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{{Infobox Nugget
{{Infobox Nugget
|name = Nugget
|name = Nugget
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|title = Hard X-ray Spikes: how fast are they and should they be?
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|title = Hard X-ray Spikes Observed by RHESSI
|number = ???
|number = ???
|first_author = Jiong Qiu
|first_author = Jiong Qiu
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Solar flares are characterized by impulsively enhanced emissions most pronounced in hard X-ray and microwave light curves. These light curves in even a simple, single, short-lived flare usually exhibit quite complicated structures consisting of numerous fast-varying bursts,  unlike the rather smooth and gradual soft X-ray emissions. Ground-based and space missions in pre-RHESSI era have revealed  the shortest time scales, from a few seconds to several tens of milli-seconds, of these bursts. A few studies correlating high cadence yet spatially unresolved hard X-ray observations with high cadence high resolution optical observations have further shown that hard X-ray bursts are correlated with optical emission on time scales of about one second, suggesting that these fast-varying bursts are thick-target emissions, and their size is within one arc-second. This is, in general, consistent with recent EUV observations showing that the size of a flare loop is about one arc-second. It may be noted that these quoted values are likely upper bounds of the tempo-spatial scales as limited by the resolving capability of instruments.
Solar flares are characterized by impulsively enhanced emissions most pronounced in hard X-ray and microwave light curves. These light curves in even a simple, single, short-lived flare usually exhibit quite complicated structures consisting of numerous fast-varying bursts,  unlike the rather smooth and gradual soft X-ray emissions. Ground-based and space missions in pre-RHESSI era have revealed  the shortest time scales, from a few seconds to several tens of milli-seconds, of these bursts. A few studies correlating high cadence yet spatially unresolved hard X-ray observations with high cadence high resolution optical observations have further shown that hard X-ray bursts are correlated with optical emission on time scales of about one second, suggesting that these fast-varying bursts are thick-target emissions, and their size is within one arc-second. This is, in general, consistent with recent EUV observations showing that the size of a flare loop is about one arc-second. It may be noted that these quoted values are likely upper bounds of the tempo-spatial scales as limited by the resolving capability of instruments.
-
Today we commonly recognize that a flare is a collection of multiple episodes of energy release on some fundamental scales. For example, the bursts' time scales may depend on the characteristic size of the elementary flux tubes (e.g., Sturrock et al. , 1984; LaRosa & Moore , 1993), or the turbulent dynamics of the reconnecting current sheets (eg., Litvinenko , 1996). As these fast-varying bursts are typically reported in hard X-ray and microwave light curves, the observed time scales are also a product of convolution with timescales of acceleration and transport of non-thermal particles in flare environment.  
+
Today we commonly recognize that a flare is a collection of multiple episodes of energy release on some fundamental scales, such as characteristic size of the elementary flux tubes, or the turbulent dynamics of the reconnecting current sheets. As these fast-varying bursts are typically reported in hard X-ray and microwave light curves, the observed time scales are also a product of convolution with timescales of acceleration and transport of non-thermal particles in flare environment. Whereas it is yet difficult to disentangle these different physical mechanisms involved in a single burst, there are basic observational questions to be addressed: what are the temporal, spatial, and spectral properties of these bursts, when compared with the gross properties of a flare?
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The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI Lin et al. , 2002) launched in early 2002 has unprecedented resolution to investigate the temporal, spectral and spatial properties of hard X-ray spikes. This nugget studies hard X-ray bursts observed by RHESS in 2002. The questions raised by these spikes are: are they the fundamental structure of flare energy release? Are flare spectral properties related to their time spatial scales?
+
RHESSI (Lin et al. , 2002) has provided unprecedented advantage to hopefully answer these questions. This nugget studies a sample of hard X-ray flares observed by RHESS using a demodulation algorithm that allows analysis of hard X-ray light curves with sub-second time resolution. It should be noted that the study focuses on the most prominent fast-varying bursts standing out of, rather than making up, the entire flare emission. We call them "hard X-ray spikes" (Figure1).
 +
[[Image:qiu_fig1.jpg|center|thumb|700px|Figure 1: Examples of two hard X-ray spikes found in a flare event in a range of photon energies from 15 to 300 keV. The hard X-ray light curves are derived after applying the demodulation algorithm with 125ms time grid. ]]
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== How Many Hard X-ray Spikes, How fast, How small ==
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== Temporal and Spectral Properties of Hard X-ray Spikes ==
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Turns out only a small fractions of flares exhibit these spikes that can be detected at up to 100 keV. They have rather symmetric rise and decay; about one third of spikes show high energy emission lagging low energy emission, and the spectrally, the spikes have slightly harder spectrum than underlying component. Therefore, the spikes are a special kind of energetic burst, but not the building blocks of flare emission in general. However, with the present
+
Having searched the entire flare catalog in 2002, it turns out that hard X-ray spikes can be detected at up to 100 keV in only a small fraction of flares. The spikes usually have symmetric rise and decay, and have durations below one second, which is independent of the photon energy (Figure 2). These results are consistent with the findings by SMM three decades ago.
 +
[[Image:qiu_fig2.jpg|left|thumb|555px|Figure 2: temporal properties of two example hard X-ray spikes. Left: duration of spikes at different energies. Right: energy-dependent spike peak times (symbol) fitted to the time-of-flight plot (lines). ]]
 +
[[Image:qiu_fig3.jpg|right|thumb|300px|Figure 3: counts spectral indices of the spikes versus those of the underlying components. ]]
 +
In terms of the spectral property of the spikes, the spectrum of integrated counts at greater than 20 keV can be fitted to a power-law distribution,
 +
a generally recognized signature of non-thermal emission. Compared with underlying components, spikes have slightly harder spectrum (Figure 3). A fraction of the spikes also exhibit energy-dependent time lag of either kind: the low-energy emission lagging high-energy emission, usually interpreted as reflecting time-of-flight of direct precipitation electrons, or high-energy emission lagging low-energy emission that is thought to be indicative of Coulomb collision effects in the trap.
 +
== Where Are the Spikes? ==
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[[Image:129fig1.jpg|center|thumb|800px|Figure 1: Maps of the Bastille Day flare, superposed on an array of positive (P) and negative (N) poles used to characterize the solar magnetic field.
 
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<i>Left:</i> hard X-rays; <i>right:</i> UV ribbons time-coded according to the bar at the bottom.
 
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Note that both emissions tend to have EW elongations lying in opposite polarities.]]
 
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== Discovery of UV "cooling" ==
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[[Image:qiu_fig4.jpg|right|thumb|400px|Figure 4: UV images from TRACE (top) and hard X-ray maps of 2s integration from RHESSI (bottom contours) before and during the time of a spike. RHESSI maps are superimposed on a longitudinal magnetogram from MDI. Grey curves in all panels indicate the polarity inversion line of magnetic fields. The inset frames on the top panel illustrate the details of the two flare kernels where the spike emission is located.]]
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Generally, and in this case as well, the hard X-ray and UV emissions have strikingly similar time variations when viewed
+
Very short timescales are usually associated with small spatial scales; it has been challenging to map fast-varying structures during the flare. A few different ways have been attempted to locate  ten spikes out of the studied sample. Direct mapping of the spikes, as confirmed by coordinated UV and optical images whenever available, shows that the spikes at above 25 keVs are most likely thick-target emissions at conjugate foot-points of flare loops (Figure 4), and observed loop length is consistent with estimate from time-of-flight analysis.  
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as a whole, but in detail they may not agree.
+
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The UV emission persists at low levels, filling out the ribbon structures even when the hard X-ray footpoints have disappeared
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or moved elsewhere.
+
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We have studied the time variation in individual pixels of the TRACE images and find a very suggestive pattern, which we illustrate in
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Figure 2.
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[[Image:129fig2.jpg|center|thumb|800px|Figure 2: UV time variations in individual pixels.]]
 
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The individual pixels show a characteristic pattern. At the beginning, there is an impulsive increase; following this, each
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Comparing hard X-ray and UV observations during and immediately before the spikes, we find that the spikes are basically inside or very closely attached to the sources of underlying components. The RHESSI image of 2s integration during the spike shows enhanced emission over the pre-spike source, yet the >4" map resolution does not reveal variations, if any, in the shape or location of the emission source. The better resolved UV images by TRACE (1" resolution) illustrate more clearly that, during the spike, UV emission is enhanced at a few flare kernels of a few arc-seconds, which have been brightened before the spike. These details would
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pixel appears to decay exponentially in brightness. We suggest that the UV emission, which rises nearly instantaneously
+
indicate that reconnection and acceleration events that produce hard X-ray spikes take place in the same magnetic environment of the underlying sources.
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with the hard X-rays, is likely produced primarily by non-thermal electron precipitation, and then undergoes an elongated decay well after the decay of hard X-rays. This could be identified with the cooling (or loss of excess energy in the form of gas pressure) of magnetic loops formed as a part of the flare arcade. The tendency for individual loops to have similar decay times would then correspond to the fact that the loops have similar dimensions.
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== Conclusions ==
== Conclusions ==
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The revisit raises some interesting questions. First, obviously, the evolution and mapping of UV and hard X-ray
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RHESSI capabilities have allowed us to continue the decades-long effort to study rapidly evolving hard X-ray bursts. These observations may hold the key to uncover fundamental scales of energy release in solar flares. The most prominent hard X-ray spikes from a sample of RHESSI flares exhibit similar temporal properties as those discovered in SMM flares in the 70s.  
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sources reflect the pattern of magnetic reconnection which is not entirely 2-dimensional, and second,
+
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the coincident rise and peak but delayed decay in UV emission with respect to hard X-ray emission
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suggests some different physics, not directly related to hard X-ray producing process, taking
+
-
place during the gradual decay of UV emission.
+
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We think that an individual point in the ribbon stays bright, even after its initial excitation (identified with the
+
Given their generally harder spectrum, hard X-ray spikes are the most energetic population of elementary bursts. The ten samples that have been mapped are almost certainly thick-target emissions, mostly from conjugate foot-points of flare loops. They are produced in nearly the same location and magnetic environment as the less impulsive and less energetic bursts in the neighborhood. Should this mean that spikes are produced by a perturbation in the same macroscopic current sheet which is the master board of clusters of  energy release events? Are we ready to directly associate the observed scales with a "fundamental" scale of flare? And, what indeed does this "fundamental" scale mean?
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hard X-ray footpoint) has faded away.
+
-
The persistence of this UV brightness allows the ribbons to have a larger extent than the footpoints, and to follow
+
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a different pattern of evolution.
+
==Acknowledgements ==
==Acknowledgements ==
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This Nugget is based on a presentation at the recent [http://aas.org/meetings/aas216/ Solar Physics Division meeting] by the author with Wenjuan Liu, Nicholas Hill, and Maria Kazachenko.
+
This Nugget is based on two manuscripts [http://adsabs.harvard.edu/abs/2012A%26A...547A..72Q Solar Flare Hard X-ray Spikes Observed by RHESSI: a Case Study] and [http://adsabs.harvard.edu/abs/2012A%26A...547A..73C Solar Flare Hard X-ray Spikes Observed by RHESSI: a Statistical Study] by the author with Jianxia Cheng, Gordon J. Hurford, Haimin Wang, Yan Xu, and Mingde Ding, published in Astronomy and Astrophysics.
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[Category: Nugget]]
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Latest revision as of 16:31, 2 January 2015


Nugget
Number:  ???
1st Author: Jiong Qiu
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Published: 2013 January
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Contents

Introduction

Solar flares are characterized by impulsively enhanced emissions most pronounced in hard X-ray and microwave light curves. These light curves in even a simple, single, short-lived flare usually exhibit quite complicated structures consisting of numerous fast-varying bursts, unlike the rather smooth and gradual soft X-ray emissions. Ground-based and space missions in pre-RHESSI era have revealed the shortest time scales, from a few seconds to several tens of milli-seconds, of these bursts. A few studies correlating high cadence yet spatially unresolved hard X-ray observations with high cadence high resolution optical observations have further shown that hard X-ray bursts are correlated with optical emission on time scales of about one second, suggesting that these fast-varying bursts are thick-target emissions, and their size is within one arc-second. This is, in general, consistent with recent EUV observations showing that the size of a flare loop is about one arc-second. It may be noted that these quoted values are likely upper bounds of the tempo-spatial scales as limited by the resolving capability of instruments.

Today we commonly recognize that a flare is a collection of multiple episodes of energy release on some fundamental scales, such as characteristic size of the elementary flux tubes, or the turbulent dynamics of the reconnecting current sheets. As these fast-varying bursts are typically reported in hard X-ray and microwave light curves, the observed time scales are also a product of convolution with timescales of acceleration and transport of non-thermal particles in flare environment. Whereas it is yet difficult to disentangle these different physical mechanisms involved in a single burst, there are basic observational questions to be addressed: what are the temporal, spatial, and spectral properties of these bursts, when compared with the gross properties of a flare?

RHESSI (Lin et al. , 2002) has provided unprecedented advantage to hopefully answer these questions. This nugget studies a sample of hard X-ray flares observed by RHESS using a demodulation algorithm that allows analysis of hard X-ray light curves with sub-second time resolution. It should be noted that the study focuses on the most prominent fast-varying bursts standing out of, rather than making up, the entire flare emission. We call them "hard X-ray spikes" (Figure1).

Figure 1: Examples of two hard X-ray spikes found in a flare event in a range of photon energies from 15 to 300 keV. The hard X-ray light curves are derived after applying the demodulation algorithm with 125ms time grid.

Temporal and Spectral Properties of Hard X-ray Spikes

Having searched the entire flare catalog in 2002, it turns out that hard X-ray spikes can be detected at up to 100 keV in only a small fraction of flares. The spikes usually have symmetric rise and decay, and have durations below one second, which is independent of the photon energy (Figure 2). These results are consistent with the findings by SMM three decades ago.

Figure 2: temporal properties of two example hard X-ray spikes. Left: duration of spikes at different energies. Right: energy-dependent spike peak times (symbol) fitted to the time-of-flight plot (lines).
Figure 3: counts spectral indices of the spikes versus those of the underlying components.

In terms of the spectral property of the spikes, the spectrum of integrated counts at greater than 20 keV can be fitted to a power-law distribution, a generally recognized signature of non-thermal emission. Compared with underlying components, spikes have slightly harder spectrum (Figure 3). A fraction of the spikes also exhibit energy-dependent time lag of either kind: the low-energy emission lagging high-energy emission, usually interpreted as reflecting time-of-flight of direct precipitation electrons, or high-energy emission lagging low-energy emission that is thought to be indicative of Coulomb collision effects in the trap.

Where Are the Spikes?

Figure 4: UV images from TRACE (top) and hard X-ray maps of 2s integration from RHESSI (bottom contours) before and during the time of a spike. RHESSI maps are superimposed on a longitudinal magnetogram from MDI. Grey curves in all panels indicate the polarity inversion line of magnetic fields. The inset frames on the top panel illustrate the details of the two flare kernels where the spike emission is located.

Very short timescales are usually associated with small spatial scales; it has been challenging to map fast-varying structures during the flare. A few different ways have been attempted to locate ten spikes out of the studied sample. Direct mapping of the spikes, as confirmed by coordinated UV and optical images whenever available, shows that the spikes at above 25 keVs are most likely thick-target emissions at conjugate foot-points of flare loops (Figure 4), and observed loop length is consistent with estimate from time-of-flight analysis.


Comparing hard X-ray and UV observations during and immediately before the spikes, we find that the spikes are basically inside or very closely attached to the sources of underlying components. The RHESSI image of 2s integration during the spike shows enhanced emission over the pre-spike source, yet the >4" map resolution does not reveal variations, if any, in the shape or location of the emission source. The better resolved UV images by TRACE (1" resolution) illustrate more clearly that, during the spike, UV emission is enhanced at a few flare kernels of a few arc-seconds, which have been brightened before the spike. These details would indicate that reconnection and acceleration events that produce hard X-ray spikes take place in the same magnetic environment of the underlying sources.

Conclusions

RHESSI capabilities have allowed us to continue the decades-long effort to study rapidly evolving hard X-ray bursts. These observations may hold the key to uncover fundamental scales of energy release in solar flares. The most prominent hard X-ray spikes from a sample of RHESSI flares exhibit similar temporal properties as those discovered in SMM flares in the 70s.

Given their generally harder spectrum, hard X-ray spikes are the most energetic population of elementary bursts. The ten samples that have been mapped are almost certainly thick-target emissions, mostly from conjugate foot-points of flare loops. They are produced in nearly the same location and magnetic environment as the less impulsive and less energetic bursts in the neighborhood. Should this mean that spikes are produced by a perturbation in the same macroscopic current sheet which is the master board of clusters of energy release events? Are we ready to directly associate the observed scales with a "fundamental" scale of flare? And, what indeed does this "fundamental" scale mean?

Acknowledgements

This Nugget is based on two manuscripts Solar Flare Hard X-ray Spikes Observed by RHESSI: a Case Study and Solar Flare Hard X-ray Spikes Observed by RHESSI: a Statistical Study by the author with Jianxia Cheng, Gordon J. Hurford, Haimin Wang, Yan Xu, and Mingde Ding, published in Astronomy and Astrophysics.

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