Two phases of X-ray emission in a solar eruptive flare

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|first_author = Bhuwan Joshi
|first_author = Bhuwan Joshi
|publish_date = 1 February 2010
|publish_date = 1 February 2010
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== Introduction ==
== Introduction ==
-
Now it is a well known fact that solar eruptive phenomena are powered by magnetic energy stored in the magnetized loops in the solar corona. There is a universal consensus that during an eruptive event this stored energy is released explosively through the process of magnetic reconnection. Decades of observations of the Sun from ground based telescopes and satellites have revealed many important observational features during flare evolution which support the reconnection scenario. These features are: separation of ribbon-like bright chromospheric structures, motion of bright kernels in HXR images, and upward movement of coronal X-ray (or loop-top) source followed by the evolution of low temperature post flare loops. RHESSI observations have detected many new aspects during the rise phase of flare evolution. R. P. Lin and co-workers studied the HXR emission of X4.8 flare on 23rd July 2002 and discovered the HXR emission from a coronal non-thermal source before the impulsive phase. RHESSI observations have established a new phenomena of apparent downward motion of loop-top source in the early rise phase which was first reported by L. Sui & G. Holman in M1.2 flare on 15th April 2002. In a few recently studied events, it has been found that during the period of descending loop-top source motion, the distance between the conjugate HXR foot points decreases.   
+
Now it is a well known fact that solar eruptive phenomena are powered by the sudden release of excess energy stored in the magnetic field of [http://en.wikipedia.org/wiki/Corona solar corona].  
 +
There is also a near-universal consensus that in an eruptive event this stored energy appears during large-scale [http://en.wikipedia.org/wiki/Magnetic_reconnection magnetic reconnection].  
 +
Decades of observations of the Sun from ground-based telescopes and satellites have revealed many important observational features during flare evolution which support the reconnection scenario.  
 +
These features are: separation of ribbon-like bright chromospheric structures, the motion of bright kernels in HXR images, and the upward movement of coronal X-ray (or loop-top) source followed by the evolution of low temperature post flare loops.  
 +
The observations made by [http://en.wikipedia.org/wiki/RHESSI the Reuven Ramaty High Energy Spectroscopic Imager] (RHESSI) have revealed many new aspects during the rise phase of flare evolution.  
 +
Lin and co-workers studied the HXR emission of X4.8 flare on 23rd July 2002 and discovered a powerful [http://adsabs.harvard.edu/abs/2003ICRC....6.3207L coronal non-thermal source] before the impulsive phase.  
 +
Another new RHESSI phenomenon is the appearance of [http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=14 downward motions of X-ray loop-top source] in the early rise phase of a flare, as first reported by Sui & Holman in an M1.2 flare on 15th April 2002.  
 +
In a few recently studied events, it has also  been found that during the period of descending loop-top source motion, the distance between the conjugate HXR foot points decreases.   
-
The RHESSI observations of GOES M7.6 class flare on 24th October 2003 provide us an excellent opportunity to study various aspects of the flare evolution with imaging and spectroscopic analyses. The TRACE images of flaring region in 195 A and 1600 A channels and H-alpha filtergrams during the decay phase supplemented RHESSI data for this study.
+
The RHESSI observations of the GOES [http://www.iop.org/EJ/abstract/0004-637X/706/2/1438/ M7.6 class flare on 24th October 2003] provided us with an excellent opportunity to study various aspects of the flare evolution with imaging and spectroscopic analyses, which we describe briefly in this Nugget.  
 +
The [http://en.wikipedia.org/wiki/TRACE TRACE] images of flaring region in 195 and 1600 &Aring; channels and H<math>\alpha</math> filtergrams during the decay phase supplemented RHESSI data for this study.
== Event overview==
== Event overview==
-
[[Image:fig1_HESSI_NUG.jpeg|center|thumb|600px|Figure 1: RHESSI and GOES light curves of the flare. RHESSI counts rates are scaled by factors of 1, 1/4, 1/5, 1/10, and 1/50 for the energy bands 6-12, 12-25, 25-50, 50-100, and 100-300 keV, respectively.]]
+
[[Image:fig1_HESSI_NUG.jpeg|center|thumb|600px|Figure 1: RHESSI and GOES light curves of the flare. RHESSI counts rates are scaled by factors of 1, 1/4, 1/5, 1/10, and 1/50 for the energy bands 6-12, 12-25, 25-50, 50-100, and 100-300 keV, respectively.
 +
There is a dropout in the low bands at about 02:34 due to attenuator motion.
 +
]]
-
Figure 1 shows GOES and RHESSI light curves of the flare in various energy bands. The HXR emission during the initial phase (indicated as the 'first phase' in Figure 1 by horizontal line) comes from a loop-top source seen outside the limb of the Sun. Figure 2 shows a few representative RHESSI X-ray images showing loop-top and footpoint sources superimposed on co-temporal TRACE 195 A images. The downward motion of the X-ray loop-top source is evident from these images. The descending loop-top source is observed for ~11 minutes in X-ray images and moves with an average speed of ~15 km/s. About 5 minutes after the appearance of X-ray loop-top source, an intense brightening started at the top of TRACE 195 A loops, located below X-ray loop-top source. Earlier studies have shown that such brightening in TRACE 195 A images correspond to flare associated high temperature features and is interpreted as being due to the Fe XXIV contribution of plasma at a temperature around 15-20 MK. The EUV loop system with bright loop-top undergoes contraction with similar speed of downward motion as that of X-ray loop-top source.
+
Figure 1 shows [http://en.wikipedia.org/wiki/GOES GOES] and RHESSI light curves of the flare in various energy bands.  
 +
The HXR emission during the initial phase (indicated as the 'Phase I' in Figure 1 on the horizontal line at the top) comes from a loop-top source seen in projection above the limb of the Sun.  
 +
Figure 2 shows a few representative X-ray images showing loop-top and footpoint sources superimposed on co-temporal TRACE 195 &Aring; images. The downward motion of the X-ray loop-top source is evident from these images (see top row of Figure 2).  
 +
The descending loop-top source is observed for ~11 minutes and moves with an average speed of ~15 km/s.  
 +
About 5 minutes after the appearance of X-ray loop-top source, an intense brightening started at the top of TRACE 195 &Aring; loops, located below the X-ray loop-top source.  
 +
Earlier studies have shown that such a brightening in TRACE 195 &Aring; images correspond to an Fe XXIV contribution from plasma at a temperature around 15-20 MK (one MK being a million degrees), rather than the nominal lower-temperature contribution at about 2 MK.  
 +
The EUV loop system with bright loop-top undergoes contraction with similar speed of downward motion as that of X-ray loop-top source.
-
[[Image:fig2_HESSI_NUG.jpeg|center|thumb|750px|Figure 2: Sequence of TRACE 195 A images overlaid by co-temporal RHESSI X-ray images at 10-25 keV (white contours) and 50-100 keV (black contours). The contour levels are 50%, 70%, and 90% of the peak flux of each image.]]
+
[[Image:fig2_HESSI_NUG.jpeg|center|thumb|600px|Figure 2: Sequence of TRACE 195 &Aring; images overlaid by co-temporal RHESSI X-ray images at 10-25 keV (white contours) and 50-100 keV (black contours). The contour levels are 50%, 70%, and 90% of the peak flux of each image.]]
-
The HXR emission (>25 keV) during the main phase of the flare (indicated as the 'second phase' in Figure 1 by horizontal line) originated from two flare kernels observed on the solar disk. These kernels lie over the two bright flare ribbons visible in TRACE 1600 A and H-alpha images. Therefore, we recognize these two sources as the footpoints of a loop system. These footpoint sources can be understood in terms of thick-target model discussed in an [http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=44 earlier nugget]. The X-ray loop-top source now shows an upward motion below which H-alpha and UV post flare loop system evolves. This picture is consistent with the standard flare model.
+
The HXR emission (>25 keV) during the main phase of the flare (indicated as the 'Phase II' in Figure 1) originated from two flare kernels observed on the solar disk. These kernels lie over the two bright flare ribbons visible in TRACE 1600 &Aring; and H<math>\alpha</math> images. Therefore, we recognize these two sources as the footpoints of a loop system. These footpoint sources can be understood in terms of thick-target model discussed in an [http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=44 earlier nugget]. The X-ray loop-top source now shows an upward motion below which H<math>\alpha</math> and UV post flare loop system evolves. This picture is consistent with the standard flare model.
-
Finally, we make a note of X-ray spectrum of flare derived at different time intervals during the flare. The spectra during the first phase of the flare suggest hot thermal emission with temperatures T > 25 MK. At X-ray energies E > 20 keV, the spectrum shows a steep power law with photon spectral index <math>\gamma</math> > 7. During the second phase of the flare, the spectra show quite different characteristics. Emission up to >200 keV from footpoints are detectable and the spectrum at high energies follows hard power laws (gamma = 2.5-2.8).
+
Finally, we make a note of the X-ray spectral evolution.  
 +
The spectra during the first phase of the flare suggest hot thermal emission with temperatures T > 25 MK. At X-ray energies E > 20 keV, the spectrum shows a steep power law with photon spectral index <math>\gamma</math> > 7.  
 +
During the second phase of the flare, the spectra show quite different characteristics: emission up to >200 keV is detectable at the footpoints, and the spectrum at high energies follows hard power laws (<math>\gamma</math> = 2.5-2.8).
== RHESSI observations and rainbow reconnection model ==
== RHESSI observations and rainbow reconnection model ==
-
RHESSI observations have revealed a new phenomenon during the rise phase of flares in which the X-ray loop-top source exhibits a downward motion (see also [http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/Coronal_implosion 'coronal implosion']). In a few events, RHESSI observations have also confirmed the converging motion of HXR footpoints during the rise phase, simultaneously with the downward motion of LT source. The explanation of 'rise phase' is beyond the scope of standard model of solar flares. Recent observations suggest that the classical picture of flares described in standard flare model is valid after the impulsive phase when we observe the upward motion of loop-top source and separation of HXR foot points. The flare event of 24th October 2003, presented in this nugget, is one of the clearest examples of this scenario. In the following paragraphs we have made an attempt to discuss the RHESSI perceptive of 'rise phase' of flare in terms of a specific model called 'rainbow reconnection model'.
+
RHESSI observations have revealed a new phenomenon during the rise phase of flares in which the X-ray loop-top source exhibits a downward motion (see also an earlier Nugget on [http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/Coronal_implosion coronal implosion]). In a few events, RHESSI observations have also confirmed the converging motion of HXR footpoints during the rise phase, simultaneously with the downward motion of loop-top source. The explanation of 'rise phase' is beyond the scope of standard model of solar flares.  
 +
The observations we describe here suggest that the classical picture of flares described in standard flare model is valid after the impulsive phase, when we observe the upward motion of loop-top source and increasing separation of HXR footpoints.  
 +
The flare event of 24th October 2003, presented in this Nugget, is one of the clearest examples of this scenario. In the following paragraphs we have made an attempt to discuss the RHESSI perspective of 'rise phase' of flare in terms of a specific model called the [http://adsabs.harvard.edu/abs/2003AdSpR..32.2439S "rainbow reconnection model"].
-
The rainbow reconnection model, suggested by B. V. Somov [http://adsabs.harvard.edu/abs/2003AdSpR..32.2439S], considers two kinds of large scale photospheric flows before a large flare. First, the shear flows, which are parallel to the photospheric neutral line (PNL), increase the length of field lines in the corona and, therefore, produce an excess of magnetic energy. Second, the converging flows, i.e., the flows directed to the photospheric neutral line (PNL), create the pre-flare current layers in the corona and provide an excess of magnetic energy sufficient to produce a large flare.   
+
The rainbow reconnection model of [http://adsabs.harvard.edu/abs/2007plas.book.....S Somov] considers two kinds of large-scale photospheric flows before a large flare. First, the shear flows, which are parallel to the photospheric neutral line (where the polarity reverses), increase the length of field lines in the corona and, therefore, produce an excess of magnetic energy. Second, the converging flows, i.e., the flows directed to the neutral line, create the pre-flare current layers in the corona and provide an excess of magnetic energy sufficient to produce a large flare.   
-
[[Image:fig3_HESSI_NUG.jpeg|center|thumb|500px|Figure 3: Schematic representation of the flare evolution in terms of the rainbow reconnection model. (a) First phase: rapidly decreasing footpoint separation dominates an increase of the distance between the footpoints. (b) Second phase: bright HXR kernels separate in opposite directions from the PNL and from earch other. the loop-top source moves upward.
+
[[Image:fig3_HESSI_NUG.jpeg|center|thumb|400px|Figure 3: Schematic representation of the flare evolution in terms of the rainbow reconnection model. (a) First phase: rapidly decreasing footpoint separation dominates an increase of the distance between the footpoints. (b) Second phase: bright HXR kernels separate in opposite directions from the neutral line and from earch other. The loop-top source moves upward.
]]
]]
-
In Figure 3, we show a schematic representation of the flare evolution. To understand the motion of loop-top and foot point sources, we consider the three-dimensional magnetic reconnection at a separator in the corona. In figure 3(a) we depict the process in which the main role of reconnection is to release the excess of magnetic energy generated before a flare as a result of photospheric shear flows. We call this process as magnetic shear relaxation. The photosphere shear mainly dominates in the vicinity of the PNL. As a consequence of magnetic shear relaxation, a magnetic loop f1 evolves into loop f2 and therefore coronal HXR source shows an apparent downward motion. During this stage the footpoints of the magnetic loops show larger movement parallel to photospheric neutral line (i.e., delta y >> delta x) and we may observe converging motion of HXR foot point sources.  
+
In Figure 3, we show a schematic representation of the flare evolution. To understand the motion of loop-top and foot point sources, we consider the three-dimensional magnetic reconnection at a separator in the corona. In figure 3(a) we depict the process in which the main role of reconnection is to release the excess of magnetic energy generated before a flare as a result of photospheric shear flows. We call this process as magnetic shear relaxation. The photosphere shear mainly dominates in the vicinity of the neutral line. As a consequence of magnetic shear relaxation, a magnetic loop f1 evolves into loop f2 and therefore coronal HXR source shows an apparent downward motion. During this stage the footpoints of the magnetic loops show larger movement parallel to photospheric neutral line (i.e., <math>\delta</math> y >> <math>\delta</math> x) and we may observe converging motion of HXR foot point sources.  
-
Figure 3(b) shows another situation related to the second phase of the flare which is consistent with standard flare models. Here, flare involves reconnection of the magnetic field lines whose FPs are located at larger distance from the PNL and, therefore, have a smaller shear or practically none. As the reconnection involves field lines rooted successively apart from the PNL, the X-ray loop-top source shows the upward motion and HXR foot points separate from each other.
+
Figure 3(b) shows another situation related to the second phase of the flare which is consistent with standard flare models.  
 +
Here the flare involves reconnection of the magnetic field lines whose footpoints are located at larger distances from the neutral line and, therefore, have a smaller shear or practically none. As the reconnection involves field lines rooted successively apart from the PNL, the X-ray loop-top source shows the upward motion and HXR foot points separate from each other.
== Biographical note ==
== Biographical note ==
-
Bhuwan Joshi works at Udaipur Solar Observatory (Physical Research Laboratory), Udaipur, India. This nugget is based on a paper published in the Astrophysical Journal [http://adsabs.harvard.edu/abs/2009ApJ...706.1438J]. This work was carried out in collaboration with Astrid Veronig, B. V. Somov, K. -S. Cho, S. -C Bong, Y. -J. Moon, Jeongwoo Lee, P. K. Manoharan, and Y. -H. Kim.
+
Bhuwan Joshi works at Udaipur Solar Observatory ([http://en.wikipedia.org/wiki/Physical_Research_Laboratory Physical Research Laboratory]), [http://en.wikipedia.org/wiki/Udaipur Udaipur], India. This Nugget is based on a [http://www.iop.org/EJ/abstract/0004-637X/706/2/1438/ research article] published in [http://en.wikipedia.org/wiki/Astrophysical_Journal The Astrophysical Journal]. This work was carried out in collaboration with Astrid Veronig, B. V. Somov, K. -S. Cho, S. -C Bong, Y. -J. Moon, Jeongwoo Lee, P. K. Manoharan, and Y. -H. Kim.
[[Category:Nugget]]
[[Category:Nugget]]

Revision as of 16:35, 22 August 2018


Nugget
Number: 120
1st Author: Bhuwan Joshi
2nd Author:
Published: 1 February 2010
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Contents

Introduction

Now it is a well known fact that solar eruptive phenomena are powered by the sudden release of excess energy stored in the magnetic field of solar corona. There is also a near-universal consensus that in an eruptive event this stored energy appears during large-scale magnetic reconnection. Decades of observations of the Sun from ground-based telescopes and satellites have revealed many important observational features during flare evolution which support the reconnection scenario. These features are: separation of ribbon-like bright chromospheric structures, the motion of bright kernels in HXR images, and the upward movement of coronal X-ray (or loop-top) source followed by the evolution of low temperature post flare loops. The observations made by the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) have revealed many new aspects during the rise phase of flare evolution. Lin and co-workers studied the HXR emission of X4.8 flare on 23rd July 2002 and discovered a powerful coronal non-thermal source before the impulsive phase. Another new RHESSI phenomenon is the appearance of downward motions of X-ray loop-top source in the early rise phase of a flare, as first reported by Sui & Holman in an M1.2 flare on 15th April 2002. In a few recently studied events, it has also been found that during the period of descending loop-top source motion, the distance between the conjugate HXR foot points decreases.

The RHESSI observations of the GOES M7.6 class flare on 24th October 2003 provided us with an excellent opportunity to study various aspects of the flare evolution with imaging and spectroscopic analyses, which we describe briefly in this Nugget. The TRACE images of flaring region in 195 and 1600 Å channels and Hα filtergrams during the decay phase supplemented RHESSI data for this study.

Event overview

Figure 1: RHESSI and GOES light curves of the flare. RHESSI counts rates are scaled by factors of 1, 1/4, 1/5, 1/10, and 1/50 for the energy bands 6-12, 12-25, 25-50, 50-100, and 100-300 keV, respectively. There is a dropout in the low bands at about 02:34 due to attenuator motion.

Figure 1 shows GOES and RHESSI light curves of the flare in various energy bands. The HXR emission during the initial phase (indicated as the 'Phase I' in Figure 1 on the horizontal line at the top) comes from a loop-top source seen in projection above the limb of the Sun. Figure 2 shows a few representative X-ray images showing loop-top and footpoint sources superimposed on co-temporal TRACE 195 Å images. The downward motion of the X-ray loop-top source is evident from these images (see top row of Figure 2). The descending loop-top source is observed for ~11 minutes and moves with an average speed of ~15 km/s. About 5 minutes after the appearance of X-ray loop-top source, an intense brightening started at the top of TRACE 195 Å loops, located below the X-ray loop-top source. Earlier studies have shown that such a brightening in TRACE 195 Å images correspond to an Fe XXIV contribution from plasma at a temperature around 15-20 MK (one MK being a million degrees), rather than the nominal lower-temperature contribution at about 2 MK. The EUV loop system with bright loop-top undergoes contraction with similar speed of downward motion as that of X-ray loop-top source.

Figure 2: Sequence of TRACE 195 Å images overlaid by co-temporal RHESSI X-ray images at 10-25 keV (white contours) and 50-100 keV (black contours). The contour levels are 50%, 70%, and 90% of the peak flux of each image.

The HXR emission (>25 keV) during the main phase of the flare (indicated as the 'Phase II' in Figure 1) originated from two flare kernels observed on the solar disk. These kernels lie over the two bright flare ribbons visible in TRACE 1600 Å and Hα images. Therefore, we recognize these two sources as the footpoints of a loop system. These footpoint sources can be understood in terms of thick-target model discussed in an earlier nugget. The X-ray loop-top source now shows an upward motion below which Hα and UV post flare loop system evolves. This picture is consistent with the standard flare model.

Finally, we make a note of the X-ray spectral evolution. The spectra during the first phase of the flare suggest hot thermal emission with temperatures T > 25 MK. At X-ray energies E > 20 keV, the spectrum shows a steep power law with photon spectral index γ > 7. During the second phase of the flare, the spectra show quite different characteristics: emission up to >200 keV is detectable at the footpoints, and the spectrum at high energies follows hard power laws (γ = 2.5-2.8).

RHESSI observations and rainbow reconnection model

RHESSI observations have revealed a new phenomenon during the rise phase of flares in which the X-ray loop-top source exhibits a downward motion (see also an earlier Nugget on coronal implosion). In a few events, RHESSI observations have also confirmed the converging motion of HXR footpoints during the rise phase, simultaneously with the downward motion of loop-top source. The explanation of 'rise phase' is beyond the scope of standard model of solar flares. The observations we describe here suggest that the classical picture of flares described in standard flare model is valid after the impulsive phase, when we observe the upward motion of loop-top source and increasing separation of HXR footpoints. The flare event of 24th October 2003, presented in this Nugget, is one of the clearest examples of this scenario. In the following paragraphs we have made an attempt to discuss the RHESSI perspective of 'rise phase' of flare in terms of a specific model called the "rainbow reconnection model".

The rainbow reconnection model of Somov considers two kinds of large-scale photospheric flows before a large flare. First, the shear flows, which are parallel to the photospheric neutral line (where the polarity reverses), increase the length of field lines in the corona and, therefore, produce an excess of magnetic energy. Second, the converging flows, i.e., the flows directed to the neutral line, create the pre-flare current layers in the corona and provide an excess of magnetic energy sufficient to produce a large flare.

Figure 3: Schematic representation of the flare evolution in terms of the rainbow reconnection model. (a) First phase: rapidly decreasing footpoint separation dominates an increase of the distance between the footpoints. (b) Second phase: bright HXR kernels separate in opposite directions from the neutral line and from earch other. The loop-top source moves upward.

In Figure 3, we show a schematic representation of the flare evolution. To understand the motion of loop-top and foot point sources, we consider the three-dimensional magnetic reconnection at a separator in the corona. In figure 3(a) we depict the process in which the main role of reconnection is to release the excess of magnetic energy generated before a flare as a result of photospheric shear flows. We call this process as magnetic shear relaxation. The photosphere shear mainly dominates in the vicinity of the neutral line. As a consequence of magnetic shear relaxation, a magnetic loop f1 evolves into loop f2 and therefore coronal HXR source shows an apparent downward motion. During this stage the footpoints of the magnetic loops show larger movement parallel to photospheric neutral line (i.e., δ y >> δ x) and we may observe converging motion of HXR foot point sources.

Figure 3(b) shows another situation related to the second phase of the flare which is consistent with standard flare models. Here the flare involves reconnection of the magnetic field lines whose footpoints are located at larger distances from the neutral line and, therefore, have a smaller shear or practically none. As the reconnection involves field lines rooted successively apart from the PNL, the X-ray loop-top source shows the upward motion and HXR foot points separate from each other.

Biographical note

Bhuwan Joshi works at Udaipur Solar Observatory (Physical Research Laboratory), Udaipur, India. This Nugget is based on a research article published in The Astrophysical Journal. This work was carried out in collaboration with Astrid Veronig, B. V. Somov, K. -S. Cho, S. -C Bong, Y. -J. Moon, Jeongwoo Lee, P. K. Manoharan, and Y. -H. Kim.

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