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We show in this article direct evidence for the presence of an inertial energy cascade, the most characteristic signature of hydromagnetic turbulence (MHD), in the solar wind as observed by the Ulysses spacecraft. After a brief rederivation of the equivalent of Yaglom's law for MHD turbulence, we show that a linear relation is indeed observed for the scaling of mixed third order structure functions involving Elsässer variables. This experimental result, confirming the prescription stemming from a theorem for MHD turbulence, firmly establishes the turbulent character of low-frequency velocity and magnetic field fluctuations in the solar wind plasma.Space flights have shown that the interplanetary medium is permeated by a supersonic, highly turbulent plasma flowing out from the solar corona, the so called solar wind [1,2]. The turbulent character of the flow, at frequencies below the ion gyrofrequency f ci ≃ 1Hz, has been invoked since the first Mariner mission [3]. In fact, velocity and magnetic fluctuations power spectra are close to the Kolmogorov's -5/3 law [2,6]. However, even if fields fluctuations are usually considered within the framework of magnetohydrodynamic (MHD) turbulence [2], a firm established proof of the existence of an energy cascade, namely the main characteristic of turbulence, remains a conjecture so far [4]. This apparent lack could be fulfilled through the evidence for the existence of the only exact and nontrivial result of turbulence [6], that is a relation between the third order moment of the longitudinal increments of the fields and the separation [5]. This observation would firmly put low frequency solar wind fluctuations within the framework of MHD turbulence. The importance of such question stands beyond the understanding of the basic physics of solar wind turbulence. For example, it is well known that turbulence is one of the main obstacles to the confinement of plasmas in the fusion devices [7,8]. The understanding of interplanetary turbulence and its effects on energetic particle transport is of great importance also for Space Weather research [9], which is a relevant issue for spacecrafts and communication satellites operations, and for the security of human beings. Finally, more theoretical problems are concerned, such as the puzzle of solar coronal heating due to the turbulent flux toward small scales [10].Incompressible MHD equations are more complicated than the standard neutral fluid mechanics equations because the velocity of the charged fluid is coupled with the magnetic field generated by the motion of the fluid itself. However, written in terms of the Elsässer variables defined as z ± = v ± (4πρ) −1/2 b (v and b are the velocity and magnetic field respectively and ρ the mass density), they have the same structure as the Navier-Stokes equations [4]where P is the total hydromagnetic pressure, while ν is the viscosity and κ the magnetic diffusivity. In particular, the nonlinear term appears as z ∓ · ∇z ± , suggesting the form of a transport process, in which Alfvé...

A deep survey of the Large Magellanic Cloud at ∼ 0.1−100 TeV photon energies with the Cherenkov Telescope Array is planned. We assess the detection prospects based on a model for the emission of the galaxy, comprising the four known TeV emitters, mock populations of sources, and interstellar emission on galactic scales. We also assess the detectability of 30 Doradus and SN 1987A, and the constraints that can be derived on the nature of dark matter. The survey will allow for fine spectral studies of N 157B, N 132D, LMC P3, and 30 Doradus C, and half a dozen other sources should be revealed, mainly pulsar-powered objects. The remnant from SN 1987A could be detected if it produces cosmic-ray nuclei with a flat power-law spectrum at high energies, or with a steeper index 2.3 − 2.4 pending a flux increase by a factor > 3 − 4 over ∼ 2015 − 2035. Large-scale interstellar emission remains mostly out of reach of the survey if its > 10 GeV spectrum has a soft photon index ∼ 2.7, but degree-scale 0.1 − 10 TeV pion-decay emission could be detected if the cosmic-ray spectrum hardens above >100 GeV. The 30 Doradus star-forming region is detectable if acceleration efficiency is on the order of 1 − 10% of the mechanical luminosity and diffusion is suppressed by two orders of magnitude within < 100 pc. Finally, the survey could probe the canonical velocity-averaged cross section for self-annihilation of weakly interacting massive particles for cuspy Navarro-Frenk-White profiles.

Abstract.Plasma and magnetic field measurements by Ulysses are used to investigate the radial evolution of hourly-scale Alfv

Abstract. Solar wind plasma and magnetic field measurements by Ulysses have been used to study magnetohydrodynamic turbulence in different hehospheric regions. Four intervals of six solar rotations have been analyzed. Two of them are on the ecliptic around 2 and 5 AU, respectively, one is at midlatitude near 5 AU, and the last one is at high latitude around 3 AU. Conditions on the ecliptic are those typical of high solar activity periods. The midlatitude interval is characterized by very strong gradients in the wind speed, due to an intermittent appearance of the wind coming from the polar coronal hole. In the high-latitude interval, fully inside the polar wind, the speed is steadily high. We investigated at three different scales (1, 4, and 12 hours) the level of correlation between velocity and magnetic field fluctuations, as given by the normalized cross-helicity, and the sharing of the fluctuation energy between its kinetic and magnetic component, as measured by the normalized residual energy. The observations on the ecliptic, while confirming previous findings based on Voyagers data, clearly indicate that the normalized crosshellcity is well different from zero also at distances as large as 5 AU. The midlatitude turbulence, when compared to that at low and high hellographic latitudes, appears much more evolved, with a remarkably lower normalized cross-helicity (in absolute value). This unambiguously highlights that processes at velocity gradients are an important factor in the turbulence evolution. For all the analyzed intervals the residual energy values indicate an imbalance in favor of magnetic fluctuations, in agreement with previous results. The strongest imbalance is observed for the high-latitude sample, where the turbulence is comparatively the least evolved. This is a quite unexpected result, probably related to the presence of interstellar pickup ion populations. In conclusion, our analysis indicates that (1) velocity gradients play a dominant role in driving the turbulence evolution in the solar wind and (2) pickup ion effects might be significant.