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MessageToEagle.com – Coronal cavities are voids in coronal emission often observed above high latitude filament channels.

Sometimes, these cavities have areas of bright X-ray emission in their centers.

Now, NASA scientists focus on this mysterious phenomenon because it seems to be strongly related to dangerous coronal mass ejections (CMEs).
And CMEs, scientists have to worry about.

Click on image to enlargeThe faint oval hovering above the upper left limb of the sun in this picture is known as a coronal cavity. NASA’s Solar and Terrestrial Relations Observatory (STEREO) captured this image on Aug. 9, 2007. A team of scientists extensively studied this particular cavity in order to understand more about the structure and magnetic fields in the sun’s atmosphere. Credit: NASA/STEREO
The Sun’s abnormal behavior is a cause for concern even we all know that activity near the Sun’s surface rises and falls through an 11-year cycle that is due to peak in 2013 or 2014.

What causes giant explosions in the sun’s atmosphere? How do they form?

The cavity as it appeared on the west limb on 2008 July 21. The location of the cavity is indicated by a box. Source
Scientists want to better understand the complexity of solar activity and its dangerous events. It’s crucial to find a way to predict them and minimize the damage they might cause.

“We don’t really know what gets these CMEs going,” says Terry Kucera, a solar scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md. “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.”

A team of researchers used NASA data to study a precursor of CMEs called coronal cavities – a larger formation, appearing from the side almost as the filament inside a large light bulb.

The bright structure around and above that light bulb is called a streamer, and the inside “empty” area is called a coronal prominence cavity.

According to scientists, the cavity is in the shape of a croissant, with a giant inner tube of looping magnetic fields.

The cavity appears to be 30% less dense than the streamer surrounding it, and the temperatures vary greatly throughout the cavity, but on average range from 1.4 million to 1.7 million Celsius (2.5 to 3 million Fahrenheit), increasing with height.

Of course, to describe a cavity, a space that appears empty from our viewpoint, from 93 million miles away – is not any easy task for scientists.

However, scientists recognize the cavities as basic building blocks of the coronal magnetic field and an important part of the development of solar activity.

The evolution of the part of the cavity over several days – visible from Earth, July 2008. Source

The Aug. 9, 2012 cavity lay at a fortuitous angle that maximized observations of the cavity itself, as opposed to the prominence at its base or the surrounding plasma.

“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,” says Sarah Gibson, a solar scientist at the High Altitude Observatory at the National Center for Atmospheric Research (NCAR) in Boulder, Colo.
Gibson is also an author on all three cavity papers.

“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.”

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