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SOI/MDI After Two Years

A presentation to NASA and ESA by Philip H. Scherrer
18 March 1998


Mode Frequencies
Mode Physics
Internal Structure
Internal Rotation
Interior Dynamics
Developing Spot Region
Oblateness Determination
Seismic Response to Flare
Fields and Supergranulation
Magnetic Carpet

Mode Frequencies

Solar p-mode frequencies are the basic input data to helioseismology inversions of interior structure and rotation. This plot shows frequencies determined from the MDI medium-l program. This figure shows both individual mode fits for lower degree l and "ridge" fits for higher l. Additional figures and information can be found here. The dynamics program allows ridge determination up to greater than l=1000. That work is in progress.

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Mode Physics

P-mode spectral lines show asymmetry that reveals information about the region of the source of acoustic noise in the Sun. A preprint of a note to ApJ is available. Figures are here.

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Internal Structure

The solar interior structure is shown as differential sound speed profiles. The form is a ratio of observed (inferred) sound speed-squared to the best current solar model (same quantity). There are three regions where the model does not yet reflect the actual Sun. In the core there is evidence of higher sound speed in the very center and a lower than expected sound speed around 0.2 R, this would suggest some "stirring" of the core. There is a bump in sound speed just beneath the base of the convection zone. Again some evidence for turbulence affecting mean molecular weight. There is also a discrepency at the surface which may be related to a lower effective seismic radius (see below).

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Analysis of the f-mode frequencies shows that the effective siesmic radius of the Sun is a bit smaller than the tau=1 radius that is traditionally used to calibrate solar models. The reason for this is not understood but the siesmic-radius leads to a difference in the sound speed profile with depth that may explain the discrepency between the observations and the model near the surface. A preprint is available.

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Internal Rotation

Inversions using mode frequency splittings reveal interior rotation. A detailed study using 8 inversion codes and testing with artifical data has resulted in greater confidence in deductions. The deep interior and regions near the rotation axis are the most uncertain. The results are shown as contour plots of omega and as a colored image of omega.

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The inferred internal rotation can also be viewed as colors representing rotation rate. Redder is faster, bluer is slower. The high latitude "jet stream" detected in the "2dRLS" inversion method is visible in this image. This image and several others were prepared for a NASA Space Science Update in August 1997.

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Analysis of the f-mode frequencies has allowed unambiguous detection of the zonal flow pattern reported a cycle ago from Mt. Wilson observations. These faster and slower "streams" migrate to the equator during the cycle. The drift has been verified with 5 72-day segments of MDI medium-l p-mode data. The plot below is the original result with the f-mode analysis. A preprint is available.

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Interior Dynamics

Time-Distance methods provide a method to infer sound-speed variations and flows in the interior. This vertical slice shows a cross-section of supergranulation. This analysis shows that supergranulation extends something like 6-8 Mm into the interior vs 30Mm diameter. The Time-Distance method is fully described in a preprint.>

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Oblateness Determination

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Developing Spot Region

Time-Distance methods also allow watching the development of an erupting active region. The following frames show several quantities from a nine-day sequence showing the development of an active region in July 1996. This is the same region that had the flare discussed below. These figures show surface continuum brightness, surface magnetic fields, interior sound speed variations, interior vector component motions.

This frame shows a cut from the end of the 9-day sequence when the region was near the west limb. As shown the "bottom" of the box is at the solar surface with depth increasing "upwards". The image plane containing the continuum data has been displaced to the "top" of the box so that the locations of the spots can be seen. The surface plane shown tha magnetogram. The depth cross-section shows sound speed variations. It is clear that just below sunspots the sund speed is reduced in a compact region. Beneath the lower wave-speed region is large diffuse region of increased wave speed. The box contains only 6-planes from the inversion so all features will appear stretched vertically. The fast speed region extends to about 6Mm.

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A similar view but without the continuum image and with different color table.

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This frame is again from a viewpoint inside the sun looking up to the north-west. In this cast the vertical cuts show velocity components. The N-S cut shows E-W velocity (+ to the west shown in red) and the E-W cut shows N-S velocity (+ is to the north and shown in red). The surface plane shows the continuum. It can be seen that in the region just beneath the spots there is a flow toward the spots while the flow is reversed in deeper layers. Horizontal cuts of these same parameters show that the flow region is constrained to coincide about with the horizontal extend of the active region.

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This frame shows a horizontal slice of vertical velocity. Red is down, blue is up. The depth is set about midway in the box. There are clear indications of downflow in the broad region surrounding the spots with a hint of an upflow around the margins of the active region. The continuum is overlaid on the velocity field.

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Seismic Response to Flare

The following pages are from a preprint describing the detection of ripples of acoustic waves from a moderately large flare.

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Fields and Supergranulation

MDI simultaneous observations of granulation and magnetic fields has allowed detailed comparisons of supergranular flow and magnetic fields. The fields are seen to be in the supergranule boundries. The field is seen to exist in small clumps which migrate along the flow-convergence lines and collect at the vertices. Small bipolar field elements can appear at any location but quickly the two components migrate to the supergranule boundries where they either cancel existing field or replace it. Some details are here.

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Magnetic Carpet

Magnetic Carpet - These images show combinations of EIT data, MDI magnetogram data and field line extrapolations. They show the high correlation between bright points in EIT and complex magnetic field clumps.

There is a more detailed description and more images here.

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Page last revised Wednesday, 05-Jan-2000 13:44:33 PST