Slowly but surely towards the huge amount of energy I

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|title = Slowly but surely towards the huge amount of energy I
|title = Slowly but surely towards the huge amount of energy I
|number = 162
|number = 162
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|first_author = Urszula Bak-Stęślicka
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|first_author = Urszula Bąk-Stęślicka
|second_author = Tomasz Mrozek, Sylwester Kołomański
|second_author = Tomasz Mrozek, Sylwester Kołomański
|publish_date = 31 October 2011
|publish_date = 31 October 2011
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|next_nugget = Slowly II
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|next_nugget={{#ask: [[Category:Nugget]] [[RHESSI Nugget Index::163]]}}
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|previous_nugget = [http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/Quasi-Periodic_Pulsations:_Fermi/GBM_Results]
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|previous_nugget={{#ask: [[Category:Nugget]] [[RHESSI Nugget Index::161]]}}
}}
}}
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Solar flares may have complicated structures and time developments, but since  
Solar flares may have complicated structures and time developments, but since  
-
[http://en.wikipedia.org/wiki/Apollo_Telescope_Mount Skylab] days we have distinguished "impulsive" and "gradual" classes.
+
[http://en.wikipedia.org/wiki/Apollo_Telescope_Mount 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.  
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.  
+
The decay phase of an LDE may last more than a day.  
-
Some of LDEs have also the unusually long rising phase that may last more than 30 minutes.  
+
Some LDEs may also have an unusually long rising phase that may lasts 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].  
+
We refer to this type of LDE as a "slow long-duration event" (SLDE).  
-
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  
+
During the rise phase of an SLDE there is no typical impulsive phase [[#References|[1]]].  
-
(see Figure 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 [http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=1 first X-class flare] was just such an event.
RHESSI's [http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=1 first X-class flare] was just such an event.
[[File:162f1.png|thumb|center|700px|'''Figure 1''':
[[File:162f1.png|thumb|center|700px|'''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.
+
RHESSI light curves (6-12 keV, 25-50 keV) for two LDE flares: Left: a long (50 min) rise phase and Right: a short (10 min) rise phase.
Note that SOL2002-04-21 (left) was RHESSI's first X-class flare observation.
Note that SOL2002-04-21 (left) was RHESSI's first X-class flare observation.
]]
]]
-
Chronologically the first studies of SLDEs were based on [http://solar.physics.montana.edu/sxt/ Yohkoh/SXT] images [2].  
+
Chronologically the first studies of SLDEs were based on [http://solar.physics.montana.edu/sxt/ Yohkoh/SXT] images [[#References|[2]]].  
These studies provided us with the basic observational characteristics of SLDEs:
These studies provided us with the basic observational characteristics of SLDEs:
-
<li> most SLDEs occur in high or mid-high structures,
+
::<li> most SLDEs occur in high or mid-high structures,
-
<li> loop-top sources are characterized by low temperature (T<10 MK), low density (N ≈ 10<sup>10</sup> cm<sup>−3</sup>) and  large size (r > 7 × 10<sup>8</sup> cm),
+
::<li> loop-top sources are characterized by low temperature (T<10 MK), low density (N ≈ 10<sup>10</sup> cm<sup>−3</sup>) and  large size (r > 7 × 10<sup>8</sup> cm),
-
<li> the heating rate of plasma is small, below 1 erg cm<sup>−3</sup>s<sup>−1</sup> ,
+
::<li> the heating rate of plasma is small, below 1 erg cm<sup>−3</sup>s<sup>−1</sup> and,
 +
 
 +
::<li> the heating rate decreases very slowly with time after reaching its maximum value.
-
<li> 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 [http://soi.stanford.edu/results/SolPhys200/Kosugi/index.html Yohkoh/HXT].  
A detailed analysis of SLDEs in the hard X-ray range requires high spectral resolution, much better than the resolution provided by the [http://soi.stanford.edu/results/SolPhys200/Kosugi/index.html 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 requirement is 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.  
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.
+
The parameters thus obtained (e.g., temperature and emission measure) can be used to investigate the energy balance/budget.
== Data Analysis ==
== Data Analysis ==
-
For our analysis we selected six limb or near-the-limb SLDEs, with rise phases lasted between 25 150 min.  
+
For our analysis we selected six limb or near-the-limb SLDEs, with rise phases lasting between 25 to 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.  
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.  
The images were reconstructed with the PIXON algorithm.  
We used time intervals of 20 – 40 s and narrow energy intervals of 1 – 2 keV.  
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.  
+
In our analysis we 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.  
+
For each such source we estimated the size (area projected on image) and altitude above the photosphere. Next, 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.  
+
These parameters were used as an input data for a 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:
+
It was assumed that a change in the thermal energy of a loop-top source is due to expansion, radiation, conduction (cooling processes) and heating. Expressed in the form of an equation,
  Observed change of thermal energy = adiabatic expansion – conductive cooling – radiative cooling + heating rate
  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 (E<sub>H</sub>) after it reaches its maximum value:
+
Knowing the change of the loop-top thermal energy and the values of the three cooling processes we can calculate how efficiently the source was heated.
 +
As an additional parameter describing the temporal evolution of the heating rate. we calculate the characteristic time, &tau;s, of the decrease of the heating rate (E<sub>H</sub>) after it reaches its maximum value:
[[File:162f1.5.png|thumb|center|600px|
[[File:162f1.5.png|thumb|center|600px|
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== Results ==
== Results ==
-
Our study (ref. [3]) dealt with six SLDEs, with the following results:
+
Our study [[#References|[3]]] dealt with six SLDEs, with the following results:
-
&#149; <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 character of the loop-top emission:</b> we find a thermal component to be dominant, however for four of the six events we observed significant non-thermal emission  
-
&#149; <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>,
+
::<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. This difference is caused by the fact that SXT had less sensitivity to higher-temperature plasma (> 10 MK).   
-
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).   
+
-
<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 &tau; is larger than 1000 s.  
+
::<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 the heating rate during this phase. In most cases the characteristic time &tau; is larger than 1000 s.  
-
<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.
+
::<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 throughout the entire rise phase and equal to its minimum measured value, (b) the loop-top volume was calculated from the mean area of the source, A, as seen in images, assuming a sphere with a constant volume 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 ==

Latest revision as of 17:10, 22 August 2018


Nugget
Number: 162
1st Author: Urszula Bąk-Stęślicka
2nd Author: Tomasz Mrozek, Sylwester Kołomański
Published: 31 October 2011
Next Nugget: X-ray and H-alpha Flare Impulses
Previous Nugget: Quasi-Periodic Pulsations: Fermi/GBM Results
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Contents

Introduction

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 event" (SLDE). During the rise 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 (10 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 and,
  • 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 is 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 to investigate the energy balance/budget.

    Data Analysis

    For our analysis we selected six limb or near-the-limb SLDEs, with rise phases lasting between 25 to 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 concentrated on coronal sources, so-called loop-top sources only. For each such source we estimated the size (area projected on image) and altitude above the photosphere. Next, we performed a spectroscopic analysis, using the standard RHESSI software, to obtain the physical parameters. These parameters were used as an input data for a calculation of the heating rate. It was assumed that a change in the thermal energy of a loop-top source is due to expansion, radiation, conduction (cooling processes) and heating. Expressed in the form of an equation,

    Observed change of thermal energy = adiabatic expansion – conductive cooling – radiative cooling + heating rate
    

    Knowing the change of the loop-top thermal energy and the values of the three cooling processes we can calculate how efficiently the source was heated. As an additional parameter describing the temporal evolution of the heating rate. we calculate the characteristic time, τs, of the decrease of the heating rate (EH) after it reaches its maximum value:

    162f1.5.png
    Figure 2: Left: RHESSI light curves in three energy bands for the flare SOL2005-09-06. Middle: a GOES/SXI image during the rise phase; the contours show RHESSI emission at 8-9 keV with 10% contour levels. Right: RHESSI spectrum of the loop-top source at 20:37 UT. Horizontal bars correspond to the energy bin widths. The fit consists of a thermal component and two spectral line complexes (at 6.7 keV and 8.0 keV).

    Results

    Our study [3] dealt with six SLDEs, with the following results:

  • The character of the loop-top emission: we find a thermal component to be dominant, however for four of the six events we observed significant non-thermal emission
  • The heating rate of the plasma: the values obtained are not high, usually below 10 erg cm3s-1, but larger than those obtained from the earlier SXT observations. This difference is caused by the fact that SXT had less sensitivity to higher-temperature plasma (> 10 MK).
  • The time evolution of the heating rate: the long duration of the rise phase of an SLDE is consistent with a very slow decrease of the heating rate during this phase. In most cases the characteristic time τ is larger than 1000 s.
  • The total thermal energy released during the rise phase: the low limit of the total energy released during the rising phase was calculated under the following assumptions: (a) the heating rate is constant throughout the entire rise phase and equal to its minimum measured value, (b) the loop-top volume was calculated from the mean area of the source, A, as seen in images, assuming a sphere with a constant volume during the whole rise phase. For the analyzed events we obtained at least about 1031 erg as the total thermal energy released during the rise phase.
  • 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"

    Facts about Slowly but surely towards the huge amount of energy IRDF feed
    RHESSI Nugget Date31 October 2011  +
    RHESSI Nugget First AuthorUrszula Bąk-Stęślicka  +
    RHESSI Nugget Index162  +
    RHESSI Nugget Second AuthorTomasz Mrozek, Sylwester Kołomański  +
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