Helioseismology of high-latitude regions Alexander Kosovichev Stanford University
> Current status of helioseismology measurements at high latitudes > Diagnostics of the tachocline > Key questions and expected results of Solar-C observations > Instrument requirements
T Differential rotation and torsional oscillations at high latitudes, polar iets Global modes |=0-250 Local helioseismology 2.  Meridional circulation, latitudinal and longitudinal structures, secondary cells, relationship to active longitudes, magnetic flux transport Local helioseismology 3.  Supergranulation and large-scale convection patterns in polar regions (super-rotation, wave-like behavior, network, tilux transport, relationship to coronal holes) Local helioseismology 4. Structure and dynamics of the high-latitude tachocline (oblateness, flows) Local helioseismology 5.  Tomography of the deep interior: stereo observations (Solar-C-SDO- eone) Local Helioseismology (time-distance, holography) Current Status of Helioseismology > The internal structure and differential rotation are measured with great precision throughout the Sun except the polar regions (75-90 deg latitude), the energy-generating core and the subsurface superadiabatic layer. (Global helioseismology) > Large-scale dynamics (supergranulation, large- scale flows around magnetic regions, zonal and meridional flows) is being investigated in details in the mid-latitude zone from -60 to 60 degrees in the upper convection zone, 30 Mm deep. (Local helioseismology)
Internal differential rotation inferred from SOHO/MDI data by two different inversion techniques (in the shaded areas the inferences are uncertain) Internal differential rotation (red – faster; blue – slower)
Birch & Kosovichev (1998)
Migrating zonal flows – “torsional oscillations" Meridional flows from equator to poles
Vorontsov et al (2002) The depth structure of the torsional oscillations -migrate to the surface along the isorotation lines, -penetrate  to the tachocline in high-latititude regions Torsional oscillations show a start of the new cycle at high latitudes where there is no significant magnetic field
Howe et al (2006)

Meridional flows can be measured from the large-scale synoptic flow maps (Haber et al. 2002
!!!!! CR1948 of 1999 !!!!!! Mm epth t t t t t – 10 0 10 20 30 40 50
CR1932 of 1998
Y Y >  In addition to the mean meridional flow from the equator to the poles we find extra meridional circulation cells converging towards the activity belts and migrating towards the equator as the solar cycle progresses >  Slowing meridional circulation at the solar maximum creates difficulties for flux transport dynamo models to explain reversals of the polar magnetic fields.
circulation
The magnetic butterfly diagram. Axisymmetrical component of the line-of-sight magnetic field averaged over a full solar rotation as a function of time. Yellow and blue colors shows positive and negative magnetic fields
Contours: toroidal fields at CZ base Gray-shades: surface radial fields (Dikpati, de Toma, Gilman, Arge & White, 2004, ApJ, 601, 1136)
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Relationship between the meridional flows and magnetic flux transport > The mean velocity of the meridional flow decreases during periods of high activity at high latitudes. > How does this affect the magnetic flux transport?
Comparison of meridional flow determined by time- distance helioseismology (solid line) and the flux transportation speed measured by the method suggested in this study (points with error-bars). It is seen that in the periods of increased magnetic activity the disagreement of both methods is quite obvious. > The recent study by Svanda et al showed that the magnetic flux transport speed does not correspond to the observed velocity of the mean meridional flow. > This indicates that it is important to take into account the longitudinal structure of the meridional flows and magnetic field. > The reason is that the speed of the mean meridional flow is decreased because of inflows around strong field regions of the leading magnetic polarity. However, for the polar field reversal the transport of the following magnetic polarity is more important, around which inflows are much weaker. Numerical simulations show multiple cells at high latitude. Are there multiple convective cells on the Sun? How  do these cells participate in the magnetic flux transport? What is the high-latitude structure of the meridional circulation? How deep is the meridional flow? View of the differential rotation from 60 degrees
What is the relationship between active longitudes and polar magnetic field? Yohkoh and EIT observations revealed giants loop structure connecting active regions and polar regions. How deep is the link between plasma and flows in polar regions? YOKHOH observations of giant coronal loops connecting  polar regions and following polarities of active regions. What is the role of these connections in the solar cycle? What is the large-scale circulation pattern in polar regions and its role the magnetic field dynamics? How well can we resolve structures and flows in the tachocline? 34
A linear code solving wave propagation equations, including only spherical degree from 0 to 170.
Surface focusing Deep focusing
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Sound-speed perturbation of 0.6% is placed at 0.7R, with a latitudinal dependence, and with a Gaussian shape. It’s symmetric along the equator. The simulation used here is 1024 minutes.
Measured travel times are displayed after a reference profile is subtracted. The reference profile is measured from a simulation that Thomas Hartlep made without perturbations.
The inversion recovers the location of the maximum perturbation well. The results are more spread out, though, especially towards the deeper interior. (may be related to relatively high realization noise (short time series)) The latitudinal variation is not recovered perfectly, but still promising. Model Inversion
Averaging kernels obtained from surface-focusing inversions when the target location is at a latitude of 18 degree, and at 0.7R, 0.8R, and 0.9R.
• Structures are not hemisphere symmetric. • Tachocline is clearly seen, pretty much latitudinal dependent. •(Zhao et al 2008,in preparation)
• Red and pink curves are from surface- and deep- focus, respectively. • Tachocline is surprisingly in good agreement! • Should keep in mind the experiments using simulated data show that results are not well localized. •The next step is investigate variations with the solar cycle. This requires careful analysis of MDI instrumental effects. Stereo-helioseismology observations of the tachocline in polar regions Major doal Knowledge of the high-latitude meridional circulation and differential rotation is crucial for understanding magnetic flux transport, polar field reversals, solar cycle models and predictions. These are the main ingredients of the solar dynamo.
> Are there multiple meridional cells predicted by numerical simulations? > How fast is the polar rotation? > How does the polar rotation changes with the solar cycle? > Are there fast variations of the polar rotation? > How deep are these variations? > Is there a link to rotation of the radiative interior? > How are these linked to the angular momentum |oss? > What are the structure, dynamics and variations with the solar cycle of the tachocline?
> Global helioseismology: 。256x256 images, 。1-min cadence, 36-72 days long time series o > Local helioseismology: 。1024x1024 images, 。1-min cadence, 。 3-hour series for the upper convection zone (depth 0-30 Mm); 。256x256 images, 1-min cadence, 36 days series for the tachocline and deep interior tomography.
Helioseismology Science Plan Basics > Low-resolution uninterrupted observations, 36-day time series. > High-resolution short campaigns, 3-hour runs > Coordinated observations with a ground- based or orbiting instrument > Link science objectives to the s/c position and telemetry
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