Slowly but surely towards the huge amount of energy I

From RHESSI Wiki

(Difference between revisions)
Jump to: navigation, search
(draft, second installment)
(more entry)
Line 32: Line 32:
• most SLDEs occur in high or mid-high structures,
• 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),
+
• 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),
• the heating rate of plasma is small, below 1 erg cm<sup>−3</sup>s<sup>−1</sup> ,
• the heating rate of plasma is small, below 1 erg cm<sup>−3</sup>s<sup>−1</sup> ,
Line 42: Line 42:
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 for an energy-balance analysis.
 +
 +
== 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 (LTS), only. For each observed LTS we estimated size (area projected on image) and altitude above the photosphere. In the next step we performed spectroscopy analysis of each LTS, using the standard OSPEX package, to obtain its physical parameters.
 +
These parameters were used as an input data for calculation of the heating rate, EH.
 +
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 EH we calculated the characteristic time τ of the EH decrease rate after reaching its maximum value:

Revision as of 15:49, 25 October 2011


Nugget
Number: 162
1st Author: Urszula Bak-Stęślicka
2nd Author: Tomasz Mrozek
Published: 31 October 2011
Next Nugget: Slowly II
Previous Nugget: [1]
List all



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.

== 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 (LTS), only. For each observed LTS we estimated size (area projected on image) and altitude above the photosphere. In the next step we performed spectroscopy analysis of each LTS, using the standard OSPEX package, to obtain its physical parameters. These parameters were used as an input data for calculation of the heating rate, EH. 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 EH we calculated the characteristic time τ of the EH decrease rate after reaching its maximum value:


References

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

Personal tools
Namespaces
Variants
Actions
Navigation
Toolbox