SM44A-01 INVITED 16:00h
The Acceleration of Fast Charged Particles in Space Plasmas
Energetic particles, some with very high energies, are observed in space whereever the ambient matter density is low enough. In most cases the observed energy spectra of the particles are inverse power laws, with indices which range over a fairly narrow range. This suggests that the same mechanism may be accelerating most of the energetic particles. Acceleration of charged particles by collisionless shock waves has the desired attributes. Strong shock waves produce power-law energy spectra with indices which are quite insensitive to the detailed parameters of the specific situation, and the indices are close to those observed. All that is required is that the scattering by the turbulent magnetic field is sufficient that the the distribution functions are not too anisotropic in pitch angle and that the energetic-particle speeds are significantly larger than the ambient flow speed of the background plasma. I will discuss the physics of shock acceleration and demonstrate that it is the likely accelerator of most observed energetic particles. I will also show that there are situations where shock accelertion doesn't seem to be the answer, and briefly discuss alternative accelerators.
SM44A-02 16:20h
Alfv\'en waves in the solar corona, the solar wind, and the magnetosphere
Observations and theoretical models show that Alfv\'en waves play an important role in many physical processes taking place in the plasma of the "local cosmos." For example, Alfven waves are still the major candidates for the acceleration and heating of the fast solar wind, since they were proposed nearly 4 decades ago. Recently, MHD waves were observed in coronal loops in the EUV with the TRACE satellite. The SOHO, and TRACE instrument provide evidence for slow magnetosonic waves in coronal plumes. In-situ Helios and Ulysses spacecraft find ample evidence for the presence of propagating Alfv\'en waves in the solar wind. The CLUSTER mission provides for the first time multi-point view of the magnetospheric and solar wind plasma environment, and in particular high cadence magnetic field measurements with the Fluxgate Magnetometer (FGM), that enables to study the properties of Alfv\'en waves in various parts of the magnetosphere simultaneously. I will discuss the observations and modeling of Alfv\'en waves starting from the low corona (loops), continuing into the solar wind, the magnetosphere, and to the auroral ionosphere. I will present the results of MHD, multifluid, and hybrid models of low-frequency (MHD), as well as kinetic Alfv\'en waves in the plasma in various parts of the "local cosmos", and I will discuss the role Alfven waves play in the energization of the plasma, and as a diagnostic tool of the plasma physical processes.
SM44A-03 16:35h
Polytropic Equations of State in a Magnetized Plasma
Closure in the low frequency MHD regime is an old problem. Polytropic one and two fluid closures have been assumed in collisionless MHD codes and low frequency dispersion analysis; double adiabatic closures have also been explored in spite of the presence of parallel electric fields and heat flow. The collisionally motivated adiabatic value for $\gamma \simeq 5/3$ is often inserted. Two years of Polar data collected in the high density regions of the magnetosheath/ entry layers have been analyzed in 15min segments to determine (i) if polytropic relations characterize the electrons and/or ions and (ii) what values of $\gamma_{e,i}$ are implied. Of the possible 7900 intervals attempted, successful fits were obtained for ions in 239 intervals, while 1552 electrons intervals had successful fits. Frequently there was insufficient dynamic range in the density to determine the polytropic index. However, only 122 intervals had good simultaneous polytropic characterizations of ions and electrons. Of those intervals with common fits there is a general trend for higher electron $\gamma_e$ to accompany increases in $\gamma_i$. Most surprising are the distribution of values for $\gamma_j$ ; the good electrons fits are especially bimodal, with a strong peak near 0 and another in the vicinity of 5/3, but a deep void in between. With poorer statistics the ions also show this behavior. The incidence of $\gamma_i = 5/3$ is rare, representing less than 5% of the good ion fits that were possible. Examples of representative observations in the newly defined regime $\gamma_e < 1; \gamma_i < 1$ that have no classical analogue will be presented. A simple model of circumstances that permit these observations will be presented.
SM44A-04 INVITED 16:50h
The distribution of MHD turbulence in the heliosphere and the charged particle radiation environment
Magnetohydrodynamic (MHD) turbulence plays an important role in cross scale couplings in the heliospheric system and is central to understanding the distribution and variations of charged particle radiation. The nonlinear turbulent cascade process acts as a conduit connecting large scale fluid-like plasma motions to small scale kinetic motions, and is thus most likely an integral part of heating processes from the coronal base to the outer boundaries of the heliosphere. Turbulence also establishes key parameters that determine the transport (and perhaps also, acceleration) of energetic charged particles. In the inner heliospheric realm of solar energetic particles, turbulence can account for scattering, field line complexity, and topological trapping, and can provide other indirect effects such as turbulent transport affecting CMEs and shocks. To understand the distribution and spectra of galactic cosmic rays, one must know the diffusion tensor and therefore local turbulence properties. Turbulence is transported outward in the supersonic solar wind, while the cosmic rays diffuse and drift inwards from the interstellar medium. Thus to understand how the spectrum of galactic cosmic rays is established at any point in interplanetary space, it is necessary to have knowledge of the turbulence everywhere in the heliosphere. Here we summarize recent progress in this challenging area. Headway has been made by employing a four equation transport model with one point nonlinear modeling of locally homogeneous turbulence. The model follows turbulence energy density, correlation scale, temperature and cross helicity under the influence of specified large scale fields. The turbulence is driven by large scale shear, and in the outer heliosphere, by pickup ions. A few constants must be estimated either from theory or observations -- the MHD Karman-Taylor constants, the shear strength, a turbulence geometry factor ("mixing term"), and the Alfven ratio. The latitudinal dependence of solar wind speed, density and large scale magnetic field are important parameters, while latitude dependence of the boundary conditions must also be established. Using parameters and boundary data that are consistent with observations, the model accounts for radial dependence of the turbulence properties from 1 to 60 AU as observed by Voyager, as well as high latitude Ulysses observations. Cosmic ray modulation models incorporating turbulence modeling also have made substantial progress in providing an "ab initio" description of the distribution of galactic cosmic rays. Support by NASA grants NAG5-11603 and NNG04GA54G, and by NSF grant ATM-0105254 is acknowledged, as are important collaborations with J. W. Bieber, B. Breech, R. A. Burger, P. A. Isenberg, J. Minnie, S. Oughton, S. Parhi, C. W. Smith, and G. P. Zank.
SM44A-05 17:10h
Laboratory Investigation of the Effect of the Lower Ionosphere Conductivity on FACs, Particles Acceleration, Auroral Luminosity, and Ionospheric Plasma Depletion
Investigation of the influence of local conductivity in the artificial lower ionosphere on distributions of the ionospheric plasma density, currents and luminosity is carried out in the model laboratory experiments. The effects of local conductivity are notable when the field-aligned current (FAC) exceeds some critical value. At this case the regions of anomalous conductivity and field-aligned electric fields appear and cause electron acceleration. The FAC have fine spatial structure and contains high-frequency oscillations. The spatial and temporal parameters of the electron fluxes are determined predominantly by these high-frequency components and to lesser degree by FAC total magnitude. When the region of reduced conductivity is located under the FAC layer, the "auroral" luminosity disappears or gets weaker. Above the region of reduced conductivity the effect of plasma depletion is observed. If spatial distribution of the conductivity under the plasma jet that generates the FAC has periodic character, a temporal modulation of the horizontal currents, which close the FAC, is revealed. These horizontal currents correspond to the Pedersen and Hall currents in the real ionosphere. The magnetic field induced by these current alternates with a frequency determined by spatial characteristics of the conductivity region and by the velocity of the plasma jet. Qualitatively this effect simulates the electrojet modulation and ELF pulsation generation by varying power in the radio heating experiments. The effect of "plasma memory" was also revealed. It consists in persistence of the "plasma bubble" when the plasma jet abandons the magnetic flux tube treaded the region of reduced conductivity. The pattern of plasma density changing in the plasma depletion regions above the regions of reduced conductivity and effect of "plasma memory" are similar an appearance of plasma bubbles observed in RF heater experiments in the ionosphere. At the same time, generally accepted interpretation of their results and the models of ionospheric ducts and plasma bubble formation cannot explain the effects observed in the laboratory conditions. We believe that effects revealed in our experiments should be taken into account at consideration of the phenomena in the real ionosphere.
SM44A-06 17:25h
IMAGE/RPI Electron-Density Determinations in Various Magnetospheric Domains
The Radio Plasma Imager (RPI) on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite has a remote-measuring capability that leads to large-scale descriptions of the magnetospheric electron density Ne (see, e.g., Reinisch et al. [2004]). It also has accurate local Ne measuring capability based on sounder-stimulated plasma resonances [Benson et al., 2003]. Here we will only discuss the latter capability. We will illustrate how it can be used to determine the local Ne as IMAGE traverses magnetospheric domains ranging from deep within the plasmasphere to near, and even beyond, the magnetopause. The spectrum of sounder-stimulated plasma resonances can vary considerably in the different regions. In some cases, the identifications of the resonances are aided by the reception of emissions of magnetospheric and solar origin, and visa versa. Benson, R. F., V. A. Osherovich, J. Fainberg, and B. W. Reinisch, Classification of IMAGE/RPI-stimulated plasma resonances for the accurate determination of magnetospheric electron-density and magnetic field values, J. Geophys. Res., 108(A5), 1207, doi:10.1029/2002JA009589, 2003. Reinisch, B. W., X. Huang, P. Song, J. L. Green, S. F. Fung, V. M. Vasyliunas, D. L. Gallagher, and B. R. Sandel, Plasmaspheric mass loss and refilling as a result of a magnetic storm, J. Geophys. Res., 109, A01202, doi:10.1029/2003JA009948, 2004.
SM44A-07 17:40h
BBELF and Electromagnetic VLF Wave Cavities in the Auroral Ionosphere
The GEODESIC sounding rocket (February 26, 2000) encountered dozens of VLF wave-filled density depletions in the auroral ionosphere at the time of a substorm onset. These cavities were similar to lower-hybrid cavities observed by many previous sounding rockets and by the Freja satellite, but with several striking differences: 1) VLF waves (near and above the lower-hybrid cutoff frequency $f_{LH}$) inside the cavities had a strong magnetic component. In fact the magnetic fields were enhanced more strongly, relative to fields outside cavities, than were VLF electric fields. 2) The cavities were found primarily in a return (downward) current region. 3) Cavity waves show no indication of counter-rotating eigenmodes that have been demonstrated previously to indicate passive scattering of externally incident VLF hiss. These points lead us to conclude that the waves were generated inside the cavities, presumably by unstable filaments of upward-drifting thermal ionospheric electrons. Broad-band electrostatic ELF waves were also strongly enhanced inside cavities, and extended up to $f_{LH}$. These waves also appear to have a geophysical origin, and likely contributed to observed ion heating up 10 eV observed in and near cavities.