Using data from NASA’s TIMED satellite, scientists have discovered the existence of hot atomic hydrogen atoms in the thermosphere (from about 56 to 311 miles, or 90-500 km, above the surface), a layer of Earth’s atmosphere. The finding changes current understanding of the hydrogen distribution and its interaction with other atmospheric constituents.

Because hydrogen atoms are very light, they can easily overcome a planet’s gravitational force and permanently escape into interplanetary space.

“Atomic hydrogen, as the dominant neutral constituent in the upper layer of the terrestrial atmosphere, is of critical importance for many disparate aspects of aeronomy and heliophysics, such as atmospheric chemistry and energetics, ion–neutral coupling and magnetospheric energy dissipation following geomagnetic storms,” the researchers said.

“Moreover, as the lightest neutral species in the atmosphere, atomic hydrogen requires the lowest energy to overcome the planet’s gravitational force and escape into interplanetary space.”

“The permanent loss of hydrogen atoms has a significant impact on long-term atmospheric evolution.”

The ongoing atmospheric escape of hydrogen atoms is one of the reasons why Mars has lost the majority of its water.

“Hot hydrogen atoms had been theorized to exist at very high altitudes, above several thousand miles, but our discovery that they exist as low as 155 miles (250 km) was truly surprising,” said co-author Dr. Lara Waldrop, from the Department of Electrical and Computer Engineering at the at the University of Illinois at Urbana-Champaign.

This discovery was enabled by the development of new numerical techniques and their application to years’ worth of remote sensing measurements acquired by NASA’s TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) satellite.

Atomic hydrogen efficiently scatters UV radiation emitted by the Sun, and the amount of scattered light sensitively depends on the amount of hydrogen atoms that are present in the atmosphere.

As a result, remote observations of the scattered hydrogen emission can be used to probe the abundance and spatial distribution of this key atmospheric constituent.

Dr. Waldrop and her colleague, Dr. Jianqi Qin, a researcher in the University of Illinois’s Department of Electrical and Computer Engineering, developed a model of the radiative transfer of the scattered emission along with a new technique that incorporated a transition region between the lower and upper extents of the hydrogen distribution.

“We show, on the basis of satellite remote sensing measurements and radiative transfer modeling, that the hydrogen atoms in the upper thermosphere are much hotter than the ambient oxygen atoms, especially under solar minimum conditions,” the researchers said.

“In contrast to the decrease of the ambient oxygen temperature, the hydrogen temperature (that is, the mean kinetic energy of the hydrogen atoms) increases significantly with declining solar activity, likely as a consequence of the drastic decrease of atomic oxygen density leading to incomplete thermalization in the presence of non-thermal energization mechanisms.”

The team’s results, published in the journal Nature Communications, also show that the presence of such hot hydrogen atoms in the thermosphere significantly affects the distribution of the hydrogen atoms throughout the entire atmosphere.

The origin of such hot hydrogen atoms, previously thought not to be able to exist in the thermosphere, is still a mystery.

“We know that there must be a source of hot hydrogen atoms, either in the local thermosphere or in more distant layers of the atmosphere, but we do not have a solid answer yet,” Dr. Waldrop said.

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Qin, J. & Waldrop, L. 2016. Non-thermal hydrogen atoms in the terrestrial upper thermosphere. Nat. Commun. 7: 13655; doi: 10.1038/ncomms13655