This is a guest post by Keighley Rockcliffe.

Coronal Mass Ejections and the Skylab Mission

On May 14th, 1973, the US’s first space station, dubbed Skylab, was launched into low-Earth orbit on the last Saturn V rocket launch. Onboard Skylab was a workshop meant for the crew to perform experiments. These experiments covered research in biomedical science, solar and space physics, geology, material science and some student proposed studies. The habitability of the station itself was an experiment.

Skylab had its own solar observatory, the Apollo Telescope Mount, that included X-ray telescopes, UV measurement instruments, two Hydrogen-alpha telescopes, and a white light coronagraph. A coronagraph allows the faint outer atmosphere of the Sun to be observed by blocking the much brighter direct light from the solar disk. The High Altitude Observatory designed the coronagraph for Skylab, using an occulting disk to block light from the Sun’s disk and allow observation of the corona.

A star’s corona is a multi-component portion of the star’s outer atmosphere. Despite being farther away from the solar surface, the Sun’s corona is actually hotter than its surface by about 1000 times. The reason for this huge temperature discrepancy has been an active research topic since its discovery in the 1930’s and will be investigated by the recently launched Parker Solar Probe. The different parts of the corona are composed of light from different sources: Thomson scattered by electrons (K corona), scattered by dust (F corona), emission from ionized atoms (E corona), and some emission from the same dust contributing the F corona (T corona). The coronagraph within Skylab’s observatory allowed observation of the K corona, which makes up the brightest parts of the corona.

Some interesting and powerful events are visible within the corona, primarily involving the ejection of solar plasma and magnetic disturbances into the heliosphere. They are called coronal mass ejections (CMEs), traveling at speeds ranging from 100 to 3000 km/s and carrying around 10¹² kg of solar plasma (approximately the mass of Mount Everest). These eruptions can and do hit the Earth’s geospace, which can cause geomagnetic storms as well as barrage satellites and spacecraft with energetic particles. This motivates our current and past study of CMEs, solar flares, and the behavior of the corona.

For a nine-month period between 1973 and 1974, Skylab’s coronagraph took images of the Sun’s corona. Those images were originally on film, and the initial analysis was done using that medium. Recently the film frames were digitized, so in the last few months, I was tasked with using these images to create a catalog of the observed coronal mass ejections and their properties (location, projected speed, width). In order to look for these events, GIFs were made for each day of the mission containing all of the images taken that day. A few things needed to be altered about an image before creating a GIF.

Even with the help of the occulting disk, the coronagraph captured a lot of light which made it difficult to see events occur and propagate. A way to bypass this would be to manually mask the bright values closest to the Sun’s disk, which would allow the image scale to show more structure. For this, I used Python and the SunPy package. The information contained within each image header contains enough metadata to create a  SunPy map. This is a spatially aware 2D array containing the image data along with metadata information and plot settings. Creating a map for each image file allowed the use of SunPy’s functions to manipulate maps. To solve the brightness problem, I used Numpy’s ‘meshgrid’ function to create a grid of pixels the size and shape of