Scientists have peered beneath the surface of the Sun to discover how large areas of stormy solar activity, called active regions, form and grow. Additionally, they’ve got their best look yet at why sunspots — dark blotches on the solar surface, often grouped in active regions — sometimes go for a spin.
“These discoveries are showing us that the Sun’s interior is much more complex and dynamic than we thought,” said Prof. Philip Scherrer of Stanford University who is Principal Investigator of the SOHO/MDI project. “This emerging picture tells us that understanding violent solar activity, which is driven by turbulence within the Sun, will be more challenging.”
Two teams of scientists used the Michelson Doppler Imager (MDI) instrument on board the Solar and Heliospheric Observatory (SOHO) spacecraft to infer the sub-surface structure of selected areas on the Sun by analyzing sound-generated ripples on its surface, using a technique similar to ultrasound diagnostics at a medical laboratory. They were able to construct a picture of magnetic structures inside the Sun because sound travels faster in solar regions with a strong magnetic field.
The results are the topic of a press conference scheduled for December 10 at 10:00 a.m. PST in the Moscone Convention Center, San Francisco, during the fall meeting of the American Geophysical Union.
Active regions are sites of fierce activity, generating explosions called solar flares and eruptions of electrified and magnetized gas (plasma) called Coronal Mass Ejections (CMEs). Scientists know this activity is driven by distorted magnetic fields that suddenly snap to a new, less energetic configuration, and that active regions are sites of strong magnetic fields.
By peering beneath the surface of “AR 9393,” the largest active region in the current solar cycle, a team led by Dr. Alexander Kosovichev of Stanford University found that such regions are comprised of many small magnetic structures that rise quickly from deep within the Sun. At one point last year, AR 9393 stretched 150,000 miles (240,000 kilometers) across the Sun, more than 18 times the diameter of the Earth.
“We thought active regions had a simple structure,” said Kosovichev. “But instead of one large tube-like magnetic structure that rises from deep inside the Sun, we find that active regions are made up of many small magnetic structures emerging at adjacent locations. Moreover, the magnetic structures are replenished by others as they emerge, which makes the active region grow.”
While clarifying the structure of active regions, the new details engender many more questions. It’s not yet known why a given region on the solar surface can suddenly erupt with magnetic structures and become active, or what causes the active region to be replenished by magnetic “reinforcements”. According to the researchers, their data extends about 62,000 miles (100,000 kilometers) inside the Sun — to the limit of the MDI — but the generation and storage of the magnetic structures probably occurs at the bottom of the Sun’s convection zone, called the tachocline, which extends another 62,000 miles down, or 124,000 miles beneath the surface.
A second team led by Junwei Zhao, also of Stanford, used SOHO MDI to explore beneath a Sunspot to understand why they sometimes start rotating. The Sunspot was located in the Sun’s northern hemisphere, in an active region designated AR 9114. Although an average-sized spot at about 18,600 miles (30,000 kilometers) across, it exhibited unusually pronounced rotation, spinning more than 200 degrees counter-clockwise in less than three days. Zhao’s team discovered that there was a strong plasma vortex beneath the rotating Sunspot and that the magnetic fields lacing the Sunspot appeared to be twisted beneath the surface.
Like Kosovichev’s research, Zhao’s observation raises new questions. “Now we have a dilemma similar to the ‘Which came first — the chicken or the egg’ question,” said Zhao. “Is it the vortex that twists the magnetic field or does the twisted magnetic field somehow create the vortex?”
Discovering the cause of twisted solar magnetic fields is important because it might eventually help predict stormy solar activity. Twisted magnetic fields on the Sun can suddenly snap to a new configuration with less energy. The excess energy is released in violent solar activity as solar flares and CMEs. These events occasionally disrupt satellites, power systems, and radio communication at Earth.
Kosovichev’s team made its observations from March to May in 2001, and Zhao’s team made its observations August 7 -11, 2000. SOHO is a cooperative project between the European Space Agency and NASA.
More information, images, and movies are available at:
http://www.gsfc.nasa.gov/topstory/20011210insidesun.html
http://sun.stanford.edu/Active_regions
