MDI shows view beneath Active Region and Rotating Spot

With the assistance of Stanford and NASA press offices the following statement was released on Dec 10, 2001 at the Fall AGU meeting in San Francisco.

FOR RELEASE MONDAY, DECEMBER 10 AT 8:30 A.M. PST

FANTASTIC VOYAGE INSIDE THE SUN REVEALS HIDDEN WORLD OF SURPRISING COMPLEXITY

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, California, 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[This page].

In the second paper, Junwei Zhao, Tom Duvall, and Alexander Kosovichev saw shearing flows in the form of reversing vortices beneath a spot pair (one large and one small) that were spinning about each other.

In each of these results we have seen details not seen before. As Douglas Gough once said, "The Sun's interior has been seen as simple only because we have not had data". Now that we have observations of the thermal structure and flows beneath interesting regions we find complexity not simplicity.

The following images and movies show these results.

Topic	Sample Image	Image or Video links	Notes
AR9393 March 2001		Mpeg(1.2Meg)	Tracked MDI Continuum Proxy
Interior of 9393		Mpeg(1.0Meg)	Sound speed beneath 9393
Life of AR9393		Mpeg(3.2meg) Quicktime(9.8meg)	Farside and Frontside for life of AR9393. March to June 2001
MDI Continuum for 9393 passage		Full field MDI movies: QuickTime, MPEG (large , small). Subfield movies: MDI QuickTime MPEG (large, small), GIF animation.	MDI Full disk and extracted latitude movies
Rotating Sunspot		Frame 1 Still Frame 92 Still Frame 302 Still Frame 530 Still MPEG(2.5meg)	Spinning spot animation prepared by NASA
Sound Speed and Flow Layers Beneath Sunspot		Frame 123 Still Frame 183 Still Frame 243 Still MPEG(2.3meg)	Sound speed and flow beneath spot. MDI time-distance analysis
Emerging Spot Aimation		MPEG (3.7meg) Quicktime (12meg)	Animation showing the conceptual picture of an emerging spot region. First a magnetic loop rises from beneath the surface (photosphere) then rises through the surface creating sunspots. Later more flux emerges and reconnects with the initial loops to cause a flare and CME.

The NOAA Space Environment Center (SEC) tracks all solar active regions and give them numbers (e.g. AR9393). Region 9393 was seen on the disk over more than 3 months with the region visible for four disk passages. It had a different number for each passage. The daily location and size of the region while seen on the front side of the Sun is in the SRS Region data for region 9393 under the names 9371, 9393, 9433, and 9461 for the four disk passages of this region in Carrington Rotations 1973, 1974, 1975, and 1976 respecively. The SRS data prepared by NOAA Space Environment Lab described in a Readme file. The NOAA SEC site contains many useful links and datasets for space weather information.

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