The Role of Solar Wind Structures in the Generation of ULF Waves in the Inner Magnetosphere

The plasma of the solar wind incident upon the Earth’s magnetosphere can produce several types of geoeffective events. Among them, an important phenomenon consists of the interrelation of the magnetospheric–ionospheric current systems and the charged-particle population of the Earth’s Van Allen radiation belts. Ultra-low-frequency (ULF) waves resonantly interacting with such particles have been claimed to play a major role in the energetic particle flux changes, particularly at the outer radiation belt, which is mainly composed of electrons at relativistic energies. In this article, we use global magnetohydrodynamic simulations along with in situ and ground-based observations to evaluate the ability of two different solar wind transient (SWT) events to generate ULF (few to tens of mHz) waves in the equatorial region of the inner magnetosphere. Magnetic field and plasma data from the Advanced Composition Explorer (ACE) satellite were used to characterize these two SWT events as being a sector boundary crossing (SBC) on 24 September 2013, and an interplanetary coronal mass ejection (ICME) in conjunction with a shock on 2 October 2013. Associated with these events, the twin Van Allen Probes measured a depletion of the outer belt relativistic electron flux concurrent with magnetic and electric field power spectra consistent with ULF waves. Two ground-based observatories apart in 90^° longitude also showed evidence of ULF-wave activity for the two SWT events. Magnetohydrodynamic (MHD) simulation results show that the ULF-like oscillations in the modeled electric and magnetic fields observed during both events are a result from the SWT coupling to the magnetosphere. The analysis of the MHD simulation results together with the observations leads to the conclusion that the two SWT structures analyzed in this article can be geoeffective on different levels, with each one leading to distinct ring current intensities, but both SWTs are related to the same disturbance in the outer radiation belt, i.e. a dropout in the relativistic electron fluxes. Therefore, minor disturbances in the solar wind parameters, such as those related to an SBC, may initiate physical processes that are able to be geoeffective for the outer radiation belt.     

Laboratory tests on soil conditioning of clayey soil

Tunnelling in difficult and challenging conditions such as soft soils in urban areas is increasing. In this condition, it is important to minimise the possible negative effect of the tunnel excavation, such as settlement or, in the worst case, collapses. To achieve this result, earth pressure balance machines are commonly used. One of the key parameters that must be considered for an optimal management of the EPB-TBM excavation is soil conditioning since the excavated muck must properly transmit the pressure to the tunnel face. Soil conditioning is also necessary to reduce the effect of the problems, such as clogging in clay layers, that can occur during the excavation and that can affect the performance of the tools and of the entire tunnelling process. For this reason, in the last decade, much research has been carried out to understand how to deal with and reduce the effects of clogging and stickiness, using different conditioning additives. These studies have proposed several different test procedures to evaluate the effect of the conditioning on the adhesion of the soil on the metallic parts of the machines. The present research has been carried out with the aim of proposing a new approach and new devices to study clay conditioning with laboratory tests, and the results of many tests carried out with the proposed device are presented and discussed.     

Effects of non-stoichiometry on the spinel structure at high pressure: Implications for Earth’s mantle mineralogy

A non-stoichiometric sample of spinel with composition ^T(Mg_0.4Al_0.6)^M(Al_1.8□_0.2)O₄ was investigated by single-crystal X-ray diffraction in situ up to about 8.7 GPa using a diamond anvil cell. The P(V) data were fitted using a third-order Birch-Murnaghan equation of state and the unit-cell volume V₀, the bulk modulus K_T0 and its first pressure derivative K′ were refined simultaneously providing the following coefficients: V₀ = 510.34(6) Å³, K_T0 = 171(2) GPa, K′ = 7.3(6). This K_T0 value represents the lowest ever found for spinel crystal structures. Comparing our data with a stoichiometric and natural MgAl₂O₄ (pure composition) we observe a decrease in K_T0 by about 11.5% and a strong increase in K′ by about 33%. These results demonstrate how an excess of Al accompanied by the formation of significant cation vacancies at octahedral site strongly affects the thermodynamic properties of spinel structure. If we consider that the estimated mantle composition is characterized by 3-5% of Al₂O₃ this could imply an Mg/Al substitution with possible formation of cation vacancies. The results of our study indicate that geodynamic models should take into account the potential effect of Mg/Al substitution on the incompressibility of the main mantle-forming minerals (olivine, wadsleyite, ringwoodite, Mg-perovskite).     

Modeling concrete exposed to high temperature: Impact of dehydration and retention curves on moisture migration

High‐performance concrete is a widely used building material for tunnels, high‐rise buildings, nuclear plants etc. When these structures are exposed to fire, high‐performance concrete is prone to spalling. Moisture migration is believed to be one of the processes directly related to this phenomenon. In this paper, moisture profiles measured experimentally from neutron radiography on heated concrete are compared with results from a numerical model implemented in the finite element code Cast3M. The water loss measured experimentally, and the numerical results suggest that the commonly used constitutive laws for dehydration and water retention curves need to be reconsidered. The influence of these constitutive laws on the moisture migration is investigated. The dehydration constitutive law plays an important role on the dehydration front but has negligible effect on the moisture accumulation behind this front. By contrast, the water retention curves do not influence the dehydration front but affect the quantity and location of water condensation. The role of the permeability is also discussed.     

Relativistic dispersion effects on SS433 jets

We here investigate the dispersion properties of radiation in the SS433 relativistic jets. We assume that the jet is composed of cold electron-proton plasma immersed in a predominantly parallel magnetic field to the jet axis. We find that for the mildly relativistic source SS433 (for which 〈ψ〉≃79°), the bulk velocity is too small (v≃0.26c) to produce significant changes in the dispersion properties of the medium. Nonetheless, in the rarefied outer regions of the jets, where radio emission dominates, even a weak magnetic field has some influence on the dispersion properties and there appear two different electromagnetic branches that are slightly sensitive to the bulk relativistic motion. In the inner, X-ray region, the magnetic field is much stronger, but in this region the high electron density preserves the isotropic character of the local plasma and no branch separation occurs. In the region of the jet where the IR and optical emission dominates, the cold plasma may be also considered isotropic, i.e., neither the magnetic field nor the bulk velocity is able to affect the propagation of the radiation. Finally, we find that the Doppler line displacement in SS433 is affected by plasma dispersion only in a narrow frequency range in the far IR. As a consequence, although the shift (z) modulation due to precession of the SS433 jets is well described by previous work, it has to be corrected by plasma dispersion effects in the far-IR range.