Modern Methods: Unlocking the Secrets of the Sun

The dawn of the space age in the 1950s, coupled with the successes of the Mount Wilson Observatory, heightened public awareness in the United States to the importance of solar and space weather research to national scientific interests. “Solar science is a maturing science,” explained Ernest Hildner, the recently retired director of NOAA's Space Environment Center. “In an immature science, one can make discoveries with one set of data. But as problems get more subtle because the field is maturing, multiple datasets are required to attack problems of interest. Therefore, you need to pool many sources of data&observatories facilitate this.” Moreover, technological advances have made systems critical to national security more vulnerable. “When we had local power generation, distribution, and consumption, the consequences of a geomagnetic storm was not so great,” Hildner noted. “But now, interconnected power grids are turning into one big antenna, vulnerable to changes in the geomagnetic field.” Smaller electronics with more and more computing capacity makes them more likely to suffer damage from space weather storms, he added. Moreover, “In a space-faring era, we are putting assets, including people, into places where the consequences of adverse space weather are much more severe,” Hildner said. As a result, several observatories were established in the U.S. to monitor space weather and the Sun. This article describes historical and current research at some of the major U.S. observatories and examines trends in ground-based scientific observation. National Solar Observatory: Kitt Peak and Sacramento Peak In 1947, the National Center for Atmospheric Research's High Altitude Observatory (HAO) received an Air Force contract to construct a solar observatory at Sacramento Peak, New Mexico. Two years later, in collaboration with Harvard University, a team of scientists started observing the Sun with a coronograph; radio arrays, supported by Cornell University, also began collecting data at 50 and 200 MHz. Six years later, a facility for additional solar observation was established at Kitt Peak, Arizona through the construction of what currently stands as the worlds’ largest unobstructed-aperture optical telescope in the world (1.5-meter diameter), the McMath-Pierce Solar Telescope. The facilities at Kitt Peak and Sacramento Peak were merged into the National Solar Observatory (NSO) in 1983. NSO (http://www.nso.edu/) became part of the National Optical Astronomy Observatory (NOAO) in 1984. The facilities are currently managed by the NSO which became autonomous from the NOAO in 1999. “The Sacramento Peak facility monitors solar activity and makes high resolution observations of solar atmospheric properties including the solar magnetic field,” explained Stephen Keil, the director of the NSO. Housing three solar telescope complexes, including coronographs and coelostats, the facility monitors solar emissions at various temperatures, providing data for space weather forecasts. Geared towards collecting high-resolution spatial images, the Sacramento Peak flagship facility, the Dunn Solar Telescope (DST), employs adaptive optics, which correct for Earth's atmospheric disturbances which can distort the collected image (Figure 1). “Adaptive optics allows telescopes to work close to their diffraction limit,” Keil said. The DST is the premier U.S. telescope for measuring the vector magnetic field of the Sun in the photosphere and chromosphere, Keil explained. It houses two advanced spectro-polarimeters as well as rapidly tunable narrow band filters to facilitate these observations. Milestones reached by DST include observing magnetic reconnection (a driving mechanism of solar activity) and mapping the vector magnetic field within the gas of solar prominences, which are magnetic loops that extend above the photosphere into the corona. The McMath-Pierce facility (Figure 2) on Kitt Peak observatory is optimized for infrared observations of the Sun and deals with the Sun more as a star, Keil said. Historic milestones reached by the McMath Pierce Solar Telescope include the discovery of helium-3, beryllium, chlorine, fluorine, and water on the Sun; new spectral emission lines, and complex magnetic fine structure in the quiet Sun. The telescope is also used at night for astronomical observations. Moreover, its advanced observational techniques have created the best atlases of the Sun's solar spectrum and the first data on magnetic vector fields in the chromosphere, Keil said, adding that it is the only solar telescope in the world to routinely collect infrared emission data beyond 2.5 microns. “Combined with the Fourier Transform Spectrometer, a very accurate spectrometer, the facilities at Kitt Peak collect data on solar lines from the ultraviolet down to 3000 angstroms,” Keil explained. The Kitt Peak facility also houses instrumentation for the Synoptic Optical Long-term Instrumentation of the Sun (SOLIS) program, which is dedicated to collecting full-disc spectroheliograms and magnetic field vector data from the photosphere through the chromosphere over three solar cycles to allow scientists to map and track solar activity on fine to broad timescales. The Owens Valley Solar Array (OVSA) is the only radio array in the United States dedicated to solar observation (http://www.ovsa.njit.edu/). Established by the California Institute of Technology (Caltech) in the 1950s, the array has been managed by the New Jersey Institute of Technology (NJIT) since 1997. OVSA supplies the space weather community with high resolution spatial, temporal and spectral observations based on microwave emissions from the solar atmosphere. Observations of the Sun's atmosphere are obtained by seven radio telescopes or antennae at the Owens Valley. Two antennae are 27.4 meters across and work in conjunction with five 1.8-meter dishes to obtain radiation maps of the Sun by measuring both the amount of solar radiation emitted and the direction from which it is traveling in the solar atmosphere. These maps are used in conjunction with solar telescopic observations from other observatories, and with satellite data. Data at the OVSA is collected at 40 frequencies between 1 and 18 GHz. “Observations at higher frequencies allow for the study of the Sun's surface,” said Dale Gary, the chair of the Department of Physics at NJIT and the director of the OVSA. “Lower frequency observations help image higher in the Sun's atmosphere, where space weather-related events originate.” One research topic focuses on energy release from the Sun and the electron acceleration and transport that results from the destabilization of large solar structures. “The non-flaring Sun produces a certain amount of energy flux. But solar bursts can be more than 1,000 times this amount of flux, and begin to disrupt radio systems on Earth at levels 100 times smaller,” Gary explained. These high energy bursts occur as often as once every 3.5 days during solar maximum and once every approximately 18 days during solar minimum, Gary noted. Monitoring and predicting these bursts is one of OSVA's missions. Other research involves the monitoring and predicting the nature and evolution of coronal magnetic fields. In 1969, Caltech began constructing the Big Bear Solar Observatory (BBSO) in the middle of Big Bear Lake (Calif.) in order to reduce the image distortion produced by convection in low-lying air as the Sun heats the ground. With more than 300 days of cloudless skies per year, the BBSO's location makes it ideal for solar observations. The BBSO's operational capacity was transferred to NJIT in 1997. NJIT owns the site's four main telescopes, which were specifically designed for solar observations and equipped with highly specialized filters and cameras that isolate small portions of the visible, near infrared, and ultraviolet portions of the Sun's spectrum (http://www.bbso.njit.edu/). With this data, scientists at BBSO can observe the Sun's different atmospheric layers, from the brightly visible photosphere at the surface, through the dynamic chromosphere higher up, to the solar corona, explained Haimin Wang, a professor of physics and associate director of the Center for Solar-Terrestrial Research at NJIT and the associate director of the BBSO. “The key to understanding these layers is to understand the magnetic field structure of the Sun and how it changes. This is best done by observing the Sun from many points around the world,” he said. As a result, BBSO is a key part of a global network that screens for the alpha spectral line of hydrogen (H-alpha) produced in the Sun's chromosphere (Figure 3). Called the Global H-alpha network, BBSO along with the Kanzelhöhe Solar Observatory in Austria, the Yunnan Observatory and Huairou Solar Observatory in China, Catania Astrophysical Observatory in Italy, and the Paris Observatory in France, produce high quality whole-Sun images every minute, ideal for the detailed study of the evolution of fine solar structures as well as large-scale solar features that can evolve into space weather-causing events. With this and other research, the BBSO provides a flare forecasting service to the world- wide solar physics community. Daily solar activity indices are also calculated from ultraviolet images; daily solar brightness calculations are also reported. Moreover, as part of the NASA-funded Earthshine experiment, BBSO scientists are monitoring how the Sun's magnetic activity cycle affects the Earth's cloud cover. BBSO is also the United States-based location for a global network of telescopes used by the Global Oscillation Network Group (GONG) and the Taiwan Oscillation Network (TON) projects. GONG has six sites around the world and is funded by the NSF and managed by the NSO; TON, with four sites, is managed by the National Tsing Hua University of Taiwan. Both projects use helioseismology to conduct detailed studies of the solar interior. Because of their spatial coverage around the globe, they are able to obtain nearly continuous observations, eliminating gaps caused by the day-night cycle, Wang explained. Several other observatories exist in the U.S. to study the Sun. Founded in the early 1960s by C. K. Mees, a businessman involved with the Kodak film company, the Mees Solar Observatory in Hawaii began during the heyday of film usage in astrophysics (http://www.solar.ifa.hawaii.edu/mees.html). “It was common then to make a movie out of solar activity,” said Jeff Kuhn, associate director of the University of Hawaii's Institute for Astronomy. “Scientists processed miles of film in order to image the Sun.” Operated by the University of Hawaii, the Mees Solar Observatory houses the Solar-C instrument. Designed by Kuhn in the 1990s, it is the world's largest optical coronograph and its mission is to advance the solar research community's understanding of how coronal magnetic fields control the dynamics of the outer sun, including the question of how mass ejections propagate through space. The Wilcox Solar Observatory, operated by Stanford University, began daily observations of the Sun's global magnetic field in 1975 (http://soi.stanford.edu/~wso/). Founded by John Wilcox and Philip Scherrer, the observatory was originally funded by the Office of Naval Research (ONR), who was interested in understanding communications disruptions caused by space weather events. Since 1976, large-scale maps of the Sun's surface have been used to compute the coronal magnetic field. “Such measurements provide the constantly-evolving big picture background,” explained Todd Hoeksema, a solar physicist with Stanford. “This is important for understanding measurements made in space.” In addition, Wilcox has a long record, spanning four successive solar minima, of polar field measurements which are useful for predicting future solar cycle strength. Other important observatories include the Mauna Loa Solar Observatory, operated by the High Altitude Observatory of the National Center for Atmospheric Research (http://mlso.hao.ucar.edu/cgi-bin/mlso_homepage.cgi), California State Northridge's San Fernando Observatory (http://www.csun.edu/sfo/), and Prairie View Observatory, operated by Texas A&M (http://www.pvamu.edu/cps/). As ground-based solar observation evolves in the United States, a clear trend can be seen: bigger is better. “Though space satellites with imaging instruments have and are producing real life ‘gee-whiz’ pictures that capture the public's imagination, to get really high resolution, high cadence spectral imaging, you need big instruments,” explained Paul Charbonneau, a professor of solar physics at the University of Montreal, Canada and a solar physics historian. “It's just not practical to put those big instruments into space.” An example of this trend is the National Solar Observatory's plan for a new facility on Mount Haleakela, Maui, Hawaii, with the help of over 22 contributing institutions, including the nearby Mees Observatory. This telescope, called the Advanced Technology Solar Telescope (ATST, Figure 3), is scheduled to see first light in 2012, and will be the largest solar telescope in the world, with a diameter of four meters. “Things on the Sun change so fast, that we need to collect more light to observe the Sun from its surface to different heights within its atmosphere. To do this, a telescope must have a large aperture. The ATST will allow for the analysis of solar magnetic fields down to a fundamental scale, providing for the first time actual data to test current theories.” explained Keil, the principle investigator for this project. He added that once ATST is operational, NSO will divest itself of the facilities at Kitt Peak and Sacramento Peak. BBSO also has plans for bigger instrumentation. “We want to understand the origins of space weather, and this requires high-resolution images,” said Phil Goode, a professor in physics and the director of the Center for Solar-Terrestrial Research at NJIT and the director of the BBSO. “To do this, we are constructing a new solar telescope at Big Bear—it will see first light a year from now.” This open, off-axis 1.6 m solar telescope, called the New Solar Telescope (NST), will be the world's largest aperture solar telescope until the ATST becomes operational. After adaptive optics corrections, NST will use visible and infrared light to calculate the intensity of the Sun's magnetic field with resolutions unattainable with current instruments, explained Goode. In addition, scientists at OVSA are working on developing new radio imaging technology called the Frequency Agile Solar Radiotelescope (FASR). This instrument will be a multifrequency (0.03–30 GHz) array composed of more than 100 antennae specifically designed for collecting high-resolution spatial and spectral images on timescales as short as a fraction of a second. Highly ranked by the National Academies’ Solar and Space Physics Decadal Survey Committee, the FASR project is in its proposal stages, with prototypes for the instrumentation currently under development. However, as some scientists turn their attention toward bigger facilities and instrumentation, others worry about the fate of smaller, more independent research facilities. For example, “Funding for Wilcox is not looking very good,” explained Hoeksema. “ONR dropped out after being very faithful for a long time. And our last two proposals to the NSF haven't been funded either. You can always say that nobody understood the value of our great work, or that we didn't write very good proposals, but I think that some have less appreciation for the importance of long-term databases. After you've been doing something for 20 years, what you are going to learn with year number 21 is sometimes a difficult argument to make in a really competitive environment.” Hoeksema explained that though other observatories measure, for example, polar field strength, “those observatories have all changed instrumentation over the years, so they don't have the same measurement from one cycle to the next.” Ironically, the quest for modernization has made related data sets more difficult to mesh into a single chronology, he said. “Wilcox does not have that problem, and there is value in that.” Such debate is natural, said Hildner, explaining that questions in solar physics and space weather exist at all scales, ranging from fine-scale subtle changes in the Sun's magnetic field, requiring advanced instrumentation, to broader questions about trends in the solar cycle which can be assessed by more modest instruments. Hildner noted, “There is a threat that the lure of the new sometimes has a natural human tendency to drive out the routine and the mundane, even thought hat is enormously important in the long term.” But scientists must keep in mind that there are different tides in the search for knowledge, he said. “As the blockage in our understanding seems in one decade to require that we characterize the fine scale better than we do today, in another decade we may decide that we need to focus on long term trends of the Sun.” Despite divisions on some topics among scientists at ground-based solar observatories in the United States, the educational and societal benefits of such institutions, seen in the numerous data products that make their way into scientific papers, space weather monitoring reports and forecasts, and the theses of solar physics graduate students, are manifold. Observatories also sponsor public visits and open houses, and publish educational materials for the general public, among many other outreach activities. In research, collaborative data collection analysis projects based out of observatories, such as GONG, the Virtual Solar Observatory (a software system linking together distributed archives of solar data into a unified whole, sponsored by NSF and NASA), will increase as more and more data are being collected both on Earth and in space, predicted Charbonneau. “And with the emergence of advanced optics, more and more numerical simulations and models will be assessed against actual solar events.” Moreover, it is the solar observatories’ accessibility that allows for continued educational benefit. “Once you put something up in space, you can perhaps reprogram its computer, but you can't actually change the lenses or the filters or anything like that—you're stuck with what you've got,” explained Hildner. “But on the ground, you can modify the instrument based on what you learned last year, and put in a new filter or a new analysis technique,” he said. Moreover, the limited lifetime of space instruments make ground instruments, with the ability to monitor for long periods of time, ideal, Hildner explained. “So the ground-based observatories absolutely have an enormous and constantly evolving role to play in advancing how the Sun and the space environment work.” Mohi Kumar is a staff writer for the American Geophysical Union.

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