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...

[1] Observed electron distribution functions of the solar wind permanently exhibit three different components: a thermal core and a suprathermal halo, which are always present at all pitch angles, and a sharply magnetic field aligned ''strahl'' which is usually antisunward moving. Whereas Coulomb collisions can explain the relative isotropy of the core population, the origin of the halo population, and more specifically the origin of its sunward directed part, remains unknown. In this study we present the radial evolution of the electron velocity distribution functions in the fast solar wind between 0.3 and 1.5 AU. For this purpose we combine data measured separately by the Helios, Wind, and Ulysses spacecraft. We compute average distributions over distance and normalize them to 1 AU to remove the effects of the solar wind expansion. Then we model separately the core, halo, and strahl components to compute their relative number density or fraction of the total electron density. We observe that, while the core fractional density remains roughly constant with radial distance, the halo and strahl fractional densities vary in an opposite way. The relative number of halo electrons is increasing, while the relative number of strahl electrons is decreasing with distance. Therefore we provide, for the first time, strong evidences for a scenario that is commonly assumed: the heliospheric electron halo population consists partly of electrons that have been scattered out of the strahl.

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NASA’s Solar Probe Plus (SPP) mission will make the first in situ measurements of the solar corona and the birthplace of the solar wind. The FIELDS instrument suite on SPP will make direct measurements of electric and magnetic fields, the properties of in situ plasma waves, electron density and temperature profiles, and interplanetary radio emissions, amongst other things. Here, we describe the scientific objectives targeted by the SPP/FIELDS instrument, the instrument design itself, and the instrument concept of operations and planned data products.

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