Hot hydrogen atoms found in Earth's atmosphere

Researchers have discovered the existence of hot atomic hydrogen atoms in an upper layer of Earth's atmosphere known as the thermosphere.
This finding significantly changes current understanding of the hydrogen (H) distribution and its interaction with other atmospheric constituents, researchers said.
Since H atoms are very light, they can easily overcome a planet's gravitational force and permanently escape into interplanetary space.
The ongoing atmospheric escape of H atoms is one reason why Earth's sister planet, Mars, has lost the majority of its water, researchers said.
H atoms play a critical role in the physics governing the Earth's upper atmosphere and also serve as an important shield for societies' technological assets, such as the numerous satellites in low earth orbit, against the harsh space environment.

"Hot H atoms had been theorised to exist at very high altitudes, above several thousand kilometres, but our discovery that they exist as low as 250 kilometres was truly surprising," said Lara Waldrop, Assistant Professor from University of Illinois' Coordinated Science Laboratory in the US.

"This result suggests that current atmospheric models are missing some key physics that impacts many different studies, ranging from atmospheric escape to the thermal structure of the upper atmosphere," said Waldrop.
The discovery was enabled by the development of new numerical techniques and their application to years' worth of remote sensing measurements acquired by NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite.

"Classical assumptions about upper atmospheric physics did not allow for the presence of hot H atoms at these heights," said Dr Jianqi Qin, research scientist at Coordinated Science Laboratory.
"Once we changed our approach to avoid this unphysical assumption, we were able to correctly interpret the data for the first time," Qin said.
Atomic hydrogen efficiently scatters ultraviolet radiation emitted by the Sun, and the amount of scattered light sensitively depends on the amount of H atoms that are present in the atmosphere.
As a result, remote observations of the scattered H emission, such as those made by NASA's TIMED satellite, can be used to probe the abundance and spatial distribution of this key atmospheric constituent.

In order to extract information about the upper atmosphere from such measurements, one needs to calculate exactly how the solar photons are scattered, which falls into Qin's unique expertise.
The researchers developed a model of the radiative transfer of the scattered emission along with a new analysis technique that incorporated a transition region between the lower and upper extents of the H distribution.
The finding was published in the journal Nature Communications.