Wolfgang finsterle, September 26, 2006 Seismology of the Solar atmosphere Seismology of the Solar...
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Transcript of Wolfgang finsterle, September 26, 2006 Seismology of the Solar atmosphere Seismology of the Solar...
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
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
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
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)
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
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
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
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.
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
Wolfgang finsterle, September 26, 2006
Seismology of the Solar atmosphere
Phase Travel Time
MDI magnetogram
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:
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
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
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
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 ]
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
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