The 2019 Solar Eclipse from 40,000 Feet
Dr. Jenna Samra takes a calibration image as the aircraft heads into totality. The faint dark line on the crescent solar disk shows the location of the spectrograph slit. Light passing through the slit enters the spectrograph and is recorded as an image. Observations of the photosphere before and after the eclipse are important for establishing the wavelength calibration of the spectrograph during the eclipse.
On July 2, scientists from the Center for Astrophysics | Harvard & Smithsonian (CfA) and the National Center for Atmospheric Research (NCAR) successfully flew a high-altitude research jet along the path of totality of the solar eclipse in the southern hemisphere in an ongoing research project to study the Sun’s corona - the million-degree outer layer of the solar atmosphere - and its associated magnetic fields. The advantage of doing coronal research during a total eclipse is that the glare of the solar disk normally overwhelms the corona, but during a total eclipse the moon’s shadow masks the disk. Scientists have long been interested in an improved understanding the Sun’s magnetic system, which is responsible for ejecting charged particles into space. When these particles reach the Earth they can disrupt electronics, block radio communication, interrupt GPS signals, and even influence the weather.
CfA astrophysicist Edward DeLuca, a principal investigator for the field campaign, explained that “The corona gives us information about the temperature and density of the plasma at the solar surface….Observing these in the infrared is relatively new and challenging because the Earth’s atmosphere gets in the way.” The corona’s super-heated, charged particles of plasma hold clues to the Sun’s invisible magnetic fields.
The research aircraft, the NSF/NCAR HIAPER Gulfstream V (GV), owned by the National Science Foundation and operated by NCAR, offers two advantages over ground-based studies: it can observe infrared radiation that is normally blocked by water vapor in the Earth’s atmosphere, and it can fly along the path of the eclipse, extending the time available to observe totality. During the flight, the scientists onboard used the Airborne InfraRed Spectrometer (AIR-Spec) instrument to obtain infrared spectra of the corona. AIR-Spec was successfully used for the first time during the Great American Eclipse of 2017 when it observed totality for four minutes. In last week’s 2019 eclipse the plane, flying along the path of totality across the South Pacific Ocean, obtained eight minutes of total eclipse observing time. While NCAR scientists have a history of observing solar eclipses with ground-based instruments stretching back decades, the 2017 eclipse was the first time the GV was used to chase an eclipse with an instrument on board.
CfA astronomer Jenna Samra, the instrument scientist on the project, explained the AIR-Spec instrument, which was successfully deployed for the first time in the 2017 campaign and detected two new infrared emission lines from very highly ionized iron atoms in the hot plasma. These detections helped scientists model the temperature, density, and other conditions in the solar corona. The AIR-Spec has undergone two major improvements since 2017, said Samra. The instrument has several mirrors that direct the image of the solar eclipse into a telescope, which then focuses onto a spectrometer held in a vacuum chamber. One of these mirrors is steered electronically to compensate for the motion of the aircraft. One improvement enabled this mirror to track the edge of the moon automatically, keeping the image of the Sun's corona steady. “The 2017 image of the Sun was very jittery,” said Samra. “Now we'll be able to have a very stable image.” Another major improvement was additional cooling inside the vacuum chamber that contains the spectrometer, improving its sensitivity by more than an order of magnitude.