Brian Fayock

Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)



Committee Chair

Gary P. Zank

Committee Member

Nikolai Pogorelov

Committee Member

Jacob Heerikhuisen

Committee Member

Gang Li

Committee Member

Qingyuan Han


Solar activity., Heliosphere (Astrophysics), Interstellar matter., Interplanetary magnetic fields., Monte Carlo method.


Computational models of the global heliospheric interaction with the local interstellar medium have evolved to include very fine and complex physical details. While these models began from theory, the observations and measurements of various spacecraft have been able to provide comparisons to simulations over time, resulting in corrections, adjustments, or additions to certain aspects, and thus a better understanding of the problem. One aspect of great interest is the interaction of neutral hydrogen within the heliosphere. Its inclusion into models is quite complex and still in need of verification. The results of our model are able to provide such a verification. With an interest in neutral hydrogen, Pioneer 10, Voyager 1, and Voyager 2 can provide a relative measure of the local density of hydrogen at the location of the spacecraft by means of Lyman-alpha intensity observations. Their continuous collection of data at increasing radii has revealed trends of decreasing intensity that can be analyzed and compared with radiative transfer simulations performed on neutral hydrogen distributions resulting from the global heliospheric models. Many simulations have been developed in the past, but none has been able to provide results that match the long-term trend, until now. This thesis describes the analysis of Lyman-alpha radiation in the heliosphere by means of a new three-dimensional Monte Carlo radiative transfer technique applied to the background neutral hydrogen distribution provided by a state-of-the-art three-dimensional magnetohydrodynamic-neutral kinetic model, both of which have been developed within the Center for Space Plasma and Aeronomic Research at the University of Alabama in Huntsville. The results of backscatter intensity are compared directly to previously reduced Pioneer 10, Voyager 1, and Voyager 2 ultraviolet data sets, which describe the anti-solar intensity of Lyman-alpha radiation out to almost 50 astronomical units. The similarity between the slopes of the simulations and observations for each spacecraft suggest that the heliospheric model used to produce the background neutral hydrogen distribution contains a fairly accurate description of the inclusion of hydrogen as a group of kinetically interacting particles. Of particular value is the nearly perfect match to the Voyager 2 data.



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