14th European Solar Physics Meeting
8th-12th September, 2014
Trinity College Dublin, Ireland
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
kuznetsov@nirfi.sci-nnov.ru
1
Introduction
Inversion 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
2
Aim of the research
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.
3
Observational data
We have used Nobeyama Radioheliograph
for analysis of observational data. This instrument
has a high temporal (0.1 sec for the flaring regime)
and spatial resolution ( 10” for 17 GHz and 5” for 34 GHz).
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Radio brightness distribution along
flaring loops
5
Inversion of polarization degree along
flaring loops
6
Size of the boxes – 10”x10”
Number of boxes - 23
7
Polarization degree distribution along
flaring loops
8
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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Kinetics of Nonthermal Electrons
in Magnetic Loops
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
f
d ln B  1   2  c   f 
  
 c
 c
f

t
s
ds   2
  0 E   
 
2 f 

1 
 S ( E ,  , s, t )


3 2
 
0    
c


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Distribution of the polarization degree
Case 1: Injection at the loop top
Increase in X-mode
polarization degree 
signature of the
perpendicular anisotropy
Melnikov, Gorbikov, Pytakov 2009
Case 2: Injection near a footpoint
Ordinary mode circular
polarization  Signature of
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the longitudinal anisotropy
Discussion
The possible reason of the inversion of the polarization degree
can be positron’s influence. But due to very small quantity of
positrons in solar flares this explanation is not possible.
Another possible reason of this effect is the twisted magnetic
field 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 longitudinal
anisotropy of accelerated electrons is a possible reason of the
inversion of the polarization degree along flaring loop.
This assumption is considered in detail in the poster of Alexander
Morgachev (s.7-07).
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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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