The August 21, 2017 Total Solar Eclipse

On 21 August 2017 a total solar eclipse will pass across the continental United States, from Oregon on the west coast to South Carolina on the east. The moon's shadow will reach the Oregon coast at 10:16 am, pass over five state capitals and several national parks, before leaving the east coast at 2:49 pm. The last total solar eclipse seen in the continental United States was in 1979, and not since 1918 did one travel from coast to coast. Such a rare occurrence is sure to capture the public's imagination, but it also affords solar scientists a unique opportunity to conduct research not otherwise possible.

2017 Total Solar Eclipse, path of totality image
2017 Total Solar Eclipse–path of totality

The outer atmosphere of the Sun, known as the solar corona, holds the key to understanding how the Sun affects life on Earth. The corona is several million degrees hotter than the surface of the Sun and dominated by magnetic fields. These magnetic fields store a huge amount of energy and, when distorted, they can release this energy in the form of solar flares and coronal mass ejections. Such space weather phenomena have a profound effect on the terrestrial environment, impacting communication satellites and the power grid. Measuring the corona is, therefore, crucial in being able to predict these events and protect resources from their effects.

2017 Total Solar Eclipse, Caspar, WY image
2017 Total Solar Eclipse passes through Caspar, WY

However, the corona is orders of magnitude dimmer than the surface of the Sun and under normal viewing conditions is completely overwhelmed by the surface brightness. It is only in the event of an eclipse that the corona becomes visible. HAO has a long history of observing the corona under eclipse conditions—both man-made and natural. We can simulate the effects of an eclipse by using an instrument known as a coronagraph—where the light from the solar surface is blocked using an occulting disk. The first director of HAO (and NCAR) installed the western hemisphere's first coronagraph in 1940, and throughout the 1950s HAO scientists flew coronagraphs in high-altitude balloons. Today's observing program includes the K-Cor and CoMP instruments at the Mauna Loa Solar Observatory. But coronagraphs can only approximate the effects of a total solar eclipse, not replicate them entirely. For this reason, in 1952, HAO began field expeditions to measure the corona during total solar eclipses. They developed the Newkirk White Light Coronal Camera, still on display in the lobby of the NCAR Mesa Lab, which was used for eclipses in Mexico (1970), Kenya (1973), India (1980), Siberia (1981), Java (1983), the Philippines (1988), Hawaii (1991), and Chile (1994).

Specialized equipment for viewing eclipse image
Specialized equipment for viewing eclipse: Fourier Transform Spectrometer (FTS) to measure the infrared emission from the corona at 2–12 microns

This tradition is continuing today as scientists from NCAR's HAO and ACOM labs come together on a project to measure the corona during the 2017 eclipse. Over the last year they have been developing a Fourier Transform Spectrometer (FTS) to measure the infrared emission from the corona at 2–12 microns. This region of the spectrum has never before been comprehensively measured but contains emission lines that are sensitive to the magnetic field. A successful experiment will, thus, provide important new constraints on the wavelength and intensity of a crucial wavelength range of the solar corona.

The instrument was originally designed for a different purpose—to measure the troposphere and stratosphere from the NCAR GV scientific aircraft. Scientists and engineers have been working to repurpose the instrument so that it meets the observing requirements of the 2017 eclipse. It is currently under testing in a lab environment before being field-tested early in 2017. For the eclipse itself, the FTS instrument will be installed in a scientific trailer and transported to the high-altitude location of Camp Wyoba in Wyoming. This site was chosen to maximize the chances of a successful mission. It has favorable weather conditions and the high altitude reduces the adverse effects of absorption from the Earth's atmosphere. Once in place, the instrument will observe the Sun for about an hour, 30 minutes either side of the 2 min 25 sec of totality.