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Why We Need the ATST

by Louise Good


Cutaway view of the ATST shows its off-axis optical design. Image by L. Phelps, courtesy of ATST.

"Our best models of what the Sun is doing don’t work," said IfA astronomer Jeff Kuhn, while explaining why solar scientists need the Advanced Technology Solar Telescope (ATST) about to be built on Maui. He was the speaker for the Frontiers of Astronomy Community Lecture, "House of the Sun: Bringing the World’s Largest Daytime Telescope to Maui," on February 16, at UH Manoa.

The ATST, he said, will provide our biggest jump in solar observing capabilities since Galileo’s time and is the largest single investment the United States has ever made in any ground-based optical/infrared telescope ($300 million). Building it on Haleakala, the best daytime astronomical site in the world, will open new windows to understanding how the Sun works and what changes in the Sun mean for changes here on Earth.

The ATST will have a unique off-axis design that will limit the glare from the Sun. With a mirror 4.2 meters (about 13 feet) in diameter, the ATST will be more than twice as large as the next-largest one, the 1.5-meter McMath solar telescope built in 1962. ATST will be the world’s largest coronagraph, which will allow us to see the faint outer solar atmosphere. In addition, it will have an adaptive optics system to compensate for blurring by Earth’s atmosphere.

The key to understanding the Sun is understanding its magnetic fields. Scientists have been measuring solar surface magnetism for many years. What they still don’t understand is how the Sun recycles "magnetic flux" (the net magnetic field) between the surface and its interior using sunspots, and how it ejects this flux out into space in the form of "flares" (brief eruptions of intense high-energy radiation from the Sun’s surface) and "coronal mass ejections" (large-scale releases into space of matter from the Sun’s corona). The ATST will enable scientists to see the details of the magnetic fields and the corona while measuring solar magnetism.

The number of sunspots on the Sun varies from "solar minimum" (few or no sunspots) to "solar maximum" (many sunspots) and back again in a cycle that averages 11 years. Although it seems counterintuitive, when there are more dark sunspots, the Sun’s total brightness is greater.

The Sun is the source of "space weather" in our solar system. The solar wind—the continuous flow of ionized particles from the Sun’s corona—interacts with the magnetospheres of Earth and other planets. The solar wind is stronger when the Sun has many sunspots, thereby providing more protection from deadly cosmic rays coming from outside the solar system. But more sunspots also mean more flares and coronal mass ejections, which can disrupt power and communication systems on Earth and endanger satellites and astronauts. Scientists would like to be able to predict these solar storms much like meteorologists can predict hurricanes, so that satellites could go into "safe mode," astronauts could be sufficiently sheltered, and terrestrial power and communication systems could take precautions.

The Sun’s brightness changes by only about 0.1 percent during an average solar cycle, and yet what happens on the Sun affects the temperature and climate on Earth. We know this from natural records such as tree rings and ice cores from the Arctic and Antarctic. For example, the period from 1645 to 1715, when there were hardly any sunspots, corresponds roughly with a lengthy cold spell known as the Little Ice Age (1550–1850). Settlements on Greenland founded during the Medieval Warm Period (approximately 950–1250) disappeared during the Little Ice Age.

The ATST is funded by the National Science Foundation and led by the National Solar Observatory with help from the IfA; the High Altitude Observatory in Boulder, Colorado; Big Bear Solar Observatory, which is part of the New Jersey Institute of Technology; and the University of Chicago. Nineteen other international organizations contribute to the ATST mission. ATST is expected to have its "first light" in 2018.