Current%20trends%20in%20coronal%20seismology

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EGU, Vienna, Austria 20/04/2007
Current trends in coronal seismology
Valery M. Nakariakov University of
Warwick United Kingdom
http//www.warwick.ac.uk/go/cfsa
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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
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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
Magnetohydrodynamic (MHD) equations ? Equilibrium
? Linearisation ? Boundary conditions
Zaitsev Stepanov, 1975- B. Roberts
and colleagues, 1981-
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Dispersion curves of coronal loop

• Main MHD modes of coronal structures
• sausage (B, r)
• kink (almost
  incompressible)
• torsional (incompressible)
• acoustic (r, V)
• ballooning (B, r)

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Observed wave phenomena (to April 2007)
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1. Transverse (kink or m1) 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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(No Transcript)
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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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Periodomap of the active region
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Higher spatial harmonics
apex
footpoints
Verwichte et al. 2004
along loop
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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?
enhanced shear viscosity (or shear viscosity
bulk viscosity), phase mixing?
dissipationless resonant absorption?
Intensive discussion
VS
But
Hmmm
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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 lt12 in intensity (lt 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.

From King et al. 2003
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Theory the evolutionary equation
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
Decorrelation
King et al. 2004
Multi-strand sub-resolution structuring?
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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 (gt 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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A possible mechanism periodic triggering of
flare by external MHD wave
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
Acceleration of non-thermal electrons
Anomalous resistivity
Triggers fast reconnection
Nakariakov et al., Quasi-periodic modulation of
solar and stellar flaring emission by
magnetohydrodynamic oscillations in a nearby
loop, AA 452, 343, 2006
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Full 2.5D finite-ß MHD simulations of the
interaction of a fast wave with a magnetic
X-point (McLaughlin Hood, 2004, 2005, 2006
Young et al. 2006)

• 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.

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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)
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, AA 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-2
  005-3