Local Helioseismology. Laurent Gizon (Stanford). Outline. Some background Time-distance helioseismology: Solar-cycle variations of large-scale flows Near surface convection Sunspots: structure and dynamics. Why is helioseismology useful?. To test the standard model of stellar structure
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Laurent Gizon (Stanford)
Millions of modes of oscillation
excited by near-surface turbulent convection.
Acoustic modes with similar wave
speeds probe similar depths.
MDI-SOHO measures Dopplergrams every minute
Figure: m-averaged medium-l
power spectrum (60-day run).
Palle et al.
GONG 1D RLS
Howe et al.
Kitt Peak magnetic data
The goal is to make 3D images of flows, temperature and density inhomogeneities,
and magnetic field in the solar interior. Local helioseismology includes different techniques that complement each other (see Gizon & Birch, Living Reviews, submitted).
Time-distance helioseismology (Duvall et al. 1993) is based on measurements of travel times for wavepackets travelling between any two points on the solar surface.
Longitudinal resolution (3D image)
Correlation between two surface locations versus distance and time-lag.
Longitudinal averages from f-mode time-distance helioseismology
Mean meridional circulation is
poleward in both hemispheres
(~20 m/s at 20 deg latitude)
Important ingredient in some
theories of the solar dynamo.
Directly observed down to 0.8R, so far (Giles 1998).
To study solar-cycle variations: subtract time average over many years…
Hathaway obtains a similar number from an analysis of sunspot drifts.
Fractional radius r/R
Plots of residuals of rotation and meridional circulation after subtraction of a temporal average. About 50 Mm deep (Beck, Gizon & Duvall, 2002).
Zonal flow residuals (torsional oscillations)
red=prograde blue= retrograde
Increased differential rotation shear at active latitudes. First discovered by Howard & Labonte (1980), agrees with global mode splittings.
May be caused by the back reaction of the Lorentz force from a propagating dynamo wave (Schuessler, 1981). [Other explanations exist.]
Meridional flow residuals
Residuals: red=equatorward, green=poleward
At 50 Mm depth, residual north-south flow diverging from the mean latitude of activity
(agrees with Chou 2001, acoustic imaging).
The opposite is seen near the surface!
(Gizon 2003, Zhao 2003).
Local 50 m/s surface flows converging toward active regions (Gizon et al. 2001).
Excellent agreement with ring-diagram analysis (Hindman, Gizon et al. 2004).
Local inflows responsible for temporal variations of surface meridional flow (Gizon 2004)
At depth of 10-15 Mm: an outflow is observed (Haber et al. 2003, Zhao et al. 2003)
Looks like a toroidal flow pattern around AR: surface outflow and deeper inflow.
Surface inflow consistent with a model by Spruit (2003).
(Duvall & Gizon)
Spatial sampling = 3 Mm
Temporal sampling = 8 hr
white = divergent flows
black = convergent flows
The supergranulation pattern appears to propagate in the form of a modulated travelling wave (Gizon, Duvall & Schou, 2003, Nature)
The direction of propagation is prograde at the equator, and slightly equatorward of the prograde direction away from the equator. An analysis in Fourier space enables to separate the background advective flows from the non-advective wave speed (65 m/s)
Red: advective flow. Green: motion of magnetic features.
Dashed: Correlation tracking with 24 hr lag.
Supergranulation may be an example of travelling-wave convection. No explanation yet. Although it is likely that the influence of rotation (or rotational shear) on convection is at the origin of this phenomenon (Busse 2003).
consistent with near-surface flows from local helioseismology
Complex flow picture.
Not always meaningful…
Zhao et al. 2003
Gizon et al. 2001
Wave-speed anomalies(Kosovichev 1999)
From MDI pipeline (almost real time)
Flares produce sunquakes (Kosovichev & Zharkova)
Linear sensitivity of travel time to small steady changes in the solar model:
In principle, have to consider all possible types ( ) of perturbations, including flows, temperature, density, magnetic field, damping and excitation…
A general recipe for computing kernels must include a physical description of the wave field generated by a stochastic source model and the details of the measurement procedure (Gizon & Birch 2002). Linearization is achieved through the Born or Rytov approximations (single-scattering). Done for sound-speed perturbations (Birch et al. 2004). Need to do it for other types of perturbations
Still some very hard problems to solve…
3D Inversion taking account
of correlations in data errors
Cut through 3D input
3D Inversion assuming
no correlation in data errors
Complex flow patterns evolving with the solar cycle in the near-surface shear layer.
Are these flows a secondary manifestation of the solar dynamo, or do they play an important role in the organization of solar magnetic fields?