During the growth phase, the magnetotail is prepared for the violent relaxation dynamics that occurs at substorm onset. Multi-satellite observations indicate the development of a thin current sheet and a rapid intensification of the cross-tail current density at near-Earth distances during a short interval (< 1 min) just before onset, after a period of sluggish growth (\sim 0.5-1.5 hr). These observational features have been accounted for recently by analysis as well as high-resolution MHD simulation of the magnetotail, including the Earth's dipole field. In the slow growth and impulsive pre-onset phase, it is shown that a thin current sheet develops spanning Y-points that stretch from the mid-tail region (\sim 30 R_E) to the near-Earth region (\sim 10 R_E). The current sheet dynamics exhibits an impulsive enhancement in amplitude and the flows are dominantly earthward, consistent with observations. When the current sheet becomes sufficiently thin, finite ion-Larmor-radius terms such as electron pressure gradients and Hall currents must be included in the theory and are shown to have a striking effect on the dynamics in the impulsive growth phase. It is shown that the thin current sheet is unstable to ideal ballooning instabilities with rapid spatial variation in the dawn-dusk direction. Ionospheric boundary conditions can have a strong influence on the linear properties of the ballooning instability, especially at near-Earth distances. Once the linear mode is triggered, nonlinear studies indicate a tendency for near-explosive growth of the instability, suggesting its possible role as a mechanism for substorm onset.
[eMopI2.02] Laboratory Simulations of Solar Prominence Eruptions
Paul M. Bellan (Caltech)
Solar prominences are large Ømega -shaped, low \beta twisted magnetic flux tubes that are line-tied to the solar surface. After remaining quiescent for days, prominences unpredicably erupt on a time-scale ranging from minutes to tens of minutes. Recent analysis (B. Vrsnak, V. Ruzdak, and B. Rompolt, Solar Phys. 136,151(1991)) suggests that eruption occurs when the magnetic flux tube becomes excessively twisted causing it to become unstable and shed its excess magnetic helicity.
This helicity-shedding instability seems analogous to the formation process of spheromak plasmas; in particular, spheromak formation similarly involves shedding of excess magnetic helicity by excessively twisted, line-tied, low \beta magnetic flux tubes.
This analogy has motivated an experiment designed to create realistic laboratory-scale simulations of erupting prominences. This experiment uses a helicity-injecting magnetized plasma gun similar to the guns used in conventional spheromak experiments, but modified to simulate the geometry and dynamics of an erupting solar prominence. Specifically, the modifications are (i) instead of being coaxial, the gun has a horseshoe shape to create Ømega -shaped, line-tied magnetic flux tubes and (ii) the gun is mounted on the wall of a vacuum chamber much larger than the gun and plasma dimensions so as to avoid wall image currents and therefore simulate the infinite half-space of the solar corona. High speed photos demonstrate the formation of structures with the morphology of a prominence and eruptive-like instability occurs at high currents. The dynamics of the instability are reproducible and the topological dynamics are reminiscent of solar eruptions.
[eMopI2.03] Theoretical Issues in the Physics of the Ionosphere
Bamandas Basu (Phillips Laboratory)
The earth's ionosphere is a partially ionized, inhomogeneous plasma in which currents and electric fields exist. It has been established by a variety of measurement techniques that plasma waves and instabilities play fundamental role in determining the physics of the ionosphere. Density and electric field fluctuations resulting from the plasma instabilities are found at all latitudes, longitudes, and at almost all altitudes. Ion-neutral collisions play important role in the excitation of the instabilities, while the ambient inhomogeneous electric fields together with the density gradient determine the topological properties of the excited modes. Moreover, the plasma modes that can be excited are intrinsically global in nature. In order to provide a realistic theoretical model for the onset of the instabilities it is important to identify the physical characteristics of these global plasma modes and their topological properties in particular. After a brief description of a few important plasma instabilities in the ionosphere, the gravity-driven instability in the equatorial F-region, which is responsible for the so-called ``Equatorial Spread'', will be discussed in somewhat details.
[eMopI2.04] Two-Dimensional Mappng of the Plasma Density in the Upper Atmosphere with Computerized Ionospheric Tomography (CIT)
P.A. Bernhardt (Plasma Physics Division, Naval Research Laboratory, Washington, DC 20375)
Tomographic imaging of the ionosphere is a recently developed technique that uses integrated measurements and computer reconstructions to determine both electron and ion densities. The integral of electron density is obtained from the phase of VHF/UFH radio transmissions from orbiting satellite beacons broadcasting to a chain of receivers on the earth's surface. The integral of oxygen ion densities is determined from optical measurements of extreme ultraviolet (EUV) emissions that are recorded with orbiting spectrometers. Radiative recombination of O^+-ions and electrons yields 91.1 nm and 135.6 nm emissions that are observable at night. During the day, photoionization of atomic oxygen yields 83.4 nm emissions that are scattered by the O^+-ion for illumination of the ionospheric densities. Each type of measurement has unique advantages and limitations. Transmissions of radio waves from satellite to ground have high spatial resolution (10 km or better) but suffer from the lack of horizontal integration paths and the requirement for ground receivers. The EUV optical signals are observable from any direction but they are the strongest when the satellite is scanning the earth's limb. The radio wave phase noise is much less than the EUV photon counting noise. Optical tomography is expected to yield 5 to 20 km vertical resolution and about 50 to 500 km horizontal resolution. The most effective way of mapping the ionosphere is to analyze the combined radio and EUV data using computerized ionospheric tomography (CIT). The radio and EUV techniques provide cross-calibration along co-aligned observation paths and provide missing data in the other regions of the tomographic scans. New ionospheric imaging instruments are scheduled for launch on a number of spacecraft including the NASA sponsored TERRIERS and the DoD sponsored ARGOS satellites. To support these instruments, algorithms are being developed for tomographic reconstructions. These algorithms must accomodate noise in the date, limited angle measurements, non-uniform scans, motion of the the medium in response to electric fields and neutral winds, and non-ideal location of the satellite orbit.