Solar Rotation Rate with Depth

Frame 61

This image is made from continuous observations over a period of twelve months, beginning in May 1996. The false colors represent speed: red material is rotating the fastest, yellow-orange is average, and dark blue the slowest. The left side of the figure shows rotation speed at the surface of the Sun; red material at the equator is moving approximately three thousand miles per hour faster than the blue material at the poles.

The cutaway on the right of the globe reveals rotation speed inside the Sun. The large dark red band is a massive fast flow of hot, electrically charged gas called plasma, beneath the solar equator. This plasma stream is approximately 300 thousand miles wide and 130 thousand miles deep. The stream moves about four percent faster than it's surrounding material.

Additionally, newly discovered but much more subtle plasma streams can be seen in the cutaway at the poles -- the light blue areas embedded in the slower moving dark blue regions. These new jet-streams occur at about 75 degrees latitude and 25,000 miles (40,000 km) below the surface. Although much smaller than the equatorial stream, these polar jets are still immense by terrestrial standards. Each is about 17,000 miles across, large enough to engulf two Earths. This material moves about ten percent faster than its surroundings.

In the Sun's outer 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 faster than the poles. The rotation can be expressed in several units of measure. The table below is in days to rotate and miles per hour (physicists tend to use nanoHz or micro-radians per second). Notice that the poles rotate very slowly - in part because the rate of rotation is intrinsically less, 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

The frames show another feature of interest recently discovered with helioseismology: a change in rotation rate with depth, just under the surface at nearly all latitudes. This shear, appearing as a very thin yellow line right near the surface at low latitudes, corresponds to an increase of about 200 mph in the top 12,000 miles (10 nano_Hz in the first few percent).

From the image you can see 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 initial result and have shown that the change actually occurs mostly just below the base of the convection zone.

Further frames show the data in ways to help enhance visibility of the important features.

(Photo Credit: Stanford University)