To investigate the universality of magnetic turbulence in space plasmas, we analyze seven time periods in the free solar wind under different plasma conditions. Three instruments on Cluster spacecraft operating in different frequency ranges give us the possibility to resolve spectra up to 300 Hz. We show that the spectra form a quasiuniversal spectrum following the Kolmogorov's law approximately k(-5/3) at MHD scales, a approximately k(-2.8) power law at ion scales, and an exponential approximately exp[-sqrt[k(rho)e]] at scales k(rho)e approximately [0.1,1], where rho(e) is the electron gyroradius. This is the first observation of an exponential magnetic spectrum in space plasmas that may indicate the onset of dissipation. We distinguish for the first time between the role of different spatial kinetic plasma scales and show that the electron Larmor radius plays the role of a dissipation scale in space plasma turbulence.

The anisotropy of turbulence in the fast solar wind, between the ion and electron gyroscales, is directly observed using a multispacecraft analysis technique. Second order structure functions are calculated at different angles to the local magnetic field, for magnetic fluctuations both perpendicular and parallel to the mean field. In both components, the structure function value at large angles to the field S{⊥} is greater than at small angles S{∥}: in the perpendicular component S{⊥}/S{∥}=5±1 and in the parallel component S{⊥}/S{∥}>3, implying spatially anisotropic fluctuations, k{⊥}>k{∥}. The spectral index of the perpendicular component is -2.6 at large angles and -3 at small angles, in broad agreement with critically balanced whistler and kinetic Alfvén wave predictions. For the parallel component, however, it is shallower than -1.9, which is considerably less steep than predicted for a kinetic Alfvén wave cascade.

Collisionless shocks are loosely defined as shocks where the transition between pre-and post-shock states happens on a length scale much shorter than the 2 collisional mean free path. In the absence of collision to enforce thermal equilibrium post-shock, electrons and ions need not have the same temperatures. While the acceleration of electrons for injection into shock acceleration processes to produce cosmic rays has received considerable attention, the related problem of the shock heating of quasi-thermal electrons has been relatively neglected.In this paper we review that state of our knowledge of electron heating in astrophysical shocks, mainly associated with supernova remnants (SNRs), shocks in the solar wind associated with the terrestrial and Saturnian bowshocks, and galaxy cluster shocks. The solar wind and SNR samples indicate that the ratio of electron temperature, (Te) to ion temperature (Tp) declines with increasing shock speed or Alfvén Mach number. We discuss the extent to which such behavior can be understood via cosmic ray-generated waves in a shock precursor, which then subsequently damp by heating electrons. Finally, we speculate that a similar mechanism may be at work for both solar wind and SNR shocks.

Shock waves are ubiquitous in astrophysics and interplanetary space. In collisionless plasmas they transform directed flow energy into thermal energy and accelerate energetic particles. The energy repartition amongst particle populations is a multi-scale process related to the spatial and temporal structure of the electromagnetic fields within the shock layer. While major features of the large scale ion heating are known, the electron heating and smaller scale fields remain poorly understood and controversial. We determine for the first time the scale of the electron temperature gradient via unprecedented high time resolution electron distributions measured in situ by the Cluster spacecraft. We discover that half of the electron heating coincides with a narrow dispersive layer several electron inertial lengths (c/ωpe) thick. Consequently, the nonlinear steepening is limited by wave dispersion. The DC electric field associated with the electron pressure gradient must also vary over these small scales, strongly influencing the efficiency of shocks as cosmic ray accelerators.

[1] Collisionless shock waves are a widespread phenomenon in both solar system and astrophysical contexts. The nature of energy dissipation at such shocks is of particular interest, especially at high Mach numbers. We use data taken by the Cassini spacecraft to investigate electron heating at Saturn's bow shock, one of the strongest collisionless shocks encountered by spacecraft to date. Measurements of the upstream solar wind ion parameters are scarce due to spacecraft pointing constraints and the absence of an upstream monitor. To address this, we use solar wind speed predictions from the Michigan Solar Wind Model. Since these model predictions are based on near-Earth solar wind measurements, we restrict our analysis to bow shock crossings made by Cassini within ±75 days of apparent opposition of Earth and Saturn. An analysis of the resulting set of 94 crossings made in 2005 and 2007 reveals a positive correlation between the electron temperature increase across the shock and the kinetic energy of an incident proton, where electron heating accounts for between ∼3% and ∼7% of this incident ram energy. This percentage decreases with increasing Alfvén Mach number, a trend that we confirm continues into the hitherto poorly explored high-Mach number regime, up to an Alfvén Mach number of ∼150. This work reveals that further studies of the Saturnian bow shock will bridge the gap between the more modest Mach numbers encountered in near-Earth space and more exotic astrophysical regimes where shock processes play central roles.

We investigate the dependence of solar wind fluctuations measured by the Wind spacecraft on scale and on the degree of alignment between oppositely directed Elsasser fields. This alignment controls the strength of the nonlinear interactions and, therefore, the turbulence. We find that at scales larger than the outer scale of the turbulence the Elsasser fluctuations become on average more antialigned as the outer scale is approached from above. Conditioning structure functions using the alignment angle reveals turbulent scaling of unaligned fluctuations at scales previously believed to lie outside the turbulent cascade in the "1/f range." We argue that the 1/f range contains a mixture of a noninteracting antialigned population of Alfvén waves and magnetic force-free structures plus a subdominant population of unaligned cascading turbulent fluctuations.

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[1] High-intensity radio emission events at 2-3 kHz were observed by the Voyager spacecraft during 1983-1984 and 1992-1993. Such events are thought to occur when shock waves associated with global merged interaction regions (GMIRs) enter a region of the outer heliosheath where the electron speed distribution is primed with a superthermal tail, generated by lower hybrid drive. Previously, this priming mechanism was combined with a theory for type II solar radio bursts to predict the flux of radio emission in the outer heliosphere. Here this theory is extended in a number of ways. First, theoretical arguments regarding the availability of Langmuir and ion sound waves are used to determine whether emission occurs via three-wave processes or processes involving wave scattering off thermal ions (STI). New expressions for conversion efficiencies into radio emission associated with STI are then implemented where appropriate. Next, the dependence of the predicted fluxes on plasma and shock parameters are determined. Lastly, dynamic spectra are calculated for the radio emission generated by shocks traveling from the inner solar wind to beyond the heliopause and into the very local interstellar medium (VLISM). It is found that the predicted fluxes of fundamental radiation are comparable with those observed for plausible shock and plasma parameters. The theory can also predict radio-quiet GMIRs to be smaller and slower and to propagate through heliosheath regions with weaker superthermal tails. The calculated dynamic spectra have predicted fluxes below the Voyager detection thresholds in the solar wind, inner heliosheath, and VLISM. However, the predicted fluxes and frequency-time behavior are very similar to the 2 kHz component observed by the Voyager spacecraft when the GMIR is in the primed region.