The outermost layer of Earth’s atmosphere, called the outer exosphere, is almost entirely made up of hydrogen. These hydrogen atoms scatter photons, producing a luminous halo called the geocorona. Observing the precise shape of the geocorona would shed light on the last phase of an important geophysical process: the escape of hydrogen atoms from Earth into interplanetary space.
The exosphere has been observed from within—distances of less than 64,000 kilometers—extensively. But, from the outside looking in, past space missions have been able to observe the geocorona only from far greater distances. For example, Mariner 5 caught a glimpse from roughly 240,000 kilometers out, and Apollo 16 observed it from the Moon—about 380,000 kilometers away.
In a recent study, Kameda et al. used the Lyman Alpha Imaging Camera on board the Proximate Object Close Flyby with Optical Navigation (PROCYON ) spacecraft to observe Earth’s geocorona from the greatest distance yet: more than 15 million kilometers away. The camera was able to capture the first image of the entire geocorona, stretching more than 240,000 kilometers: 38 times the length of Earth’s radius. (In comparison, partial images captured by past observation revealed roughly 100,000 kilometers, or less than 16 times the length of Earth’s radius.)
In addition to this comprehensive image—which proved the ecliptic north–south symmetry of the geocorona for the first time—the team used a mathematical model to determine the distribution of the geocoronal emission’s intensity. From this model, they found that the production of hot hydrogen in the magnetized plasmasphere (a layer of dense plasma surrounding Earth) is likely not the main process involved in shaping the outer exosphere, although it may still be involved somehow.
This study is a step forward in the geophysical and space sciences and the first successful attempt since the 1970s era Apollo mission to paint a picture of the outermost reaches of Earth’s atmosphere. (Geophysical Research Letters, https://doi.org/10.1002/2017GL075915, 2017)
—Sarah Witman, Freelance Writer