wolfgang finsterle, september 26, 2006 seismology of the solar atmosphere seismology of the solar...
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
Seismology of the Solar Atmosphere
HELAS Roadmap Workshop, OCA NiceWolfgang Finsterle, PMOD/WRC, Davos, Switzerland
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
Seismology of the Solar Atmosphere● Conceptual ideas
Traveling waves Wave travel times Many different types of waves (MAG, Alfvén,
etc.)● Techniques
Multi-height observations “Doppler”-grams Cross-correlation analysis
● Scientific potential Dispersion relation of the solar atmosphere Diagnostics of magnetic fields Chromospheric heating
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
The Atmospheric Wave Field
● Solar eigenmodes oscillate in phase at all heights in the solar atmosphere
● Traveling waves produce a relative phase shift which is characteristic to the observation height and depends on the sound speed structure
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
Acoustic Probing of the Sun's Lower Atmosphere
● By cross-correlating the wave fields at different heights, we can estimate the wave paths and sound speed between the observed heights
● The results naturally link to the solar interior, where seismic models are well established
● Sound waves interact with magnetic fields (absorption, wave conversion/transmission)
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
Basic Model for Sound Waves
observe
Waves propagate when > 0
Standing waves
Traveling waves
Wave equation
d2/dt2 = v2 d2/dz2 - 02
(where v has dimensions of velocity)
Solution
= Re{A exp[i(t-kz)]}
Dispersion relation
2=c2k2+02
(0 is the cut-off frequency)
Acoustic pressure: v2~ P/
Magnetic pressure: v2~ B2/4
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
Multi-height ObservationsMOTH observations:
time
Fit correlation using:
time series FT-1
Na
KFT
Power
filter cross correlate
Power
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
Group Travel Time K→Na
Group time (tg)
Green “islands” coincident with magnetic regions
“➢“Quiet Sun”:
➢ Eveanescent-like behaviour for < 0➢ upward propagating waves for > 0
➢“Mangetic Regions”➢ “islands” of evanescent-like behaviour➢ Upward propagating waves for < 0
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
Phase Travel Time K→Na
Phase time (tp)
Qualitatively the same structures as in the group travel time, but numerically much more stable, hence less noisy.
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
Quiet Sun - Dispersion Relation
tg: group travel time (model)tp: phase travel time (model)
Tg: group travel time (measured)Tp: phase travel time (measured)
Dispersion relation
2=c2k2+02
(0 is the cut-off frequency)
,, t p=
z/k
t g= z
∂/∂k
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
Phase Travel Time
MDI magnetogram
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
Tp(B,ν)
phase time
1.Acoustic “portals”: Lower acoustic cut-off in magnetized regions
2.Plasma-ß canopy: Wave reflection at the boundary layer between “thermal” and “magnetic” atmosphere
3.What are we looking at?
Possible Explanation:
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
1. Acoustic “Portals”
● Inclined magnetic field lines at the boundaries of supergranules locally lower the acoustic cut-off frequency
➔ Acoustic portals for low-frequency waves (<5 mHz) to propagate into the solar atmosphere
➔ Chromospheric heating
Jefferies et al. 2006, ApJ 648, L151
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
2. The Plasma-ß Canopy
Rosenthal et al. (2002, ApJ 564, 508)
tim
e
Below magnetic canopy:propagating wave
Above magnetic canopy:evanescent tail
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
Height of the ß Canopy
reflecting surfacereflecting surface
MOTH Na Doppler Power
MOTH K Doppler Power
MDI Ni Doppler Power
Potential Field Extrapolation
=5
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
cross phasecross phase
contours=5
Height of the ß Canopy
0 100 200 300 400 500 600
z Na−z canopy [km ]
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
Height of the ß Canopy
0 100 200 300 400 500 600
z 1−z canopy [km ]
K→Na Ni→Na
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Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
3. What are we looking at?Some Thoughts about “Doppler”-Grams
● Line-of-sight velocities of the observed medium introduce Doppler shifts
● Dopplergrams filter for anti-parallel intensity changes in the red and blue wings of absorption lines
● The red- and blue-wing probes observe different heights in the solar atmosphere
● At high frequencies, the acoustic wavelengths become comparable to this separation
● → Frequency-dependent “Doppler”-grams