Hard X-rays in Descent

From RHESSI Wiki

(Difference between revisions)
Jump to: navigation, search
 
(11 intermediate revisions not shown)
Line 2: Line 2:
|name = Nugget
|name = Nugget
|title = Hard X-rays in Descent
|title = Hard X-rays in Descent
-
|number = 200
+
|number = 201
|first_author = Aidan O'Flannagain
|first_author = Aidan O'Flannagain
|second_author = John Brown and Peter Gallagher
|second_author = John Brown and Peter Gallagher
-
|publish_date = May 6, 2013
+
|publish_date = May 27, 2013
-
|next_nugget =  
+
|next_nugget = [[Mars Odyssey/HEND and RHESSI]]
-
|previous_nugget = [[]]
+
|previous_nugget = [[A huge gamma-ray burst]]
}}
}}
== Electron Beams in the Chromosphere ==
== Electron Beams in the Chromosphere ==
-
[[File:Nugget200_figure1.png|right|thumb|Figure 1: An illustration of thick target model-based HXR emission in a semicircular flare loop. From top to bottom, the injected spectral index drops from 5 to 4, which results in a descent of peak emission.]]
+
[[File:Nugget200_figure1.png|right|thumb|Figure 1: A model illustration of HXR emission, showing thick-target interactions in a semicircular magnetic loop.  
 +
From top to bottom, the injected spectral index drops from 5 to 4, which results in an apparent descent of the locus of peak emission.]]
-
The most widely-used model used to interpret solar flare hard X-rays (HXRs) is the collisional thick target model (CTTM, Brown 71, Hudson 73). In this model, a population of electrons are accelerated in the corona, and propagate down to the chromosphere where they lose their energy to collisions and also emit bremsstrahlung as HXRs.
+
Solar flares almost universally involve hard X-ray bursts, which many Nuggets in this collection have discussed.
 +
These bursts are detected via [http://en.wikipedia.org/wiki/Bremsstrahlung bremsstrahlung], which contributes only negligibly to the slowing-down of the fast
 +
electrons that produce the radiation.
 +
Accordingly, a much larger amount of energy may remain undetected directly.
 +
This realization led to the most widely-used model used to interpret solar flare hard X-rays (HXRs), the collisional thick target model.  
 +
In this model, a population of electrons is accelerated in the corona; an electron beam forms and propagates down to the chromosphere where the energy can be lost their collisions.
 +
The HXR bremsstrahlung appears during this interaction.
-
However, there remain a number of predictions based on this model which have yet to be observed. One example is that of HXR source motion in response to variation in the spectrum of injected electrons; as a beam 'hardens', the bulk of HXR emission should descend, especially at low photon energies [fig 1]. In this nugget, we outline an attempt to make an observation of this previously unseen phenomenon.
+
However, there remain a number of predictions based on this model which have yet to be observed.  
 +
One example is that of HXR source motion in response to variation in the spectrum of injected electrons; as a beam 'hardens', the bulk of HXR emission should descend, especially at low photon energies (Figure 1).
 +
Such a spectral evolution is normal: it is the [http://solar.physics.montana.edu/nuggets/2001/010601/010601.html "soft-hard-soft"] pattern in which the peak of the hard X-ray burst usually has the hardest spectrum.
 +
In this Nugget, we outline an attempt to make an observation of this previously unseen phenomenon: the height variation of the HXR centroid during the burst evolution.
== Detection of Hard X-rays ==
== Detection of Hard X-rays ==
-
As this effect should be most easily observed at low photon energies – where thermal X-rays usually dominate the spectrum – we need to look at a specific type of flare known as an early impulsive event (Sui et al 2003). In these events, little plasma preheating takes place, and so early on, the spectrum is dominated at almost all energies by nonthermal bremsstrahlung. One such event, previously studied by Sui et al 2006, occurred on 28 November 2002.
+
As this effect should be most easily observed at low photon energies – where thermal X-rays usually dominate the spectrum – we need to look at a specific type of flare.
 +
The best candidates are known as [http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=47 "early impulsive"] events.  
 +
In these events, little plasma preheating may take place, and so, early on, the spectrum should be dominated at almost all energies by nonthermal bremsstrahlung.  
 +
One such event (already described by its discoverers; see Ref. [1]) is the event SOL2002-11-28.
-
As shown by RHESSI observations in Fig 2, this event did exhibit unique source motion. A source appears at the apparently looptop, splits into two, and descends down both legs of the loop until they reach the footpoints, around the time of the peak in HXRs. Spectral analysis of this event tells us that over this time interval, the index of the injection spectrum also drops from ~5 to ~4, constituting the commonly-observed spectral hardening seen in the early phase of a flare (e.g., Parks  1969). It is also indicated that the spectrum contains a very strong nonthermal component, and so the images are expected to reflect the behaviour of thick target emission.
+
As shown by RHESSI observations in Figure 2, this event did exhibit unique source motions.  
 +
A source initially appears at the apparently looptop, splits into two, and descends down both legs of the loop until the two parts reach the footpoints, around the time of the peak in HXRs.
 +
Spectral analysis of this event tells us that over this time interval, the index of the injection spectrum also drops from ~5 to ~4, consistent with the commonly-observed spectral hardening seen in the early phase of a flare.  
 +
The spectral analysis also shows that a very strong nonthermal component, and so the images are expected to reflect the behaviour of thick target emission.
 +
This is the advantage of an "early impulsive" event, since the bright thermal X-ray sources have not yet appeared.
[[File:Nugget200_figure2.png|center|frame|Figure 2: Top: Lightcurve of soft (3-6 keV, red) and hard (12-25 keV, blue) X-rays. Also shown is the spectral index of the injected electron distribution (dashed line). Bottom: RHESSI images of 3-6 keV emission, taken at the time indicated by vertical bars in the lightcurve.
[[File:Nugget200_figure2.png|center|frame|Figure 2: Top: Lightcurve of soft (3-6 keV, red) and hard (12-25 keV, blue) X-rays. Also shown is the spectral index of the injected electron distribution (dashed line). Bottom: RHESSI images of 3-6 keV emission, taken at the time indicated by vertical bars in the lightcurve.
Line 29: Line 46:
== Explanation ==
== Explanation ==
-
In order to understand this observed descent, a collisional model of electron beam propagation was used, very similar to that outlined in e.g. Brown et al 2002. Using this model, a comparsion was made between expected location of HXR peak emission, and to the observations made by RHESSI [Fig 3]. Because only collisions were taken into account, the only variable in this modelling process was the density model of the chromosphere. In order to explain the difference in descent rates between each photon energy, a variable scale height was required. This resulted in a density structure which was in agreement with previous studies.
+
[[File:Nugget201_figure4.png|right|thumb|350px|Figure 4: Density and scale height models required to match our model to the observed descending sources. The range of observed source heights is marked by the shaded region, with vertical lines denoting the approximate stopping depth of electrons in each of the modelled energy bands.]]
-
[[File:Nugget200_figure3.png|right|thumb|400px|Figure 3: Three phase loop lifetime shown by plots of plasma temperature, X-ray emission, loop width/corpulence and thermal pressure for each flare (left:23-Aug-2005, middle:14/15-Apr-2002 and right:21-May-2004). The pattern repeats for the 14/15-Apr-2002 flare due to the multiple X-ray peaks. The shaded orange bars denote each phase.]]
+
In order to understand this observed descent, we used a standard collisional model of electron beam propagation.
 +
This model allowed for a ready  comparison between the expected location of HXR peak emission, and to the observations made by RHESSI (Figure 3).
 +
Because only collisions were taken into account, the only assumption required in this modelling process is the density distribution of the chromosphere.
 +
In order to explain the difference in descent rates between each photon energy, a variable scale height was required.
 +
This resulted in a density structure which was in agreement with previous studies (Figure 4).
 +
 
 +
[[File:Nugget200_figure3.png|center|thumb|400px|Figure 3: Observed and modeled source heights during the increasing phase of the early impulsive burst of SOL2002-11-28. Note the systematic apparent downward motion.]]
== Closer to an Answer ==
== Closer to an Answer ==
-
Analysis of this event has allowed us to finally verify a prediction of the CTTM which has, until now,  not been observed. This observation is consistent with flare models that require a beam of electrons progpegating from the corona to the chromosphere. Alternative models exist, such as those which take into account transport of energy via Alfvén waves originating in the corona (e.g., Emslie & Sturrock 1982, Fletcher & Hudson 2008). The observations presented here can be used in future tests of these more recent theories. For a much more complete discussion, see O'Flannagain et al, A&A, accepted.
+
Analysis of this event has allowed us to finally verify a prediction of the collisional thick-target model which had previously not been observed.  
 +
This observation is consistent with flare models that require a beam of electrons propagating from the corona to the chromosphere.  
 +
Alternative models exist, such as those which take into account transport of energy via Alfvén waves originating in the corona, and studies such as this one are vital in trying to understand which mechanisms are at work.
 +
The observations presented here can be used in future tests of these more recent theories.  
 +
For a much more complete discussion, please see the full paper (Ref. [2]).
-
== References and links in the article ==
+
== References ==
-
[1] [http Title]
+
[1] [http://adsabs.harvard.edu/abs/2006ApJ...645L.157S Motion of 3-6 keV Nonthermal Sources along the Legs of a Flare Loop]
-
Acknowledgement.
+
[2] [http://adsabs.harvard.edu/abs/2013arXiv1305.1574O Solar Flare X-ray Source Motion as a Response to Electron Spectral Hardening]

Latest revision as of 17:31, 22 August 2018


Nugget
Number: 201
1st Author: Aidan O'Flannagain
2nd Author: John Brown and Peter Gallagher
Published: May 27, 2013
Next Nugget: Mars Odyssey/HEND and RHESSI
Previous Nugget: A huge gamma-ray burst
List all



Contents

Electron Beams in the Chromosphere

Figure 1: A model illustration of HXR emission, showing thick-target interactions in a semicircular magnetic loop. From top to bottom, the injected spectral index drops from 5 to 4, which results in an apparent descent of the locus of peak emission.

Solar flares almost universally involve hard X-ray bursts, which many Nuggets in this collection have discussed. These bursts are detected via bremsstrahlung, which contributes only negligibly to the slowing-down of the fast electrons that produce the radiation. Accordingly, a much larger amount of energy may remain undetected directly. This realization led to the most widely-used model used to interpret solar flare hard X-rays (HXRs), the collisional thick target model. In this model, a population of electrons is accelerated in the corona; an electron beam forms and propagates down to the chromosphere where the energy can be lost their collisions. The HXR bremsstrahlung appears during this interaction.

However, there remain a number of predictions based on this model which have yet to be observed. One example is that of HXR source motion in response to variation in the spectrum of injected electrons; as a beam 'hardens', the bulk of HXR emission should descend, especially at low photon energies (Figure 1). Such a spectral evolution is normal: it is the "soft-hard-soft" pattern in which the peak of the hard X-ray burst usually has the hardest spectrum. In this Nugget, we outline an attempt to make an observation of this previously unseen phenomenon: the height variation of the HXR centroid during the burst evolution.

Detection of Hard X-rays

As this effect should be most easily observed at low photon energies – where thermal X-rays usually dominate the spectrum – we need to look at a specific type of flare. The best candidates are known as "early impulsive" events. In these events, little plasma preheating may take place, and so, early on, the spectrum should be dominated at almost all energies by nonthermal bremsstrahlung. One such event (already described by its discoverers; see Ref. [1]) is the event SOL2002-11-28.

As shown by RHESSI observations in Figure 2, this event did exhibit unique source motions. A source initially appears at the apparently looptop, splits into two, and descends down both legs of the loop until the two parts reach the footpoints, around the time of the peak in HXRs. Spectral analysis of this event tells us that over this time interval, the index of the injection spectrum also drops from ~5 to ~4, consistent with the commonly-observed spectral hardening seen in the early phase of a flare. The spectral analysis also shows that a very strong nonthermal component, and so the images are expected to reflect the behaviour of thick target emission. This is the advantage of an "early impulsive" event, since the bright thermal X-ray sources have not yet appeared.

Figure 2: Top: Lightcurve of soft (3-6 keV, red) and hard (12-25 keV, blue) X-rays. Also shown is the spectral index of the injected electron distribution (dashed line). Bottom: RHESSI images of 3-6 keV emission, taken at the time indicated by vertical bars in the lightcurve.

Explanation

Figure 4: Density and scale height models required to match our model to the observed descending sources. The range of observed source heights is marked by the shaded region, with vertical lines denoting the approximate stopping depth of electrons in each of the modelled energy bands.

In order to understand this observed descent, we used a standard collisional model of electron beam propagation. This model allowed for a ready comparison between the expected location of HXR peak emission, and to the observations made by RHESSI (Figure 3). Because only collisions were taken into account, the only assumption required in this modelling process is the density distribution of the chromosphere. In order to explain the difference in descent rates between each photon energy, a variable scale height was required. This resulted in a density structure which was in agreement with previous studies (Figure 4).

Figure 3: Observed and modeled source heights during the increasing phase of the early impulsive burst of SOL2002-11-28. Note the systematic apparent downward motion.

Closer to an Answer

Analysis of this event has allowed us to finally verify a prediction of the collisional thick-target model which had previously not been observed. This observation is consistent with flare models that require a beam of electrons propagating from the corona to the chromosphere. Alternative models exist, such as those which take into account transport of energy via Alfvén waves originating in the corona, and studies such as this one are vital in trying to understand which mechanisms are at work. The observations presented here can be used in future tests of these more recent theories. For a much more complete discussion, please see the full paper (Ref. [2]).

References

[1] Motion of 3-6 keV Nonthermal Sources along the Legs of a Flare Loop

[2] Solar Flare X-ray Source Motion as a Response to Electron Spectral Hardening

Facts about Hard X-rays in DescentRDF feed
RHESSI Nugget Date27 May 2013  +
RHESSI Nugget First AuthorAidan O'Flannagain  +
RHESSI Nugget Index201  +
RHESSI Nugget Second AuthorJohn Brown and Peter Gallagher  +
Personal tools
Namespaces
Variants
Actions
Navigation
Toolbox