Particle Acceleration and Intermittent Turbulence in Coronal Loops

A test particle numerical experiment is performed to simulate particle acceleration in low-frequency turbulence generated by footpoint motions in a coronal loop. The turbulence is modeled within the reduced MHD theory. Only the effect of the resistive electric field E|| is retained, which is mainly parallel to the axial magnetic field. In its spectrum, the contribution of small scales is dominant. The spatial structure of E|| is obtained by a synthetic turbulence method (p-model), which allows us to reproduce intermittency. By solving the relativistic motion equations, the time evolution of particle distribution is calculated. Electrons can be accelerated to energies of the order of 50 keV in less than 0.3 s, and the final energy distribution can exhibit a power-law range. A correlation is found between the heating events in the MHD turbulence and particle acceleration that is qualitatively similar to what is observed in solar flares. Spatial intermittency plays a key role in acceleration, enhancing both the extension of a power-law range and the maximum energy

Exact hybrid-kinetic equilibria for magnetized plasmas with shearing flows

Context. Magnetized plasmas characterized by shearing flows are present in many natural contexts, such as the Earth's magnetopause and the solar wind. The collisionless nature of involved plasmas requires a kinetic description. When the width of the shear layer is of the order of ion scales, the Hybrid Vlasov-Maxwell approach can be adopted. Aims. The aim of the paper is to derive explicit forms for stationary configurations of magnetized plasmas with planar shearing flows,within the Hybrid Vlasov-Maxwell description. Two configurations are considered: the first with a uniform magnetic field obliquely directed with respect to the bulk velocity; and the second with a uniform-magnitude variable-direction magnetic field. Methods. Stationary ion distribution functions are obtained by combining single-particle constant of motions, which are derived studying particle dynamics. Preliminary information about the form of the distribution functions are analytically derived considering a local approximation for the background electromagnetic field. Then, a numerical method is set up to obtain a solution for general profiles. Results. The explicit distribution functions that are found allow to obtain profiles of density, bulk velocity, temperature and heat flux. Anisotropy and agyrotropy in the distribution function are also evaluated. Stationarity of the solution during numerical simulations is checked in the uniform oblique magnetic field case. Conclusions. The considered configurations can be used as models for the Earth's magnetopause in simulations of the Kelvin-Helmholtz instability.Comment: 13 pages, 12 figure

Large-Amplitude Velocity Fluctuations in Coronal Loops: Flare Drivers?

Recent space observations of coronal lines broadening during a flare occurrence suggest that unresolved nonthermal velocity rises well above the background level before the start of the flare, defined as the start of hard X-ray emission. Using a new shell model to describe the Alfvenic turbulence inside a coronal loop, it is shown that the occurrence of high values (of the order of 100 km s-1) of the large-scale fluctuating velocity can represent an efficient trigger to a nonlinear intermittent turbulent cascade and then to the generation of a burst of dissipated energy. The numerical results of the model furnish a well-supported physical explanation for the reason why large velocity fluctuations represent the flare trigger rather than the result of the later energy deposition

Evolution of Magnetohydrodynamic Waves in Low Layers of a Coronal Hole

Although a coronal hole is permeated by a magnetic field with a dominant polarity, magnetograms reveal a more complex magnetic structure in the lowest layers, where several regions of opposite polarity of typical size of the order of 104 km are present. This can give rise to magnetic separatrices and neutral lines. MHD fluctuations generated at the base of the coronal hole by motions of the inner layer of the solar atmosphere may interact with such inhomogeneities, leading to the formation of small scales. This phenomenon is studied on a 2D model of a magnetic structure with an X-point, using 2D MHD numerical simulations. This model implements a method of characteristics for boundary conditions in the direction outer-pointing to Sun surface to simulate both wave injection and exit without reflection. Both Alfvenic and magnetosonic perturbations are considered, and they show very different phenomenology. In the former case, an anisotropic power-law spectrum forms with a dominance of perpendicular wavevectors at altitudes ~104 km. Density fluctuations are generated near the X-point by Alfven wave magnetic pressure and propagate along open fieldlines at a speed comparable to the local Alfven velocity. An analysis of energy dissipation and heating caused by the formation of small scales for the Alfvenic case is presented. In the magnetosonic case, small scales form only around the X-point, where a phenomenon of oscillating magnetic reconnection is observed to be induced by the periodic deformation of the magnetic structure due to incoming waves

Turbulence generation during the head-on collision of Alfvénic wave packets.

The description of the Moffatt and Parker problem recently revisited by O. Pezzi et al. [Astrophys. J. 834, 166 (2017)1538-435710.3847/1538-4357/834/2/166] is here extended by analyzing the features of the turbulence produced by the interaction of two colliding Alfvénic wave packets in a kinetic plasma. Although the approach based on the presence of linear modes features is still helpful in characterizing some low-energy fluctuations, other signatures, which go beyond the pure linear modes analysis, are recovered, such as the significant weakening of clear dispersion relations and the production of zero frequency fluctuations

Electron Heating by Kinetic Alfvén Waves in Coronal Loop Turbulence

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Highly efficient smart photovoltachromic devices with tailored electrolyte composition

Driven by the tremendous opportunities offered by dye solar cells technology in terms of building integration, a new generation of smart multifunctional photoelectrochemical cells has the potential to attract the interest of a rapidly growing number of research institutions and industrial companies. Photovoltachromic devices are capable to produce a smart modulation of the optical transmittance and, at the same time, to generate electrical power by means of solar energy conversion. In this work, a specifically designed bifunctional counterelectrode has been realized by depositing a C-shaped platinum frame which bounds a square region occupied by a tungsten oxide (WO3) film onto a transparent conductive substrate. These two regions have been electrically separated to make possible distinct operations on one or both of the available circuits. Such an unconventional counterelectrode makes it possible to achieve a twofold outcome: a smart and fast-responsive control of the optical transmittance and a relatively high photovoltaic conversion efficiency. In particular we investigated the effect of the electrolyte composition on both photoelectrochromic and photovoltaic performances of such devices by systematically tuning the iodide content in the electrolyte. The best result was obtained by filling the cell with an iodine concentration of 0.005 M: a coloration efficiency of 61.10 cm(2) C-1 at a wavelength of 780 nm and, at the same time, a photovoltaic conversion efficiency of 6.55% have been reported