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Signs of Sun’s ripples reported, after 30-year search

May 3, 2007
Courtesy ESA
and World Science staff

Scientists say they may have glimpsed long-sought rip­ples on the Sun’s sur­face. These would re­veal in­for­ma­tion about the Sun’s core—in par­tic­u­lar, that it seems to be spin­ning faster than the rest of the star—and how the Sun formed, the re­search­ers add.

Artist's con­cep­tion of the SO­HO space­craft prob­ing sig­nals from the so­lar in­te­ri­or. (Cour­te­sy SO­HO)

Astronomers believe un­der­stand­ing the so­lar co­re could help clar­i­fy how the whole So­lar Sys­tem formed. This is be­cause the co­re rep­re­sents the hub of ro­ta­tion for the dusty gas cloud that even­tu­al­ly formed the Sun, 4.6 bil­lion years ago, and the sur­round­ing plan­ets.

The rip­ples would ap­pear as mi­nus­cule vari­a­tions in the over­all move­ment of the sur­face of the hot gas ball that is the Sun. As­tro­no­mers have been seek­ing them since the 1970s, when they first de­tected that the so­lar sur­face was os­cil­lat­ing in and out.

The rip­ples, called “g-modes,” are driv­en by grav­i­ty and thought to oc­cur when gas churn­ing in­side the star plunges even deeper and slams in­to dens­er ma­te­ri­al. This sends waves through the Sun and to the sur­face.

Un­for­tu­nate­ly for ob­servers, these waves are bad­ly de­grad­ed along their trip. By the time they break sur­face, they’re lit­tle more than a few me­tres (yards) high. That’s less than one hundred-millionth of the Sun’s own width. To make mat­ters harder, the rip­ples take be­tween two and se­ven hours to rise up and down just once.

Now, as­tro­no­mers us­ing the So­lar and He­lio­spheric Ob­serv­a­to­ry (SO­HO), a space­craft of NASA and the Eu­ro­pe­an Space Agen­cy, think they may have caught glimpses of this be­hav­iour. In­stead of look­ing for in­di­vid­ual os­cil­la­tions, they looked for a sig­na­ture of the cu­mu­la­tive ef­fect of many. By anal­o­gy, im­ag­ine the Sun as a mas­sive pia­no play­ing all notes si­mul­ta­ne­ous­ly. In­stead of look­ing for one note, such as the mid­dle C, it would be eas­i­er to search for all the “C’s”, play­ing to­geth­er. Eve­ry 12th key is a C.

“We must be cau­tious but if this de­tec­tion is con­firmed, it will open a brand new way to study the Sun’s co­re,” said said Raf­a­el A. Gar­cia, a mem­ber of the re­search team. The group used an in­stru­ment aboard the space­craft called Glob­al Os­cil­la­tion at Low Fre­quen­cy, or GOLF.

Un­til now, the so­lar co­re’s ro­ta­tion rate was un­cer­tain. If the de­tec­tion is con­firmed, it will show the co­re is spin­ning faster than the sur­face, Gar­cia said. “We hope to im­prove this de­tec­tion and open up a new branch of so­lar sci­ence,” he added.

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Scientists report that they may have glimpsed long-sought ripples on the Sun’s surface. These would reveal information about the Sun’s core—in particular, that it seems to be spinning faster than the rest of the star—and how the Sun formed, the invest igators say. Researchers think understanding the star’s core could help clarify how the whole Solar System formed. This is because the core represents the hub of rotation for the dusty gas cloud that eventually formed the Sun, 4.6 billion years ago, and the surrounding planets. The ripples would appear as extremely subtle variations in the overall movement of the surface of the hot gas ball that is the Sun. Astronomers have been seeking them since the 1970s, when they first detected that the solar surface was oscillating in and out. The ripples, also called “g-modes,” are driven by gravity and thought to occur when gas churning inside the star plunges even deeper and slams into denser material. This sends waves through the Sun and to the surface. Unfortunately for observers, these waves are badly degraded one their way out. By the time they break surface, they’re little more than a few metres (yards) high. That’s less than one hundred-millionth of the Sun’s total width. To make matters harder, the ripples take between two and seven hours to rise up and down just once. Now, astronomers using the Solar and Heliospheric Observatory (SOHO), a spacecraft of NASA and the European Space Agency, think they may have caught glimpses of this behaviour. Instead of looking for indi vidual oscillations, they looked for a signature of the cumulative effect of many. By analogy, imagine the Sun as a massive piano playing all notes simultaneously. Instead of looking for one note, such as the middle C, it would be easier to search for all the “C’s”, playing together. Every 12th key is a C. “We must be cautious but if this detection is confirmed, it will open a brand new way to study the Sun’s core,” said said Rafael A. Garcia, a member of the research team. The group used an instrument aboard the spacecraft called Global Oscillation at Low Frequency, or GOLF. Until now, the rotation rate of the solar core was uncertain. If the detection is confirmed, it will show that the solar core is rotating faster than the surface, Garcia said. “We hope to improve this detection and open up a new branch of solar science,” he added.