Preliminary theoretical and computational analyses of the Combined Release and Radiation Effects Satellite (CRRES) magnetospheric barium releases are presented. The focus of the studies is on the evolution of the diamagnetic cavity which is formed by the barium ions as they expand outward, and on the structuring of the density and magnetic field during the expansion phase of the releases. Two sets of simulation studies are discussed. The first set is based upon a two-dimensional ideal MHD code and provides estimates of the time and length scales associated with the formation and collapse of the diamagnetic cavity. The second set uses a nonideal MHD code; specifically, the Hall term is included. This additional term is critical to the dynamics of sub-Alfvenic plasma expansions, such as the CRRES barium releases, because it leads to instability of the expanding plasma. We performed detailed simulations of the G4 and G10 releases. In both cases the expanding plasma rapidly structured: the G4 release structured at time t ≲ 3 s and developed scale sizes ∼ 1-2 km, while the G10 release structured at time t ≲ 22 s and developed scale sizes ∼ 10-15 km. We also find that the diamagnetic cavity size is reduced from those obtained from the ideal MHD results because of the structure. On the other hand, the structuring allows the formation of plasma blobs which appear to free stream across the magnetic field; thus, the barium plasma can propagate to larger distances transverse to the magnetic field than the case where no structuring occurs. Finally, we also discovered a new normal mode of the system which may be excited at the leading edge of the expanding barium plasma. This mode is a magnetic drift wave which propagates azimuthally around the barium cloud in the frequency range Ωi ≪ ω ≪ Ωe.

Preliminary study of the CRRES magnetospheric barium releases

The optical properties of laser-induced plasma generated firm solid (Al alloy) and liquid (Mn, Cr, Mg, or Ti solutions) samples expanded across an external, steady magnetic field have been studied by atomic-emission spectroscopy. Various line emissions obtained from the constituents of the Al alloy and of the aqueous solution show an enhancement in intensity in the presence of an approximately 5-kG magnetic field. The enhancement of the signal was nearly a factor of 2 for the minor constituents of the solid samples and a factor of 1.5 for the elements in liquid phase. Temporal evolution of the emission from the solid sample showed maximum enhancement in emission intensity at 3-10-micros time delay after plasma formation in the laser energy range 10-50 mJ. However, for the liquid sample the maximum signal was for a gate delay of 3-25 micros the energy range 50-200 mJ. This enhancement in the emission intensity was found to be due to an increase in effective density of the plasma as a result of magnetic confinement when the plasma cooled after expansion. This enhanced emission was due to an increase in the rate of radiative recombination in the plasma.

Optical emission from laser-induced breakdown plasma of solid and liquid samples in the presence of a magnetic field.

Over the past fifteen years it has become increasingly clear that Hall magnetohydrodynamics plays a crucial role in many space and laboratory plasma processes: magnetic reconnection, sub-Alfenic plasma expansions, and plasma opening switches to name a few. Hall magnetohydrodynamics is important for plasma dynamics on length scales less than the ion inertial scale length but greater than the electron inertial length. On these scales the ion and electron motions are decoupled; the electrons remain frozen to the magnetic field but the ions are not. In this paper we provide a basic overview of Hall magnetodydrodynamics with an emphasis on numerical methods. We also provide several concrete examples of Hall dynamics: whistler waves, Hall drift waves, plasma opening switch dynamics, and three dimensional magnetic reconnection.

Hall Magnetohydrodynamics - A Tutorial

Scaled experiments on the nonlinear Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) instabilities are described for two limiting conditions. First, at high Reynolds number, the mixing layer is found to grow self-similarly ∼αiAgt2 for a constant acceleration and as a power law tθi for an impulsive acceleration g=Uδ(t). The growth coefficients αi and exponents θi are measured over a comprehensive range of Atwood number A. Second, with non-Newtonian materials, the critical wavelength and amplitude for RT instability associated with the shear modulus and tensile yield of the material is observed over a variety of conditions. The results are applied to naturally occurring supernova explosions and volcanic islands.

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Nonlinear evolution of the Rayleigh–Taylor and Richtmyer–Meshkov instabilities

A large ion Larmor radius plasma undergoes a particularly robust form of Rayleigh–Taylor instability when sub‐Alfvenically expanding into a magnetic field. Results from an experimental study of this instability are reported and compared with theory, notably a magnetohydrodynamic (MHD) treatment that includes the Hall term, a generalized kinetic lower‐hybrid drift theory, and with computer simulations. Many theoretical predictions are confirmed while several features remain unexplained. New and unusual features appear in the development of this instability. In the linear stage there is an onset criterion insensitive to the magnetic field, initial density clumping (versus interchange), linear growth rate much higher than in the ‘‘classic’’ MHD regime, and dominant instability wavelength of order of the plasma density scale length. In the nonlinear limit free‐streaming flutes, apparent splitting (bifurcation) of flutes, curling of flutes in the electron cyclotron sense, and a highly asymmetric expansion are found. Also examined is the effect on the instability of the following: an ambient background plasma (that adds collisionality and raises the expansion speed/Alfven speed ratio), magnetic‐field line tying, and expansion asymmetries (that promotes plasma cross‐field jetting).

Sub-Alfvenic plasma expansion

A theoretical treatment of the linear stability of sub-Alfvenic plasma expansion is developed. The theory is fully kinetic and includes finite-beta effects, collisional effects, and neutral gas flow. A variety of results are obtained, and are applied to the the AMPTE magnetotail release, the NRL laser experiment, and the upcoming CRRES GTO releases.

Stability of sub‐Alfvénic plasma expansions

The equilibrium of a nonneutral plasma in a toroidal vessel with a toroidal magnetic field is analyzed. In the zero inertia limit it is heuristically shown from force balance considerations that there is an electrostatic hoop force and a force due to diamagnetism along the major radius. The problem of equilibrium is formulated in terms of solutions of a 2D partial difference equation. This equation is solved in the large-aspect-ratio limit and a general expression for the shift of the potential axis is obtained which shows that the shift is approximately epsilon and that it depends solely on the internal capacitance of the cloud. The simulation study is based upon the modified MHD equations and the nonlocal nature of the mode is investigated. Applications to sub-Alfvenic plasma expansions, electromagnetic waves in the earth's magnetosphere, and plasma switches are discussed.

J. D. Huba

Papers

Stability of sub‐Alfvénic plasma expansions

Theory and simulation of a high‐frequency magnetic drift wave