not only the decay. Energy is also released during the rise.
A powerful example
As an example of how much energy can be generated during the whole flare we took the SOL2007-01-25T07:14 event. This was so-called slow long duration event (SLDE), i.e. it had not only slow decay but also slow rise. In total the flare lasted almost 18 hours (GOES 1-8 Å lightcurve). In the first part of “Slowly but surely towards the huge amount of energy I” we estimated that during the rise of the flare there was released at least 1.5×1031 erg of thermal energy. During the decay there was additional 2.2×1031 erg of thermal energy. Thus in total the event had to generate at least about 4×1031 erg, although it was ‘just’ C6.3 x-ray class flare.
Acknowledgements
We are grateful to members of our group Urszula Bąk-Stęślicka and Zbigniew Kołtun for their work, inspiring discussions and many critical comments.
References
[1] "RHESSI observations of long-duration flares with long-lasting X-ray loop-top sources"
RHESSI Nugget Date | 28 November 2011 + |
RHESSI Nugget First Author | Sylwester Kołomański + |
RHESSI Nugget Index | 164 + |
RHESSI Nugget Second Author | Tomasz Mrozek + |