NG33A-01
Measurement of Energy Cascade Rate and Dissipation Rate in the Solar Wind: Theoretical Approach Based on Third Order Moments in Axisymmetric MHD Turbulence
The energy dissipation rate of solar wind turbulence has recently been measured by a technique based on Politano and Pouquet's law for incompressible MHD turbulence. Politano and Pouquet's law is valid for isotropic turbulence but not for anisotropic turbulence in magnetized plasmas such as the solar wind. A generalization of Politano and Pouquet's law to anisotropic incompressible magnetohydrodynamic (MHD) turbulence is used to develop a theory for the measurement of the turbulent cascade rates of energy and cross-helicity assuming that the turbulence is statistically axisymmetric about the mean magnetic field. The theory is based on measurements of third order moments of the velocity and magnetic field fluctuations at two points separated by a distance r (two point measurements). In the inertial range the third order moments are proportional to the separation r and to the energy cascade rate \varepsilon. For axisymmetric MHD turbulence the third order moments are also functions of the angle θ between the displacement vector \bm r and the mean magnetic field \bm B0 and the dependence on this angle must be measured to determine the cascade rates from experimental data. The theory provides a relationship between the cascade rates and the angular dependence of the third order moments.
NG33A-02
Radial (L) profiles of MHD wave power and energetic electron flux during high-speed streams: dependence on IMF Bz
During the passage of solar wind high-speed streams, normally characterized by a weak IMF, historic observations [e.g. Rostoker et al., JGR 1998] have shown that a peak of both ULF wave power and electron flux is formed at high L (5-6), measurable from geosynchronous orbit (6.6). Recurrence of the streams leads to a driven oscillation in the magnetospheric field and particle distributions between an excited and a quiet state, with a delay time of 2 days following the solar wind speed. Here we show that, in the presence of Southward IMF Bz, the new peak will form instead at low L (~3) and in some events within the slot L range (2-3). In addition to the change in the magnetospheric configuration resulting from a steady IMF BSouth, it is the fluctuations in the field power and therefore in the wave-particle interaction that are significant in changing the wave power and particle flux profiles. The timescale for those interactions is reduced from 2 days to several hours or tens of minutes. The effect is compared both to effects during low-IMF high-speed streams and to interplanetary coronal mass ejections (Vassiliadis et al., submitted to GRL).
NG33A-03
Nonlinear Mirror Waves in Space Plasmas
A unified theory of finite-amplitude mirror type waves in non-Maxwellian space plasmas is developed. The collisionless kinetic theory in a guiding center approximation, modified for accounting the effects of the finite ion Larmor radius effects, is used as the starting point. The model equation governing the nonlinear dynamics of mirror waves near instability threshold is derived. In the linear approximation it describes the classical mirror instability with the linear growth rate expressed in terms of an arbitrary ion distribution function. In the nonlinear regime the mirror waves form solitary structures that have the shape of magnetic holes. The formation of such structures and their nonlinear dynamics has been analyzed both analytically and numerically. The main nonlinear mechanism responsible for mirror instability saturation is associated with modification (flattening) of the shape of the background ion distribution function in the region of small parallel particle velocities. The width of this region is of the order of the particle trapping zone in the mirror hole. Near the mirror instability threshold the saturation arises before its width reaches the ion thermal velocity. The nonlinear mode coupling effects in this approximation are smaller and unable to take control over evolution of the space profile of saturated mirror waves or lead to their magnetic collapse. This results in the appearance of quasi-stable solitary mirror structures having the form of deep magnetic depressions. The relevance of the theoretical results to recent satellite observations is stressed.
NG33A-04
Theory, Simulations, and Laboratory Experiments of Auroral Kilometric Radiation
Auroral kilometric radiation (AKR) is generated when electron beams are accelerated along the Earth's magnetic field lines into the auroral region. The mechanism is thought to be due to a cyclotron maser-type instability. There are two branches of this instability – the first is a beam instability and the second is a kinetic instability. In the Earth's case, the electrons moving into a stronger field region form a horseshoe-shaped velocity distribution and the radiation is generated by the kinetic branch of the instability. The beam instability is difficult to arrange in a natural plasma and is therefore unlikely to occur. A laboratory experiment reproducing AKR is used to investigate the process of radiation generation due to an energetic electron beam moving into a region with stronger magnetic field. Theory and simulations demonstrate that the kinetic instability is the one that generates AKR. We investigate a number of issues associated with the generation of AKR, namely the ratio of plasma to cyclotron frequency, the saturation state and the propagating mode.
NG33A-05
Experimental Study of the Nonlinear Evolution of Plasma Filaments
We investigate experimentally the nonlinear motion and structure of toroidally symmetric plasma filaments propagating through neutral gas. Propagating structures, often denoted `blobs', have been observed in diverse plasmas ranging from the low-latitude F region of the ionosphere [1] to the edge of many magnetically-confined laboratory plasmas [2]. We use the Versatile Toroidal Facility (VTF), a well-diagnosed basic plasma physics experiment, to study blobs systematically. We observe for the first time the characteristic mushroom shape of the blob, which is predicted from nonlinear simulations [3]. Furthermore, we measure the internal electrostatic structure and show that the blob propagation velocity is inversely proportional to the background neutral density. Further evidence that the neutrals provide drag on the blob is given by measurements showing that the electron temperature decreases as the plasma loses energy to the neutrals. [1] Park J et al, Geophys. Res. Lett. 30 (2003) 2114 [2] Zweben S et al, Nucl. Fusion, 44 (2004) 134 [3] Garcia O et al, Phys. Plasmas, 12 (2005) 090701 Work supported by DOE Junior Faculty Award DE-FG02-06ER54878. N. Katz partly funded by a Fusion Energy Sciences Fellowship, administered by ORISE. W. Fox and N. Katz partly funded by CMPD Award DE-FC02- 04ER54786.
NG33A-06
Electron energization by field line annihilation in whistler spheromaks
A loop antenna is inserted into a large laboratory plasma with its axis along the dc magnetic field. A large oscillating loop current is applied which in one polarity produces a field-reversed configuration, in the other a strong mirror field. This arrangement excites nonlinear whistler modes with topologies alternating each half cycle between that of a spheromak and a mirror [http://link.aps.org/abstract/PRL/v96/e095004/html]. The propagation speed and damping of whistler spheromaks and whistler mirrors are amplitude dependent. In whistler spheromaks, electrons are strongly accelerated along the toroidal magnetic null line (separator), resulting in magnetic energy loss by field line annihilation at the O-point. In whistler mirrors, electrons perform \mathbf{E} × \mathbf{B} drifts, hence are not energized. Wave-wave interactions also differ: Counter-propagating mirrors pass through each other without interaction, whistler spheromaks collide inelastically and merge into stationary field-reversed configuration. These nonlinear properties are important for understanding strong whistler turbulence, fast reconnection, and whistler wave excitation from loop antennas. Work supported by NSF/DOE.
NG33A-07
Electrostatic Solitary Waves in the vicinity of the Terrestrial Bow Shock
Nonlinear, small scale electric field structures are recognized as an ubiquitous phenomenon in space plasma. However, their role in the processes of energy, momentum, and charge redistribution is still unclear. Using Cluster EFW data we analyze a number of instances of uni/bi/tri-polar electrostatic solitary waves observed in the weakly magnetized foot region of a quasi-perpendicular shock to investigate their fundamental wave properties, generation mechanisms, significance in the plasma dynamics and their contribution to the overall of shock structure. The properties of the solitary waves (e.g., wave propagation direction, velocity, potential scale and amplitude) are experimentally derived by means of the phase-differencing method applied to two sets of parallel electric field components. The observed solitary waves are found to correspond to ion depletion structures such as BGK ion holes. The derived wave properties are compared with the predictions from analytical theory and numerical computations to study their excitation mechanisms. These parameters are also compared with observations from other space plasma regions to study the parametric dependence of solitary waves in different plasma regimes.
NG33A-08
BGK Waves: Nonlinear Saturated States in Space Plasmas?
There has been renewed interest in the theory of Bernstein-Greene-Kruskal (BGK) waves, motivated by recent identifications of electrostatic solitary waves (ESWs) in space plasmas from spacecraft such as Geotail, Fast, Polar, Cassini, and Cluster. These observations make it evident that the observed waves cannot be described by the classical one-dimensional theory. While one-dimensional BGK theory is quite mature, there appear to be no exact three-dimensional solutions in the literature except for the limiting case when the magnetic field is sufficiently strong so that one can apply the guiding-center approximation. We show, in fact, that two- and three- dimensional solutions that depend only on energy do not exist [Phys. Rev. Lett., 95, 245004, 2005]. However, if there exists an ignorable coordinate and solutions depend on both energy and a constant of the motion such as angular momentum, it is possible to construct exact two-dimensional solutions in the presence of a finite magnetic field. The latter are shown to be exact, fully electromagnetic solutions of the steady-state Vlasov- Poisson-Ampère system. Width-amplitude relation of such BGK wave solutions as well as the spatial electric field structures are shown to be consistent with observations. These exact results impose strong constraints on theoretical models that interpret observations of ESWs on the basis of BGK wave theory.