The Sun shrinks a teensy bit when it’s feel­ing ac­tive

Iran Daily - - Science & Technology -

How big is the Sun? Well, that de­pends on when you’re mea­sur­ing.

The Sun slightly shrinks and ex­pands as it goes through a so­lar cy­cle, a roughly 11-year pe­riod of high and low mag­netic ac­tiv­ity, a new study finds, sci­ence­ wrote.

When the Sun is the most ac­tive, its ra­dius de­creases by one or two kilo­me­ters, two re­searchers re­ported in a pa­per ac­cepted in the Astro­phys­i­cal Jour­nal. Given that the Sun’s full ra­dius is about 700,000 kilo­me­ters, that’s a tiny change.

As­tronomer Jeff Kuhn of the Uni­ver­sity of Hawaii in Maui said, “Un­like a planet, the Sun has no solid sur­face, so defin­ing the ab­so­lute size isn’t sim­ple. “It’s a slip­pery con­cept: What does it mean, the ra­dius of the Sun?”

One way of defin­ing the Sun’s width is based on how the bright­ness of the Sun de­creases from its cen­ter.

In 2010, Kuhn and col­leagues found no signs that the Sun’s ra­dius var­ied dur­ing the so­lar cy­cle, when based on bright­ness.

But the new study con­sid­ered a dif­fer­ent bench­mark called the seis­mic ra­dius, which is mea­sured via seis­mic waves that travel through the Sun’s in­te­rior. Any shrink­ing or ex­pand­ing of the Sun will change the fre­quency of those waves.

That dif­fer­ent yard­stick has some ad­van­tages.

Alexan­der Koso­vichev of the New Jersey In­sti­tute of Tech­nol­ogy in Ne­wark, who coau­thored the pa­per with Jeanpierre Rozelot of Univer­sité Côte d’azur in Nice, France, said, “By us­ing the seis­mic ra­dius, we can mea­sure more ac­cu­rately.”

To tease out the seis­mic ra­dius, the as­tro­physi­cists used 21 years’ worth of data on the waves’ fre­quen­cies col­lected by two space­craft.

The amount of ex­pan­sion or con­trac­tion var­ied by depth: Some lay­ers within the Sun con­tracted while oth­ers ex­panded. But the fi­nal re­sult was an over­all de­crease in seis­mic ra­dius for a more ac­tive Sun.

The new mea­sure­ment, how­ever, is not a re­place­ment for mea­sur­ing the ra­dius of the Sun’s bright­ness.

Kuhn said, “I think that’s a sep­a­rate ques­tion.”

The two mea­sure­ments rely on dif­fer­ent tech­niques, and there­fore probe dif­fer­ent as­pects of the Sun’s be­hav­ior.

Astro­physi­cist Sa­batino Sofia, who is re­tired from Yale Uni­ver­sity, said, “Still, the Sun’s seis­mic ra­dius may help sci­en­tists un­der­stand the fluc­tu­at­ing strengths of mag­netic fields at dif­fer­ent depths within the Sun, a po­ten­tial cause of the shrink­age.”

While there pre­vi­ously have been hints of vari­a­tion in the Sun’s seis­mic ra­dius, the ad­di­tional data re­ally con­firms that dur­ing the ac­tiv­ity cy­cle, the seis­mic ra­dius of the Sun is chang­ing.

sci­ence­ The ra­dius of the Sun gets a smidge smaller dur­ing pe­ri­ods when the Sun is most ac­tive, a new study re­ported.

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