The proton temperature anisotropy in the solar wind is known to be constrained by the theoretical thresholds for pressure anisotropy-driven instabilities. Here we use approximately 1 million independent measurements of gyroscale magnetic fluctuations in the solar wind to show for the first time that these fluctuations are enhanced along the temperature anisotropy thresholds of the mirror, proton oblique firehose, and ion cyclotron instabilities. In addition, the measured magnetic compressibility is enhanced at high plasma beta (β 1) along the mirror instability threshold but small elsewhere, consistent with expectations of the mirror mode. The power in this frequency (the 'dissipation') range is often considered to be driven by the solar wind turbulent cascade, an interpretation which should be qualified in light of the present results. In particular, we show that the short wavelength magnetic fluctuation power is a strong function of collisionality, which relaxes the temperature anisotropy away from the instability conditions and reduces correspondingly the fluctuation power.PACS numbers: 96.50. Ci, 52.35.Ra, 95.30.Qd The physical processes that regulate the expansion of the super-Alfvénic solar wind into space include adiabatic particle motion, plasma instabilities, and binary particle collisions. As the wind expands, plasma density n p and magnetic field |B| decrease radially. The Chew-Goldberger-Low (CGL) relations [1] predict that the plasma ions should become anisotropic in the sense of T > T ⊥ if the particle motion is adiabatic and collisionless; here T is the ion temperature parallel and perpendicular to the background magnetic field. However, Coulomb collisions and pressure-anisotropy instabilities act to pitch-angle scatter the plasma back towards isotropy [2]. At 1 AU, the most probable value of the proton temperature anisotropy is T ⊥ /T ≈ 0.89 (see Figure 1, top panel below). If CGL were valid, this would imply a proton temperature anisotropy of T ⊥ /T ≥ 200 at 5 solar radii.The same pressure-anisotropy instabilities that operate in the solar wind are believed to operate in other low-collisionality astrophysical plasmas, including clusters of galaxies [3] and some accretion disks onto black holes [4,5]. In the latter environment, these instabilities not only modify the thermodynamics of the plasma (as in the solar wind), but they also play a crucial dynamical role, regulating the anisotropic stress that helps transport angular momentum, allowing accretion to proceed. vice, where isotropization and magnetic fluctuations were observed corresponding to the Alfvén ion cyclotron instability [6]. Similar results were obtained in the solar wind at 1 AU [7] (for T ⊥ /T > and solar wind speeds greater than 600 km/s), suggesting that the proton cyclotron instability plays a role. Both mirror mode and ion cyclotron anisotropy instabilities appear to be active in the terrestrial magnetosheath, inferred from observed constraints on the temperature anisotropy [8,9]. More recently, Kasper et al. Growth of ion...

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We present in situ measurements in a space plasma showing that thin current sheets the size of an ion inertial length exist and are abundant in strong and intermittent plasma turbulence. Many of these current sheets exhibit the microphysical signatures of reconnection. The spatial scale where intermittency occurs corresponds to the observed structures. The reconnecting current sheets represent a type of dissipation mechanism, with observed dissipation rates comparable to or even dominating over collisionless damping rates of waves at ion inertial length scales (x100), and can have far reaching implications for small-scale dissipation in all turbulent plasmas.

Turbulence in fluids and plasmas is a ubiquitous phenomenon driven by a variety of sources-currents, sheared flows, gradients in density and temperature, and so on. Turbulence involves fluctuations of physical properties on many different scales, which interact nonlinearly to produce self-organized structures in the form of vortices. Vortex motion in fluids and magnetized plasmas is typically governed by nonlinear equations, examples of which include the Navier-Stokes equation, the Charney-Hasegawa-Mima equations and their numerous generalizations. These nonlinear equations admit solutions in the form of different types of vortices that are frequently observed in a variety of contexts: in atmospheres, in oceans and planetary systems, in the heliosphere, in the Earth's ionosphere and magnetosphere, and in laboratory plasma experiments. Here we report the discovery by the Cluster satellites of a distinct class of vortex motion-short-scale drift-kinetic Alfvén (DKA) vortices-in the Earth's magnetospheric cusp region. As is the case for the larger Kelvin-Helmholtz vortices observed previously, these dynamic structures should provide a channel for transporting plasma particles and energy through the magnetospheric boundary layers.