NASA Spacecraft Finds the Sun is Not a Perfect Sphere

Oct 02, 2008
In this diagram, the sun's oblateness has been magnified 10,000 times for easy visibility. The blue curve traces the sun's shape averaged over a three month period. The black asterisked curve traces a shorter 10-day average. The "wiggles" in the 10-day curve are real, caused by strong magnetic ridges in the vicinity of sunspots.(NASA Goddard Space Flight Center)

(PhysOrg.com) -- Scientists using NASA’s RHESSI spacecraft have measured the roundness of the sun with unprecedented precision. They find that it is not a perfect sphere. During years of high solar activity the sun develops a thin “cantaloupe skin” that significantly increases its apparent oblateness: the sun’s equatorial radius becomes slightly larger than its polar radius. Their results appear the Oct. 2nd edition of Science Express.

“The sun is the biggest and therefore smoothest object in the solar system, perfect at the 0.001% level because of its extremely strong gravity,” says study co-author Hugh Hudson of UC Berkeley. “Measuring its exact shape is no easy task.”

The team accomplished the task by analyzing data from the Reuven Ramaty High-Energy Solar Spectroscopic Imager, RHESSI for short, an x-ray/gamma-ray space telescope launched in 2002 on a mission to study solar flares. Although RHESSI was never intended to measure the roundness of the sun, it has turned out ideal for the purpose. RHESSI observes the solar disk through a narrow slit and spins at 15 rpm. The spacecraft’s rapid rotation and high data sampling rate (necessary to catch fast solar flares) make it possible for investigators to trace the shape of the sun with systematic errors much less than any previous study. Their technique is particularly sensitive to small differences in polar vs. equatorial radius or “oblateness.”

“We have found that the surface of the sun has rough structure: bright ridges arranged in a network pattern, as on the surface of a cantaloupe but much more subtle,” describes Hudson. During active phases of the solar cycle, these ridges emerge around the sun’s equator, brightening and fattening the “stellar waist.” At the time of RHESSI’s measurements in 2004, ridges increased the sun’s apparent equatorial radius by an angle of 10.77 +- 0.44 milli-arcseconds, or about the same as the width of a human hair viewed one mile away.

“That may sound like a very small angle, but it is in fact significant,” says Alexei Pevtsov, RHESSI Program Scientist at NASA Headquarters. Tiny departures from perfect roundness can, for example, affect the sun’s gravitational pull on Mercury and skew tests of Einstein’s theory of relativity that depend on careful measurements of the inner planet’s orbit. Small bulges are also telltale signs of hidden motions inside the sun. For instance, if the sun had a rapidly rotating core left over from early stages of star formation, and if that core were tilted with respect to its outer layers, the result would be surface bulging. “RHESSI’s precision measurements place severe constraints on any such models.”

The “cantaloupe ridges” are magnetic in nature. They outline giant, bubbling convection cells on the surface of the sun called “supergranules.” Supergranules are like bubbles in a pot of boiling water amplified to the scale of a star; on the sun they measure some 30,000 km across (twice as wide as Earth) and are made of seething hot magnetized plasma. Magnetic fields at the center of these bubbles are swept out to the edge where they form ridges of magnetism. The ridges are most prominent during years around Solar Max when the sun’s inner dynamo “revs up” to produce the strongest magnetic fields. Solar physicists have known about supergranules and the magnetic network they produce for many years, but only now has RHESSI revealed their unexpected connection to the sun’s oblateness.

“When we subtract the effect of the magnetic network, we get a ‘true’ measure of the sun’s shape resulting from gravitational forces and motions alone,” says Hudson. “The corrected oblateness of the non-magnetic sun is 8.01 +- 0.14 milli arcseconds, near the value expected from simple rotation.”

Further analysis of RHESSI oblateness data may help researchers detect a long-sought type of seismic wave echoing through the interior of the sun: the gravitational oscillation or “g-mode.” Detecting g-modes would open a new frontier in solar physics—the study of the sun’s internal core.

The paper reporting these results, “A large excess in apparent solar oblateness due to surface magnetism,” was authored by Martin Fivian, Hugh Hudson, Robert Lin and Jabran Zahid, and appears in the Oct. 2nd issue of Science Express.

Provided by NASA, by Dr. Tony Phillips

Explore further: Heavy metal frost? A new look at a Venusian mystery

add to favorites email to friend print save as pdf

Related Stories

Galileo duo handed over in excellent shape

8 hours ago

A pair of fully functioning Galileo navigation satellites was recently delivered to its operators, as preparations get underway for the next round of launches.

Study of electrons in space could help weather forecasting

Oct 14, 2014

Researchers have discovered a formerly undetected impact of space weather on the polar atmosphere, which may explain some previously unexplained variations in winter weather patterns. Their results, published today (Tuesday ...

Recommended for you

Heavy metal frost? A new look at a Venusian mystery

8 hours ago

Venus is hiding something beneath its brilliant shroud of clouds: a first order mystery about the planet that researchers may be a little closer to solving because of a new re-analysis of twenty-year-old ...

Hot explosions on the cool sun

14 hours ago

(Phys.org) —The Sun is more spirited than previously thought. Apart from the solar eruptions, huge bursts of particles and radiation from the outer atmosphere of our star, also the cooler layer right below ...

Europe secures new generation of weather satellites

14 hours ago

Contracts were signed today to build three pairs of MetOp Second Generation satellites, ensuring the continuity of essential information for global weather forecasting and climate monitoring for decades to ...

User comments : 5

Adjust slider to filter visible comments by rank

Display comments: newest first

skye
1 / 5 (2) Oct 03, 2008
Because there is a nearby small brown dwarf star pulling at it. And i mean nearby as in it is already making its path back near the edge of our solar system. And guess what, thats what is also causing global warming, because its pulling at our orbit. I'm guessing that 2012 will probably be the year it is the closest to us and at peak destructive gravitational pulling of the earth.
You dont have to realize the truth, but if you want to verify what I think, then please start researching nibiru, and the legends and stories of the original tribes and people on earth. They all say the same thing. That people descended from the heavens and created them to help them with their work. If you understand what they are saying, then you should start to understand that the Illuminati are not satanist, they worship their gods, the aliens from nibiru. All the blood sacrifices on both sides of the worlds to their gods, the pyramids, advanced civilizations, they were all guided by a higher presence. Life is AMAZING, don't you feel like that getting a job, paying taxes is going against the grain of life? Its basically slavery..A cage built of fear, loneliness, money, and peoples expectations. Everyone should be allowed to live their life doing whatever they want to do, unless that involves hurting someone else ;). Getting up and going to work every day feels so pointless, its depressing.. seriously. I would love to be able to do what I want to do, not WORK MOST OF MY LIFE. But you need money to do anything in society.. what a stupid system. Thank god nature and life still provide plenty of free pleasures and enjoyments.
wonko
not rated yet Oct 04, 2008
Oh, I think you missed something...

Ni.Bi.Ru came out 2005. It even got some quite nice reviews. But point-and-click is not everyones taste...
Alizee
Oct 08, 2008
This comment has been removed by a moderator.
VPXL
1 / 5 (1) Oct 09, 2008
Does it really matter whether the Sun is a perfect sphere or not?
http://www.fdadrugstore.org - my favorite web site
bmcghie
not rated yet Oct 09, 2008
I was kind of surprised at the headline. I always assumed that, like the earth, objects undergoing rotation are going to be distorted. Unless you have infinite gravity, there is going to be some distortion around the equator. That said, the sunspot distortions are cool!
pills
not rated yet Nov 16, 2008