Hard X-ray Emission from Partially Occulted Solar Flares
From RHESSI Wiki
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- | + | == Introduction == | |
Hard X-ray emission of solar flares can enable insights | Hard X-ray emission of solar flares can enable insights | ||
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- | [[File:291f1.png| | + | [[File:291f1.png|500px|thumb|center| |
Figure 1: The solar disk positions of the partially occulted flares | Figure 1: The solar disk positions of the partially occulted flares | ||
selected (C-Class: dark green; M-Class: dark blue; X-Class: light | selected (C-Class: dark green; M-Class: dark blue; X-Class: light | ||
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]] | ]] | ||
- | + | == Occulted Flare Selection and Results == | |
An earlier analysis studied 55 partially occulted flares from solar cycle | An earlier analysis studied 55 partially occulted flares from solar cycle | ||
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X-rays. | X-rays. | ||
- | + | == Spectral Analysis == | |
Two kinds of spectral analysis were performed: | Two kinds of spectral analysis were performed: | ||
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spectrum. | spectrum. | ||
- | [[File:291f2.png| | + | [[File:291f2.png|500px|thumb|center| |
Figure 2: Correlation between the broken power-law photon spectral | Figure 2: Correlation between the broken power-law photon spectral | ||
index γ and the electron κ index values of the thermal | index γ and the electron κ index values of the thermal | ||
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fitting parameters gives δ = 0.83κ -1.15 (black dashed line). | fitting parameters gives δ = 0.83κ -1.15 (black dashed line). | ||
- | == Spatial separation of thermal and non-thermal emission == | + | |
+ | === Spatial separation of thermal and non-thermal emission === | ||
Images in a low 6-14 keV and high-energy (>20 keV) range | Images in a low 6-14 keV and high-energy (>20 keV) range | ||
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between the maximum of both sources (see Figure 2). | between the maximum of both sources (see Figure 2). | ||
- | + | [[File:291f3.png|700px|thumb|center| | |
- | [[File:291f3.png| | + | |
Figure 3: SDO/AIA 131 Å: emission and RHESSI X-ray at 614 keV | Figure 3: SDO/AIA 131 Å: emission and RHESSI X-ray at 614 keV | ||
(red) and 2230 keV (blue) contours for the M1.4 class flare | (red) and 2230 keV (blue) contours for the M1.4 class flare | ||
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corona. | corona. | ||
- | == Light-curves and the Neupert effect == | + | === Light-curves and the Neupert effect === |
We analyzed the time evolution of the hard X-ray flux measured by | We analyzed the time evolution of the hard X-ray flux measured by | ||
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SOL2014-09-11 (M2.1) | SOL2014-09-11 (M2.1) | ||
at four energy ranges (red, blue, | at four energy ranges (red, blue, | ||
- | purple and green). The | + | purple and green). The GOES high energy (0.5-4 Å) and low |
energy (1-8 Å) fluxes are plotted as solid and dashed grey | energy (1-8 Å) fluxes are plotted as solid and dashed grey | ||
lines, while their derivatives are given by the respective black | lines, while their derivatives are given by the respective black | ||
- | lines. The counts in the two high energy | + | lines. The counts in the two high energy RHESSI channels are |
multiplied by 20 and 15, respectively, to make them comparable in | multiplied by 20 and 15, respectively, to make them comparable in | ||
magnitude. Right: Correlation coefficients for the complete ensemble | magnitude. Right: Correlation coefficients for the complete ensemble | ||
of partially occulted flares as calculated from the light curve | of partially occulted flares as calculated from the light curve | ||
- | cross-correlation analysis of the | + | cross-correlation analysis of the GOES soft X-ray time |
derivative (low channel: red; high channel: blue) and the | derivative (low channel: red; high channel: blue) and the | ||
]] | ]] | ||
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flares in the sample. | flares in the sample. | ||
- | == | + | == Conclusions == |
+ | |||
The three parts of the study of occulted flares allow us to conclude | The three parts of the study of occulted flares allow us to conclude | ||
- | + | ||
- | + | • Spectra of occulted flares tend to be softer than general disk flares with the relation between the photon and electron indexes in rough agreement with that expected in a thin-target model; | |
- | + | ||
- | + | • There is no clear trend for large positive or negative radial separations between low and high energy hard X-ray components. There are, however, notable exceptions with larger separations and a richer coronal source structure; | |
+ | |||
+ | • There is a significant correlation between the time derivative of the soft X-ray and the observed hard X-rays light curves for a large fraction of our sample, consistent with earlier studies for on-disk flares (Ref. [5]). | ||
This latter conclusions indicates the | This latter conclusions indicates the | ||
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can play important roles. | can play important roles. | ||
- | + | == References == | |
[1] [http://adsabs.harvard.edu/abs/2016arXiv161202856E "Hard X-Ray Emission from Partially Occulted Solar Flares: RHESSI Observations in Two Solar Cycles"] | [1] [http://adsabs.harvard.edu/abs/2016arXiv161202856E "Hard X-Ray Emission from Partially Occulted Solar Flares: RHESSI Observations in Two Solar Cycles"] |
Revision as of 17:08, 27 January 2017
Nugget | |
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Number: | 291 |
1st Author: | Frederic Effenberger |
2nd Author: | and Fatima Rubio da Costa |
Published: | 30 January 2017 |
Next Nugget: | Eclipses |
Previous Nugget: | GOES Hard X-rays? |
List all |
Contents |
Introduction
Hard X-ray emission of solar flares can enable insights into the non-thermal energetic particle properties created during these eruptive events. Depending on the coronal densities and plasma properties, a part of the non-thermal electron population can be trapped at the loop-top. Theory suggest that the coronal region at the loop-top is the main acceleration site for electrons. At high energies, however, the bright footpoint emission from the flare loops can prevent a detailed analysis of the usually weaker loop-top source due to the limited dynamic range of instruments like RHESSI. Thus, flares close to the solar limb, where the footpoints are occulted, are interesting events to study because they can reveal the emission in this region in isolation. Here, we report on recent findings of a statistical study of such events, comprising of about 120 flares during solar cycles 23 and 24 (Ref. [1]).
It is interesting to also study the so-called Neupert effect (Ref. [2]) in this context of coronal sources. This is an empirically observed correlation in solar flares, which relates the changes (i.e. the time derivative) in the soft X-ray (SXR) flux to the temporal hard X-ray (HXR) variations. It is usually assumed that the HXRs originate in strong footpoint emissions from accelerated electron beams hitting the transition region and chromosphere. This in turn can lead to heating processes, increasing the overall SXR and extreme ultraviolet (EUV) emission, as observed by the GOES satellites. Here, we interestingly found this relation to hold also for a large fraction of flares when only observed as coronal sources above the limb.
Occulted Flare Selection and Results
An earlier analysis studied 55 partially occulted flares from solar cycle 23 were already analyzed in Ref. [3]. We extended this list with selection criteria based on flares from the RHESSI list with significant counts at energies of 25 keV and occulted footpoints, having their centroid position above the solar limb. We studied 61 flares from solar cycle 24 in addition to the 55 from cycle 23. Figure 1 shows their distribution on the solar disk.
Using GOES, SDO/AIA, and RHESSI we performed a spectral analysis, a light curve correlation between different wavelength ranged, and X-ray imaging to study the spatial relation between soft and hard X-rays.
Spectral Analysis
Two kinds of spectral analysis were performed:
The first is a fit of the observed photon spectrum by a thermal plus broken power-law model. The second method fits the observed photon spectrum to a thermal plus a kappa spectral model for the flux of accelerated electrons.
Figure 2 shows the resulting spectral index κ versus the photon index γ. The mean values are <γ> 5.7, similar to the previously reported value of 5.4, and <κ> = 5.4. These are softer than what is found for the high-energy index of disk flares, which contain the footpoint emission with harder spectrum.
The relation between the electron flux spectral index δ and the electron kappa distribution index $\kappa$ is δ = κ. In the thin-target case, we expect δ = κ - 1 (the green dotted line in Figure 2). A linear-least-square fit to the fitting parameters gives δ = 0.83κ -1.15 (black dashed line).
Spatial separation of thermal and non-thermal emission
Images in a low 6-14 keV and high-energy (>20 keV) range allow us to estimate the radial separation between the thermal and non-thermal emission, by measuring the distance dmax between the maximum of both sources (see Figure 2).
Although we did not find a clear tendency toward positive or negative separations between both sources,<dmax> = 0.3 Mm, which may imply that higher-energy emission is radially farther out in the corona.
Light-curves and the Neupert effect
We analyzed the time evolution of the hard X-ray flux measured by RHESSI and compared it with the temporal derivative of the soft X-ray flux measured by GOES at (0.5-4 Å). We calculated the linear correlation between soft and hard X-rays.
In Figure 4 we find that the 25-50 keV RHESSI energy channel correlates well with the GOES time derivative during the rise phase, with a small temporal lag. The statistical results for the correlation coefficient are significant for a large fraction of flares in the sample.
Conclusions
The three parts of the study of occulted flares allow us to conclude
• Spectra of occulted flares tend to be softer than general disk flares with the relation between the photon and electron indexes in rough agreement with that expected in a thin-target model;
• There is no clear trend for large positive or negative radial separations between low and high energy hard X-ray components. There are, however, notable exceptions with larger separations and a richer coronal source structure;
• There is a significant correlation between the time derivative of the soft X-ray and the observed hard X-rays light curves for a large fraction of our sample, consistent with earlier studies for on-disk flares (Ref. [5]).
This latter conclusions indicates the presence of the simple Neupert effect for purely coronal sources and supports the scenario that the main source of non-thermal particles is produced near the looptop. The lags found in some flares indicate that additional processes like thermal conduction can play important roles.
References
[1] "Hard X-Ray Emission from Partially Occulted Solar Flares: RHESSI Observations in Two Solar Cycles"
[2] "Comparison of Solar X-Ray Line Emission with Microwave Emission during Flares"
[3] "Hard X-ray emission from the solar corona"
[4] "Kappa distribution and hard X-ray emission of solar flares"
[5] "Investigation of the Neupert effect in solar flares"
RHESSI Nugget Date | 30 January 2017 + |
RHESSI Nugget First Author | Frederic Effenberger + |
RHESSI Nugget Index | 291 + |
RHESSI Nugget Second Author | and Fatima Rubio da Costa + |