The Big Bear Solar Observatory (BBSO) exploits the excellent climatic conditions of Big Bear Lake to study the Sun, source of life on Earth. The observatory is located in the middle of Big Bear Lake to reduce the image distortion which usually occurs when the Sun heats the ground and produces convection in the air just above the ground. Turbulent motions in the air near the observatory are also reduced by the smooth flow of the wind across the lake instead of the turbulent flow that occurs over mountain peaks and forests. These conditions, combined with the usually cloudless skies over Big Bear Lake and the clarity of the air at 2,000 meters (6,750 feet) elevation, make the observatory a premier site for solar observations. The observatory was built by the California Institute of Technology in 1969. Management of the observatory, and an array of solar radio telescopes at Owens Valley Radio Observatory (OVRO) in Owens Valley, California, was transferred to the New Jersey Institute of Technology on July 1, 1997. Funding for the operation of the observatory is from the National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF), the United States Air Force, the United States Navy and other agencies.

Figure 1. Big Bear Solar Observatory

We study the Sun for a number of reasons. The Sun is the ultimate source of virtually all energy on Earth, and therefore all life on Earth. Studying the Sun provides many insights into purely scientific questions in solar astronomy, nuclear physics, and plasma physics, as well as providing techniques in the prediction of solar flares and other forms of solar activity that impact our daily life on Earth.

Solar activity directly affects radio communications, electric power systems, the safety of astronauts outside their space ships and the lifetime of satellites in orbit, to name just a few. A better understanding of the Sun may allow for more effective management of various aspects of our ecological resources, including issues related to global and local climate changes, atmospheric ozone depletion and even crop management. The Sun is the only star on which we are able to observe surface features; therefore solar research is a key to understanding other stars.

Figure 2. 65 cm vacuum reflector, 25 cm vacuum refractor, and 20 cm full disk telescope.

The observatory's telescopes are specially designed for solar observations. The top floor of the observatory contains a single fork mount supporting the four main telescopes: a 65 cm (26 inch) reflector, a 25 cm (10 inch), a 20 cm (8 inch), and a 15 cm (6 inch) refractor for Earthshine observations. The telescopes are equipped with highly specialized filters and cameras that isolate small portions of the visible, as well as the near infrared and ultraviolet portions, of the Sun's spectrum.

Images in different wavelengths (colors) allow us to view the Sun at different depths or layers in its atmosphere. The apparent surface of the Sun is called the photosphere. The thickness of this layer is only about 500~km which explains why the Sun has a sharp edge in the visible light. Layers outward from the photosphere are referred to as the atmosphere of the Sun. Sunspots and granulation are best seen in observations of the photosphere. The chromosphere lies above the photosphere. The total thickness of the chromosphere is 2,000 to 3,000 km. Filaments, prominences, and a cell-like structure called the network originate in the chromospheric layer of the Sun.

Figure 3. H-alpha image of a solar flare - 5 November 1998.

The 25 cm and 65 cm telescopes observe sunspots and active regions as well as filaments, network and tiny intra-network elements. Sunspots, active regions, filaments and prominences depend on the presence of strong magnetic fields. The outermost layer of the solar atmosphere is called the corona. The corona has an extremely high temperature - 1,000,000 to 2,000,000 degree C. The periodic variation of solar activity (active regions, sunspots, flares, filaments and prominences) is referred to as the 11-year solar cycle. Solar activity and sunspot numbers peak about every 11 years. Sunspots are great loops of magnetic fields that emerge from below the Sun's surface. As sunspots evolve, they rearrange and twist their magnetic fields until the fields break, much like winding up a rubber band until it breaks. Suddenly, a great amount of energy at all wavelengths, X-rays through radio waves, is released in great solar eruptions called flares. Flares shower the Earth with high-energy particles, affecting communications and in some cases, power transmission. Light from a flare takes eight minutes to make the 150 million kilometer trip from the Sun to Earth. High-energy particles take less than half an hour. A large flare can release enough energy to supply the entire United States energy consumption for about 50,000 years. BBSO provides a flare forecasting service to the world wide solar physics community in the form of Solar Activity Reports on the World Wide Web (WWW).

The 20 cm telescope monitors the whole Sun. The H-alpha full disk images are obtained with a large format digital CCD and enhanced with modern image processing techniques. Every clear day the 20 cm full disk telescope observes the Sun from sunrise to sunset, obtaining an image every 30 seconds. The high quality of the final full disk image is ideal for the detailed study of the evolution of fine solar structures as well as large-scale solar features. The full disk telescope is part of a network with observing stations at the Kanzelhöhe Solar Observatory in Austria and the Yunnan Observatory in China.

Figure 4. H-alpha full disk image - 30 January 1999.

Attached to the underside of the 20 cm telescope is a 9 cm telescope that obtains daily full disk visible light images from the photosphere, and near ultraviolet light images originating in the chromosphere. A daily solar activity index is calculated from the ultraviolet images and displayed on our WWW page.

A 15 cm refractor mounted on the side of the 65 cm telescope is used to obtain detailed measurements of earthshine. Earthshine is sunlight reflecting from Earth to the crescent Moon's dark portion and back to Earth again. Measuring the brightness of earthshine can give us a measure of how much sunlight is absorbed by Earth's atmosphere. Such measurements can reveal the Earth's temperature, and much about global warming and changes in the chemical composition of Earth's atmosphere - especially changes in the ozone layer - and how the Sun's magnetic activity cycle affects Earth's cloud cover.

Figure 5. Active region magnetogram - 5 November 1998.

Both the 65 cm and 25 cm telescopes feed light to three separate optical benches, and the 65 cm also feeds a spectrograph. The spectrograph allows for more detailed spectral analysis. The three main telescopes are independently pointed at the Sun by individual photoelectric servo-guiders. The 65 cm and 25 cm telescopes are vacuum systems to avoid air turbulence inside the telescope tubes caused by the solar beam heating air molecules. Furthermore, the special white paint used inside and outside the observatory diffusely reflects sunlight and radiates heat away to reduce turbulence due to solar heating. A large fan and special ventilation system evacuate warm air from the dome so that the interior of the dome remains at the same temperature as the outside air. All of these features reduce distortion of the solar images caused by small temperature differences in the air. Heat-producing telescope and camera control electronics and computers are located in the telescope control room one floor below, and thermally insulated from the telescopes. Adjacent to the telescope control room and attached to the south side of the observatory is another telescope used to obtain daily measurements of the brightness of the Sun. Small differences in brightness between different parts of the solar surface can give information on the nature of the solar cycle.

Scientist from NJIT also study the Sun in radio wavelengths. Observations of the Sun's atmosphere are obtained by five radio telescopes or antennae at the Owens Valley Radio Observatory. The five antennae (two are 27.4 m and three are 1.8 m in diameter) obtain radiation maps of the Sun by measuring the amount of solar radiation coming off it at specific places. Images from BBSO's solar telescopes and radio maps from OVRO's solar radio telescopes are coordinated and combined.

Figure 6. Owens Valley Radio Observatory.

Big Bear Solar Observatory 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. The GONG project is federally funded by the NSF and managed by the National Solar Observatory. The TON project is managed by the National Tsing Hua University of Taiwan. Both the GONG and TON projects are conducting detailed studies of the solar interior using the science of helioseismology. Helioseismology uses naturally occurring sound waves that travel inside the Sun to measure the internal structure of the Sun. The six-site GONG and four-site TON network obtain nearly continuous observations, and therefore eliminate the noise in the data caused by the day-night cycle.

by W. Marquette and C. Denker

Address:	Big Bear Solar Observatory 40386 North Shore Lane Big Bear City, CA 92314
Phone:	909-866-5791
WWW:	http://www.bbso.njit.edu/
FTP:	ftp.bbso.njit.edu
E-Mail:	webmaster@bbso.njit.edu