Research

Clint Sheehan’s Research Page

1. Research Interests: Space and Plasma Physics

The ionosphere is the region of the Earth’s atmosphere beginning around 100 km above the surface. This region of the atmosphere differs from the lower regions due to the relatively large concentration of ions. For this reason, the dynamics of the ionosphere are considerably more complicated. While the dynamics of the lower atmosphere are governed by pressure gradients, both pressure gradients, and electric and magnetic fields drive the dynamics of the ionosphere. Understanding our ionosphere is of tremendous importance. The interaction of the solar wind with our ionosphere produces one of the most striking phenomena on earth, the aurora (northern and southern lights). The interaction of the solar wind with the ionosphere also produces very large electrical currents around an altitude of 100 km. These currents are strong enough to disrupt electrical networks on the earth’s surface, to knock out communications and to cause the corrosion of pipelines. For example, on March 13, 1989, ionospheric disturbances triggered the collapse and blackout of the Hydro Quebec systems which left millions of people without electricity. Late in the fall of 2003 several large coronal mass ejections from the Sun triggered fears of possible similar blackouts. One of these ejections produced a radio blackout on October 19 at 12:50 pm EDT. My current research interests focus on two main areas, Chemical Kinetics of Planetary Ionospheres and Kinetic Theory Studies of the Auroral Ionosphere.

A. Chemical Kinetics of Planetary Ionospheres

Studying ionospheres can be particularly challenging due to the complex interplay between composition and dynamics. Our understanding of the Earth’s ionosphere as a whole must begin with a solid understanding of the individual ionospheric processes. For example, to understand the specific composition of any ionosphere we must first understand the relevant chemical processes. One of the most important processes in determining the specific composition of any ionosphere is the dissociative recombination of molecular ions. In this process, a low energy collision occurs between an electron and a molecular ion. The ion captures the electron, resulting in the formation of an unstable neutral molecule that subsequently stabilizes by dissociating. This process for an arbitrary molecular ion AB⁺ is illustrated by the equation

Dissociative recombination is effectively the only electron loss mechanism in planetary ionospheres and thus it plays a key role in the diurnal variations in ionospheric electron densities. It is also the primary loss mechanism for molecular ions, and is responsible for significant alterations in the populations of the various atomic and molecular species present. Dissociative recombination is, however, not the only possible interaction. In recent years, molecular physicists have made considerable progress in the study of a variety of molecular ion-electron processes both theoretically and experimentally. One of these processes in particular, electron impact dissociative excitation, is potentially of considerable significance for ionospheric studies. Dissociative excitation for an arbitrary molecular ion AB⁺ is illustrated by the equation

Experiments have consistently shown that as collision energy increases, dissociative excitation cross sections begin to dominate over dissociative recombination cross sections. Like dissociative recombination, dissociative excitation is a source of excited atoms and so both processes will contribute to emissions. Unlike dissociative recombination, dissociative excitation is a source of atomic ions and after a dissociative excitation interaction there is still a free electron. These differences have potential implications for a variety of studies, including those which involve ionospheric composition, and those which involve electron cooling rates.

The primary focus of my research involves studying the reaction rates and their implications for these processes for a variety of species of molecular ions of importance to the study of planetary ionospheres.

B. Kinetic Theory Studies of the Auroral Ionosphere

The starting point for understanding a variety of physical and chemical processes in planetary ionospheres is an accurate knowledge of the ion and electron velocity distributions. To correctly extract the temperature from radar or satellite measurements, we must first know the form of the distribution function. Many ion-neutral chemical reactions are strongly temperature dependent and so accurate reaction rates require an accurate knowledge of the distribution function. Plasma parameters such as resistivity and the diffusion coefficient, as well as plasma dispersion relations are temperature dependent. This too necessitates an accurate knowledge of the distribution function. Successfully understanding transport processes also requires an accurate knowledge of the distribution function. This is because each form of the distribution function has an associated set of transport equations. For example, Navier-Stokes transport equations do not apply for non-Maxwellian distributions.

In the upper atmosphere, a number of factors can cause ion velocity distributions to depart significantly from Maxwellian. Rapidly varying forces, the collisionless nature of the medium, and forces that act differently on minor gas species than on the major species are all common examples of physical conditions that give rise to non-Maxwellian distributions. Using a combination of analytical techniques and numerical techniques (in particular, a Monte Carlo approach) I study auroral ion velocity distributions under various conditions. Currently I am investigating ion velocity distributions in the presence of time varying electric fields in the auroral ionosphere.

2. Publications

1.) C. Sheehan, A Merged Beam Analysis of the Dissociative Recombination of Helium Hydride Ions, M.Sc. Thesis (London, ON: The University of Western Ontario) 1996.

2.) A. Le Padellec, C. Sheehan et al., A Merged Beam Study of the Dissociative Recombination of HCO⁺, J. Phys. B 30, 319 (1997).

3.) A. Le Padellec, C. Sheehan and J. B. A. Mitchell, The Dissociative Recombination of CN⁺, J. Phys. B 31, 1725 (1998).

4.) C. Sheehan, A. Le Padellec et al., Merged Beam Measurement of the Dissociative Recombination of HCN⁺ and HNC⁺, J. Phys. B 32, 3347 (1999).

5.) C. Sheehan, W. Lennard and J. B. A. Mitchell, Measurement of the Efficiency of a Silicon Surface Barrier Detector for Medium Energy Ions Using a Rutherford Backscattering Experiment, Meas. Sci. Technol. 11, L5 (2000).

6.) C. Sheehan, A Merged Beam Analysis of the Dissociative Recombination of Molecular Ions of Importance to Ionospheric and Interstellar Chemistry, PhD Thesis (London, ON: The University of Western Ontario) 2000.

7.) C. H. Sheehan and J. P. St. Maurice, The Dissociative Recombination of N₂⁺, O₂⁺, and NO⁺ with Electrons: Rate coefficients for ground state and vibrationally excited ions, J. Geophys. Res., 109, A03302, March 13, 2004.