Researchers discover hot hydrogen atoms in Earth's upper atmosphere

A team of researchers from University of Illinois discovered the existence of hot atomic hydrogen (H) atoms in an upper layer of Earth's atmosphere, known as the thermosphere.

The finding was recently published in the report titled Nature Communications.

The discovery significantly changes current understanding of the distribution of hot atomic hydrogen and its interaction with other atmospheric constituents.

Key highlights of the finding

• 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.

• Atomic hydrogen efficiently scatters ultraviolet radiation emitted by the sun. The amount of scattered light sensitively depends on the amount of Hydrogen atoms that are present in the atmosphere. As an outcome, remote observations of the scattered H emission can be used to probe the abundance and spatial distribution of H.

• In order to extract information about the upper atmosphere from such measurements, one needs to calculate exactly how the solar photons are scattered.

• The researchers, under support from the National Science Foundation and NASA, developed a model of the radiative transfer of the scattered emission. This was done along with a new analysis technique that incorporated a transition region between the lower and upper extents of the H distribution.

• The analysis of the TIMED data led to the counter-intuitive finding that the temperature of the H atoms in the thermosphere increases significantly with declining solar activity. The finding is in contrast to the ambient atmospheric temperature, which decreases with declining solar activity.

• It was also found that the presence of such hot H atoms in the thermosphere significantly affects the distribution of the H atoms throughout the entire atmosphere.

Comment

Hot H atoms had been theorized to exist at very high altitudes, above several thousand kilometres. However, the new finding suggests that they exist as low as 250 kilometres. The origin of such hot H atoms, previously thought not to be able to exist in the thermosphere, is still a mystery.

The ongoing atmospheric escape of Hydrogen atoms is the reason why Mars has lost the majority of its water.

H atoms play a critical role in the physics governing the Earth's upper atmosphere. Also, it serves as an important shield for satellites in low-earth orbit against the harsh space environment.