Date of Award

Winter 2016

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Nathan A Schwadron

Second Advisor

Charles W Smith

Third Advisor

Harlan Spence

Abstract

I provide a characterization of the radiation environment of the inner heliosphere from mid-2009 to present using measurements made by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) aboard the Lunar Reconnaissance Orbiter (LRO) and modelling provided by the Earth-Moon-Mars Radiation Environment Module (EMMREM). In the course of this study, I analyze solar energetic particle (SEP) radiation in the form of four major solar events that occurred during this time range as well as the evolution of galactic cosmic ray (GCR) modulation over a period in which relatively calm solar conditions have resulted in the highest GCR fluxes measured in the space age. Using CRaTER measurements taken during three major solar events that occurred in 2012, I demonstrate a validation of the online PREDICCS system (Predictions of radiation from REleASE, EMMREM, and Data Incorporating CRaTER, COSTEP, and other SEP measurements), which uses EMMREM to provide near real-time radiation modelling at the Earth, Moon and Mars, finding PREDICCS to be quite accurate in modelling the peak dose rates and total accumulated doses for major solar events. Having demonstrated the accuracy of PREDICCS/EMMREM in modelling SEP events, EMMREM is used to provide an analysis of the potential radiation hazard of the extreme solar event observed by STEREO A on 23 July 2012, an event which has drawn comparisons to the historic Carrington event due to the exceptional size and record speed of the interplanetary coronal mass ejection associated with it. Such an event might be viewed as something like a worst case scenario in terms of the threat of SEP radiation to astronauts, however the evidence shown here suggests that, with the benefit of heavy protective shielding, astronauts would not have been exposed to levels of radiation that approach NASA's permissible exposure limits. These findings add to a mounting set of evidence which suggests that, contrary to conventional wisdom, the largest radiation threat to future manned space missions is not extreme solar events, but GCRs, which represent a constant source of radiation for which shielding is much less effective. With this in mind, CRaTER measurements taken over the course of the LRO mission are used to model the modulation of GCRs over this time period, which is then used as input into a new atmospheric radiation model that has been developed to compute dose rates as a function of altitude in the atmospheres of the Earth and Mars. I compare the modelled dose rates to the nearest available measurements including balloon and aircraft based measurements for the Earth dose rates and measurements made by the Mars Science Laboratory for the Mars dose rates, finding the modelled results to be reasonable. For airline altitudes in particular, the model is able to reproduce measurements made aboard commercial and research aircraft as a part of the Automated Radiation Measurements for Aviation Safety (ARMAS) project to within the uncertainty limits recommended by the International Commission on Radiation Units and Measurements (ICRU).

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