Geometry of Magnetic Fluctuations near the Sun from the Parker Solar Probe

Solar wind magnetic fluctuations exhibit anisotropy due to the presence of a mean magnetic field in the form of the Parker spiral. Close to the Sun, direct measurements were not available until the recently launched Parker Solar Probe (PSP) mission. The nature of the anisotropy and geometry of the magnetic fluctuations play a fundamental role in dissipation processes and in the transport of energetic particles in space. Using PSP data, we present measurements of the geometry and anisotropy of the inner heliosphere magnetic fluctuations, from fluid to kinetic scales. The results are surprising and different from 1 au observations. We find that fluctuations evolve characteristically with size scale. However, unlike 1 au solar wind, at the outer scale, the fluctuations are dominated by wavevectors quasi-parallel to the local magnetic field. In the inertial range, average wavevectors become less field aligned, but still remain more field aligned than near-Earth solar wind. In the dissipation range, the wavevectors become almost perpendicular to the local magnetic field in the dissipation range, to a much higher degree than those indicated by 1 au observations. We propose that this reduced degree of anisotropy in the outer scale and inertial range is due to the nature of large-scale forcing outside the solar corona.

Optical Turbulence Profile Forecasting and Verification in the Offshore Atmospheric Boundary Layer

A backpropagation neural network (BPNN) approach is proposed for the forecasting and verification of optical turbulence profiles in the offshore atmospheric boundary layer. To better evaluate the performance of the BPNN approach, the Holloman Spring 1999 thermosonde campaigns (HMNSP99) model for outer scale, and the Hufnagel/Andrew/Phillips (HAP) model for a single parameter are selected here to estimate profiles. The results have shown that the agreement between the BPNN approach and the measurement is very close. Additionally, statistical operators are used to quantify the performance of the BPNN approach, and the statistical results also show that the BPNN approach and measured profiles are consistent. Furthermore, we focus our attention on the ability of the BPNN approach to rebuild integrated parameters, and calculations show that the BPNN approach is reliable. Therefore, the BPNN approach is reasonable and remarkable for reconstructing the strength of optical turbulence of the offshore atmospheric boundary layer.

Oxidation Behavior and Oxide Transformation of a Pt-Modified Aluminide Coating at Moderate High Temperature

The oxidation performance of a single-phase Pt-modified aluminide coating was assessed in oxidation test at 980 °C in comparison with the single crystal superalloy. The results suggested that the Pt-modified aluminide coating exhibited superior oxidation resistance. During oxidation, the oxide scale formed on bare alloy changed constantly followed by the constitution of the multi-layer scale structure: An outer scale mainly consisted of Cr2O3 + NiCr2O4 + TiO2 with scarce protection, and an internal scale mainly consisted of Al2O3. The thickness of the outer oxide scale increased with time, where the scale became looser and more porous. Meanwhile, the internal scale was discontinuous. Oxygen and nitrogen inwardly diffused into substrate, forming Ta2O5 and TiN particles. In contrast to the complex constitution of oxide scale, a uniform and continuous Al2O3 scale formed on Pt-modified aluminide-coated samples after oxidation at 980 °C for 1500 h, which showed no spallation and cracking. Interestingly, θ-Al2O3 and α-Al2O3 phases remained after such a long oxidation time. It is the relatively lower temperature and the presence of Pt retarded θ-α transformation. The degradation rate from β-NiAl to γ′-Ni3Al was very slow in the coating. The various development of oxide scale on the coating and substrate was individually studied.

Heightened Faraday complexity in the inner 1 kpc of the galactic centre

ABSTRACT We have measured the Faraday rotation of 62 extra-galactic background sources in 58 fields using the CSIRO Australia Telescope Compact Array (ATCA) with a frequency range of 1.1–3.1 GHz with 2048 channels. Our sources cover a region $\sim 12\, \times 12\, \mathrm{deg^{ 2}}$ (∼1 kpc) around the Galactic Centre region. We show that the Galactic Plane for |l| < 10° exhibits large Rotation Measures (RMs) with a maximum |RM| of $1691.2 \pm 4.9\, \mathrm{rad}\, \mathrm{m}^{-2}$ and a mean $|\mathrm{RM}| = 219 \pm 42\, \mathrm{rad}\, \mathrm{m}^{-2}$. The RMs decrease in magnitude with increasing projected distance from the Galactic Plane, broadly consistent with previous findings. We find an unusually high fraction (95 per cent) of the sources show Faraday complexity consistent with multiple Faraday components. We attribute the presences of multiple Faraday rotating screens with widely separated Faraday depths to small-scale turbulent RM structure in the Galactic Centre region. The second-order structure function of the RM in the Galactic Centre displays a line with a gradient of zero for angular separations spanning 0.83°–11° (∼120–1500 pc), which is expected for scales larger than the outer scale (or driving scale) of magneto-ionic turbulence. We place an upper limit on any break in the SF gradient of 66 arcsec, corresponding to an inferred upper limit to the outer scale of turbulence in the inner 1 kpc of the Galactic Centre of 3 pc. We propose stellar feedback as the probable driver of this small-scale turbulence.

On the determination of the atmospheric outer scale length of turbulence using GPS phase difference observations: the Seewinkel network

Microwave electromagnetic signals from the Global Navigation Satellite System (GNSS) are affected by their travel through the atmosphere: the troposphere, a non-dispersive medium, has an especial impact on the measurements. The long-term variations of the tropospheric refractive index delay the signals, whereas its random variations correlate with the phase measurements. The correlation structure of residuals from GNSS relative position estimation provides a unique opportunity to study specific properties of the turbulent atmosphere. Prior to such a study, the residuals have to be filtered from unwanted additional effects, such as multipath. In this contribution, we propose to investigate the property of the atmospheric noise by using a new methodology combining the empirical mode decomposition with the Hilbert–Huang transform. The chirurgical “designalling of the noise” aims to filter both the white noise and low-frequency noise to extract only the noise coming from tropospheric turbulence. Further analysis of the power spectrum of phase difference can be performed, including the study of the cut-off frequencies and the two slopes of the power spectrum of phase differences. The obtained values can be compared with theoretical expectations. In this contribution, we use Global Positioning System (GPS) phase observations from the Seewinkel network, specially designed to study the impact of atmospheric turbulence on GPS phase observations. We show that (i) a two-slope power spectrum can be found in the residuals and (ii) that the outer scale length can be taken to a constant value, close to the physically expected one and in relation with the size of the eddies at tropospheric height.