Transcript Sampling the quiet Sun with coronal global waves

Global coronal seismology
and EIT waves
Istvan Ballai
SP2RC, University of Sheffield, UK
Coronal seismology
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Local seismology: using waves propagating in magnetic structures (coronal
loops, filaments, solar wind, etc)
Started with Roberts et al (1983),
Aschwanden et al. 1999, Nakariakov et
al. 1999; the development is accelerated
and diversified by a large number of highresolution observations
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Global seismology: using waves propagating over very large distances in the
quiet Sun, e.g. EIT waves and the connection
between global and local waves
Started with Meyer 1968, Uchida 1970,
Ballai et al. 2005, Ballai 2007; is backed
by observations og global waves by, e.g.
SOHO, TRACE, STEREO
EIT waves
• Generated by sudden energy releases (flares, CMEs); very well
correlated to CMEs, weakly to flares
• Observed to propagate over large distances, sometimes comparable
to the solar radius; the shape is almost circular (in many cases)
• Large span of velocities (100-400 km/s)
• Able to carry information about the quiet Sun
• Problems with EIT waves
 There is no unified concept about EIT waves
 Most of observations during solar minima
 Not properly observed (see, e.g. Wills-Davey, 2006)
Observation of EIT waves
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Although there is a very good correlation not every impulsive event is
associated with an EIT wave
Causes: 1. Observational
SOHO/EUV
 poor temporal resolution (1 frame/12-15 min)
 not able to record EIT waves for flares/CMEs near the limb
TRACE/EUV
 Much better resolution but limited FOV (observation of EIT
waves is merely a matter of luck)
 Wave front too faint to be observed
2. Theoretical
 If the idea of guided trapped waves is OK, waves
become evanescent very quickly
EIT wave seen by SOHO/EIT
(courtesy of M. Wills-Davey)
EIT wave seen by STEREO/EUVI
(Courtesy of G. Attrill)
Propagation speed: 288±55 km/s
EIT waves observed by TRACE/EUV
The 13 June 1998 event
(Wills-Davey and Thompson 1999,
Ballai, Erdélyi and Pintér 2005)
15:25 UT– 15:44 UT
TRACE 195 A (1.5 MK)
Oscillatory motion with periods of about 400 seconds
(Ballai, Erdélyi and Pintér 2005)
Generation of EIT waves
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For simplicity suppose a
magnetic-free environment, and
study the propagation of waves at
a single spherical interface
Generation of EIT waves
• The difference in the
pressure perturbation in
the two regions could
generate a siphon flow
which drives much denser
material in the outer region
• In the exterior (right hand
side), the dimming
propagates away from the
source (as observed)
Sampling the magnetic field (vertical)
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Suppose that EIT waves are FMW propagating perpendicular to the ambient
magnetic field
c=(cS2+vA2)1/2
The propagation height of EIT waves is important since many physical
parameters (temperature, density, pressure) have height-dependence
Suppose a simple atmosphere such that (Sturrock et al. 1996)
 7 / 2 7 R  F0 
1 
T ( x )  T 0 
1  
2 
x 

2/7
n( x) 
n 0T0
T ( x)
F0: inward heat flux (1.8×105 erg/cm2s)
x: normalized height coordinate (=r/R☼)
T0: temperature at the base of the model (=1.3MK)
κ: coefficient of thermal conductivity
δ: a constant
 
exp   T ( x )
5/2
 T0
5/2

Sampling the magnetic field (vertical) contd...
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x
With the sound speed and density calculated at each height, values of the
magnetic field (via the Alfvén speed) are obtained to be
T
n
cS
vA (1) B(1)
vA(2)
B(2)
[T]: MK
1.00 1.30 3.60 1.72 1.81 1.57 3.61 3.13
1.02 1.41 3.30 1.80 1.73 1.44 3.57 2.97
1.04 1.50 3.10 1.85 1.67 1.34 3.54 2.85
1.06 1.58 2.95 1.90 1.61 1.27 3.51 2.76
1.08 1.64 2.83 1.94 1.57 1.21 3.49 2.69
1.10 1.70 2.73 1.97 1.52 1.15 3.47 2.63
[n]: 108 cm-3
[cS],[vA]: 107 cm/s
[B]: G
(a): c=250 km/s
(b): c=400 km/s
Flare and magnetic field diagnostics
EIT waves interact with loops transferring part of their energy to loops
 loop oscillations
Supposing that the entire energy of EIT waves is transferred to loops,
the minimum energy of EIT waves is
 L  i R   e / 
2
E 
2
2
e
  x
max
 t
 max
 x1 

 t1 
2
For the event on 13 June 1998, we obtain
E=3.8×1018 J, for the event on 14 July 1998
(Nakariakov et al. 1999) we obtain E=1019 J.
Since λe-1 contains the Alfvén speed, it is possible
to derive a formula giving the magnetic field in the
oscillating loop provided the energy of the EIT wave
can be measured.
Flare and magnetic field diagnostics
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Time
980714
980714
981123
990704
991025
000323
000412
010321
010322
010412
010415
010513
010515
010615
L(Mm)
168
204
190
258
166
198
78
406
260
226
256
182
192
146
R(Mm) n×108(cm-3) E(J)
7.2
5.7
2.2x1017
7.9
6.2
9.7x1018
16.8
3
1.3x1019
7
6.3
3.9x1016
6.3
7.2
1.6x1018
8.8
17
5.2x1016
6.8
6.9
2.5x1016
9.2
6.2
7.4x1016
6.2
3.2
1.9x1016
7
4.4
1.4x1018
8.5
5.1
1.4x1016
11.4
4
2.2x1018
6.9
2.7
1.6x1019
15.8
3.2
1.1x1017
Lengths, width and
densities taken from
Aschwanden et al. (2001)
Time given in yymmdd
format
E: the minimum energy of
EIT waves to generate the
observed dislocation of
loops
No particular correlation
between the energy and
geometrical sizes of loops
but a relative good
agreement between energy
and 1/n
Sampling the magnetic field (tangential)
Magnetic map of the quiet Sun
Magnetic tomography
of the quiet Sun
Conclusions
• EIT waves are very good candidates for sampling the coronal
magnetic field in the quiet Sun
• More observations are needed with higher resolution
• Since EIT waves relate flare/CMEs with oscillations in coronal loops,
they are very useful tools to diagnose the magnetic field on a larger
scale and connect CMEs and loop oscillations
• After all, the magnitude of the magnetic field is not the most
important factor, instead the of structure (sub-structure) of the
magnetic field could be more interesting and important