Nation & World

El sol es más delgado de lo que se pensaba

Un grupod e investigadores ha encontrado que la parte media del sol es considerablemente más delgada delos que se creía. No solo eso, la forma del sol en general no cambia tanto durante el ciclo de manchas solares aunque los científicos pensaron que una pequeña sección de su parte externa podría estar rotando con más lentitud delo esperado.

Las fuerza de la rotación hacen que cualquier burbuja de gas se alise enlos polos y la parte más sobresaliente del ecuador, creando una forma distinta. En nuestro sisema solar, Satoruno, que se mueve a gran velocidad, es el más delgado de los planetas, y su diámetropolar es de 12,000 kilómetros. LA tierra, que rota una vez cada 24horas, tiene una prominencia en su parte media de 43 kilómetros.

Aunque el sol rota relativamente poco una vez cada 27 days or so-it, too, is oblate, says Jeffrey Kuhn, a solar physicist at the University of Hawaii’s Institute for Astronomy in Pukalani. But the distance of the sun’s surface from its center depends on several factors other than its gravitational field, he notes, including the sun’s inner rotation; the convection and turbulence in its outer layers; and magnetic forces, particularly those around sunspots. Previous data have suggested that the sun’s oblateness varies with the solar cycle, with the overall shape subtly changing as the number of sunspots waxes and wanes during the 11-year period.

But data recently gathered by sensors onboard NASA’s Solar Dynamics Observatory (SDO), a satellite that continuously watches the sun from geosynchronous orbit, reveals the sun’s shape is steadier and more nearly spherical than previously thought, Kuhn and his colleagues report online Thursday in Science. SDO takes more than 15,000 images of the sun each day, but for this new study the team analyzed images taken only twice each year, when the satellite’s instruments are calibrated. During these times, SDO continues to stare at the sun but rotates slowly through a full circle-a maneuver that lets scientists analytically remove any distortions caused by the camera’s optics, presumably revealing the sun’s true shape.

Analyses of data gathered during five calibration cycles over the past 2 years-an interval when solar activity has risen from near nil to near maximum, with 90 or more sunspots visible on the surface each day-indicate that the sun’s oblateness is remarkably steady despite the large changes in magnetic activity at its surface. Results suggest the sun’s equatorial bulge is only 12 kilometers or so, compared with its average diameter of nearly 1.4 million kilometers. Previous data suggesting that the sun’s shape varies during a solar cycle, which were gathered by ground-based instruments, may have been affected by atmospheric turbulence-the same phenomenon that makes distant stars twinkle, Kuhn notes.

Besides the unexpected steadiness of the sun’s shape, the team found that the sun’s oblateness is about 25% smaller than previously estimated, Kuhn says. The oddly low measurement may be caused by misunderstandings about how turbulence or magnetic forces influence the outermost layers of the sun, he notes, “but neither of those adjustments would agree with our current models.” Instead, he suggests, it’s most likely that the layers of the sun just below its visible surface are rotating between 3% and 10% more slowly than expected.

“For 50 years we’ve been working to understand how oblateness changes over the solar cycle,” says Philip Goode, a solar physicist at Big Bear Solar Observatory in California who was not involved in the study. The new findings present a strong argument that whatever changes are making oblateness smaller than expected are taking place in the outermost layers of the sun, he says.

The work “is the best attempt so far” at measuring the sun’s oblateness, says Douglas Gough, an astrophysicist at University of Cambridge in England. Also, the team’s lower-than-expected estimate for the sun’s oblateness “is a fascinating result.” However, he adds, “it appears to me that a change in rotation is unlikely to explain this.”

An alternate explanation for the result, Gough suggests, might be that the atmosphere over the sun’s polar regions is slightly different than that over the sun’s equator. Such a disparity, he notes, could cause distortions in the paths of light headed to Earth from the various regions, thereby skewing the estimate of oblateness lower than expected.

- - -

This is adapted from ScienceNOW, the online daily news service of the journal Science.

bc-sun AMX-2012-08-17T21:13:00-04:00