Astronomers Look Long At Coronal Mass Ejections
September 20, 2012

Astronomers Look To CME’s Coronal Cavity For Better Understanding

Lee Rannals for — Your Universe Online

Astronomers reported in The Astrophysical Journal they have a better understanding of the temperatures of the coronal cavities in Coronal Mass Ejections (CMEs).

Terry Kucera, a solar scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, along with colleagues, set out to understand the structure of CMEs, even before they erupt.

CMEs are eruptions that take place on the sun that can travel toward Earth and disrupt human technologies in space.

These explosive plasma structures sometimes shoot off completely into space, while other times they fall back down under their own weight, back into the sun.

"We don't really know what gets these CMEs going," Kucera said in a statement. "So we want to understand their structure before they even erupt, because then we might have a better clue about why it's erupting and perhaps even get some advance warning on when they will erupt."

The bright structure around and above the place in which the CME takes off is called a streamer, and the inside "empty" area the CME creates is called a coronal prominence cavity.

Having an understanding of temperatures, geometry and the density of cavities seen in CMEs can help scientists better understand the space weather that can disrupt technologies near Earth.

Their latest paper is the third in a series of papers, the first of which discussed cavity geometry, while the second its density.

"Our first objective was to completely pin down the morphology," Sarah Gibson, a solar scientist at the High Altitude Observatory at the National Center for Atmospheric Research (NCAR), said in a statement. "When you see such a crisp clean shape like this, it´s not an accident. That shape is telling you something about the physics of the magnetic fields creating it, and understanding those magnetic fields can also help us understand what´s at the heart of CMEs."

The team collected as much data from as many instruments and perspectives as they could for their research. They collected this information for the cavity's entire trip across the face of the sun along the sun's rotation.

Figuring out why the cavity was visible to the left of the sun, but not as visible on the right, helps identify clues about the structure´s orientation. This suggests a tunnel shape that could be viewed head on from one perspective, but was misaligned for proper viewing from the other.

In one image of a CME taken on August 9, 2007, the cavity looked like a tunnel in a crescent shape. Magnetic fields loop through the tunnel in giant circles to support the shape. The team first reported this observation in The Astrophysical Journal on December 1, 2010.

During research detailed in the second paper, the team used a variety of techniques to tease density out from temperature of the same CME they looked at for the first study.

They determined that the cavity was 30% less dense than that of the surrounding streamer. This means that there is quite a bit of material in the cavity, but it appears dim to our eyes when compared with the denser, brighter areas. This paper was published in the same journal on May 20, 2011.

"With the morphology and the density determined, we had found two of the main characteristics of the cavity, so next we focused on temperature," Kucera said in the statement. "And it turned out to be a much more complicated problem. We wanted to know if it was hotter or cooler than the surrounding material — the answer is that it is both."

The team found during their latest research that the temperature of the cavity was not hotter or cooler than the surrounding plasma. However, Kucera thinks the cavity links the much colder 17,000 degrees Fahrenheit temperature at the bottom, to the 1.8 to 3.6 million degrees Fahrenheit corona at the top.

The three papers focused on just the one cavity from 2007, but the scientists have already begun comparing this test case to other cavities and have found that the characteristics are fairly consistent.

"Our point with all of these research projects into what might seem like side streets, is ultimately to figure out the physics of magnetic fields in the corona," Gibson said in a statement. "Sometimes these cavities can be stable for days and weeks, but then suddenly erupt into a CME. We want to understand how that happens. We´re accessing so much data, so it´s an exciting time — with all these observations, our understanding is coming together to form a consistent story."