Probing the Solar Dynamo by observing the Sun’s high latitude field, flows and seismic waves
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Transcript of Probing the Solar Dynamo by observing the Sun’s high latitude field, flows and seismic waves
35th COSPAR Scientific Assembly 18-25 July 2004
Probing the Solar Dynamo by observing the Sun’s high latitude field,
flows and seismic waves
Thierry Corbard
Observatoire de la Côte d’Azur
France
35th COSPAR Scientific Assembly 18-25 July 2004
Plan
Links between the Solar interior and magnetic cycle Observed Surface Magnetic fields Solar Dynamo models
Current helioseismic probe of the dynamo and limitations Internal angular velocity from Global helioseismology 3D Subsurface flows from local helioseismology
Perspectives for Solar Orbiter
35th COSPAR Scientific Assembly 18-25 July 2004
Observed Surface magnetic Fields
Courtesy: D. Hathaway; NASA/MSFC
Equatoward migration of sunspot-belt
Variation of amplitudes, duration etc..
35th COSPAR Scientific Assembly 18-25 July 2004
Observed Surface magnetic Fields
Courtesy: D. Hathaway; NASA/MSFC
•Weak diffuse fields outside the sunspot drift poleward•Polar reversal takes place during sunspot maximum•Polar field take the sign of the following spots of the current cycle
35th COSPAR Scientific Assembly 18-25 July 2004
6 Ingredients for Solar Dynamo Models
Parker 1970
Helical Turbulence
(-effect)
Differential rotation
Ω-effect
Poloidal Field
Toroidal Field
+ Meridional Circulation
.
+ Flux Tube dynamics, buoyancy and stabilityParker 75, Fan et al. 93, D’Silva & Choudhouri 93, Caligari et al. 95…
35th COSPAR Scientific Assembly 18-25 July 2004
6 Ingredients for Solar Dynamo Models
Helical Turbulence
(-effect)
Differential rotation
Ω-effect
Poloidal Field
Toroidal Field
+ Meridional Circulation
+ Flux Tube dynamics, buoyancy and stability
2 Main Class of Dynamo models
Flux transport Dynamo (Wang et al. 91, Choudhuri et al. 95, Durney 95…)
Interface Dynamo (Parker 93, Tobias 96, Charbonneau & Mc Gregor 97….)
Dikpati & Charbonneau 99, Petrovay & Kerekes 2004
35th COSPAR Scientific Assembly 18-25 July 2004
Evolution of Magnetic Fields in a Flux Transport Solar Dynamo Model
Courtesy M. Dikpati
+ Addition of a tachocline -effect is needed to satisfy Hale’s polarity rule
(Dikpati & Gilman 2001)
35th COSPAR Scientific Assembly 18-25 July 2004
Global Helioseismology
Angular degree l
Fre
qu
en
cy (
mH
z)
n=0
n=1n=2n=3
:
drdθθ)Ω(r,θ)(r,Km
ωωnlm
nlmm-nl
Internal Rotation breaks spherical symmetry
=> Multiplets of 2l+1 frequencies
=> 2D Inverse Problem
observed Standard Solar Model
Unknown
35th COSPAR Scientific Assembly 18-25 July 2004
Global Helioseismology Results: Internal Rotation
Tachocl
ine
Equator
Pole
Period (days)
“Polar Jet” ?
Corbard 98 Schou et al. 2002
35th COSPAR Scientific Assembly 18-25 July 2004
Global Helioseismology Results: Torsional Oscillations
•Pattern found to persist from the surface down to (0.82R) Howe et al. 2000
•Indications of even deeper penetration at high latitudes Vorontsov et al. 2002
•likely a back reaction from the Dynamo generated magnetic field
Adapted from Corbard & Thompson 2001
35th COSPAR Scientific Assembly 18-25 July 2004
Global HelioseismologyLimitations
Sensitive only to the part of the rotation that is symmetric about the equator.
Kernels Independent of longitude (e.g. cannot detect effect of active region on rotation)
Limited access to the polar zone
Impossible to separate the spherically asymmetric effects other than rotation (meridional circulation, magnetic fields, structural asphericity)
35th COSPAR Scientific Assembly 18-25 July 2004
Local HelioseismologyAnalysis Principles
Small areas (16ºx16º) are tracked (typically between 8hrs and 28hrs)over the solar disk at a rate depending on the latitude of their center.
These areas are remapped using a Postel or transverse cylindrical projection that tend to preserve the distance along great circles.
=> Data cubes (Latitude – Longitude – time)
35th COSPAR Scientific Assembly 18-25 July 2004
Local Helioseismology Main Methods
1. Time-distance analysis (Time domain)(Duvall et al. 1993, 1996)
2. Ring-Diagram analysis (Fourier domain)(Gough & Toomre 1983; Hill 1988)
3. Acoustic Holography (Phase sensitive)
(Lindsey & Braun, 1990, 1997)
Based on the local observation of global acoustic modes.
35th COSPAR Scientific Assembly 18-25 July 2004
Time-distance helioseismology Principles
First developed by Duvall 1993
Employs waves travel times observed between different surface location on the Sun.
These travel times are then “inverted” to deduce:
Flow speed and direction, Sound speed perturbations [Magnetic field perturbations]
Along the ray paths of the observed modes.
35th COSPAR Scientific Assembly 18-25 July 2004
Time-distance helioseismology Measuring Travel times
These travel times are obtained by cross-correlating the observed surface oscillations.
Because of the stochastic nature of excitation of the oscillations, the cross-covariance function must be averaged over some areas on the solar surface to achieve a S/N sufficient for measuring travel times.
=>Averages are made on rings around the central point
T
dtrtfrtf0
21 ),(),(),(
Kosovichev & Duvall., 1997
35th COSPAR Scientific Assembly 18-25 July 2004
Time-distance helioseismology Interpretation of travel time perturbations
ds.kδ1
Fermat’s Principle and ray approximation theory
Phase travel time perturbation
Ray path in the unperturbated medium
Perturbation of the wave number
Gough 1993Kosovichev 1996
Bogdan 97: wavepacket are not confined to the raypath Development of wave-theoretical sensitivity kernels: Jensen et al. (2000), Birch &
Kosovitchev (2000), Jensen & Pijpers (2002), Gizon & Birch (2002) First comparisons (e.g. Couvidat et al. (2004)) show reasonable agreement for depths
above 15Mm
35th COSPAR Scientific Assembly 18-25 July 2004
Time-distance helioseismology Interpretation of travel time perturbations
22222222
222 /).(4)(
2
k)v.k(ω kckccccc AAAac
Dispersion relation:
Temperature perturbations => do not depend on the direction of propagation Flow perturbations => waves move faster along the flow Magnetic fields perturbations => waves traveling perpendicular to the field lines are
the most sensitive to B (wave speed anisotropy not yet detected) Ryutova & Scherrer 1998
4B
cA
3D velocity field Acoustic cut off frequency
Alvén velocity
(Kosovichev et al. 1997)
35th COSPAR Scientific Assembly 18-25 July 2004
Time-distance helioseismology Inverting Travel Times Maps
),(rdiff
2
),(rmean
Annulus ranges: 1º19 - 1º598
Zhao 2004
dsrdiff )(c
n(r).v2
2o
ds
r
r
omean ω
k
)(c
)(c o
+Different annulus ranges sound
different depths
Inversions for sound speed perturbations and 3D flows as a function of
depth
35th COSPAR Scientific Assembly 18-25 July 2004
Time-distance helioseismology Results: 3D Flows below sunspot
0-3 Mm 6-9 Mm
9-12 Mm
Zhao, Kosovichev,Duvall 2001
Downward flows
Upward flows
35th COSPAR Scientific Assembly 18-25 July 2004
Time-distance helioseismology Sub sunspot dynamics and structure
Courtesy SoHO/MDI
The cluster Model of Sunspot
Parker 1979
Illustration of sound speed variations and
subsurface flow patterns of a sunspot
35th COSPAR Scientific Assembly 18-25 July 2004
Ring Diagram Analysis
kx
ky
3D FFT
(, ,t) (kx,ky,)
3D Power spectra Fitting
dzzzKU yx )(v),k,ω()k,(ω yx,hoho,
2 x 1D Inversions
Vz may then be computed using the divergence of the horizontal flow and assuming mass conservation. (Komm, Corbard et al. 2004)
35th COSPAR Scientific Assembly 18-25 July 2004
Ring Diagram Analysis Flow Maps
35th COSPAR Scientific Assembly 18-25 July 2004
Synoptic Maps of Vorticity – Kinetic Helicity etc..
Komm, Corbard et al. 2004 (ring diagrams)
Zhao & Kosovichev 2003 (time-distance)
35th COSPAR Scientific Assembly 18-25 July 2004
Time and depth variations ofMeridional Flows
Basu & Antia 2002
Gonzalez Hernandez et al.
2004
35th COSPAR Scientific Assembly 18-25 July 2004
Controversial detection of a counter cell at “high latitudes” in the Northern hemisphere
Haber et al. 2002
Not found by Basu and Antia 2003
And Time-distance analysis Zhao and Kosovichev 2003
35th COSPAR Scientific Assembly 18-25 July 2004
Dynamo Models and Polar Reversal
N-S asymmetry in meridional flow speed during 1996-2002 and the appearance of a reverse, high-latitude flow cell in the N-hemisphere during 1998-2001 caused the N-pole to reverse ~1 yr before the S- pole
Dikpati et al. 2003
35th COSPAR Scientific Assembly 18-25 July 2004
Perspectives for The Solar Orbiter
The Visible Light-imager and Magnetograph (VIM) will provide high resolution Magnetograms and Dopplergrams from outside the ecliptic plane (up to 30º).
Immediate benefit: Reduced foreshortening and Increased sensitivity to Magnetic Fields (factor 5)
Possibility of stereoscopic observations with SDO or Earth (120º)
Observations will provide the missing parts of solar dynamo models
Polar rotation rate and torsional oscillation pattern(High latitude Jets?, lower rotation bands migrating poleward ?)
Polar meridional flows (counter cell, returning flow ?) Maps of high latitude vorticity / kinetic helicity / Small scale Dynamo Direct observations of polar reversal and flux cancellation processes Access to the deep layers (tachocline) from stereoscopic observations Access to the far side of the Sun (continuous probe of spots / far side imaging
technique)
35th COSPAR Scientific Assembly 18-25 July 2004
Acoustic HolographyFar Side Imaging
Lindsey & Braun (1997)
A local acoustic depression at the focal point will shift the phase of the ingression-egression correlation.
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Concluding Remarks
The field of Local Helioseismology is fairly new
A lot of efforts have been made during the last few years in:
Understanding the forward problem (wave-theoretical sensitivity kernels / Building of artificial data)
Making comparison between the different Methods (group LoHCO) Understanding errors, methods resolution and correlations Understanding effects of image misalignment / geometric effects (P-angle /
Bo angle)
First Results are very encouraging and positive First attempt of “archeo-helioseismology” using Mt Wilson data in order to
infer the meridional circulation in the past two solar cycles
There is no doubt that a lot more progress will be achieved in developing and understanding these methods before the launch of Solar Orbiter