Shaping up the sun

The most accurate measurements yet account for magnetic activity that gives the star a more oval appearance.

With its turbulent structure, tangled magnetic fields and a propensity to expel billion-ton clouds of charged particles, the sun makes it difficult to measure its exact shape.

Now researchers using a NASA spacecraft called RHESSI, the Reuven Ramaty High Energy Solar Spectroscopic Imager, have measured with unprecedented accuracy just how out-of-round the sun is.

The measurements reveal that the sun’s magnetic fields, already known to figure prominently in solar outbursts, also make the sun appear slightly more elongated, or flattened, during times of high solar activity. During the sun’s 11-year activity cycle, the magnetic activity and explosive nature of the star waxes and wanes.

Martin Fivian and Hugh Hudson of the University of California, Berkeley and their colleagues were initially perplexed by the RHESSI measurements. A visible-light telescope on the craft indicated that during 2004 — the most recent peak in the solar activity cycle — the sun increased its roughly 1.4-million–kilometer waistline by about 13 kilometers, the team reports online October 2 in Science.

Although that increase is only a tiny percent, it’s still significantly more than can be accounted for by the sun’s rate of rotation, which varies from equator to pole. Because the sun is a giant ball of gas, rather than a solid object, different parts of the star rotate at different rates. The equator rotates fastest, completing a full rotation in a period of about 25 days, while the poles rotate more slowly, taking about 36 days.

This difference causes the sun to bulge ever so slightly, endowing the star with an equatorial diameter, or waistline, that’s just a tiny bit bigger than its polar diameter. However, RHESSI’s measurements of the diameter of the equator couldn’t be fully explained by the faster rotation.

The team then compared the RHESSI observations with extreme-ultraviolet images taken by another spacecraft, the Solar and Heliospheric Observatory. Those images track magnetic activity at the solar surface. In combination with the RHESSI measurements, the extreme-ultraviolet images suggest that the sun has a rough structure of bright ridges arranged in a network, like the surface of a cantaloupe.

At solar maximum, the ridges, which are created by the sun’s magnetic field, emerge and are most prominent at the sun’s equator, both brightening and adding heft to the star’s waist.

By subtracting the effect of this magnetic network, Fivian, Hudson and their colleagues obtained a true measure of the sun’s shape, which is indeed more rounded and matches the geometry produced by the sun’s rotation. Accurately assessing the sun’s shape is critical for measuring the object’s gravitational pull on Mercury and testing Einstein’s theory of general relativity, as well as for probing the activity within the star’s roiling interior.

“The RHESSI measurements show that the sun only appears to be oval,” comments Gary Chapman of the CaliforniaStateUniversity, Northridge. “In reality, the quiet sun — the sun devoid of magnetic features — is oval, or oblate, by about 8 parts per million, which is what one expects from the known rotation of the sun,” he adds. “The excess oblateness determined from RHESSI is from the brightness associated with magnetic activity” and suggests that the rotation of the inner part of the sun is probably close to the surface rate, Chapman notes.

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