Posted on Friday 22 November 2002

Sharpest ever view of the Sun

The first images from the Swedish 1-m Solar Telescope on the Canary Island of La Palma are presented in Nature on November 14. The images are the most detailed ever obtained of the Sun - among the new solar features uncovered are hitherto unknown phenomenae in sunspots.

The telescope was opened in March this year and is operated by the Institute for Solar Physics of the Royal Swedish Academy of Sciences. Its objective lens has a diameter of one meter and the telescope is designed to minimize problems from turbulent air that blur the images. The telescope tube is evacuated and a mirror in the beam adjusts its shape a thousand times a second to counteract the atmospheric blurring. This makes the images the sharpest ever of the Sun. The resolution achieved is 1,200 times better than normal eyesight (20/20 vision).

The new images show thin dark cores in the thread-like structures that surround the darkest part of a sunspot. The nature of these cores is unknown. Sunspots are regions with strong magnetic fields. Solar magnetic fields can disturb telecommunications and satellite operations.

Sunspot umbrae—the dark central regions of the spots—are surrounded by brighter filamentary penumbrae, the existence of which remains largely inexplicable1. The penumbral filaments contain magnetic fields with varying inclinations2 and are associated with flowing gas3–5, but discriminating between theoretical models6–8 has been difficult because the structure of the filaments has not hitherto been resolved. Here we report observations of penumbral filaments that reveal dark cores inside them. We cannot determine the nature of these dark cores, but their very existence provides a crucial test for any model of penumbrae. Our images also reveal other very small structures, in line with the view that many of the fundamental physical processes in the solar photosphere occur on scales smaller than 100 km.

The best spatial resolution attained in solar observations has typically been about 0.2 arcsec (150 km). The Swedish 1-m Solar Telescope9 at the Roque de los Muchachos on the Canary island of La Palma is a newly installed (spring 2002), evacuated refractor designed to significantly improve this. The main obstacle to diffraction-limited imaging is rapidly varying aberrations from temperature inhomogeneities in the Earth’s atmosphere. These effects, referred to as ‘seeing’, are corrected for by low-order adaptive optics (15 corrected modes), real-time frame selection (picking the best images from a continuous stream of exposures), and subsequent image restoration using a variant10 of the phase-diversity technique11,12. The resolution achieved is better than 0.12 arcsec.

Nature, November 2002

What is a sun spot?

According to George Fischer, a solar astronomer at the University of California, "A sunspot is a dark part of the sun's surface that is cooler than the surrounding area. It turns out it is cooler because of a strong magnetic field there that inhibits the transport of heat via convective motion in the sun. The magnetic field is formed below the sun's surface, and extends out into the sun's corona."

While it's easy to understand gas pressure (as gas is heated it expands, increasing pressure, and as it cools, it contracts, decreasing pressure), magnetic pressure may be a tougher concept to grasp. David Dearborn explains, "If you take those places where there are concentrations of magnetic field and put them together, they have pressure of their own. You can feel magnetic pressure when you take two magnets and take the ends of the same polarity and try to put them together. The just don't quite want to go together. That's magnetic pressure."

Think of a sunspot as a bubble of magnetic pressure, surrounded, by the gas pressure of the photosphere. For the sunspot to exist, the total pressure must be in balance between the region inside and the region outside of the sunspot. David Dearborn elaborates on how magnetic fields keep sunspots cooler: " Outside a sunspot, you have only gas pressure, which depends on the temperature. In the sunspot you have both gas pressure and magnetic field pressure combined." Since the pressure must be in balance, magnetic pressure inside the sunspot allows the gas pressure (and thus the temperature) to remain lower than the areas outside of the sunspot.