Fire Burn and Cauldron Bubble
March 13, 1997
Watch a pot of spaghetti sauce bubbling on the stove and you will see a model of how much of the solar wind streams from the sun across the solar system.
"There's been an open question for some time about the origins of the solar wind," said Dr. Steve Suess, a solar physicist in the Space Sciences Laboratory (SSL) here at NASA's Marshall Space Flight Center. Almost a quarter-century ago, X-ray telescope images from Marshall's Skylab space station showed coronal "holes" that appeared to be the source of the solar wind. Where the corona - the sun's outer atmosphere - is heated to a million degrees or so, it glows in X-rays. The holes were cooler regions, only a few hundred thousand degrees hot, where the solar atmosphere would spill out into interplanetary space.
"The model now is that it continually leaks out of the streamers," Suess said. "Streamers are what you see that's bright during an eclipse," or by using a telescope called a coronagraph which blocks the bright disk of the sun so the relatively faint corona can be seen.
Blue circle at left highlights bubble about to break loose in a solar streamer. Click to get the whole picture (230K) and an explanation of the two images in it.
First you build a model...
In recent work with Shi-Tsan Wu and Ai-Hua Wang at the Center for Space Plasma, Aeronomy, and Astrophysics Research (CSPAR) at the University of Alabama in Huntsville, Suess used computers to model the activity the flow of gases in the corona. When extra heat was pumped into the model, the gases started to form bubbles that would pull away from the solar magnetic field and sail into space. Suess, Wu, and Wang published a paper predicting this effect and at the same time, the bubbles were discovered by the Solar and Heliospheric Observatory (SOHO) built by the European Space Agency and operated by NASA's Goddard Space Flight Center.
They will present their results at the spring meeting of the American Geophysical Union in Baltimore this May in a session on the origin of the slow solar wind and dynamics of the equatorial streamer belt.
"This was such a surprise because the sun's at solar minimum," the low point of its 11-year sunspot cycle," Suess said. "Big ejections almost never happen at this time of the cycle."
Emissions from the sun gained new attention in January when some scientists suggested that the Telstar 401 communications satellite was zapped by a coronal mass ejection (CME; also visible in the full version of the image above). But a clue came in 1994 when the Ulysses probe soared over the sun's poles. While Ulysses was over the mid-latitudes, it saw the solar wind blazing out at 700 to 800 kmilometers per second (km/s, about 1.6 million to 1.8 million mph), but from about 70 degrees latitude to the poles the wind sputtered at 400 to 500 km/s (about 900,000 to 1.1 million mph).
Suess said this leakage is similar to the bubbling at the top of a pot of spaghetti sauce or oat meal. As the bubbles form they are held in place by viscous material above. In the case of the sun, the viscous material actually is the solar magnetic field which is aligned up-down, rather than horizontal, near the poles.
...and bring to a full boil
"There's no place for the heat to go in magnetically closed regions," Suess continued. "It starts puffing up the streamer, and after a while it bubbles away." As in the sauce, enough heat collects in a small spot to break the surface -- the magnetic field in the sun's case -- and a puff of hot gas escapes.
"The bubbling explains why they're so irregular. They're not being blown out, they're really being ejected, so I'm hesitant to call it a real coronal mass ejection," he added.
The coronal bubbling was found by other astronomers using SOHO's Large Aperture and Spectrometric Coronal Observatory (LASCO) provided by the Naval Research Laboratory. It has greater sensitivity and resolution than previous coronagraphs, and thus was able to pick out the bubbles in the corona."What really brought it home was LASCO," Suess said. In addition, the sun is at solar minimum, so the equatorial regions have less activity that might blur the bubbles.
For those who want to dig in, the papers are
Dynamical evolution of a coronal streamer-bubble system I: A self-consistent MHD simulation. Solar Phys. 157, 325. 1995.
Dynamical evolution of a coronal streamer-flux rope system II: A self consistent non-planar MHD simulation. Solar Phys. 1997 (in press).