This project will examine how energetic particle precipitation affects the electrical conductivity and fields in the upper and middle atmosphere. It will examine how solar energetic protons access the middle atmosphere and will utilize the gamma rays emitted by the incoming protons to determine the energy spectrum of the incoming particles. It will also investigate the mechanisms which accelerate, transport, and precipitate relativistic electrons.
The project will analyze data from two balloon data sets that contain multiple episodes of energetic particle precipitation. Models will be constructed to describe the effects these energetic particles produce in the upper and middle atmosphere. Model results will be compared with measured middle atmosphere radiation, conductivity, and electric fields. To determine the mechanisms which accelerate, transport and precipitate relativistic electrons the balloon observations will be combined with ground and spacecraft observations to test wave-particle interaction theories for precipitation.
The results from the project will improve our ability to predict energetic precipitation events that can affect a variety of satellites and can alter the chemical composition of the atmosphere. The project is a collaborative one involving faculty and students at Dartmouth College, the University of Washington and the University of Houston.
Processes at the sun frequently send bursts of accelerated protons earthward. These highly energized protons are ionizing radiation, capable of penetrating deeply into Earth's atmosphere or passing through metallic shielding on spacecraft. As they penetrate matter, energetic protons knock electrons off of atoms and alter atomic nuclei. This research project investigated an interesting sun-Earth interaction that proceeds through two kinds of direct effects of solar energetic protons upon Earth's atmosphere. One objective was to investigate solar energetic proton effects on atmospheric electrical properties. A second objective was to investigate energetic proton-induced nuclear reactions involving atmospheric gases (nitrogen and oxygen). These investigations were possible due to the fortuitous combination of an x-ray spectrometer and an electric field instrument flying in the antarctic stratosphere during a major episode of solar activity (15--20 Jan 2005). One value of this work is in understanding how different parts of our solar system are connected. Thus protons from an explosion on the sun modify the flow of electrical current in the middle atmosphere 10 hours after, perhaps influencing cloud formation even later. We modeled the effects of observed solar energetic protons upon the ability of the atmosphere to conduct electricity (conductivity), and compared model results with measured conductivity. One thinks of air as an electrical insulator, yet air's conductivity increases with altitude, both because air becomes thinner, and because the number of mobile charges increases. Mobile charges are produced by ultraviolet light and cosmic rays detaching electrons from air molecules (ionization). As energetic protons pierce the atmosphere, they rapidly add mobile charges by ionizing air molecules, thereby increasing atmospheric conductivity. We used spacecraft instruments to estimate the influx of solar protons near our stratospheric instruments, approximated the ionization from the protons as they moved into the atmosphere, and inserted this ionization into an ion chemistry computational model that models conductivity development over the 20 to 120 km altitude range. The model confirms that observed stratospheric conductivity changes are explained well by solar energetic protons ionization, and predicts that these protons have maximum impact on atmospheric conductivity at 60 km altitude. The agreement between model and observations further validates the model, indirectly supporting the model computation of chemical species abundances and their interactions in the atmosphere. A related electrical effect that we studied was a mechanism for changing the stratospheric vertical electric field, that is, voltage difference between two close altitudes. Above thunderstorms, the vertical field is directed upward, and away from thunderstorms (fair weather) downward. Before the proton event, we measured a typical fair weather electric field. At the time of the event, the vertical electric field disappeared. Later it reversed direction, eventually returning to its typical downward direction and strength. We hypothesized that energetic protons were penetrating to an altitude below the observation platform and depositing a layer of positive charge, which could cancel and even reverse the vertical electric field. We modeled this idea, similarly to the conductivity modeling, and found that far too few protons are deposited at lower altitude to cancel or reverse the vertical electric field. This computation eliminated one mechanism as an explanation for the vertical electric field reversal, yet it leaves unanswered the question of how the vertical electric field reversed. We did not complete the analysis and modeling of nuclear interactions caused by solar energetic protons in the atmosphere. Although the resulting gamma rays are clearly evident in the dataset, the spectrometer was not designed for studying gamma rays arising from atmospheric nuclear reactions. It was necessary to better characterize the spectrometer response and its detection efficiency. We accomplished these goals first in the laboratory using a spare spectrometer, and the latter was done using a physics modeling code. The proton-nucleus interactions proved more difficult to model, and so this work continues under alternate support. For broader impacts, this project has contributed importantly to hands-on education of scientists. It supported research experiences for two entering undergraduate students, and partially supported three graduate students. The undergrads are now majoring in physics and chemical engineering while one graduate student, after completing his doctoral degree, became a Project Manager at Jet Propulsion Laboratory.
