Current trends in coronal seismology
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Transcript of Current trends in coronal seismology
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Current trends in coronal seismology
Valery M. Nakariakov
University of WarwickUniversity of Warwick
United KingdomUnited Kingdomhttp://www.warwick.ac.uk/go/cfsa
EGU, Vienna, Austria 20/04/2007
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Wave and oscillatory processes in the solar corona:
• Observational evidence of coronal oscillations (or quasi-periodic pulsations) is abundant (major contribution by SOHO,TRACE and NoRH).
•Possible relevance to coronal heating and solar wind acceleration problems.
• Possible role in the physics of solar flares.
• Plasma diagnostics.
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Mechanisms for (Quasi) Periodicity:
• Resonance (characteristic spatial scales)
• Dispersion
• Nonlinearity / self-organisation
Characteristic scales: 1 Mm-100 Mm,
MHD speeds: Alfvén speed 1 Mm/s, sound speed 0.2 Mm/s
→ periods 1 s – several min - MHD waves
Seismological information
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(MHD) coronal seismology – diagnostics of solar coronal plasmas with the use of coronal MHD waves and oscillations
Main differences with helioseismology:
• Transparent medium
• Usually only local diagnostics of the oscillating structures and their nearest vicinity (e.g. magnetic field in the oscillating loop (c.f. time-distance helioseismology).
• Three wave modes (fast, slow magnetoacoustic and Alfven) – more constrains and more toys to play with.
• C.f. MHD spectroscopy of tokamaks.
Local (various coronal structures) vs Global (AR, CH)(Roberts et al. 1984) (Uchida 1970, Ballai 2004)
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Basic theory: Dispersion relations of MHD modes of a magnetic flux tube:
2 2 2 2 2 200 0 0
0
'( ) '( )( ) ( ) 0
( ) ( )m m e
e z Ae z A em m e
I m a K m ak C m k C m
I m a K m a
Magnetohydrodynamic (MHD) equations
Equilibrium
Linearisation
Boundary conditions
Zaitsev & Stepanov, 1975- B. Roberts and colleagues, 1981-
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Main MHD modes of coronal structures:
• sausage (|B|, )
• kink (almost incompressible)
• torsional (incompressible)
• acoustic (, V)
• ballooning (|B|, )
Dispersion curves of coronal loop:Dispersion curves of coronal loop:
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Observed wave phenomena (to April 2007):
1. Kink oscillations of coronal loops (Aschwanden et al. 1999, Nakariakov et al. 1999)
2. Propagating longitudinal waves in polar plumes and near loop footpoints (De Forest & Gurman, 1998; Berghmans & Clette, 1999)
3. Standing longitudinal waves in coronal loops (Kliem at al. 2002; Wang & Ofman 2002)
4. Global sausage mode (Nakariakov et al. 2003)
5. Propagating fast wave trains. (Williams et al. 2001, 2002; Cooper et al. 2003; Katsiyannis et al. 2003; Nakariakov et al. 2004, Verwichte et al. 2005)
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1. Transverse (kink or m=1) mode:
• Decaying kink-like oscillations of coronal loops, excited by a nearby flare.
• Periods are several minutes (e.g. 256 s), different for different loops.
• Decay times are about a few wave periods.
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Estimation of the magnetic field:
One of the aims of SDO/AIA
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Challenges:
• to minimise the errors
• automated detection of oscillations in imaging data cubes
Recent achievements:
(Van Doorsselaere et al. 2007)
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Automated detection techniques (for SDO/AIA):
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“Per
iodo
map
of
the
activ
e re
gion
”
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along loop
Higher spatial harmonics:
apex footpoints
Verwichte et al. 2004
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A number of theoretical papers on P2/P1 ratio:
• Andries et al. (2005)
• McEwan et al. (2006)
• Dymova et al. (2007)
Estimation of
• density scale height
• flux tube divergence
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Van Doorsselaere et al. 2007 :
The hydrostatic estimation: H = 50 Mm
(c.f. Aschwanden et al. 2000: “over-dense loops”)
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Mechanism responsible for the decay?
Intensive discussion:
enhanced shear viscosity (or shear viscosity = bulk viscosity), phase mixing?
dissipationless resonant absorption?VS
But…
Hmmm…
4/3PM:
RA:
decay
decay
P
P
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Kink oscillations?
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Open questions:
• Excitation mechanism. Options are: a flare-generated coronal blast (fast) wave; a chromospheric wave exciting loop footpoints.
• Decay mechanisms. Options are: resonant absorption, phase mixing with enhanced sheer viscosity; possibly leakage in the corona in multi-thread systems.
• Selectivity of the excitation: why some loops respond to the excitation while others do not?
• The role of nonlinear effects (the displacement is greater than the loop width). Do the oscillations change the loop cross-section shape?
• Coupling of oscillations of neighbouring loops, oscillations of AR.
Spectral information is crucial (EIS).
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2. Propagating Longitudinal Waves = Slow Waves
Observed near in legs of loops and in plumes:
• Upwardly propagating perturbations of EUV
emission intensity.• With constant speed about 25-165 km/s.
• Amplitude is <12% in intensity (< 6% in density),
• The periods are about 130-600 s.
• No manifestation of downward propagation.
• A number of examples.
• No correlation between the amplitudes, periods and speeds. F
rom
Kin
g et
al.
2003
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2
2
1 1( ) 0
2 ( ) 2 s
V V VV V S
S H S C
Theory: the evolutionary equation:
stratification nonlinearity
dissipation
radiative losses - heating
Theory VS Observations:
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Main mechanisms affecting the vertical dependence of the amplitude:
• Stratification (can be estimated, relative density change is needed),
• Thermal conduction (can be estimated if temperature is known),
• Magnetic flux tube divergence (can be estimated from images)
• Radiative damping (can be estimated if temperature is known, e.g. RTV approximation),
• Unknown coronal heating function.
- can be estimated from the observations of the waves!
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Multi-wavelength observations:
TRACE 171 A and 195 A:
Multi-strand sub-resolution structuring?
Decorrelation
Kin
g et
al.
2004
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A probe of the sub-resolution structuring of the coronal temperature
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Open questions:
• What is their origin and driver? (Options: thermal overstability, leakage of p-modes, connection with running penumbra waves).
• What determines the periodicity and coherency of propagating waves?
• What is the physical mechanism for the abrupt disappearance of the waves at a certain height (Options: dissipation and density stratification, magnetic field divergence, phase mixing).
• Are the waves connected with the running penumbra waves?
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3. Similar periodicities are often detected in flares:
E.g., in microwave emission: (NoRH)
Period about 40 s
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Often QPP are seen in both microwave (GS) and hard X-ray : e.g. Asai et al. (2001)
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Also, stellar flaring QPP:
EQ Peg B flare VL emission (Mathioudakis et al. 2004) :
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Suppose that QPP are connected with some MHD oscillations (the same periods!).
The model has to explain:
• the modulation of both microwave and hard X-ray (and possibly WL) emission simultaneously and in phase; (are there any observations which contradict this?)
• the modulation depth (> 50% in some cases, while the amplitudes of known coronal MHD waves are usually just a few percent);
• the observed 2D structure of the pulsations.
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MHD oscillation in the external
loop (very small amplitude)
Fast wave perpendicular to B approaches X-point
Electric currents build up (time variant)
Current driven micro-instabilities
Anomalous resistivity
Triggers fast reconnection
Acceleration of non-thermal electrons
Nakariakov et al., Quasi-periodic modulation of solar and stellar flaring emission by magnetohydrodynamic oscillations in a nearby loop, A&A 452, 343, 2006
A possible mechanism: periodic triggering of flare by external MHD wave
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• The fast wave experiences refraction.
• The fast wave energy is accumulated near the separatrix.
• The current density near the X-point experiences building up.
• Incoming periodicity is reflected in current periodicity.
• The amplitude of the generated variations of current density is orders of magnitude higher than the amplitude of the driving fast wave.
Full 2.5D finite-β MHD simulations of the interaction of a fast wave with a magnetic X-point (McLaughlin & Hood, 2004, 2005, 2006; Young et al. 2006):
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Thus, the electric current density at the null-point varies periodically in time:
The amplitude of the source fast wave is just 1%.
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Current-driven plasma microinstabilities were suggested as a triggering mechanism for fast reconnection (e.g. Ugai, Shibata):
classical anomalousthreshold classical
threshold anomalous ,,
,
jj
jj
thersholdj
Periodic variation of the current density causes periodic triggering of fast reconnection
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There is some observational evidence:
(Foullon et al., X-ray quasi-periodic pulsations in solar flares as MHD oscillations, A&A 420, L59, 2005)
Unseen kink oscillations of the faint trans-equatorial EUV loop cause modulation of the hard X-ray emission near the magnetically conjugate points.
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Conclusions:• MHD waves are a common feature of the solar corona.
• The waves contain information about physical parameters in the corona (sometimes unique) – MHD coronal seismology.
• If understood in the solar corona – very interesting perspectives in stellar coronae.
• Several MHD modes have been directly observed in solar coronal structures, mainly in EUV.
• Very interesting perspectives in the microwave band.
• Flaring QPP can be cause by MHD waves too – there are simple mechanisms for the modulation of hard X-ray and microwave.
• Nakariakov & Verwichte, Living Reviews of Solar Physics, 2005, http://www.livingreviews.org/lrsp-2005-3