Radio polarization signatures in twisted flare loops

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|second_author = Alexei Kuznetsov
|second_author = Alexei Kuznetsov
|publish_date = 25 April 2016  
|publish_date = 25 April 2016  
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|next_nugget = TBD
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|next_nugget={{#ask: [[Category:Nugget]] [[RHESSI Nugget Index::272]]}}
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|previous_nugget = [http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/An_Unreported_White-light_Prominence A previously unreported white-light prominence]
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|previous_nugget={{#ask: [[Category:Nugget]] [[RHESSI Nugget Index::270]]}}
}}
}}
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=== Introduction ===
+
== Introduction ==
The RHESSI view of a solar flare, via hard X-rays and gamma-rays, is quite
The RHESSI view of a solar flare, via hard X-rays and gamma-rays, is quite
direct in one sense, but quite indirect in another: these radiations
direct in one sense, but quite indirect in another: these radiations
-
can only come from a [dense target medium].
+
can only come from a [http://solarphysics.livingreviews.org/open?pubNo=lrsp-2008-1&page=articlesu7.html dense target medium].
This basic problem means that model understanding of the high-energy
This basic problem means that model understanding of the high-energy
emissions must include an understanding of the particle transport.
emissions must include an understanding of the particle transport.
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particle acceleration in a solar plasma atmosphere.
particle acceleration in a solar plasma atmosphere.
-
By contrast, [gyrosynchrotron] radio emissions do not require the presence  
+
By contrast,  
 +
[http://www.oxfordreference.com/view/10.1093/acref/9780199609055.001.0001/acref-9780199609055-e-1593 gyrosynchrotron]  
 +
radio emissions do not require the presence  
of a dense target medium.  
of a dense target medium.  
Instead, the emission process requires the existence of a magnetic field,
Instead, the emission process requires the existence of a magnetic field,
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Where the magnetic field dominates, on the other hand, the equilibrium
Where the magnetic field dominates, on the other hand, the equilibrium
condition would be essentially an isobaric one (a smoothly varying magnetic pressure,
condition would be essentially an isobaric one (a smoothly varying magnetic pressure,
-
or "isomagnetobaric"), where
+
a state that could be termed "isomagnetobaric"), where
-
only the [https://en.wikipedia.org/wiki/Maxwell_stress_tensor Maxwell stress tensor] should produce a smooth, large-scale
+
only the [https://en.wikipedia.org/wiki/Maxwell_stress_tensor Maxwell stress tensor] should define the large-scale
-
pattern.
+
structure.
Recall that we believe these flaring regions to have low [https://en.wikipedia.org/wiki/Beta_(plasma_physics) plasma beta], i.e. that magnetic pressure dominates gas pressure.
Recall that we believe these flaring regions to have low [https://en.wikipedia.org/wiki/Beta_(plasma_physics) plasma beta], i.e. that magnetic pressure dominates gas pressure.
This means that the gyrosynchrotron emissivity, all else being equal,
This means that the gyrosynchrotron emissivity, all else being equal,
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What does this imply for an observable, such as the polarization pattern,
What does this imply for an observable, such as the polarization pattern,
of a coronal magnetic structure containing fast electrons?
of a coronal magnetic structure containing fast electrons?
-
We have thus carried out numerical experiments [1] using the GX Simulator
+
We have thus carried out numerical experiments [1] using the ''GX Simulator''
software package [2], finding that interesting specific signatures
software package [2], finding that interesting specific signatures
may appear.
may appear.
-
New microwave polarimetric imaging facilities such as [E-OVSA] and  
+
New microwave polarimetric imaging facilities such as  
-
[Mingantu] are now appearing, and our experiments suggest that the  
+
[http://www.ovsa.njit.edu E-OVSA] (see also our
 +
[http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/A_New_Day_Dawns earlier Nugget]) and  
 +
[http://www.skyandtelescope.com/astronomy-news/astro-sightseeing-in-innermongolia/ Mingantu] are now appearing, and our experiments suggest that the  
polarization patterns will be informative about not just the energy-loss
polarization patterns will be informative about not just the energy-loss
domains (hard X-rays) but possibly also the acceleration regions
domains (hard X-rays) but possibly also the acceleration regions
of high-energy particles.
of high-energy particles.
-
=== The Simulations ===
+
== The Simulations ==
These simulations consist of analytic approximations to the behavior  
These simulations consist of analytic approximations to the behavior  
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models, but the present ones serve to illustrate at least one interesting conclusion (described below).
models, but the present ones serve to illustrate at least one interesting conclusion (described below).
Our magnetic model follows that designed by Titov and Demoulin [3] in a  
Our magnetic model follows that designed by Titov and Demoulin [3] in a  
-
commonly-used description of [non-potential magnetic field]  
+
commonly-used description of [https://en.wikipedia.org/wiki/Force-free_magnetic_field non-potential magnetic field]  
structures in the solar corona.
structures in the solar corona.
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]]
]]
-
In these models the fields colored in magenta are the ["strapping fields"]
+
In these models the fields colored in magenta are the "strapping" fields
-
introduced to confine the twisted structure via the [magnetic tension]
+
introduced to confine the twisted structure via the magnetic tension, one
-
component of the [Maxwell stress]. The gold field domain connects  
+
component of the  
-
opposite polarities across the [photospheric neutral line] and carries
+
[https://en.wikipedia.org/wiki/Maxwell_stress_tensor Maxwell stress tensor].
 +
The gold field domain connects  
 +
opposite polarities across the photospheric neutral line (the line defined by the locus of zero vertical field component) and carries
a current to generate the twist.
a current to generate the twist.
-
We imbed the test particles in these structures, specifying
+
We imbed the particle fluxess in these structures, specifying
-
[pitch-angle distributions], plasma properties such as density and
+
pitch-angle distributions (relative to the direction of the magnetic field), plasma properties such as density and
temperature, and particle sources.
temperature, and particle sources.
-
Then we "turn the crank" on GX_Simulator to see what happens.
+
Then we "turn the crank" on ''GX Simulator'' to see what happens.
[[File:271f2.png|500px|thumb|center|Figure 2:
[[File:271f2.png|500px|thumb|center|Figure 2:
-
Results of GX_Simulator runs on the fields described in Figure 1,
+
Results of ''GX Simulator'' runs on the fields described in Figure 1,
with particle inputs as described in [1].
with particle inputs as described in [1].
The alternation of polarization sense clearly appears, correlated
The alternation of polarization sense clearly appears, correlated
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these numerical experiments.
these numerical experiments.
-
=== Conclusions ===
+
== Conclusions ==
The main conclusions from our paper [1] reads as  
The main conclusions from our paper [1] reads as  
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Beyond this greater complexities can be imagined and explored with experiments of this type.
Beyond this greater complexities can be imagined and explored with experiments of this type.
-
=== References ===
+
== References ==
[1] [http://arxiv.org/abs/1604.05618 "Modelling of Nonthermal Microwave Emission From Twisted Magnetic Loops"]
[1] [http://arxiv.org/abs/1604.05618 "Modelling of Nonthermal Microwave Emission From Twisted Magnetic Loops"]
-
[2] [http://adsabs.harvard.edu/abs/2012AAS...22020451N Integrated Idl Tool For 3d Modeling And Imaging Data Analysis]
+
[2] [http://adsabs.harvard.edu/abs/2012AAS...22020451N Integrated IDL Tool For 3d Modeling And Imaging Data Analysis]
[3] [http://adsabs.harvard.edu/abs/1999A%26A...351..707T "Basic topology of twisted magnetic configurations in solar flares"]
[3] [http://adsabs.harvard.edu/abs/1999A%26A...351..707T "Basic topology of twisted magnetic configurations in solar flares"]
 +
 +
[[Has article subject:: simulations| ]]
 +
[[Has article subject:: radio| ]]

Latest revision as of 13:25, 21 September 2018


Nugget
Number: 271
1st Author: Ivan Sharykin
2nd Author: Alexei Kuznetsov
Published: 25 April 2016
Next Nugget: Extreme events, stellar evolution, and magnetic reconnection
Previous Nugget: An Unreported White-light Prominence
List all



Contents

Introduction

The RHESSI view of a solar flare, via hard X-rays and gamma-rays, is quite direct in one sense, but quite indirect in another: these radiations can only come from a dense target medium. This basic problem means that model understanding of the high-energy emissions must include an understanding of the particle transport. Specifically, one cannot "see" the acceleration site if it has low density, which might be an important requirement for a successful theory of particle acceleration in a solar plasma atmosphere.

By contrast, gyrosynchrotron radio emissions do not require the presence of a dense target medium. Instead, the emission process requires the existence of a magnetic field, of course, but the distribution of magnetic field and plasma have very different equilibrium processes. For plasma or gas, hydrostatic equilibrium can dominate; at a minimum the solar atmosphere typically has a dense chromosphere and a tenuous corona. Where the magnetic field dominates, on the other hand, the equilibrium condition would be essentially an isobaric one (a smoothly varying magnetic pressure, a state that could be termed "isomagnetobaric"), where only the Maxwell stress tensor should define the large-scale structure. Recall that we believe these flaring regions to have low plasma beta, i.e. that magnetic pressure dominates gas pressure. This means that the gyrosynchrotron emissivity, all else being equal, will have a smoother distribution in space than (say) the hard X-ray emissivity.

What does this imply for an observable, such as the polarization pattern, of a coronal magnetic structure containing fast electrons? We have thus carried out numerical experiments [1] using the GX Simulator software package [2], finding that interesting specific signatures may appear. New microwave polarimetric imaging facilities such as E-OVSA (see also our earlier Nugget) and Mingantu are now appearing, and our experiments suggest that the polarization patterns will be informative about not just the energy-loss domains (hard X-rays) but possibly also the acceleration regions of high-energy particles.

The Simulations

These simulations consist of analytic approximations to the behavior of a specified particle population in a fixed magnetic geometry This is far from realistic in the sense that it does not describe the dynamics of energy exchange between particles and field, which we know to be important in the flare process. One could also study the polarization distribution in more complete models, but the present ones serve to illustrate at least one interesting conclusion (described below). Our magnetic model follows that designed by Titov and Demoulin [3] in a commonly-used description of non-potential magnetic field structures in the solar corona.

Figure 1: Titov-Demoulin models of coronal magnetic fields with small twist (left column) and large twist (right column).

In these models the fields colored in magenta are the "strapping" fields introduced to confine the twisted structure via the magnetic tension, one component of the Maxwell stress tensor. The gold field domain connects opposite polarities across the photospheric neutral line (the line defined by the locus of zero vertical field component) and carries a current to generate the twist. We imbed the particle fluxess in these structures, specifying pitch-angle distributions (relative to the direction of the magnetic field), plasma properties such as density and temperature, and particle sources. Then we "turn the crank" on GX Simulator to see what happens.

Figure 2: Results of GX Simulator runs on the fields described in Figure 1, with particle inputs as described in [1]. The alternation of polarization sense clearly appears, correlated with the twist of the flux-rope model.

The advantage of these numerical experiments is that one can vary the parameters, which one cannot do with observations - we observe only what Nature gives us, and sometimes not very well! Thus one can make possible discoveries in the simulations just as one can make discoveries in real observations. These need to confirmed by theoretical and observational work in any case, but the initial germ of discovery may be in one of these numerical experiments.

Conclusions

The main conclusions from our paper [1] reads as

"• Inversion of the polarization sign of the radio emission, generated by non-
thermal electrons in the twisted loop located in the center of the solar disk,
has form of the line inclined relatively to the loop axis. Polarization of 
radio emission from twisted loop on the solar limb experiences change of 
its sign along its axis."

This experiment predict slanting lines of polarity inversion that cut across the flux-rope structure, which may or may not be recognizable in direct images. Beyond this greater complexities can be imagined and explored with experiments of this type.

References

[1] "Modelling of Nonthermal Microwave Emission From Twisted Magnetic Loops"

[2] Integrated IDL Tool For 3d Modeling And Imaging Data Analysis

[3] "Basic topology of twisted magnetic configurations in solar flares"


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