Session BO2: Space and Astrophysical Plasmas

Solitons in the Auroral Upward Current Region

We have studied the evolution of auroral plasmas --- specifically the boundary between the auroral cavity and ionosphere --- using 1-D and 2-D dynamic Vlasov simulations. These simulations are initialized with a strong observation-based double layer and result in many features observed with FAST such as a persistent density cavity, an anti-earthward ion beam, a quasi-stable parallel electric field, and ion phase space holes. However, a suprising new result from the simulation is the formation of an ion-acoustic soliton that forms in the auroral cavity and is associated with a population of earthward-traveling ions. I will show FAST observations of solitons and earthward-traveling ions, which compare favorably with the simulation. I will also show how solitons and phase space holes differ both theoretically and observationally. Using a pseudo-potential model [Goldman et al., this meeting], I will show that the term ``soliton'' is appropriate for the structures that are seen in the simulation. Finally, kinetic simulations with a model soliton will be presented in order to better understand the evolution of the soliton in the more realistic auroral simulations.   

Simulations of Jetted Relativistic Blastwaves in Astrophysics

We present relativistic hydrodynamic simulations of jetted blastwaves using the Cosmos++ astrophysics code. We post-process these simulations by integrating the radiative transfer equation thru a observer's space-time slices of the data, assuming relativistic self-absorbed synchrotron emission, to derive detailed multi-frequency lightcurves for the jet as viewed at arbitrary inclination angle. In particular, we simulate the asymmetric outflow resulting from the giant flare of December 27, 2004 from SGR 1806-20 and obtain excellent agreement with the data. We find that the asymmetric radio nebula that was observed to expand over the months following the flare cannot be explained by a simple ballistic ejection of material during the flare, but requires angular dependence of the energy injection with respect to the jet axis. In addition, we present simulations of jetted blastwaves of the relativistic afterglows resulting from gamma-ray bursts. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.   

Influence of Episodic Mass Ejection on Hydrodynamic Jet Evolution

The Laboratory for Laser Energetics OMEGA laser has been used to create plasma jets formed by two mass ejections. Two 1-ns laser pulses, separated by 10 ns, launched strong shocks into a 220-\textit{$\mu $}m-thick, mid-$Z$ metal plug. The first laser pulse consisted of three laser beams and the second pulse four beams. Material unloading from the back of the plug traveled 300 \textit{$\mu $}m through a high-$Z$ pipe and launched a jet into a low-$Z$ foam. Control experiments were performed with single mass ejections created by either three or seven laser beams. All experimental results were compared to 1-D and 2-D simulations. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Production of Cumulative Jets Generated by Laser-Driven Collapsing Hollow Cones and Wedges

Cumulative plasma jets formed by imploding hollow cones and wedges are observed. The cones, made from 4-10 mg/cm$^{2 }$Ni or Al foils, with a base diameter of 500 $\mu $m and 90 - 130$^{o}$ apex angles, are imploded by a 1.054-$\mu $m wavelength, $\sim $4 ns FWHM laser pulse focused to $\sim $ 2*10$^{13}$ W/cm$^{2}$ on their outer surface. Jet shape, location, and densities are measured with monochromatic radiography utilizing 0.65 keV x-rays. For certain cone geometries, cumulative jets with ion densities $>$10$^{19}$ cm$^{-3}$ propagate at velocities $>$10 km/sec. The interaction of such jets with an ambient medium and the resulting shock structures can be used for lab simulation of aspects of various astrophysical phenomena such as jets produced in supernovae explosions or accretion of plasma onto compact objects (black hole, neutron star or white dwarf).   

Statistical Theory of a Magnetized Accretion Disk Corona

We present a statistical description of a stochastic magnetic field in the force-free corona of a turbulent accretion disk. We represent the field by an ensemble of magnetic loops tied to the disk and introduce the distribution function of loops over their sizes. Each loop evolves under several physical processes, e.g., Keplerian shearing, random walk of the footpoints due to disk turbulence, and reconnection with other loops. To represent these processes statistically, we construct a loop kinetic equation for the evolution of the distribution function. This is similar to Boltzmann's kinetic equation, with reconnection represented by a binary collision integral. We solve the equation numerically and obtain a statistical steady state. Once the loop distribution function is known, we can calculate important integral characteristics of the coronal magnetic field, such as the overall magnetic energy and the magnetic dissipation rate; their distribution with height above the disk; and the rate of angular momentum transfer by the coronal loops. We also access the efficiency of the reconnective inverse cascade in producing a population of very large loops.   

3-D MHD Simulation of the Accretion Disk Corona

Evolution of a magnetic loop in an accretion disk corona is studied by using the resistive MHD code MAB. Axisymmetric corona and infinitesimally-thin accretion disk with the Keplerian velocity profile is used as the initial state. In the accretion disk, conservation of angular momentum prevents the accretion. The microscopic resistivity and viscosity are too small to explain the accretion rate inferred from observations. In this work, we test an idea that the evolution of coronal magnetic fields might make differential rotation flows in the disk to be unstable by leading to the development of coronal magneto- rotational instability (MRI) and enhancement of angular momentum transport in the disk. In our computer simulations, the MHD equations for the accretion disk and its corona are modeled separately. The poloidal component of magnetic field and the velocity field in the disk are used as a boundary condition to advance the coronal flows. The toroidal and radial components of magnetic field are computed in the corona simulation and their boundary values are used in turn to advance the accretion disk flows. This provides a feedback loop between the MHD flows in the accretion disk and its corona. In this report, the evolution of a single coronal magnetic loop and the corresponding angular momentum transport in the disk are considered.   

Generation of secondary whistler emissions

We present initial results from a numerical study of whistler wave generation in the radiation belt plasma by energetic electrons. The long-term goal of this investigation is to understand the mechanisms of trigering of intense secondary waves by an initial whistler. This phenomenon has been observed in a number of experiments performed on the SIPLE station in Antarctica. These observations show that the triggering depends in a complex way on the frequency and amplitude of the pump wave, and on the conditions of the ambient plasma and the geomagnetic field as well. Thus to get new insight into this problem a comprehensive numerical model has been developed and simulations of wave-particle interactions have been performed for different parameters of the energetic particles distribution function and pump wave.   

Nonlinear electron dynamics in large-amplitude whistlers

The nonlinear evolution of whistlers in the radiation belt depends on trapping and phase bunching of resonant electrons in the wave field B$_{w}$, and is sensitive to spatial variation of the geomagnetic field B$_{0}$ (z) along a field line and to the finite spatial extent of a wave packet. The scale length L for these inhomogeneities is 10$^{2}$ to 10$^{3}$ times longer than the wavelength $\lambda $, making nonlinear evolution a global problem which strains computational resources. Our particle simulation code HEMPIC partially addresses this problem by eliminating the plasma frequency and the speed-of-light timescales, thereby permitting gyrofrequency time steps. Here we use the small parameters $\lambda $/L and B$_{w}$/B$_{0}$ to integrate the electron trajectories with $>$100 times longer time steps that characterize either the trapping frequency or the time for an electron to traverse the inhomogeneities. We use this approach analytically to elucidate electron phase trapping by a wave packet in the presence of a varying magnetic field, and numerically to elucidate the phase bunching that initiates secondary waves.   

Meteor Plasma Trails in E-Region Ionosphere: Diffusion, Electric Fields, and Disturbances

Meteoroids penetrating the Earth's ionosphere frequently leave behind trails of dense plasma in the region between 130km and 75km. We will present the first quantitative model of the fields and density evolution which accounts for both the geomagnetic field and the background plasma. Using both simulations and 2D analytical theory, we can accurately model trail evolution for a broad range of conditions. Ambipolar diffusion of trails gives rise to polarization electric fields, which generate electron density disturbances and may drive plasma instabilities, both in the trail and in a vast area in the background ionosphere. In addition, the strong electric fields typically found in the equatorial and high-latitude E-region electrojets will polarize the highly conducting meteor trail resulting in substantial spatial redistribution of the electric potential around the trail. A 3D analytical theory shows that the electric field in the near-trail region can be drastically amplified, which may result in strong electron heating and associated effects. Combining our theory with radar observations yields useful information about meteor trails and the surrounding atmosphere.   

Generation and Evolution of Intense Ion-Cyclotron Turbulence by Artificial Plasma Cloud in the Magnetosphere

It is shown that intense ion-cyclotron turbulence can be induced in the near-Earth space environment by shaped release of neutral gas such as lithium. Release of one ton in the Earth's equatorial plane at L=2 can introduce about 30 GJ of energy to pump intense turbulence around the ion cyclotron harmonics that readily evolves into the turbulent state. The energy is obtained by converting the orbital kinetic energy of the neutral lithium atoms into free energy for the electromagnetic waves through photo-ionization and creation of a ring distribution. The distribution function is highly unstable to the generation of shear Alfven waves near the lithium cyclotron harmonics. Additionally these waves lead to intense pitch angle scattering of the trapped electrons in a broad energy band.   

Formation of a Heavy-Ion Induced ULF Cavity in the Earth's Magnetosphere

The injection of an easily ionized vapor (lithium) from a satellite with an anisotropic and population inverted velocity distribution is highly unstable fot the spontaneous growth of ULF waves. The growth of these waves occurs near the harmonics of the lithium gyrofrequency. We show that the waves generated during this process will be trapped between two turning points forming a cavity, prolonging the lifetime of the turbulence. In the presence of a second ion species, in this case Helium, the generated waves will reflect when the frequency of the waves meets the Buchsbaum frequency. For a proton-electron-Helium plasma, with a small percentage of Helium, the Buchsbaum frequency is near the Helium cyclotron frequency. This situation where there is a turning point near the absorbtion point has been analyzed and we compute the potential loss of wave energy via tunneling. With typical percentages of Helium there is virtually no loss of wave energy. Thus as waves travel back and forth between turning points, they continue to amplify as they pass through the instability region.   

Multiscale phenomena in the magnetosphere: Power-law and Weibull distributions

The power law behaviour of the probability distribution of burst lifetime duration of magnetospheric substorms, studied earlier by Freeman et. al. Geophys. Res. Lett, 2000), was modelled as a combination of a power law with an exponential cutoff and a lognormal distribution. We study the interburst time or the waiting time between substorms above a threshold as quantified by the AL index. The AL index data set from January 1978 until June 1988 (1 min resolution, 5520960 data points) is analyzed and the waiting time distribution is found to exhibit a similar behaviour as that of the burst lifetime. Recent Bunde et. al( Phys. Rev. Lett., 2005) have shown the streched exponential (Weibull distribution) behavior in the recurrence time statistics of extreme events in long term climate records. Similar result has been shown for the earthquake statistics by Turcotte (2006). The existence of such statistiscal order in the magnetospheric multiscale phenomena will be presented.   

Turbulent Alfven waves and fast magnetosonic waves in the solar corona

High-frequency Alfven waves and fast magnetosonic waves may play an important role in the heating of the solar corona and in the acceleration of energetic particles in solar flares. This presentation describes how MHD turbulence can generate such high-frequency waves through a cascade of wave energy from low frequencies to high frequencies. The three principle physical mechanisms governing this energy cascade will be discussed, and analytic and numerical results based on weak turbulence theory will be presented. These results show that MHD turbulence is a promising mechanism for explaining the anisotropic heating of minor ions in the solar corona, and for generating high-frequency MHD waves in solar flares that can accelerate both electrons and ions.   

Inertial range physics of solar wind turbulence as revealed by 3 second plasma measurements

The 3DP instrument on-board the Wind spacecraft provides the highest time resolution plasma measurements currently available with a time resolution of 3 seconds. This instrument enables almost the entire inertial range to be probed using both velocity and magnetic field data although the dissipation range of the velocity fluctuations is still beyond reach. Analysis of power spectra and structure functions have shown that while the magnetic energy spectrum of the solar wind is a power law with an exponent near 5/3, the velocity or kinetic energy spectrum often exhibits an exponent near 3/2. Another important discovery is that the Elsasser ratio, the ratio of energy in the two Elsasser fields, approaches unity at the smallest measurable scales. Thus, as the energy and cross-helicity cascade through the inertial range the fluctuations in the two Elsasser fields evolve toward a state of equipartition, a process called dynamic mixing as opposed to dynamic alignment. These and other results that are improving our knowledge of solar wind turbulence shall be discussed.   

3D Magnetospheric Structure Prior to Substorm Onset and Onset Mechanism

Space and ground-based observations have indicated that the substorm onset occurs in the near-Earth plasma sheet region ($\sim 8-10 R_E$)and is associated with a low frequency (in the Pi 2 range) instability. To understand substorm onset process we model the global magnetospheric structure prior to substorm onset realistically by 3D quasi-static equilibrium solutions which consist of a current sheet with an enhanced cross-tail current density with thickness of $\sim 1 R_E$ around the local midnight and with a longitudinal extent of $\sim 60-70^{\circ}$ at $X \sim – (7-11) R_E$. The associated ionospheric Birkeland current moves equatorward with an enhanced current density shrinking in latitudinal width, consistent with the observed ionospheric growth phase signatures. The observed low frequency instability has been explained as kinetic ballooning modes (KBMs) which are excited when $\beta_{eq}$ increases from $\sim$ 20 to above 50. To study the onset instability, we present theoretical analysis and numerical solutions of KBMs, which include kinetic effects of particle trapping, finite ion gyroradii, and wave-particle resonances. The results indicate that a new branch of unstable KBM is destabilized through wave- ion magnetic drift resonance. The KBM has a real frequency in the Pi2 frequency range. The kinetic calculations will be compared with the MHD theory.