The propagation, refraction, or reflection of radio waves through weakly ionized plasma (e.g., the natural ionosphere) is most succinctly understood by comparison of the frequency of the radio wave to the critical frequency of the plasma, a function of the electron density. The electron density in turn is determined by the chemistry of charged species present in the plasma. Decades of research into the ion chemistry of the ambient ionosphere have provided a solid foundation for modeling and prediction of radio wave propagation under a range of conditions. More recent interest has been focused on “artificial” ionospheric conditions, either those produced incidentally by effects of hypersonic objects (e.g., re-entry vehicles) or those produced intentionally via ionospheric modification. Much of the basic chemistry under the temperature and pressure conditions required to model these environments is unstudied. This includes ion-molecule, electron attachment, mutual neutralization, and dissociative recombination processes at elevated temperatures, a range of pressures, and over non-thermal conditions.

We are particularly interested in reactions resulting in spontaneous ionization, offering a means to increase the local electron density over a region of the lower ionosphere. A number of the lanthanide rare earth metals along with a handful of transition metals possess energetics where chemi-ionization (e.g., A + B → AB⁺ + e) is exothermic with either atomic oxygen or oxygen-containing species. Release of these metals in the E-region of the ionosphere, where atomic oxygen is a dominant neutral species, provides an avenue to creating a cloud of enhanced electron density. However, the kinetics of these processes has been only minimally explored and the energy dependences generally unexplored. Additionally, the related ion chemistry is in nearly all cases unknown. Vaporization of these metals in an oven source coupled to a flow tube reactor provides a route to studying the chemi-ionization kinetics and products. Electrospray ionization of solutions of these metals allows for production of the ionized metal and metal oxides in the gas phase, and determination of the kinetics of those species through standard techniques. The results of these measurements can be directly transitioned to models of chemical releases in the ionosphere, aiming to account for both the associated physics and chemistry, to interpret results of prior chemical release experiments and to determine the relative efficacy of different species under consideration for future experiments.

 

References

Shuman NS, Hunton DE, Viggiano A: Ambient and modified atmospheric ion chemistry: From top to bottom. Chemical Reviews 115: 4542-4570, 2015

Cox RM, et al: Evaluation of the exothermicity of the chemi-ionization reaction Sm + O → SmO⁺ + e-. The Journal of Chemical Physics 142: 134307, 2015

 

key words

Plasma chemistry; Ionosphere; Electron attachment; Chemical physics; Radicals; Ion-molecule; Flow tube; Kinetics; Dissociative recombination;

Citizenship:  Open to U.S. citizens

Level:  Open to Postdoctoral and Senior applicants