Locating Lost Hydrogen

Hydrogen is quite a rare gas in our atmosphere due to how light it is. Its low mass means it has a low density and so it rises above all other gasses and has the most opportunity to escape from the Earth’s gravitational pull. Hydrogen is only really found in the exosphere (it goes troposphere, stratosphere, mesosphere, thermosphere, exosphere) and high parts of the thermosphere, collectively known as the geocorona, where gas densities are so low that the atoms act more like individual particles following trajectories due to how rarely they may interact with another particle, travelling hundreds of kilometres without doing so. Being the primary component atmospheric fringe means that hydrogen has a part to play when it comes to how our atmosphere interacts with incoming effects of outer space. Also the fact we are losing hydrogen from the atmosphere at the fastest rate of any gas, with about 3kg of hydrogen escaping every second could have a long term impact on atmospheric evolution.

The conundrum when it comes to atmospheric hydrogen is the fact that all the models and prediction that are made, no matter how well though out, always undercount based on the ultraviolet sensing used to experimentally gather the number of hydrogen molecules. A paper just released has attempted to explain this  by demonstrating a factor that was not included in the original mathematical models. By using NASA’s Global Ultraviolet Imager device which is currently loaded onto a satellite, scientists were able to observe Lyman alpha photons (photons emitted when a hydrogen’s electron moves from n=2 to n=1). By calculating the average angle of the photons received an estimate can be found for the height of the atmosphere where hydrogen exists in the greatest density and so is scattering the photons from that point. The height in the atmosphere can then be used to calculate the average kinetic energy of the hydrogen as with more energy the hydrogen will have a greater speed and so on average will exist at a grater height. The unnerving thing that was discovered is the fact that the times when hydrogen kinetic energy and therefore temperature was highest was when solar activity was at its lowest. This seems inherently contradictory that when the sun shines the hydrogen appears to get colder and this can only be explained based on a buffer system being set up with oxygen as a likely culprit. The conclusion that has been drawn is that there a hot to cold transition as hydrogen rises into the exosphere from the thermosphere and so this cold hydrogen could account for the missing portion in the models. This study hopefully provides some essential developments in geocoronal theory and allows us to better understand our atmospheric composition in the future.


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