A higher-order multiscale analysis of the dissipation range of collisionless plasma turbulence is presented using in-situ high-frequency magnetic field measurements from the Cluster spacecraft in a stationary interval of fast ambient solar wind. The observations, spanning five decades in temporal scales, show a crossover from multifractal intermittent turbulence in the inertial range to non-Gaussian monoscaling in the dissipation range. This presents a strong observational constraint on theories of dissipation mechanisms in turbulent collisionless plasmas.PACS numbers: 94.05. Lk, 52.35.Ra, 96.60.Vg, 95.30.Qd The solar wind provides an ideal laboratory for the study of plasma turbulence [1]. In-situ spacecraft observations suggest well-developed turbulence at 1 AU with a magnetic Reynolds number ∼ O 10 5 [2,3]. These show an inertial range of Alfvénic turbulence on magnetohydrodynamic (MHD) scales which is an anisotropic and possibly compressible energy cascade [4,5,6] with intermittent magnetic field fluctuations described by statistical multifractals and a power spectral density (PSD) with a scaling exponent close to −5/3 [1]. An outstanding problem is how, in the absence of collisional viscosity in the solar wind, this inertial range of MHD turbulence terminates at smaller scales where one anticipates a cross-over to dissipative and/or dispersive processes via wave-particle resonances. Understanding the nature of the dissipation processes may also inform open questions such as how the solar wind and solar coronal plasmas are heated [7,8,9].It has long been known [10,11] that in collisionless plasmas there is a transition in the PSD at high wavenumber k from MHD to kinetic physics at approximately the ion gyroscale. High resolution in-situ magnetic field observations reveal that at these scales the turbulent solar wind shows a transition from a ∼ −5/3 power law in the inertial range to a steeper power-law at higher k with spectral exponents in the range (−4, −2) [12,13]. However, the relevant physical mechanism is much debated; having implications for phenomena as diverse as magnetic reconnection [14,15], neutron stars and accretion disks [16]. Theories which have been proposed range from nonlinear turbulent-like cascade processes [17,18,19] to weak turbulence theories with wave dispersion and resonant plasma interactions [20]. As well as studies of in-situ spacecraft measurements in the solar wind [21], foreshock [22] and magnetosheath [23,24] regions, these theories are explored using simulations ranging from Hall-MHD [25], electron-MHD [16,26], gyrokinetics [27], particle-in-cell simulations of whistler turbulence [28] and Vlasov-hybrid simulations [29].Both neutral fluid and MHD turbulence share a 'classic' statistical signature -namely an intermittent mul- Figure 1: PSD plots of the components of the magnetic field from both FGM (at frequencies lower than 1 Hz) and STAFF-SC (at frequencies above 1Hz) instruments. The PSD values for Bx and By have been shifted up for clarity. The 95% confidence intervals...

The anisotropic nature of solar wind magnetic turbulence fluctuations is investigated scale-by-scale using high cadence in-situ magnetic field measurements from the Cluster and ACE spacecraft missions. The data span five decades in scales from the inertial range to the electron Larmor radius. In contrast to the inertial range, there is a successive increase towards isotropy between parallel and transverse power at scales below the ion Larmor radius, with isotropy being achieved at the electron Larmor radius. In the context of wave-mediated theories of turbulence, we show that this enhancement in magnetic fluctuations parallel to the local mean background field is qualitatively consistent with the magnetic compressibility signature of kinetic Alfvén wave solutions of the linearized Vlasov equation. More generally, we discuss how these results may arise naturally due to the prominent role of the Hall term at sub-ion Larmor scales. Furthermore, computing higher-order statistics, we show that the full statistical signature of the fluctuations at scales below the ion Larmor radius is that of a single isotropic globally scale-invariant process distinct from the anisotropic statistics of the inertial range.