The Sun Goes Round and (Less) Round

OK, I’ll admit it: With so NASA many missions under way right now, it’s hard to keep track of what they’re all up to. That’s why it was a little surprising to learn this week of an intriguing new result from a scientific spacecraft that’s been off my radar screen for some time.

The Reuven Ramaty High-Energy Solar Spectroscopic Imager &mdash called RHESSI, though it was just HESSI before being renamed to honor a pioneering solar physicist — rocketed into space on February 5, 2002. Since then it’s been scrutinizing the Sun’s face with X- and gamma-ray imagers, primarily to probe how energy is released and propagates during solar flares.

The Sun's changing shape
The Sun is a not-quite-perfect sphere, as determined recently by a NASA spacecraft called RHESSI. Its measurements, combined with prior ones obtained by the SOHO spacecraft and a balloon-borne experiment, show that the Sun's oblateness varies throughout the rise and fall of the 11-year-long solar cycle (as indicated by its radio flux).
M. Fivian and others / Science Express
Apparently RHESSI has been moonlighting as a solar yardstick, measuring the Sun’s dimensions with unprecedented accuracy. Our star’s spin rate, once every 25.3 days at the equator, causes its midsection to bulge outward ever so slightly relative to its poles. The predicted value of this oblateness is 7.8 milli-arcseconds, about the apparent size of a dime in Boston as seen from San Francisco. It’s only about 0.0001% of the Sun’s diameter — not the sort of deviation that’s easy to detect, let alone measure.

Yet in this week’s Science Express, where Science trumpets articles before they appear in print, a quartet of researchers led by Martin Fivian (University of California, Berkeley) announced that the Sun is a little more oblate than predicted, a hair more than 8 milli-arcseconds. Moreover, when the team combined RHESSI’s measurements with those acquired previously, it found that the polar flattening becomes even more pronounced, by another 10.8 milli-arcseconds, during times of high solar activity.

Photographs filtered for the blue emission line of singly ionized calcium show the magnetic granules responsible for the Sun's unexpected flattening.
National Solar Observatory, Sunspot, New Mexico
These results might seem trivial, but solar physicists assure us they are not. The changing girth arises in magnetic ridges on the Sun’s surface that mimic, subtly, the texture of a cantaloupe’s skin. The deviation from a perfect sphere has implications for how the Sun pulls on Mercury, how the solar core is shaped, and perhaps how acoustic waves propagate throughout the solar interior.

Fortunately, the variable oblateness is far too inconsequential to affect predictions for the durations of solar eclipses — except for the most ardent purists. As diehard “umbraphile” Glenn Schneider points out, eclipse calculations usually consider the shape of the solar and lunar disks only to a precision of about 1 arcsecond, 50 to 100 times greater than what RHESSI measured.

5 thoughts on “The Sun Goes Round and (Less) Round

  1. Richard Carroll

    I’d like to hear more about how this affects Mercury’s orbit. Does it affect the rate of change of the direction to Mercury’s aphelion, a test of relativity? Does it increase the degree to which Mercury’s orbital plane is pulled toward the plane of the sun’s equator? Perhaps the 2/3 resonance of Mercury’s rotation and revolution?

  2. Daniel Berke

    So what, exactly, is causing the extra oblateness? Powerful magnetic fields flattening the sun? If the “changing girth arises in magnetic ridges on the Sun’s surface that mimic, subtly, the texture of a cantaloupe’s skin” is it really the oblatness of the Sun that is affected, or is RHESSI just measuring local variations on the Sun’s surface?

    Also, I too would like to hear about how Mercury’s orbit is affected.

  3. Daniel Berke

    So what, exactly, is causing the extra oblateness? Powerful magnetic fields flattening the sun? If the “changing girth arises in magnetic ridges on the Sun’s surface that mimic, subtly, the texture of a cantaloupe’s skin” is it really the oblatness of the Sun that is affected, or is RHESSI just measuring local variations on the Sun’s surface?

    Also, I too would like to hear about how Mercury’s orbit is affected.

  4. Rod Bernitt

    Does this report hint that current solar interior models according to stellar evolution theory may not be accurate, at least in some parts?

  5. Astronomical Unit

    It is not mentioned that in everyday units the swelling of the Sun by 10.8 mas is just about 5 miles! Indeed, a very tiny increase in the radius of the Sun. For comparison, a similar change in the Earth’s radius would amount to 80 meters or 250 feet: locally impressive waves, but even more impressive is the ability to measure them from so far away.

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