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



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



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



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)



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



Multi-height ObservationsMOTH observations:

time

Fit correlation using:

time series FT-1

Na

KFT

Power

filter cross correlate

Power



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



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.



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



Phase Travel Time

MDI magnetogram



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:



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



2. The Plasma-ß Canopy

Rosenthal et al. (2002, ApJ 564, 508)

tim

e

Below magnetic canopy:propagating wave

Above magnetic canopy:evanescent tail



Height of the ß Canopy

reflecting surfacereflecting surface

MOTH Na Doppler Power

MOTH K Doppler Power

MDI Ni Doppler Power

Potential Field Extrapolation

=5



cross phasecross phase

contours=5


0 100 200 300 400 500 600

z Na−z canopy [km ]




0 100 200 300 400 500 600

z 1−z canopy [km ]

K→Na Ni→Na



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

wolfgang finsterle, september 26, 2006 seismology of the solar atmosphere seismology of the solar...

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