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Dunn Solar Telescope's New AO System Captures Solar Flare

Sunspot, New Mexico – May 31, 2004:   New clues to what happens inside a solar flare have been provided by a new adaptive optics system that recently became operational at the United States' premier telescope for high-resolution solar observing.
  The new high-order adaptive optics system (AO76) in the National Science Foundation's Dunn Solar Telescope in Sunspot, NM, produced the data as the sunspot at the heart of an active region erupted into a flare.

"A stunning H-alpha flare movie was made and shows, to our knowledge for the first time, flare structure at scales of 0.2 arc-seconds," said Dr. Thomas Rimmele, project scientist for the NSO's Adaptive Optics (AO) project. In addition, the new Diffraction Limited Spectropolarimeter (DLSP) captured high-resolution polarization maps that are essential to studying the fine structure of magnetic activity on the Sun.

"We were extremely lucky to capture this flare with the new adaptive optics system running," explained Rimmele, the principal investigator for the AO76, "We are very excited to be able to study the dynamical events during the flare at this unprecedented
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resolution over a period of about one hour. The data provide some new clues as to what might have triggered the flare and also show the great potential of this new instrumentation for understanding solar activity."

The AO76 system provided crisp, stable images to the new Diffraction Limited Spectropolarimeter (DLSP) and the older Universal Birefringent Filter (UBF) camera. The DLSP is a joint project of National Solar Observatory and the High Altitude Observatory in Boulder, Colorado. Kasiviswanathan Sankarasubramanian of NSO and Bruce Lites of HAO are co-principal investigators.
Solar Flare H-alpha image of a flare. The small sunspot observed here close to the limb on Oct. 24 2003 was part of the big active region that produced the X17.2 flare on Oct. 28 2003. The sequence shows a super-penumbral loop system erupt. It appears that the foot points of the loops as well as some loop tops became bright during the flare. The data indicate that the flare was triggered by new flux emerging in the lower right corner of the image. Tick marks are 1 arc-sec. Structure on spatial scales of 0".2 arsec is visible in this image.
  Image:   Thomas Rimmele, NSO/AURA/NSF.

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The Dunn, completed in 1969, is still the nation's premier telescope for high-resolution solar observing. It has a 76 cm (30 in.) aperture that, like other ground-based telescopes, has been limited to a resolution of 0.5 to 1.0 arc-second, depending on atmospheric seeing conditions, even though the telescope has a theoretical diffraction limit of 0.2 arc-second (at 600 nm). The telescope now can reach that limit for extended periods under good to moderate seeing conditions thanks to the AO76 system.

The high-order AO76 system, in advanced engineering tests since April 2003, compensates for much of the blurring caused by turbulence in Earth's atmosphere. It analyzes distortion within an image and then calculates how to reshape a deformable mirror to cancel most or all of the distortion.

Using the Shack-Hartmann technique, the AO76 system divides the incoming view into 76 subimages. A computer determines how much to shift the subimages so they align with each other, then reshapes a deformable mirror 2500 times a second. This is the forerunner of a highly complex AO system, with about 1,200 subimages, that will be used by the NSO's planned 4-meter Advanced Technology Solar Telescope. The AO76 became operational in March 2004. A few weeks later, the prototype AO24 system was upgraded to AO76 so the Dunn now has two high-order adaptive optics systems on call.

During Oct. 23–25, 2003, Rimmele and Sankarasubramanian used active region 10486 as a target during engineering tests. (The same active region produced a huge flare on Oct. 29.) The region was near the limb (18° latitude, –66° longitude). The results are being presented today to the American Astronomical Society meeting in Denver by Thomas Rimmele and Kasiviswanathan Sankarasubramanian of the National Solar Observatory in Sunspot, NM.

The Universal Birefringent Filter captured a movie of the flare in iron I (543.4 nm) in the upper photosphere (the lowest visible layer of the solar atmosphere) and in H-alpha (656.3 nm) in the chromosphere (the hot layer above the photosphere). H-alpha (656.3 nm) is emitted by hot, neutral hydrogen and outlines active regions on the Sun. The iron line reveals the speed of gas flows in the photosphere.

During the dynamic H-alpha core flare sequence, the observers were able to resolve fine structure in the flare ribbons at scales of 0.2 arc-second. They saw the footpoints of loops as well as individual loop tops brighten as the flare erupted.

"With this kind of resolution we can see individual H-alpha loops and study their dynamic evolution during the flare in a way not possible before", Rimmele said. "We see highly structured flare ribbons that propagate at speeds of 30 km/s both in the chromosphere and the photosphere. We were also able to observe magnetic field changes from before and after the flare."

Pre- and post-flare data from the DLSP show evidence of magnetic flux cancellation at small spatial scales that could not have been observed before the AO76 system was installed. (The DLSP only captured before-and-after data, 18:15-18:41 and 19:08-19:34 UT, because it was off line for a calibration period that coincided with the flare.) The DLSP was optimized to observe around the 630.15 and 630.25 nm iron lines associated with magnetic activity about 200 km (120 miles) above the photosphere. Analyzing the polarization and intensity of these lines determines the strength and direction of the magnetic fields at that altitude.

"The data indicate that the flare may have been triggered by these opposite polarities coming together and canceling each other out," Rimmele said. Analysis of the observations is continuing, and that he and colleagues hope to extract more information that will lead to a better understanding of the underlying physics of a flare.

"We are looking forward to using the AO76 and DLSP for extended, high-resolution observations of new solar activities," he added.

The observations were made during engineering test runs with two new instruments at the National Science Foundation's Dunn Solar Telescope in Sunspot, NM. The NSO is owned and funded by the National Science Foundation, and operated under contract by the Association of Universities for Research in Astronomy. AO is funded through NSF's Major Research Instrumentation division.

K. Sankarasubramanian, T.R. Rimmele and B.W. Lites,
"Diffraction limited spectro-polarimetry at the Dunn Solar Telescope,"
American Astronomical Society Meeting 204, #20.06; Bulletin of the American Astronomical Society 36, 686 (2004)

T. Rimmele and K. Sankarasubramanian,
"High resolution flare observations using adaptive optics,"
American Astronomical Society Meeting 204, #27.01; Bulletin of the American Astronomical Society 36, 693 (2004)

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