Earlier notes can be found on abc.html and old.
Text for the SSU Press Release.
Text for SSU Figure Caption 1.
Text for SSU Figure Caption 2.
Text for SSU Figure Caption 3.
(Postscript version)
A new image showing the main results combined into one figure
shows the difference between observed surface rotation and the
smooth fit, the cut into the sun showing rotation, and the
streamlines showing observed poleward flow and the expected return flow.
The movie in srot.mv and srot.mpg consists of 214 frames showing several aspects of solar rotation. NOTE: the frame counts have changed. There is now a set of 100, 10, 10, 10, 100, 5 for 235 frames total. The 5 images shown here are now frames 0, 110, 120, 130, 230. The animation sequences are frames 0-110 and 121-230. The tiff directory is still the old sequence numbers.
srot.mv (30.1Mb is an SGI movie format file.
srot.mpg (2.5Mb) is an attempt to convert the movie to mpeg. The mpeg version has poor colors, but is much much smaller and can be accessed by most web browsers.
The image frames are available as SGI .rgb files and as .tiff files. See the directories srot_rgb and srot_tiff respectively. The "tiff" files are also in a tar file.
Postscript versions are available for the 5 main images with both white and dark blue backgrounds. The white may be best for printed copies. The blue background images are numbers 0, 110, 120, 130, 230 with the corresponding white background in reverse order in frames 235, 234, 233, 232, 231 matching the 5 images below.
The primary data shown in the movie are derived from the angular velocity data from the 2dRLS inversion of solar oscillation frequencies. For detailed discussions of the methods used see: "Structure and Rotation of the Solar Interior: Initial Results from the MDI Medium-l Program", Solar Physics, 1997, in press.
The data are shown in false color projected onto a sphere.
Frame 1
shows the rotation of the solar sphere. Red color
represents faster-than-average flows, yellow is slower,
and blue is slower yet. The bulk of the sun rotates at
a rate colored between yellow and the orange-red color at
about 30 degrees latitude.
Frames 2 to 61 show the sphere rotating to reveal a cut into the sphere. The sides of the cut show the rotation rate as it varies with depth into the sun and with latitude.
Frame 61
shows the primary result of this study. Further
frames will show the data in ways to help enhance visibility
of the important features.
Note first that the inner 70% or so of the sun rotates at nearly the same rate. The variations in the deep interior are as yet uncertain. The values in the outer half by radius are more secure. The nearly "solid" rotation of the inner 70% of the Sun was an early result from helioseismology. SOHO/MDI observations have confirmed the earlier result and have shown that the change actually occurs mostly just below the base of the convection zone.
In the upper 30%, which corresponds to the solar convection zone where the Sun's energy is carried upwards by convection rather than by radiation, there is marked differential rotation. The equator rotates rather faster than the poles. The rotation can be expressed in several units of measure. While we tend to use nanoHz or micro-radians per second the table below is in days to rotate and miles per hour. Note that expressed in miles/hour the poles rotate very slowly - in part because the rate of rotation is smaller, but also because the surface near the poles is closer to the Sun's axis - i.e. the distance around the Sun is shorter.
Solar Rotation from MDI 2dRLS Inversion Latitude Days Speed (mph) -------------------------------------- Equator | 25.67 | 4410 15 degrees | 25.88 | 4230 30 degrees | 26.64 | 3680 45 degrees | 28.26 | 2830 60 degrees | 30.76 | 1840 75 degrees | 33.40 | 880
In frame 61 one can also see two features of interest that have recently been discovered with helioseismology. The first is a change with depth just under the surface at nearly all latitudes. It can be seen in frame 61 as a very thin yellow line right near the surface at low latitudes. It corresponds to an increase of about 10 nano_Hz in the first few percent, i.e. about 200 mph in the top 12,000 miles.
The second feature is a discovery made with SOI/MDI data. This new feature is a jet-stream at about 75 degrees latitude and about 40,000 km, or 25,000 miles below the surface. This feature appears as a light blue oval surrounded by darker blue on all sides. It is just under the surface of the slow blue polar region.
With the MDI observations we have also been able to measure the average poleward flow in the solar interior for the first time. The measurements show that in the top 15,000 miles (at least) there is a poleward motion in both northern and southern hemispheres with the flow speed of about 55 mph. We have NOT yet detected a return flow deeper in the convection zone, but expect that it must be present at some very slow speed. These poleward flow measurements were made be Peter Giles and Thomas Duvall using a new technique called "Time-Distance Helioseismology", developed by Duvall.
Frames 62-71 illustrate the effect of the poleward flow, by showing the streamlines of flow. These are the paths that will be followed by individual blobs of solar plasma. They are drawn from the persective of an observer at 30 degrees latitude, which corresponds to the rotation rate of the bulk of the Sun. This is also a typical latitude where solar activity forms. The combination of differential rotation and poleward flow has been the explanation for the stretched out shapes of magnetic field regions when these regions migrate to the poles and get stretched to the left. These new observations demonstrate for the first time that the poleward flow is a property of at least 12 percent of the convection zone. It is not just a surface phenomenon.
Frames 72 to 81
show a different view of the Sun's rotation. In this
figure we have removed the general variation from the equator to
the poles. We computed the smooth variation of differential
rotation in the Sun's surface, and subtracted that from the value at each latitude and
depth. The result is shown as a speed of motion rather than rotation
rate. Red-yellow is faster than average, and blue is slower than
average. This highlights the smaller scale variations in the
change in rotation from the equator to the poles.
[ For those who care, the term subtracted is of the form
A + B*sin^2(latitude) + C*sin^4(latitude)
where A = 450.78, B = -48.55, C = -67.76 all in nano-Hz]
There are clear bands of faster and slower-than-average rotation. These bands persist through at least the upper 12,000 miles.
Similar bands were detected on the surface about 15 years ago by Robert Howard and Barry LaBonte at Mt Wilson Observatory, California. They reported alternating fast and slow bands which moved from high latitudes towards the equator during the 11 years of the solar sunspot cycle. In fact they saw several bands in each hemisphere, and showed that the sunspots erupt near the poleward side of the faster streams. The streams move to the equator during the cycle, following (or at least connected with) the movement to the equator of the locations at which new sunspots appear.
The new observations just recently published (Kosovichev and Schou, "Detection of Zonal Shear Flows Beneath the Sun's Surface from F-Mode Frequency Splitting", ApJ letters, accepted, preprint) show that these bands penetrate at least 15,000 miles into the Sun. Detailed comparisons with the Mt Wilson surface observations (continued by Roger Ulrich of UCLA) show that these interior bands are indeed the same as the surface bands seen before. We now know that these bands are real and deeply rooted. They are not due to some obscure systematic observing error in the surface data. We can now go back and re-evaluate the Mt Wilson data to learn more about the solar cycle.
Frames 82 to 141 zoom and then pause. Frames 142 to 152
are a close-up
of the slice into the sun showing the difference in speed between the
local rotation and the smoothed surface rotation. The two features
described above, the polar jet stream and the general speed-up
just under the surface, are both more visible in this view.
The polar jet-stream shows up as a red oval here, showing that it is faster than the material above, below, to the north, and to the south. In fact, it is about 10% faster. Note that there is no hint of the jet on the surface. This is a feature that was not expected, and could not be detected without helioseismology.
The fine detail seen as the thin yellow strip at all latitudes from the equator to about 75 degrees shows that the surface moves slower than the regions just below over nearly the entire Sun. There were hints of this from earlier helioseismological work, but only the SOHO observations allow a clean look at this thin near-surface region.
The final frames just say goodbye to the Sun and put it back into the sky.
In addition to new analyses, we have available images and movies to show what helioseismology and SOHO/MDI are all about.
In particular there are sequences of MDI "Medium-l" observations that show the raw data used for this work. Some are available as mpeg movies or a much larger SGI movie or as a set of 300 "gif" files in one "tar" file.
Figure
3 shows the difference between observed and model sound speed
as a function of depth in the Sun. The "bump" just below 0.7 R
indicates the possible location of excess turbulence.
Figure
4 shows the inferred rotation rate as a function of depth and
latitude. Evidently the convection zone rotates uniformly along a
radius with all depths showing the differential rotation seen at the
surface. Below the convection zone is a layer of shear below which the
radiative interior seems to rotate rigidly. This shear zone which
coincides with the sound speed excess could be the region where the
solar cycle dynamo operates.
Internal ray paths Black Internal ray paths
Frame 62 to 71
