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Yi-Jiun
Su, Adjunct Assistant Professor I. Summary My research has been primarily focused on the investigation of plasma phenomena in the near-earth and planetary environments, issues fundamental to the understanding of plasma physics. My expertise is in performing comprehensive data analyses and the development of model simulations, particularly those involving Earth's magnetosphere-ionosphere (MI) coupling. Recently, I have concentrated on charged particle acceleration and wave propagation on Jupiter-Io flux tubes using gyrofluid and test particle models that have been validated in the Earth's environment and used to reproduce various electron and field signatures from satellite observations. The ultimate goal of our current effort is to determine whether the physical auroral generation mechanisms are universal or unique to Jupiter or Earth.
II. Research Experiences and Interests A. Auroral Physics The Fast Auroral SnapshoT (FAST) mission was designed with sufficient resolution to address key questions on particle acceleration, which, in most cases, required improvements to instruments in order to provide two to three orders of magnitude higher time resolution than was available from earlier auroral missions. The categorization of auroras into three distinct types of acceleration regions is well established. Of these regions: (1) an upward current region; (2) a downward current region; and (3) an Alfvénic acceleration region, I have chosen to focus on the Alfvénic acceleration region near the open-closed field line boundary where particle acceleration is dominated by waves rather than the quasi-static potential structures found in upward and downward current regions. Based on the data analysis, highly irregular electron structures in the dayside auroral/cusp region have been found to be associated with small-scale electric field variations, which are believed to be propagating Alfvén waves. I was also able to show that the highest low-frequency wave powers and ion energy fluxes were obtained in the polar cap boundary acceleration region, where Alfvén waves were known to be present. A linear gyrofluid code, originally developed for use in nuclear fusion research, was adapted for use on the Earth's cusp magnetic field line to study Alfvén wave propagation. Additionally, I established a test particle scheme to load electrons with wave fields generated by the gyrofluid model to study electron behaviors. With this approach, not only were we able to reproduce the energy-time dispersion signature studied extensively by modelers, but we also had the ability to generate the most frequently observed electron bursts. Our simulation results from this effort very closely resemble FAST observations. An increased mass density (significant O+ density) in the acceleration region is an essential prerequisite in the generation of electron bursts. The primary effect of the O+ is to decrease the phase speed of the Alfvén wave. Furthermore, the full gyro-kinetic effects of the O+ act to produce a region in which the Alfvén speed profile is gradually slowing allowing electrons to remain within the wave to lower altitudes. B. Jovian Magnetosphere-Ionosphere Coupling The knowledge gained from studying the Earth's MI coupling can be applied to explain Jovian auroral phenomenon. In this effort, my focus has been on understanding the acceleration processes which cause intense auroral emissions at the magnetic footprint of Io, the most volcanically active satellite in our solar system. In this study, we have suggested that each of the three types of auroral acceleration regions observed in the Earth's environment by FAST are active MI coupling processes on Jupiter as well. The bright emissions at the Io magnetic footprint are caused by Alfvén-dominated precipitation, while the extended tail emissions in Io's wake are due to electron acceleration from parallel electric field in an upward current region between Jupiter and Io. The downstream electron acceleration is investigated using a static Vlasov code under constraints of quasi-neutrality and an applied potential drop. Results suggest that the proton and the hot electron population in Io's plasma torus control the electron current densities and thus may control the energy flux and the brightness of the aurora downstream from Io's magnetic footprint. Parallel electric fields are expected to create an unstable shell electron distribution inside the auroral cavity, which may lead to the electron cyclotron maser instability. Hence, this may be the source mechanism of Io-controlled decametric radio emissions. I am currently adapting a gyrofluid code developed for use in the modeling of Earth's magnetosphere for application to a Jupiter-Io flux tube. The primary differences when comparing the environments of Earth and Jupiter are Io's dense plasma torus and the strong Jovian magnetic field. The majority of the Alfvén wave energy is reflected due to the density gradient of the plasma torus. The strong magnetic field and low density at high latitudes cause the Alfvén speed to reach the speed of light, where the displacement current in Ampere's law needs to be taken into account. We would like to understand precisely where and how electron acceleration occurs and how the electrons are influenced by propagating Alfvén waves. An Alfvén eignemode, called the ionospheric Alfvén resonator, can develop between the conducting boundary of Jupiter and the exponential increase in Alfvén velocity on the topside of ionosphere. At Earth, reflections of Alfvén waves give rise to pulsations with a frequency near 0.1-1 Hz. We are attempting to understand the Alfvén resonator in Jupiter's ionosphere which may be associated with the periodicity of S-bursts. The long term goal of this investigation is to
study the similarities and differences of MI coupling between the terrestrial
and Jovian environments. Scientists are eager to determine which of the
physical mechanisms are universal and which may be unique to a particular
celestial body. This research will be guided by this ambitious goal making
use of all available data or simulation techniques to unveil the mysteries of
our space environment. C. Plasmasphere My primary research interest while serving as a postdoctoral research associate at Los Alamos National Laboratory was the fate of the plasmasphere. The near-Earth region of the magnetosphere populated by dense, cold plasma is known as the plasmasphere. When the solar wind pressure is strong and the interplanetary magnetic field (IMF) is southward, plasmaspheric material can be stripped away following newly formed open drift trajectory toward the dayside magnetopause. During a comprehensive examination of 8 years of data obtained from magnetospheric plasma analyzers (MPAs) onboard five geosynchronous satellites, I was able to characterize multiple cases, although rare, where cold plasmaspheric ions and warm magnetosheath ions were simultaneously present on the same flux tube. The majority of obtainable events with available IMF information have velocity space signatures that are consistent with expectations based on our understanding of the reconnection process. In an additional study involving an investigation of Interball/Hyperboloid and Polar/TIDE data, we were able to show evidence for the existence of cold plasmaspheric material on high-latitude open magnetic field lines. Observed phase-space densities are compared with modeled magnetosheath and plasmaspheric phase-space densities in order to distinguish plasmaspheric material from the low-energy portion of entering magnetosheath plasma from the solar wind. Furthermore, by comparing MPA data with observations from Millstone Hill's incoherent scatter radar, we demonstrated that plasmaspheric drainage plumes and polar ionization patches follow the same convection pattern leading to a conclusion that the draining plasmasphere should follow a similar trajectory transporting material over the polar cap and into the tail plasma sheet. This scenario was introduced by myself and my co-authors prior to the collection of data by the IMAGE satellite. These findings have been further confirmed by IMAGE, radar, and GPS observations. I have additionally completed a comprehensive survey of plasmasphere refilling at geosynchronous orbit using 11 years of data available from MPAs. During this investigation, I was able to demonstrate how the refilling rate varies with solar cycle, geomagnetic activity, and season. My postdoctoral research on the plasmasphere resulted in 5 published papers with myself as first author, one of which was selected as the highlighted article in an issue of the EOS magazine. These papers have been cited quite frequently in later plasmaspheric related papers. D. Ionospheric Outflows/Polar Winds The polar wind is an ambipolar
outflow of thermal plasma from the terrestrial high altitude ionosphere to
the magnetosphere along magnetic field lines. The polar wind was the topic of
interest for my Ph.D. research, guided by my advisor, Professor James
Horwitz, at the During this data analysis effort, I introduced an innovative technique for overcoming an instrument limitation which caused ions with the lowest energy to be shielded by the spacecraft potential. I later acted as a consultant helping graduate students in the continuation of the development of a time-dependent polar wind model, also known as a dynamic fluid-kinetic (DyFK) model. An additional study involved a systematical investigation of soft-electron precipitation effects on topside ionospheric upflows through the use of a dynamic multi-fluid model. Conclusions from this research demonstrated that a declining characteristic energy at constant energy flux increases the number of precipitating electrons available to heat the thermal electrons, thus enhancing the thermal electron temperature and, in turn, the ambipolar electric field for propelling the upward oxygen ion flows. E. The Evolution of Nonlinear Compressed Alfvén Waves My first research experience was supervised by
Professor Ling-Hsiao Lyu at Current Group Members: Research
Associate: Dr. Sam Jones Graduate Student:
Mr. Debrup Hui Former Students: M.S. in 2007: Mr.
Lun Ma B.S. in 2007: Mr.
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