Session TO8: Space and Astrophysical Plasmas I

Anomalous conductances caused by plasma turbulence in the high-latitude ionosphere

During periods of intense geomagnetic activity, strong electric fields penetrating from the Earth's magnetosphere form electrojets and excite plasma instabilities in the high-latitude E-region ionosphere. These instabilities give rise to plasma density turbulence coupled to electrostatic field fluctuations. This turbulence induces nonlinear currents, while the field fluctuations cause strong electron heating. These two effects increase ionospheric conductivities that play an important role in magnetosphere-ionosphere coupling. A quantitative understanding of turbulent conductivities and energy conversion is important to accurately model magnetic storms and substorms. Our theoretical analysis, supported by fully kinetic 3-D simulations, allows one to quantify energy deposits in the electrojet, particle heating, and anomalous conductivities. Our estimates show that during strong geomagnetic storms the inclusion of the anomalous effects may nearly double the total Pedersen conductance. This helps explain why existing global MHD codes developed for predictive modeling of space weather systematically overestimate the cross-polar cap potentials by approximately a factor of two.   

Modeling generation of ULF electromagnetic waves by modulated heating of the ionospheric F2 region

We present a theoretical and numerical study of the generation of ultra-low frequency (ULF) waves by the modulation of the electron pressure at the F2-region with an intense high-frequency electromagnetic wave. We develop a cold Hall MHD model that governs the dynamics of the shear Alfven and magnetosonic modes generated in the F2 region, of the damped modes in the diffusive Pedersen layer, and of the weakly damped helicon wave mode in the Hall-dominated E-region. A realistic profile of the ionospheric conductivities was used in the model. We studied the generation and dynamics of the low-frequency electromagnetic waves for different frequencies of the ULF waves. In particular we study the propagation and penetration of ULF electromagnetic fields and currents through the ionospheric layers to the ground. Different magnetic field configurations were studied and comparison between simulation with HAARP observation is also presented. The concept may constitute a means of injecting electromagnetic waves into the earth-ionosphere waveguide. This work is supported by ONR MURI grant.   

Energization and Injection of Ring Current Ions during Magnetic Storms

During storms, induced electric and magnetic fields within the magnetosphere lead to the build up of energetic particles within the ring current and radiation belts. Using single-particle tracking with time-dependent global magnetic and electric fields, we explore the mechanisms responsible for the acceleration and injection of plasma sheet ions into the inner magnetosphere. We examine the contribution from various ionospheric source regions to the storm-time ring current. Solar wind boundary conditions are used as inputs for a self-consistent 3D multi-fluid model, which produces time-dependent global electric and magnetic fields that are read into our single-particle code. Ionospheric H$^{+}$ and O$^{+}$ are injected into the simulation from various ionospheric regions. Results show that the energization and trapping of ionospheric H$^{+}$ and O$^{+}$ are highly dependent on the location where the outflowing ions are initialized, and small scale structures in the current sheet are correlated with particle convection and energization. The same magnetic features that produce intensification of auroral current lead to the injection of energetic particles into the inner magnetosphere.   

Propagation of kinetic Alfv\'{e}n waves at the plasma sheet boundary layer

Satellite observations have indicated that strong Alfv\'{e}nic Poynting fluxes are present at the plasma sheet boundary layer, and that these Alfv\'{e}n waves are associated with the field-aligned acceleration of electrons. The ionospheric signature of these Poynting fluxes are broad-band electron distributions, characterized as the Alfv\'{e}nic aurora. In locations such as the boundary layer where there are gradients in the Alfv\'{e}n speed perpendicular to the magnetic field, phase mixing can rapidly decrease the perpendicular wavelength of the Alfv\'{e}n waves to the point where kinetic effects lead to a significant parallel electric field. A numerical model for this region has been developed based on a two-fluid approach to the modeling of kinetic Alfv\'{e}n waves, with the inclusion of a term modeling Landau damping using an approximation for the plasma dispersion function. This model is applicable not only to the boundary layer but to other situations where there are perpendicular Alfv\'{e}n speed gradients, such as the Large Plasma Device at UCLA.   

Magnetic Reconnection or Alfvenic Interaction in Collisionless Plasmas?

The magnetic reconnection hypothesis emphasizes the importance of the breakdown of the frozen-in condition, explains the strong dependence of the geomagnetic activity on the interplanetary magnetic field (IMF) and approximates an average qualitative description for many IMF controlled effects in magnetospheric physics. However, the crucial components of such models, such as the well-accepted X-line reconnection picture and the broadly-used explanations of the breakdown of the frozen-in condition, lack complete theoretical support. In fact, the generation of parallel electric field, which is necessary to break down the frozen-in condition, cannot be described by a purely dissipative or passive process and must be the result of Alfvenic interaction. The important Alfvenic interaction has often been overlooked in most reconnection models. We demonstrate how the Alfvenic interaction of MHD mesoscale wave packets at current sheets and in the auroral acceleration region can create and support parallel electric fields, causing the breakdown of the frozen-in condition and plasma acceleration. The Alfvenic interaction scenario not only explains some observational facts explained by the traditional reconnection model but also provides new and different interpretations and predictions for aspects of physical processes occurring at current sheets that are not given by previous models.   

The density fluctuation spectrum in the solar wind plasma

The density fluctuation spectrum in the solar wind reveals a Kolmogorov-like scaling with a spectral slope of -5/3 in wavenumber space. The energy transfer process in the magnetized solar wind, characterized typically by magnetohydrodynamic turbulence, over extended length-scales remains an unresolved paradox of modern turbulence theories, raising the question of how a compressible magnetofluid exhibits a turbulent spectrum that is a characteristic of an incompressible hydrodynamic fluid. To address these questions, we have undertaken three-dimensional time-dependent numerical simulations of a compressible magnetohydrodynamic fluid describing super-Alfv\'enic, supersonic and strongly magnetized plasma fluid. It is shown that a Kolmogorov-like density spectrum can develop by plasma motions that are dominated by Alfv\'enic cascades whereas compressive modes are dissipated.   

Nonlinear Plasma Effects in Natural and HF-Perturbed Subauroral Ionosphere

This presentation describes nonlinear plasma effects in the plasmasphere and subauroral ionosphere during magnetospheric substorms and injections of high-power HF radio waves. First, we present magnetically-conjugate Cluster-DMSP-Polar satellite observations of substorm Sub-Auroral Ion Drifts (SAID), showing that the SAID channel is a turbulent boundary layer formed via a short circuit of the substorm-injected plasmoid by the plasmasphere. Then, an overview of HF modification experiments at the High-Frequency Active Auroral Program (HAARP) heating facility is given, including recently discovered artificial plasma layers. We show that their formation can be explained in terms of an ionization wave sustained by suprathermal electrons accelerated by the excited plasma turbulence.   

Global Hybrid and Fully Kinetic Simulations of the Magnetosphere

Currently global magnetospheric simulations are predominantly based on MHD. MHD simulations have proven useful in studies of the global dynamics of the magnetosphere with the goal of predicting eminent features of substorms and other global events. But it is well known that the magnetosphere is dominated by ion and electron kinetic effects, which are ignored in MHD simulations, and many key aspects of the magnetosphere relating to transport and structure of boundaries await global kinetic simulations. Taking full advantage of petascale computing and a number of innovations, we have been able to conduct 3D global hybrid (electron fluid, kinetic ions) and 2D global full PIC simulations. Our full PIC simulations are used to develop a better understanding of reconnection under realistic conditions in the magnetosphere and to develop better models of resistivity to be used in the global hybrid simulations. Here we show several specific science issues that we have been able to address for the first time. This includes formation of flux transfer events at the dayside magnetopause and associated flows, plasma depletion layer, and flux ropes in the magnetotail.   

MPI parallelization of full PIC simulation code with Adaptive Mesh Refinement

A new parallelization technique developed for PIC method with adaptive mesh refinement (AMR) is introduced. In AMR technique, the complicated cell arrangements are organized and managed as interconnected pointers with multiple resolution levels, forming a fully threaded tree structure as a whole. In order to retain this tree structure distributed over multiple processes, remote memory access, an extended feature of MPI2 standards, is employed. Another important feature of the present simulation technique is the domain decomposition according to the modified Morton ordering. This algorithm can group up the equal number of particle calculation loops, which allows for the better load balance. Using this advanced simulation code, preliminary results for basic physical problems are exhibited for the validity check, together with the benchmarks to test the performance and the scalability.