I used to study space plasma physics and nuclear fusion plasmas. My Ph.D. was also in this field. Here I describe the main areas I was involved in, and list the papers we published in each.
My main interest in ionospheric physics was on the Farley-Buneman instability, a major source of turbulence in the equatorial electrojet, a huge electrical current that flows high above the Earth's equator.
|Density fluctuations in a computer simulation of the Farley-Buneman instability. Note that the higher density fluctuations associated with the wave (in red) tend to drift downward as the whole pattern propagates to the right, while the lower density fluctuations (in blue) drift upward. This behavior is consistent with our nonlinear theory of the instability.|
What causes the Northern Lights (also known as the Aurora Borealis)? This much was known at the time I did this research: Particles streaming from the Sun get trapped by the Earth's magnetic field. Then, somehow, some of the electrons that are part of this trapped particle population are accelerated towards the Earth along magnetic field lines that originate near the Earth's north and south magnetic poles. When the electrons crash into the Earth's ionosphere, they excite the atoms and molecules there, producing the colors of the aurora. The question then becomes, how are these electrons accelerated? It must be due to an electric field oriented parallel to the magnetic field lines along which the electrons travel. But it is well-known that, in classical magnetohydrodynamics (MHD), electric fields are perpendicular to magnetic fields. So my research concentrated on models that included physics that was beyond the scope of MHD. Localized structures callled double layers and large-scale waves called inertial Alfvén waves were among the candidates I examined.
|Computer simulation of the formation of double layers and subsequent electron acceleration caused by the presence of inertial Alfvén waves. First, note the formation of a strong disturbance in the ion population (green dots, vertical axis: velocity, horizontal axis:position) about 1/6 of the distance across the system from the left boundary. This disturbance is a cluster of double layers, driven by the leftward drift of electrons (red dots) associated with the Alfvén wave. Later, another large double layer appears at the extreme right of the system, driven by the rightward traveling electrons. Other weaker double layers driven by rightward traveling electrons are visible just to the left of this double layer. Each double layer is accompanied by a characteristic spike in the electric potential (yellow trace). The potential drops abruptly from left to right for the case of double layers driven by leftward traveling electrons and rises left-to-right when driven by rightward traveling electrons. Also note that these rightward traveling electrons are accelerated by the Alfvén wave and later steepen up into a significant front racing from left to right, most obvious in the center of the system, at the top of the movie frame, late in the simulation. These electrons are trapped in and are essentially surfing on the rightward-propagating Alfvén wave, identifiable as the rightward progression of all the field quantities graphed in the bottom half of the movie frame. It is this population of electrons that can potentially travel downward along the Earth's magnetic field lines (to the right in our simulation) to produce the Aurora Borealis. From Silberstein and Otani, J. Geophys. Res. (1994).|
Plasmas that contain charged dust particles are intriguing because they can, at times, behave like both a solid and a gas at the same time. Specifically, the plasma component behaves like a gas (or more specifcally, like a plasma), while, simultaneously, the dusty component often forms a lattice structure characteristic of a crystal. My research focused on this interesting state of matter.
Plasma physics is involved in several aspects of the Sun's behavior. In the Sun itself, I studied the dynamo action created by plasma flow, which maintains the Sun's magnetic field, a field that otherwise would have disappeared long ago. In the solar corona, the Sun's extremely hot but tenuous outer atmosphere, I studied the process by which magnetic lines reconnect, a process that also generates intense sheets of electrical current and releases tremendous amounts of energy, visible in the X-ray spectrum of the Sun as "hot spots." In the solar wind, a colleague of mine, Perry Gray, and I studied the physics of the unusual distribution of velocities often exhibited by the ions.
|Colorscale depiction of how plasma convection amplifies the embedded magnetic field as obtained from a two-dimensional cross-section of a three-dimensional computer simulation. From Otani, J. Fluid. Mech. (1993).|
When I was a graduate student, the so-called "tandem mirror machine" was one of the major concepts for bottling up plasma long enough so that controlled nuclear fusion could take place. If it had been successful, the machine would have been a relatively clean way (yes, really!) to generate electrical power on a large scale. I examined two plasma instabilities that were preventing the concept from working: the Alfvén ion-cyclotron instability, and the interchange instability. With respect to the latter, I also looked the possibility that externally generated radio-frequency waves might serve as a stabilizing influence. The mirror machine concept has since been abandoned in favor of toroidal-shaped devices known as tokamaks.
While conducting research on plasmas, I also had occasion to look at and solve a number of computational problems. These had mainly to do with the particle-in-cell method we used for modeling plasmas. I examined problems having to do with computatonal speed and numerical instabilites arising from the artificially small number of particles we were required to use in the simulations.