Electron acceleration and hard X-ray emission from SOL2013-11-09
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|second_author = Galina Motorina | |second_author = Galina Motorina | ||
|publish_date = 5 May 2016 | |publish_date = 5 May 2016 | ||
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- | + | == Introduction == | |
The acceleration of high-energy particles has long been recognized as | The acceleration of high-energy particles has long been recognized as | ||
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Until now it has not been clear exactly where the electron acceleration region | Until now it has not been clear exactly where the electron acceleration region | ||
is located during the flare energy release. | is located during the flare energy release. | ||
- | It is usually considered that electrons are accelerated in the corona. | + | It is usually considered that electrons are accelerated in the |
- | In particular, they can be accelerated in the region of the | + | [https://en.wikipedia.org/wiki/Corona solar corona]. |
+ | In particular, they can be accelerated in the region of the top | ||
+ | of a [https://en.wikipedia.org/wiki/Coronal_loop magnetic loop] structure | ||
(models with a compact acceleration region) or in the whole loop | (models with a compact acceleration region) or in the whole loop | ||
(models with an extended acceleration region). | (models with an extended acceleration region). | ||
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This Nugget is devoted to the analysis of the hard X-ray emission | This Nugget is devoted to the analysis of the hard X-ray emission | ||
from different parts of a flare loop based on RHESSI observations | from different parts of a flare loop based on RHESSI observations | ||
- | for the well-studied | + | for the well-studied solar flare of [[Has event date::2013-Nov-09 06:26:09]], |
+ | [http://sprg.ssl.berkeley.edu/~tohban/browser/?show=grth1+grth3+qlpcr+qli02+qli04+fergo+rms4a&date=20131109&time=062609&bar=1 SOL2013-11-09 (C1.2)] [1,2]. | ||
- | + | == Relationships between hard X-ray emissions from different parts of a flare loop == | |
The hard X-ray images (Figure 1) of the solar flare SOL2013-11-09 (C2.6) | The hard X-ray images (Figure 1) of the solar flare SOL2013-11-09 (C2.6) | ||
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SDO/AIA 171 Å image (in inverted colors) | SDO/AIA 171 Å image (in inverted colors) | ||
overlaid with RHESSI images at 4.1-4.9~keV (white) near the peak | overlaid with RHESSI images at 4.1-4.9~keV (white) near the peak | ||
- | of the impulsive phase, with contours at 50, 70 and 90% of the | + | of the impulsive phase, with contours at 50, 60, 70 and 90% of the |
maximum of each image, and at 23.0-27.5 keV (blue, levels 50, | maximum of each image, and at 23.0-27.5 keV (blue, levels 50, | ||
- | 60, 70, | + | 60, 70, and 90% of the image maximum) integrated for ~300 s, |
covering the entire main impulsive phase (time intervals | covering the entire main impulsive phase (time intervals | ||
indicated in the figure legend). | indicated in the figure legend). | ||
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The RHESSI imaging appears to resolve this looptop source as a | The RHESSI imaging appears to resolve this looptop source as a | ||
- | dense [1], [2] | + | dense structure [1], [2], with estimates of n<sub>e</sub> ~ 3 x 10<sup>11</sup> cm<sup>-3</sup> |
and a characteristic size L ~ 9-18 arcsec. | and a characteristic size L ~ 9-18 arcsec. | ||
- | These | + | These estimates apply at the time of maximum of the hard X-ray emission (06:25:41~UT) |
in terms of the simultaneous | in terms of the simultaneous | ||
[http://global.jaxa.jp/projects/sat/solar_b/ Hinode]/[http://solarb.mssl.ucl.ac.uk/SolarB/Solar-B.jsp EIS] | [http://global.jaxa.jp/projects/sat/solar_b/ Hinode]/[http://solarb.mssl.ucl.ac.uk/SolarB/Solar-B.jsp EIS] | ||
observations [1]. | observations [1]. | ||
The high plasma density suggests that accelerated | The high plasma density suggests that accelerated | ||
- | electrons | + | electrons must lose the kinetic energy quite rapidly while |
propagating from the looptop to the footpoints. | propagating from the looptop to the footpoints. | ||
In particular, supposing the | In particular, supposing the | ||
- | coronal loop column density | + | coronal loop column density N = nL/2 = 1-2 x 10<sup>20</sup> cm<sup>-2</sup>, and that |
- | 1-2 x 10<sup>20</sup> cm<sup>-2</sup>, and that | + | |
the cosine between the direction of the | the cosine between the direction of the | ||
magnetic field and the electron velocity μ = 0.5, we find that | magnetic field and the electron velocity μ = 0.5, we find that | ||
accelerated electrons will be collisionaly stopped in the coronal | accelerated electrons will be collisionaly stopped in the coronal | ||
- | part of a flare loop if their energy | + | part of a flare loop if their energy E < E<sub>loop</sub> ~ 10 (N<sub>19</sub>/μ)<sup>1/2</sup> = 45-63 keV, |
- | E<sub>loop</sub> ~ 10< | + | where N<sub>19</sub> = N/10<sup>19</sup> |
- | + | ||
- | + | ||
The relationship between the spectral fluxes of hard X-ray | The relationship between the spectral fluxes of hard X-ray | ||
- | emission at the coronal part | + | emission at the coronal part I<sub>lp</sub> and the footpoints I<sub>fp</sub>, |
- | as | + | as predicted by the |
+ | [http://solarphysics.livingreviews.org/open?pubNo=lrsp-2008-1&page=articlesu7.html collisional thick target] model, can be | ||
represented as [3] | represented as [3] | ||
- | [[File:273f2.png|500px|thumb|center | + | [[File:273f2.png|500px|thumb|center]] |
- | ]] | + | |
- | where B(a, b, c) is an incomplete beta function and delta is the | + | where B(a, b, c) is an incomplete |
- | spectral index of the | + | [https://en.wikipedia.org/wiki/Beta_function beta function] and δ is the |
- | Then, adopting delta = 5.6 and E<sub>loop</sub> = 0.5, we | + | spectral index of the electron flux. |
+ | Then, adopting delta = 5.6 and ε/E<sub>loop</sub> = 0.5, we | ||
find I<sub>lp</sub>/ I<sub>fp</sub> ~ 11. | find I<sub>lp</sub>/ I<sub>fp</sub> ~ 11. | ||
The estimate thus obtained contradicts the | The estimate thus obtained contradicts the | ||
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solar flare model requires at least some modifications. | solar flare model requires at least some modifications. | ||
- | + | == Conclusions == | |
The discrepancy between the model estimates and hard X-ray | The discrepancy between the model estimates and hard X-ray | ||
observations suggests that electron acceleration can occur not | observations suggests that electron acceleration can occur not | ||
only in the coronal part of a loop but also in the footpoints. | only in the coronal part of a loop but also in the footpoints. | ||
- | This inference | + | This inference agrees well with the idea of re-acceleration [4] |
- | + | and in addition with the solar flare model proposed by Zaitsev and | |
Stepanov [5]; see also [6]. | Stepanov [5]; see also [6]. | ||
Note that an extended acceleration region in the corona does not | Note that an extended acceleration region in the corona does not | ||
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in this case. | in this case. | ||
+ | == References == | ||
- | + | # [http://adsabs.harvard.edu/abs/2015A%26A...577A..68S "Direct observation of the energy release site in a solar flare by SDO/AIA, Hinode/EIS, and RHESSI"] | |
- | + | # [http://adsabs.harvard.edu/abs/2015SoPh..290.3573S "Impulsive heating of solar flare ribbons above 10 MK"] | |
- | + | # [http://adsabs.harvard.edu/abs/2004ApJ...603L.117V "A coronal thick-target interpretation of two hard X-ray loop events"] | |
- | + | # [http://adsabs.harvard.edu/abs/2009A%26A...508..993B "Local re-acceleration and a modified thick target model of solar flare electrons"] | |
- | + | # [http://adsabs.harvard.edu/abs/2015SoPh..290.3559Z "Particle acceleration and plasma heating in the chromosphere"] | |
- | + | # [http://adsabs.harvard.edu/abs/1998ARep...42..275T "The stochastic acceleration of upper chromospheric electrons"] | |
- | + | ||
- | + | ||
- | + | ||
- | [ | + | [[Has observation by:: RHESSI| ]] |
+ | [[Has observation by:: SDO AIA| ]] |
Latest revision as of 14:52, 21 September 2018
Nugget | |
---|---|
Number: | 273 |
1st Author: | Yuri Tsap |
2nd Author: | Galina Motorina |
Published: | 5 May 2016 |
Next Nugget: | Data-driven radiative hydrodynamic modeling of SOL2014-03-29 |
Previous Nugget: | Extreme events, stellar evolution, and magnetic reconnection |
List all |
Contents |
Introduction
The acceleration of high-energy particles has long been recognized as one of the fundamental characteristics of a solar flare.
Until now it has not been clear exactly where the electron acceleration region is located during the flare energy release. It is usually considered that electrons are accelerated in the solar corona. In particular, they can be accelerated in the region of the top of a magnetic loop structure (models with a compact acceleration region) or in the whole loop (models with an extended acceleration region). In our view, we can shed light on this issue based on the relationship between hard X-ray emissions from the coronal part (looptop) of a flare loop and its footpoints.
This Nugget is devoted to the analysis of the hard X-ray emission from different parts of a flare loop based on RHESSI observations for the well-studied solar flare of 2013-Nov-09 06:26:09, SOL2013-11-09 (C1.2) [1,2].
Relationships between hard X-ray emissions from different parts of a flare loop
The hard X-ray images (Figure 1) of the solar flare SOL2013-11-09 (C2.6) show three sources [1]. One of them was located between two others and characterized by the strong hard X-ray emission dominated in some time intervals. This appears to correspond to the looptop.
The RHESSI imaging appears to resolve this looptop source as a dense structure [1], [2], with estimates of ne ~ 3 x 1011 cm-3 and a characteristic size L ~ 9-18 arcsec. These estimates apply at the time of maximum of the hard X-ray emission (06:25:41~UT) in terms of the simultaneous Hinode/EIS observations [1]. The high plasma density suggests that accelerated electrons must lose the kinetic energy quite rapidly while propagating from the looptop to the footpoints. In particular, supposing the coronal loop column density N = nL/2 = 1-2 x 1020 cm-2, and that the cosine between the direction of the magnetic field and the electron velocity μ = 0.5, we find that accelerated electrons will be collisionaly stopped in the coronal part of a flare loop if their energy E < Eloop ~ 10 (N19/μ)1/2 = 45-63 keV, where N19 = N/1019
The relationship between the spectral fluxes of hard X-ray emission at the coronal part Ilp and the footpoints Ifp, as predicted by the collisional thick target model, can be represented as [3]
where B(a, b, c) is an incomplete beta function and δ is the spectral index of the electron flux. Then, adopting delta = 5.6 and ε/Eloop = 0.5, we find Ilp/ Ifp ~ 11. The estimate thus obtained contradicts the observed intensities of the hard X-ray sources in the energy range 23.0-27.5 keV (see Fig.1) and demonstrates that the standard solar flare model requires at least some modifications.
Conclusions
The discrepancy between the model estimates and hard X-ray observations suggests that electron acceleration can occur not only in the coronal part of a loop but also in the footpoints. This inference agrees well with the idea of re-acceleration [4] and in addition with the solar flare model proposed by Zaitsev and Stepanov [5]; see also [6]. Note that an extended acceleration region in the corona does not significantly improve the relationship (1) since the generation of the hard X-ray emission in the coronal part will be more effective in this case.
References
- "Direct observation of the energy release site in a solar flare by SDO/AIA, Hinode/EIS, and RHESSI"
- "Impulsive heating of solar flare ribbons above 10 MK"
- "A coronal thick-target interpretation of two hard X-ray loop events"
- "Local re-acceleration and a modified thick target model of solar flare electrons"
- "Particle acceleration and plasma heating in the chromosphere"
- "The stochastic acceleration of upper chromospheric electrons"
Has event date | 9 November 2013 06:26:09 + |
Has observation by | RHESSI +, and SDO AIA + |
RHESSI Nugget Date | 5 May 2016 + |
RHESSI Nugget First Author | Yuri Tsap + |
RHESSI Nugget Index | 273 + |
RHESSI Nugget Second Author | Galina Motorina + |