Spectrum of kinetic plasma turbulence at 0.3-0.9 AU from the Sun

<p>We investigate the spectral properties of the turbulence in the solar wind which is a weakly collisional astrophysical plasma, accessible by in-situ observations. Using the Helios search coil magnetometer measurements in the fast solar wind, in the inner heliosphere, we focus on properties of the turbulent magnetic fluctuations at scales smaller than the ion characteristic scales, the so-called kinetic plasma turbulence. At such small scales, we show that the magnetic power spectra between 0.3 and 0.9 AU from the Sun have a generic shape ~f<sup>-8/3</sup>exp(-f/f<sub>d</sub>) where the dissipation frequency f<sub>d</sub> is correlated with the Doppler shifted frequency f<sub>ρe</sub> of the electron Larmor radius. This behavior is statistically significant: all the observed kinetic spectra are well described by this model, with f<sub>d</sub>=f<sub>ρe</sub>/1.8. These results provide important constraints on the dissipation mechanism in nearly collisionless space plasmas.</p>

Fully kinetic simulations of electron-scale plasma turbulence in the inner heliosphere: a pathfinder for future spacecraft missions

<p>We present numerical results from high-resolution fully kinetic simulations of plasma turbulence under the near-Sun conditions encountered by Parker Solar Probe during its first perihelion, characterized by a low plasma beta and a large level of turbulent fluctuations. The recovered spectral properties are in agreement with those from PSP observations and recent high-resolution hybrid simulations just below the ion characteristic scales, i.e., the spectrum of the magnetic field exhibits a steep transition region with a spectral index compatible with -11/3. When the electron scales are reached a spectral break is observed and the spectrum steepens while still showing a clear power law. We discuss theoretical predictions for such a spectral behavior, based on a two-fluid model which assumes that a self-similar energy transfer across scales is occurring, without the need to include any kinetic process. We also analyse the role of magnetic reconnection and the statistics of reconnection events, as well as signatures in the proton and electron distribution functions hinting at mechanisms for energy dissipation. The results of this work represent a step forward in understanding the processes responsible for particle heating and acceleration and therefore on the origin of the solar wind and coronal heating. Furthermore, they allow for reliable predictions for future spacecraft missions investigating electron-scale physics in low-beta plasmas.</p>

Preparation and Characterization of SrAl2O4: Eu2+, Dy3+ Luminescent Nanofibers

PVP/[Sr (NO3)2+Al (NO3)3+Eu (NO3)3+Dy (NO3)3] composite nanofibres were prepared by electrospinning technique. SrAl2O4:Eu2+, Dy3+ luminescent nanofibers were synthesized by calcination under a reducing atmosphere at 1100°C. The luminescent nanofibers were characterized by XRD, SEM, FTIR, TG, fluorescence spectrophotometer, respectively. The experimental results showed that the products are with monoclinic SrAl2O4 crystal structure, the diameter of nanofibers is about 100nm and the size is uniformly distributed. The luminescent nanofibers emit bright blue fluorescence of 470nm in wavelength of Eu2+ ion characteristic emission under the excitation of 365nm in wavelength of ultraviolet ray. The position of emission peak exhibit blue shift in some degree compared with that of bulk SrAl2O4:Eu2+, Dy3+.

Spectra and anisotropy of magnetic fluctuations in the Earth's magnetosheath: Cluster observations

We investigate the spectral shape, the anisotropy of the wave vector distributions and the anisotropy of the amplitudes of the magnetic fluctuations in the Earth's magnetosheath within a broad range of frequencies [10−3, 10] Hz which corresponds to spatial scales from ~10 to 105 km. We present the first observations of a Kolmogorov-like inertial range of Alfvénic fluctuations δB2⊥}~f−5/3 in the magnetosheath flanks, below the ion cyclotron frequency fci. In the vicinity of fci, a spectral break is observed, like in solar wind turbulence. Above the break, the energy of compressive and Alfvénic fluctuations generally follows a power law with a spectral index between −3 and −2. Concerning the anisotropy of the wave vector distribution, we observe a clear change in its nature in the vicinity of ion characteristic scales: if at MHD scales there is no evidence for a dominance of a slab (kł>>k⊥) or 2-D (k⊥>>kł) turbulence, above the spectral break, (f>fci, kc/ωpi>1) the 2-D turbulence dominates. This 2-D turbulence is observed in six selected one-hour intervals among which the average ion β varies from 0.8 to 10. It is observed for both the transverse and compressive magnetic fluctuations, independently on the presence of linearly unstable modes at low frequencies or Alfvén vortices at the spectral break. We then analyse the anisotropy of the magnetic fluctuations in a time dependent reference frame based on the field B and the flow velocity V directions. Within the range of the 2-D turbulence, at scales [1,30]kc/ωpi, and for any β we find that the magnetic fluctuations at a given frequency in the plane perpendicular to B have more energy along the B×V direction. This non-gyrotropy of the fluctuations at a fixed frequency is consistent with gyrotropic fluctuations at a given wave vector, with k⊥>>kł, which suffer a different Doppler shift along and perpendicular to V in the plane perpendicular to B.

Solar wind vs magnetosheath turbulence and Alfvén vortices

In this paper we give firstly a broad review of the space plasma turbulence around the ion characteristic space and temporal scales within two natural laboratories, the solar wind and the Earth magnetosheath. In both regions power law spectra of magnetic fluctuations are observed. In both regions these spectra have a break in the vicinity of the ion cyclotron frequency. A distinctive feature of the magnetosheath turbulence is the presence of Alfvén vortices at scales of the spectral break. The Alfvén vortices are multi-scale nonlinear structures. We give a review of the main theoretical features of incompressible Alfvén vortsices in the second part of the paper. Finally, we analyze the spectral properties of the Alfvén vortex solution and of the network of such vortices. We show that the observed magnetosheath spectrum in presence of the Alfvén vortices can be described, at least partially, by the vortex network model.

Raman and Photoluminescence Spectral Study of Eu(DBM)3•2H2O at High Pressures

The photoluminescence and Raman spectra of the tris(dibenzoylmethane) europium(III) dihydrate (Eu(DBM)3•2H2O) are investigated at high pressures with a diamond anvil cell. For pressures lower than 3 GPa, the luminescent intensities of the Eu3+ ion 5D0→7FJ (J=0, 1, 2, 3, 4) characteristic peaks increase with the pressure. However, above 3 GPa the intensities decrease. The phenomena indicate that there is energy transition between the Eu3+ ion and the DBM ligand. With the increasing pressure, the intensity for the vibration bands becomes more obvious compared with that of the Eu3+ ion characteristic peaks, as the result of the crystal distortion at high pressures. The Raman bands at low wavenumbers have not shown significant changes with pressure, which manifests that the Eu3+ ion site symmetries are not varied but that the crystal is distorted at high pressures.