Analysis of the polarization degree distribution along limb flaring loop
of July 19, 2012
S. Kuznetsov1, A. Morgachev1
V. Melnikov2
Radiophysical Research Institute, Nizhny Novgorod, Russia1
Pulkovo Observatory, Saint-Petersburg, Russia2
14th European Solar Physics Meeting8th-12th September, 2014
Trinity College Dublin, Ireland
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IntroductionInversion of the polarization sign of microwave gyrosynchrotron emission (extra-ordinary to ordinary) is a signature of some important peculiarities in solar flaring loops.
Possible reasons for the inversion of polarization degree along solar flaring loops:- Anisotropy of accelerated electrons (Melnikov, Gorbikov, Pytakov 2009);- positron’s emission (Fleishman 2013);- twisted magnetic field lines
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The main goal of the research is to find and analyze solar flares with inversion of the polarization along flaring loops. As an example we have considered the event of July 19, 2012. This event is M 7.7 class.
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Aim of the research
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Observational data
We have used Nobeyama Radioheliographfor analysis of observational data. This instrumenthas a high temporal (0.1 sec for the flaring regime)and spatial resolution ( 10” for 17 GHz and 5” for 34 GHz).
Radio brightness distribution along flaring loops
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Inversion of polarization degree along flaring loops
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Size of the boxes – 10”x10”Number of boxes - 23
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Polarization degree distribution along flaring loops
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Dynamics of the pol. degree along flaring loop
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Dynamics of the pol. degree along flaring loop
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Dynamics of the pol. degree along flaring loop
Loop Top
Southern Leg
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In a magnetic loop, a part of injected electrons are trapped due to magnetic mirroring and the other part directly precipitates into the loss-cone. The trapped electrons are scattered due to Coulomb collisions and loose their energy and precipitate into the loss-cone.
A real distribution strongly depends on the injection position in the loop and on the pitch-angle dependence of the injection function S(E,,s,t), and also on time (Melnikov et al. 2006; Gorbikov and Melnikov 2007).
Non-stationary Fokker-Plank equation (Lu and Petrosian
1988):
f
E
cf
ds
Bdc
s
fc
t
f
0
2
2
1ln
),,,(1 223
0
tsESfc
Kinetics of Nonthermal ElectronsKinetics of Nonthermal Electronsin in Magnetic Magnetic LoopsLoops
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Distribution of the polarization degree
CCase ase 11: Injection at the loop top: Injection at the loop top CCase ase 22: Injection near a footpoint: Injection near a footpoint
Ordinary mode circular polarization Signature of the longitudinal anisotropy
Increase in X-mode polarization degree signature of the perpendicular anisotropy
Melnikov, Gorbikov, Pytakov 2009
Discussion
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The possible reason of the inversion of the polarization degreecan be positron’s influence. But due to very small quantity ofpositrons in solar flares this explanation is not possible.
Another possible reason of this effect is the twisted magneticfield lines. The twist makes the viewing angle changing along a loop (for example, from less than 90 deg to more than 90deg). But, for our flaring loop we have regions where the polarization degree changes its sign with time. So, this reason is also unlikely.
Anisotropy is a kinetic effect. We suppose that longitudinalanisotropy of accelerated electrons is a possible reason of theinversion of the polarization degree along flaring loop.This assumption is considered in detail in the poster of AlexanderMorgachev (s.7-07).
Conclusions
• The inversion of the polarization degree during the flaring loop has been found.
• Polarization degree is negative for the footpoints.• Polarization degree is positive for the loop top and for the leg of
the flaring loop.• Temporal dynamics of the polarization degree is different for
different parts of the flaring loop.• Changing the polarization degree along the flaring loop can be
explained by the longitudinal pitch-angle distribution of emitting mildly relativistic electrons.
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