Sharp images of solar storms
Posted: December 23, 2003

As last October's solar flares blossomed into a coronal mass ejections, scientists at the National Solar Observatory used a new set of instruments to record the sharpest-ever images of the heart of the storms.

"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. The DSLP and the AO projects are funded by the National Science Foundation (AO is funded through NSF's Major Research Instrumentation division).

The small sunspot observed here close to the limb on Oct. 24 was part of the big active region that produced the X17.2 flare on Oct. 28. 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. Credit: NSO/AURA/NSF
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. The AO76 system was described in an earlier NSO story. (76 refers to the number of subapertures in this AO system.)

The DLSP, a joint project of the NSO and the High Altitude Observatory in Boulder, CO, is the first instrument designed to take advantage of the AO76 system. The systems are installed in the NSO's Dunn Solar Telescope here. The 0.2 arc-second resolution is very close to the telescope's maximum of 0.18 arc-second.

NSO scientists were testing the two systems when active regions on the Sun erupted during Oct. 23-25.

Although the Sun is blindingly bright to the human eye, it is quite faint for many science studies. Examining just a small region in just a narrow spectral line at the end of a complex optical system leaves a relatively faint light flux arriving at the instrument's sensor. It takes up to 50 minutes for the DLSP to scan across the target, hence the need for the AO76 system to provide sharp, stable images. Otherwise, atmospheric turbulence would blur the data.

"During much of the observing time the high-order AO delivered excellent and consistent image quality and we were able to scan a sunspot with the DLSP's highest spatial resolution mode," Rimmele said.

The telescope's Universal Birefringent Filter (UBF) recorded high resolution H-alpha filtergrams and spectral scans of an iron line. H-alpha (656.3 nm) is emitted by hot, neutral hydrogen and outlines active regions on the Sun. The iron line (Fe I 543.4 nm) reveals the speed of gas flows in the photosphere. Images also were produced in G-band (430.3 nm), which includes absorptions by several types of molecules that reveal magnetic structures in the photosphere.

"We believe that these G-band images show evidence for dark penumbral cores in the sunspots," Rimmele said. "Observations of these were recently reported from the new 1-meter Swedish Telescope on La Palma. The UBF data is likely to give more information as to the physical origin of these newly discovered features."

Finally, the new DLSP obtained its first ultra high-resolution Stokes profiles of a sunspot before and after eruptions on Oct. 24. The Stokes parameters -- four aspects of how the light is polarized -- determine the strength and direction of the magnetic field in and around a spot, explained Dr. K. "Sankar" Sankarasubramanian, the DLSP project scientist.

While measuring the Stokes parameters is a standard technique, the combination of AO and DLSP will reveal structures about with an area 16 times finer than what can be done with the older Advanced Stokes Polarimeter. This is part of the primary mission of the DLSP, measuring the strength and direction of magnetic fields in and around a sunspot in order to understand the flow of energy through these magnetically intense regions.

The DLSP looks at two spectral lines of interest, centered on 630.25 nm. The lines come from iron atoms about 200 km (~120 mi) above the photosphere, the "visible surface" of the Sun.

"If you have two spectral lines the magnetic field strength measurements are more accurate than with a single spectral line," Sankar said. "The field strength determination of unresolved magnetic field structures are more accurate with two lines."

In addition to greater sensitivity and resolution than previous instruments, the DLSP will be a stable instrument at the DST and thus easier to set up and run observations.

The AO76 and DLSP are to become fully operational in March 2004, and will be available to the solar physics community soon after that. The DSLP and the AO projects are funded by the National Science Foundation (AO is funded through NSF's Major Research Instrumentation division).