[1] We present a comparison between WIND/SWE observations (Kasper et al., 2006) of b kp and T ?p /T kp (where b kp is the proton parallel beta and T ?p and T kp are the perpendicular and parallel proton temperatures, respectively; here parallel and perpendicular indicate directions with respect to the ambient magnetic field) and predictions of the Vlasov linear theory. In the slow solar wind, the observed proton temperature anisotropy seems to be constrained by oblique instabilities, by the mirror one and the oblique fire hose, contrary to the results of the linear theory which predicts a dominance of the proton cyclotron instability and the parallel fire hose. The fast solar wind core protons exhibit an anticorrelation between b kc and T ?c /T kc (where b kc is the core proton parallel beta and T ?c and T kc are the perpendicular and parallel core proton temperatures, respectively) similar to that observed in the HELIOS data (Marsch et al., 2004). Citation: Hellinger, P., P. Trávníček, J. C. Kasper, and A. J. Lazarus (2006), Solar wind proton temperature anisotropy: Linear theory and WIND/SWE observations, Geophys. Res. Lett., 33, L09101,

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[1] We report an analysis of the proton temperature anisotropy evolution from 0.3 to 2.5 AU based on the Helios and Ulysses observations. With increasing distance the fast wind data show a path in the parameter space (b kp , T ?p /T kp ). The first part of the trajectory is well described by an anticorrelation between the temperature anisotropy T ?p /T kp and the proton parallel beta, while after 1 AU the evolution with distance in the parameter space changes and the data result in agreement with the constraints derived by a fire hose instability. The slow wind data show a more irregular behavior, and in general it is not possible to recover a single evolution path. However, on small temporal scale we find that different slow streams populate different regions of the parameter space, and this suggests that when considering single streams also the slow wind follows some possible evolution path. Citation: Matteini, L., S. Landi, P. Hellinger,

[1] The proton thermal energetics in the fast solar wind between 0.3 and 1 AU is reinvestigated using the Helios 1 and 2 data. Closer to the Sun, it is estimated that, to account for the observed radial profiles of the proton parallel and perpendicular temperature, nonnegligible parallel cooling and perpendicular heating are necessary. Around 1 AU heating is needed in both directions. We also calculate the corresponding rates and find that in total significant interplanetary heating is necessary, in agreement with previous results. The possible influence that deceleration of fast solar wind streams due to interaction with slow ones has on the proton thermodynamics is evaluated.Citation: Hellinger, P., L. Matteini, Š. Štverák, P. M. Trávníček, and E. Marsch (2011), Heating and cooling of protons in the fast solar wind between 0.3 and 1 AU: Helios revisited,

Abstract. Two instabilities could take place in plasma with a bi-Maxwellian proton distribution function with Tvi > Tvñ, where T•i I and Tvñ are proton temperatures, parallel and perpendicular, respectively, to the background magnetic field. The first instability is the fire hose (or whistler fire hose), generating low-frequency whistler waves at parallel propagation. We found a new, second instability, the Alfv6n fire hose, that generates zero-frequency waves of the Alfv6n branch at strongly oblique propagation. The Alfv6n fire hose has a linear growth rate comparable to or even greater than that of the whistler fire hose. The two instabilities with the same initial plasma parameters are examined via one-dimensional hybrid simulations and turn out to have behavior very different from each other. The whistler fire hose has an overall quasi-linear evolution, while the evolution of the Alfv6n fire hose is more complicated: Initially, unstable zero-frequency waves are gradually transformed into propagating Alfv6n waves; during this process the waves are strongly damped and heat protons in a perpendicular direction. Consequently, the Alfv6n fire hose is very efficient at destroying proton anisotropy.

We investigate properties of plasma turbulence from magneto-hydrodynamic (MHD) to sub-ion scales by means of two-dimensional, high-resolution hybrid particle-in-cell simulations. We impose an initial ambient magnetic field, perpendicular to the simulation box, and we add a spectrum of largescale magnetic and kinetic fluctuations, with energy equipartition and vanishing correlation. Once the turbulence is fully developed, we observe a MHD inertial range, where the spectra of the perpendicular magnetic field and the perpendicular proton bulk velocity fluctuations exhibit power-law scaling with spectral indices of −5/3 and −3/2, respectively. This behavior is extended over a full decade in wavevectors and is very stable in time. A transition is observed around proton scales. At sub-ion scales, both spectra steepen, with the former still following a power law with a spectral index of ∼ −3. A −2.8 slope is observed in the density and parallel magnetic fluctuations, highlighting the presence of compressive effects at kinetic scales. The spectrum of the perpendicular electric fluctuations follows that of the proton bulk velocity at MHD scales, and flattens at small scales. All these features, which we carefully tested against variations of many parameters, are in good agreement with solar wind observations. The turbulent cascade leads to on overall proton energization with similar heating rates in the parallel and perpendicular directions. While the parallel proton heating is found to be independent on the resistivity, the number of particles per cell and the resolution employed, the perpendicular proton temperature strongly depends on these parameters.

A new path for the generation of a sub-ion scale cascade in collisionless space and astrophysical plasma turbulence, triggered by magnetic reconnection, is uncovered by means of high-resolution two-dimensional hybrid-kinetic simulations employing two complementary approaches, Lagrangian and Eulerian, and different driving mechanisms. The simulation results provide clear numerical evidences that the development of powerlaw energy spectra below the so-called ion break occurs as soon as the first magnetic reconnection events take place, regardless of the actual state of the turbulent cascade at MHD scales. In both simulations, the reconnection-mediated small-scale energy spectrum of parallel magnetic fluctuations exhibits a very stable spectral slope of ∼ −2.8, whether or not a large-scale turbulent cascade has already fully developed. Once a quasi-stationary turbulent state is achieved, the spectrum of the total magnetic fluctuations settles towards a spectral index of −5/3 in the MHD range and of ∼ −3 at sub-ion scales.

Abstract. Using 5 years of Cluster data, we present a detailed statistical analysis of magnetic fluctuations associated with mirror structures in the magnetosheath. We especially focus on the shape of these fluctuations which, in addition to quasi-sinusoidal forms, also display deep holes and high peaks. The occurrence frequency and the most probable location of the various types of structures is discussed, together with their relation to local plasma parameters. While these properties have previously been correlated to the β of the plasma, we emphasize here the influence of the distance to the linear mirror instability threshold. This enables us to interpret the observations of mirror structures in a stable plasma in terms of bistability and subcritical bifurcation. The data analysis is supplemented by the prediction of a quasistatic anisotropic MHD model and hybrid numerical simulations in an expanding box aimed at mimicking the magnetosheath plasma. This leads us to suggest a scenario for the formation and evolution of mirror structures.