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Magnetic turbulence is classified as weak or strong based on the relative amplitude of the magnetic field fluctuations compared to the mean field. These two classifications have different energy transport properties. This study analyzes interstellar turbulence based on this classification. Specifically, we examine the ISM of simulated galaxies to detect evidence of strong magnetic turbulence and provide statistics on the associated magnetic coherent structures (MCoSs), such as current sheets, that arise in this context. We analyzed MHD galaxy simulations with different initial magnetic field structures (ordered or random) and studied the magnetic field fluctuations ($\delta B/B_0$) and the MCoSs, defined here as regions where the current density surpasses a certain threshold. We also studied the MCoS sizes and kinematics. The magnetic field disturbances in both models follow a log-normal distribution, peaking at values close to unity, which turns into a power-law at large values ($\rm \delta B/B_0 > 1$). The current densities are widely distributed, with deviations from a log-normal at the largest values. These deviating values of the current density define MCoSs. We find that, in both models, MCoSs are fractally distributed in space, with a typical volume-filling factor of about 10 percent, and tend to coincide with peaks of star formation density. Their fractal dimension is close to unity below kpc scales and between 2 and 3 on larger scales. Our work challenges the prevailing paradigm of weak magnetic turbulence in the ISM by demonstrating that strong magnetic disturbances occur even when the initial magnetic field is initially ordered due to differential rotation and supernova feedback. Our findings provide a foundation for a strong magnetic turbulence description of the galactic ISM. (abridged), Comment: Accepted for publication in A&A

Atmospheric composition of exoplanets is often considered as a probe of the planet's formation condition. How exactly the initial chemical memory may be altered from the birth to the final state of the planet, however, remains unknown. Here, we develop a simple model of pollution of planetary atmosphere by the vaporization of infalling planetesimal of varying sizes and composition (SiO$_2$ inside 1 au and H$_2$O outside 1 au), following their trajectory and thermal evolution through the upper advective and radiative layers of a sub-Neptune class planet during the late stage of disk evolution. We vary the rate of pollution by changing the solid content of the disk and by dialing the level of disk gas depletion which in turn determines the rate of planetary migration. We find that pollution by silicate grains will always be limited by the saturation limit set by the thermal state of the atmosphere. By contrast, pollution by water ice can lead to $\sim$2--4 orders of magnitude variation in the atmospheric water mass fraction depending on the solid and gas content of the disk. Both cases suggest that post-formation pollution can erase the initial compositional memory of formation. Post-formation pollution can potentially transform sub-Neptunes with H/He-dominated envelope that initially formed beyond the iceline to waterworlds (water-enriched envelope) when the disk gas is depleted by $\gtrsim$2 orders of magnitude, allowing gentle migration. We additionally discuss the expected C/O ratio profile under pollution by water and refractory carbon species., Comment: Accepted to ApJ

We review the current literature on the formation of Coherent Structures (CoSs) in strongly turbulent 3D magnetized plasmas. CoSs (Current Sheets (CS), magnetic filaments, large amplitude magnetic disturbances, vortices, and shocklets) appear intermittently inside a turbulent plasma and are collectively the locus of magnetic energy transfer (dissipation) into particle kinetic energy, leading to heating and/or acceleration of the latter. CoSs and especially CSs are also evolving and fragmenting, becoming locally the source of new clusters of CoSs. Strong turbulence can be generated by the nonlinear coupling of large amplitude unstable plasma modes, by the explosive reorganization of large scale magnetic fields, or by the fragmentation of CoSs. A small fraction of CSs inside a strongly turbulent plasma will end up reconnecting. Magnetic Reconnection (MR) is one of the potential forms of energy dissipation of a turbulent plasma. Analysing the evolution of CSs and MR in isolation from the surrounding CoSs and plasma flows may be convenient for 2D numerical studies, but it is far from a realistic modeling of 3D astrophysical, space and laboratory environments, where strong turbulence can be exited, as e.g. in the solar wind, the solar atmosphere, solar flares and Coronal Mass Ejections (CMEs), large scale space and astrophysical shocks, the magnetosheath, the magnetotail, astrophysical jets, Edge Localized Modes (ELMs) in confined laboratory plasmas (TOKAMAKS), etc., Comment: 27 pages, 31 figures; review; accepted for publication in Physics of Plasmas 2023

Parker Solar Probe and Solar Orbiter data are used to investigate the radial evolution of magnetic turbulence between $0.06 ~ \lesssim R ~\lesssim 1$ au. The spectrum is studied as a function of scale, normalized to the ion inertial scale $d_{i}$. In the vicinity of the Sun, the inertial range is limited to a narrow range of scales and exhibits a power-law exponent of, $\alpha_{B} = -3/2$, independent of plasma parameters. The inertial range grows with distance, progressively extending to larger spatial scales, while steepening towards a $\alpha_{B} =-5/3$ scaling. It is observed that spectra for intervals with large magnetic energy excesses and low Alfv\'enic content steepen significantly with distance, in contrast to highly Alfv\'enic intervals that retain their near-Sun scaling. The occurrence of steeper spectra in slower wind streams may be attributed to the observed positive correlation between solar wind speed and Alfv\'enicity., Comment: Accepted to APJ letters with minor revisions

We present test-particle simulations of electrons during a nonlinear MHD simulation of a type-I edge localized mode (ELM) to explore the effect of an eruptive plasma filament on the kinetic level. The electrons are moderately heated and accelerated during the filamentary eruption on a fast time scale of the order of 0.5 ms. A clearly non-thermal tail is formed in the distribution of the kinetic energy that is of power-law shape and reaches 90 keV for some particles. The acceleration is exclusively observed in the direction parallel to the magnetic field, i.e. with a clear preference in counter-current direction, and we show that the parallel electric field is the cause of the observed acceleration. Most particles that escape from the system leave at one distinct strike-line in the outer divertor leg at some time during their energization. The escaping high energy electrons in the tail of the energy distribution are not affected by collisions, they thus show characteristics of runaway electrons. The mean square displacement indicates that transport in energy space clearly is super-diffusive, and interpreting the acceleration process as a random walk, we find that the distributions of energy-increments exhibit exponential tails, and transport in energy space is equally important of convective (systematic) and diffusive (stochastic) nature. By analyzing the MHD simulations per se, it turns out that the histograms of the parallel electric field in the edge region exhibit power-law shapes, and this clearly non-Gaussian statistics is ultimately one of the reasons for the moderately anomalous phenomena of particle transport that we find in energy space., Comment: Accepted for publication in Physics of Plasmas, Oct. 19, 2022. Regular article, 21 pages, 24 figures

$PSP$ and $SolO$ data are utilized to investigate magnetic field intermittency in the solar wind (SW). Small-scale intermittency $(20-100d_{i})$ is observed to radially strengthen when methods relying on higher-order moments are considered ($SF_q$, $SDK$), but no clear trend is observed at larger scales. However, lower-order moment-based methods (e.g., PVI) are deemed more appropriate for examining the evolution of the bulk of Coherent Structures (CSs), $PVI \ge 3$. Using PVI, we observe a scale-dependent evolution in the fraction of the dataset occupied by CSs, $f_{PVI \ge 3}$. Specifically, regardless of the SW speed, a subtle increase is found in $f_{PVI\ge3}$ for $\ell =20 d_i$, in contrast to a more pronounced radial increase in CSs observed at larger scales. Intermittency is investigated in relation to plasma parameters. Though, slower SW speed intervals exhibit higher $f_{PVI \geq 6}$ and higher kurtosis maxima, no statistical differences are observed for $f_{PVI \geq 3}$. Highly Alfv\'enic intervals, display lower levels of intermittency. The anisotropy with respect to the angle between the magnetic field and SW flow, $\Theta_{VB}$ is investigated. Intermittency is weaker at $\Theta_{VB} \approx 0^{\circ}$ and is strengthened at larger angles. Considering the evolution at a constant alignment angle, a weakening of intermittency is observed with increasing advection time of the SW. Our results indicate that the strengthening of intermittency in the inner heliosphere is driven by the increase in comparatively highly intermittent perpendicular intervals sampled by the probes with increasing distance, an effect related directly to the evolution of the Parker spiral.

We use data from the first six encounters of Parker Solar Probe and employ the Partial Variance of Increments ($PVI$) method to study the statistical properties of coherent structures in the inner heliosphere with the aim of exploring physical connections between magnetic field intermittency and observable consequences such as plasma heating and turbulence dissipation. Our results support proton heating localized in the vicinity of, and strongly correlated with, magnetic structures characterized by $PVI \geq 1$. We show that on average, such events constitute $\approx 19\%$ of the dataset, though variations may occur depending on the plasma parameters. We show that the waiting time distribution ($WT$) of identified events is consistent across all six encounters following a power-law scaling at lower $WTs$. This result indicates that coherent structures are not evenly distributed in the solar wind but rather tend to be tightly correlated and form clusters. We observe that the strongest magnetic discontinuities, $PVI \geq 6$, usually associated with reconnection exhausts, are sites where magnetic energy is locally dissipated in proton heating and are associated with the most abrupt changes in proton temperature. However, due to the scarcity of such events, their relative contribution to energy dissipation is minor. Taking clustering effects into consideration, we show that smaller scale, more frequent structures with PVI between, $1\lesssim PVI \lesssim 6$, play the major role in magnetic energy dissipation. The number density of such events is strongly associated with the global solar wind temperature, with denser intervals being associated with higher $T_{p}$., Comment: Accepted for publication in APJ

Experimental observations of thermionic electron emission demonstrate a smooth transition between TL and FSCL regions of the emitted-current-density-versus-temperature (J-T) (Miram) curve and the emitted-current-density-versus-voltage (J-V) curve. Knowledge of the temperature and shape of the TL-FSCL transition is important in evaluating the thermionic electron emission performance of cathodes, including predicting the lifetime. However, there have been no first-principles physics-based models that can predict the smooth TL-FSCL transition region for real thermionic cathodes without applying physically difficult to justify a priori assumptions or empirical phenomenological equations. Previous work detailing the nonuniform thermionic emission found that the effects of 3-D space charge, patch fields, and Schottky barrier lowering can lead to a smooth TL-FSCL transition region from a model thermionic cathode surface with a checkerboard spatial distribution of work function values. In this work, we construct a physics-based nonuniform emission model for commercial dispenser cathodes for the first time. This emission model is obtained by incorporating the cathode surface grain orientation via electron backscatter diffraction (EBSD) and the facet-orientation-specific work function values from density functional theory (DFT) calculations. The model enables construction of two-dimensional emitted current density maps of the cathode surface and corresponding J-T and J-V curves. The predicted emission curves show excellent agreement with experiment, not only in TL and FSCL regions but, crucially, also in the TL-FSCL transition region. This model improves the understanding on the relationship between thermionic emission and cathode microstructure, which is beneficial to the design of vacuum electronic devices.

Determining for the first time the Darwin operator contribution for the non-leptonic charm-quark decays and using new non-perturbative results for the matrix elements of $\Delta C=0$ four-quark operators, including eye-contractions, we present a comprehensive study of the lifetimes of charmed mesons and inclusive semileptonic decay rates as well as the ratios, within the framework of the Heavy Quark Expansion (HQE). We find good agreement with experiment for the ratio $\tau(D^+)/\tau(D^0)$, for the total $D_s^+$-meson decay rate, for the semileptonic rates of all three mesons $D^0$, $D^+$ and $D_s^+$, and for the semileptonic ratio $\Gamma_{sl}^{D^+}/\Gamma_{sl}^{D^0}$. The total decay rates of the $D^0$ and $D^+$ mesons are underestimated in our HQE approach and we suspect that this is due to missing higher-order QCD corrections to the free charm quark decay and the Pauli interference contribution. For the $SU(3)_F$ breaking ratios $\tau (D_s^+) / \tau (D^0) $ and $\Gamma_{sl}^{D_s^+}/\Gamma_{sl}^{D^0} $ our predictions lie closer to one than experiment. This might originate from the poor knowledge of the non-perturbative parameters $\mu_G^2$, $\mu_\pi^2$ and $\rho_D^3$ in the $D^0$ and $D_s^+$ systems. These parameters could be determined by experimental studies of the moments of inclusive semileptonic $D$ meson decays., Comment: 50 pages, 18 Tables, 7 Figures

Parker (1983) suggested a mechanism for the formation of current sheets (CSs) in the solar atmosphere. His main idea was that the tangling of coronal magnetic field lines by photospheric random flows facilitates the continuous formation of CSs in the solar atmosphere. This part of his idea represents one of the many ways by which the turbulent convection zone drives the formation of coherent structures and CSs in the solar atmosphere. Other mechanisms include emerging magnetic flux, interaction of current filaments, and explosive magnetic structures. However, there are two unproven assumptions in the initial idea of Parker for the coronal heating through nanoflares that must be re-examined. They are related to his suggestion that {ALL CSs formed are led to magnetic reconnection and that magnetic reconnection heats the plasma in the solar atmosphere. Let us discuss these two assumptions briefly in this short comment: (1) Are ALL coherent structures and CSs formed by the turbulent convection zone reconnecting? Does turbulence associated with non-reconnecting CSs play a role in the heating of the corona? (2) Does magnetic reconnection heat the plasma?

In this article, we study the physics of charged particle energization inside a strongly turbulent plasma, where current sheets naturally appear in evolving large-scale magnetic topologies, but they are split into two populations of fractally distributed reconnecting and non-reconnecting current sheets (CS). In particular, we implement a Monte Carlo simulation to analyze the effects of the fractality and we study how the synergy of energization at reconnecting CSs and at non-reconnecting CSs affects the heating, the power-law high energy tail, the escape time, and the acceleration time of electrons and ions. The reconnecting current sheets (RCS) systematically accelerate particles and play a key role in the formation of the power-law tail in energy distributions. On the other hand, the stochastic energization of particles through their interaction with non-reconnecting CSs can account for the heating of the solar corona and the impulsive heating during solar flares. The combination of the two acceleration mechanisms (stochastic and systematic), commonly present in many explosive events of various sizes, influences the steady-state energy distribution, as well as the transport properties of the particles in position- and energy-space. Our results also suggest that the heating and acceleration characteristics of ions and electrons are similar, the only difference being the time scales required to reach a steady state., Comment: in press at Astronomy & Astrophysics, July 2021

A naive application of the heavy quark expansion (HQE) yields theory estimates for the decay rate of neutral $D$ mesons that are four orders of magnitude below the experimental determination. It is well known that this huge suppression results from severe GIM cancellations. We find that this mismatch can be solved by individually choosing the renormalisation scale of the different internal quark contributions. For $b$ and $c$ hadron lifetimes, as well as for the decay rate difference of neutral $B$ mesons the effect of our scale setting procedure lies within the previously quoted theory uncertainties, while we get enlarged theory uncertainties for the semi-leptonic CP asymmetries in the $B$ system., Comment: 6 pages, 3 figures

We analyze the stochastic acceleration of particles inside a fully developed turbulent plasma. It is well known that large-amplitude magnetic fluctuations and coherent structures in such an environment obey a fractal scaling, and our specific aim is to study for the first time the effects of the fractality of these environments on stochastic acceleration. We have shown that an injected Maxwellian energy distribution is heated and forms a high energy tail in a very short time. Using standard parameters for the low solar corona, the injected Maxwellian distribution of electrons gets heated from the initial 100 eV to 10 KeV, and the power-law index of the high energy tail is about -2.3-4.0. The high energy tail starts around 100 keV, and reaches 10 MeV. The index of the power-law tail depends on the system size, and it is in good agreement with observed values for realistic system sizes. The heating and acceleration process is very fast (\sim 2 s). The reason why the acceleration time is so short is that the particles are trapped within small scale parts of the fractal environment, and their scattering mean free path reduces drastically. The presence of small scale activity also pulls easily particles from the thermal pool, so there is no need for a seed population. The mean square displacement in space and energy is superdiffusive for the high energy particles., Comment: 8 pages, 7 figures, Accepted for publication in The Astrophysical Journal Letters

We analyze the transport properties of charged particles (ions and electrons) interacting with randomly formed magnetic scatterers (e.g.\ large scale local ``magnetic fluctuations'' or ``coherent magnetic irregularities'' usually present in strongly turbulent plasmas), using the energization processes proposed initially by Fermi in 1949. The scatterers are formed by large scale local fluctuations ($\delta B/B \approx 1$) and are randomly distributed inside the unstable magnetic topology. We construct a 3D grid on which a small fraction of randomly chosen grid points are acting as scatterers. In particular, we study how a large number of test particles are accelerated and transported inside a collection of scatterers in a finite volume. Our main results are: (1) The spatial mean-square displacement $<(\Delta r)^2>$ inside the stochastic Fermi accelerator is superdiffusive, $<(\Delta r)^2> \sim t^{a_{r}},$ with $a_r \sim 1.2-1.6$, for the high energy electrons with kinetic energy $(W)$ larger than $1 MeV$, and it is normal ($a_r=1$) for the heated low energy $(W< 10 keV)$ electrons. (2) The transport properties of the high energy particles are closely related with the mean-free path that the particles travel in-between the scatterers ($\lambda_{sc}$). The smaller $\lambda_{sc}$ is, the faster the electrons and ions escape from the acceleration volume. (3) The mean displacement in energy $<\Delta W> \sim t^{a_{W}}$ is strongly enhanced inside the acceleration volume $(a_W=1.5- 2.5)$ for the high energy particles compared to the thermal low energy particles ($a_W=0.4$), i.e.\ high energy particles undergo an enhanced systematic gain in energy.(4) The mean-square displacement in energy $$ is superdiffusive for the high energy particles and normal for the low energy, heated particles., Comment: 11 pages, 8 figures, accepted for publication in MNRAS

The interaction between emerging and pre-existing magnetic fields in the solar atmosphere can trigger several dynamic phenomena, such as eruptions and jets. A key element during this interaction is the formation of large scale current sheets and, eventually, their fragmentation that leads to the creation of a strongly turbulent environment. In this paper, we study the kinetic aspects of the interaction (reconnection) between emerging and ambient magnetic fields. We show that the statistical properties of the spontaneously fragmented and fractal electric fields are responsible for the efficient heating and acceleration of charged particles, which form a power law tail at high energies on sub-second time scales. A fraction of the energized particles escapes from the acceleration volume, with a super-hot component with temperature close to $150\,$MK, and with a power law high energy tail with index between -2 and -3. We estimate the transport coefficients in energy space from the dynamics of the charged particles inside the fragmented and fractal electric fields, and the solution of a fractional transport equation, as appropriate for a strongly turbulent plasma, agrees with the test particle simulations. We also show that the acceleration mechanism is not related to Fermi acceleration, and the Fokker Planck equation is inconsistent and not adequate as a transport model. Finally, we address the problem of correlations between spatial transport and transport in energy space. Our results confirm the observations reported for high energy particles (hard X-rays, type III bursts and solar energetic particles) during the emission of solar jets., Comment: in press at Astrophysical Journal, 2019

The physical processes, which drive powerful solar eruptions, play an important role in our understanding of the Sun-Earth connection. In this Special Issue, we firstly discuss how magnetic fields emerge from the solar interior to the solar surface, to build up active regions, which commonly host large-scale coronal disturbances, such as coronal mass ejections (CMEs). Then, we discuss the physical processes associated with the driving and triggering of these eruptions, the propagation of the large-scale magnetic disturbances through interplanetary space and the interaction of CMEs with Earth's magnetic field. The acceleration mechanisms for the solar energetic particles related to explosive phenomena (e.g. flares and/or CMEs) in the solar corona are also discussed. The main aim of this Issue, therefore, is to encapsulate the present state-of-the-art in research related to the genesis of solar eruptions and their space-weather implications. This article is part of the theme issue 'Solar eruptions and their space weather impact'., Comment: 3 pages

Solar Energetic Particles (SEP) are an integral part of the physical processes related with Space Weather. We present a review for the acceleration mechanisms related to the explosive phenomena (flares and/or CMEs) inside the solar corona. For more than 40 years, the main 2D cartoon representing our understanding of the explosive phenomena inside the solar corona remained almost unchanged. The acceleration mechanisms related to solar flares and CMEs also remained unchanged and were part of the same cartoon. In this review, we revise the standard cartoon and present evidence from recent global MHD simulations that supports the argument that explosive phenomena will lead to the spontaneous formation of current sheets in different parts of the erupting magnetic structure. The evolution of the large scale current sheets and their fragmentation will lead to strong turbulence and turbulent reconnection during solar flares and turbulent shocks. In other words, the acceleration mechanism in flares and CME-driven shocks may be the same, and their difference will be the overall magnetic topology, the ambient plasma parameters, and the duration of the unstable driver., Comment: Accepted at Philosophical Transactions A, 2019

Turbulence, magnetic reconnection, and shocks can be present in explosively unstable plasmas, forming a new electromagnetic environment, which we call here turbulent reconnection, and where spontaneous formation of current sheets takes place. We will show that the heating and the acceleration of particles is the result of the synergy of stochastic (second order Fermi) and systematic (first order Fermi) acceleration inside fully developed turbulence. The solar atmosphere is magnetically coupled to a turbulent driver (the convection zone), therefore the appearance of turbulent reconnection in the solar atmosphere is externally driven. Turbulent reconnection, once it is established in the solar corona, drives the coronal heating and particle acceleration., Comment: 20 pages, 19 figures, in press at Plasma Physics and Controled Fusion (PPCF), 2018

Diffusive Shock Acceleration (DSA) cannot efficiently accelerate particles without the presence of self-consistently generated or pre-existing strong turbulence ($ \delta B/B \sim 1 $) in the vicinity of the shock. The problem we address in this article is: if large amplitude magnetic disturbances are present upstream and downstream of a shock then Turbulent Reconnection (TR) will set in and will participate not only in the elastic scattering of particles but also in their heating and acceleration. We demonstrate that large amplitude magnetic disturbances and Unstable Current Sheets (UCS), spontaneously formed in the strong turbulence in the vicinity of a shock, can accelerate particles as efficiently as DSA in {\bf large scale systems and on long time scales}. We start our analysis with "elastic" scatterers upstream and downstream and estimate the energy distribution of particles escaping from the shock, recovering the well known results from the DSA theory. Next we analyze the additional interaction of the particles with active scatterers (magnetic disturbances and UCS) upstream and downstream of the shock. We show that the asymptotic energy distribution of the particles accelerated by DSA/TR has very similar characteristics with the one due to DSA alone, but the synergy of DSA with TR is much more efficient: The acceleration time is an order of magnitude shorter and the maximum energy reached two orders of magnitude higher. We claim that DSA is the dominant acceleration mechanism in a short period before TR is established, and then strong turbulence will dominate the heating and acceleration of the particles. In other words, the shock serves as the mechanism to set up a strongly turbulent environment, in which the acceleration mechanism will ultimately be the synergy of DSA and TR., Comment: 12 pages, 8 figures, Accepted for publication in MNRS

The important characteristic of turbulent reconnection is that it combines large scale magnetic disturbances $(\delta B/B \sim 1)$ with randomly distributed Unstable Current Sheets (UCSs). Many well known non linear MHD structures (strong turbulence, current sheet(s), shock(s)) lead asymptotically to the state of turbulent reconnection. We analyze in this article, for the first time, the energization of electrons and ions in a {\bf large scale} environment that {\bf combines} large amplitude disturbances propagating with sub-Alfv\'enic speed with UCSs. The magnetic disturbances interact stochastically (second order Fermi) with the charged particles and they play a crucial role in the heating of the particles, while the UCS interact systematically (first order Fermi) and play a crucial role in the formation of the high energy tail. The synergy of stochastic and systematic acceleration provided by the mixture of magnetic disturbances and UCSs influences the energetics of the thermal and non-thermal particles, the power law index, and the time the particles remain inside the energy release volume. We show that this synergy can explain the observed very fast and impulsive particle acceleration and the slightly delayed formation of a super-hot particle population., Comment: 6 pages, 3 figures, Accepted for publication in ApJ

We consider a large scale environment of turbulent reconnection that is fragmented into a number of randomly distributed Unstable Current Sheets (UCS), and we statistically analyze the acceleration of particles within this environment. We address two important cases of acceleration mechanisms when the particles interact with the UCS: (a) electric field acceleration, and (b) acceleration through reflection at contracting islands. Electrons and ions are accelerated very efficiently, attaining an energy distribution of power-law shape with an index $1-2$, depending on the acceleration mechanism. The transport coefficients in energy space are estimated from the test-particle simulation data, and we show that the classical Fokker-Planck (FP) equation fails to reproduce the simulation results when the transport coefficients are inserted into it and it is solved numerically. The cause for this failure is that the particles perform Levy flights in energy space, the distributions of energy increments exhibit power-law tails. We then use the fractional transport equation (FTE) derived by Isliker et al., 2017, whose parameters and the order of the fractional derivatives are inferred from the simulation data, and, solving the FTE numerically, we show that the FTE successfully reproduces the kinetic energy distribution of the test-particles. We discuss in detail the analysis of the simulation data and the criteria that allow judging the appropriateness of either an FTE or a classical FP equation as a transport model., Comment: in press at Astrophysical Journal, 2017 (17 pages,22 figures)

We analyze statistically the energization of particles in a large scale environment of strong turbulence that is fragmented into a large number of distributed current filaments. The turbulent environment is generated through strongly perturbed, 3D, resistive MHD simulations, and it emerges naturally from the nonlinear evolution, without a specific reconnection geometry being set up. Based on test-particle simulations, we estimate the transport coefficients in energy space for use in the classical Fokker-Planck (FP) equation, and we show that the latter fails to reproduce the simulation results. The reason is that transport in energy space is highly anomalous (strange), the particles perform Levy flights, and the energy distributions show extended power-law tails. Newly then, we motivate the use and derive the specific form of a fractional transport equation (FTE), we determine its parameters and the order of the fractional derivatives from the simulation data, and we show that the FTE is able to reproduce the high energy part of the simulation data very well. The procedure for determining the FTE parameters also makes clear that it is the analysis of the simulation data that allows to make the decision whether a classical FP or a FTE is appropriate., Comment: in press at Physical Review Letters, 2017 (6 pages, 7 figures)

The aim of this study is to analyze the interaction of charged particles (ions and electrons) with randomly formed particle scatterers (e.g.\ large scale local "magnetic fluctuations" or "coherent magnetic irregularities"), using the set up proposed initially by \cite{Fermi49}. These scatterers are formed by the explosive magnetic energy release and propagate with the Alfv\'en speed along the irregular magnetic fields. They are large scale local fluctuations ($\delta B/B \approx 1$), randomly distributed inside the unstable magnetic topology and will here be called {\bf Alfv\'enic Scatterers (AS)}. We constructed a 3D grid on which a small fraction of randomly chosen grid points are acting as AS. In particular, we study how a large number of test particles evolve inside a collection of AS, analyzing the evolution of their energy distribution and their escape time distribution. We use a well established method to estimate the transport coefficients directly from the trajectories of the particles. Using the estimated transport coefficients and solving the Fokker-Planck (FP) equation numerically, we can recover the energy distribution of the particles. We have shown that the Stochastic Fermi Energization (SFE) of mildly relativistic and relativistic plasma can heat and accelerate the tail of the ambient particle distribution as predicted by \citet{Parker58} and \citet{Ramaty79}. The temperature of the hot plasma and the tail of the energetic particles depend on the mean free path ($\lambda_{\rm sc}$) of the particles between the scatterers inside the energization volume., Comment: 30 pages, 25 figures. Accepted for publication in the Astrophysical Journal (2016)

The limitation of the Quasilinear Theory (QLT) to describe the diffusion of electrons and ions in velocity space when interacting with a spectrum of large amplitude electrostatic Langmuir, Upper and Lower hybrid waves, is analyzed. We analytically and numerically estimate the threshold for the amplitude of the waves above which the QLT breaks down, using a test particle code. The evolution of the velocity distribution, the velocity-space diffusion coefficients, the driven current, and the heating of the particles are investigated, for the interaction with small and large amplitude electrostatic waves, i.e. in both regimes, there where QLT is valid and there where it clearly breaks down., Comment: 21 pages, 14 figures, accepted at Physics of Plasmas (2016)

List of contributions from the Cherenkov Telescope Array (CTA) Consortium presented at the 6th International Symposium on High-Energy Gamma-Ray Astronomy (Gamma 2016), July 11-15, 2016, in Heidelberg, Germany., Comment: Index of CTA conference proceedings for the Gamma 2016, Heidelberg, Germany

Turbulent flows in the solar wind, large scale current sheets, multiple current sheets, and shock waves lead to the formation of environments in which a dense network of current sheets is established and sustains "turbulent reconnection". We constructed a 2D grid on which a number of randomly chosen grid points are acting as scatterers (i.e. magnetic clouds or current sheets). Our goal is to examine how test particles respond inside this large scale collection of scatterers. We study the energy gain of individual particles, the evolution of their energy distribution and their escape time distribution. We have developed a new method to estimate the transport coefficients from the dynamics of the interaction of the particles with the scatterers. Replacing the "magnetic clouds" with current sheets, we have proven that the energization processes can be more efficient depending on the strength of the effective electric fields inside the current sheets and their statistical properties. Using the estimated transport coefficients and solving the Fokker-Planck (FP) equation we can recover the energy distribution of the particles only for the sstochastic Fermi process. We have shown that the evolution of the particles inside a turbulent reconnecting volume is not a solution of the FP equation, since the interaction of the particles with the current sheets is "anomalous", in contrast to the case of the second order Fermi process., Comment: 6 pages, 8 figures, in press at Astrophysical Journal Letters (2016)

Coronal heating through the explosive release of magnetic energy remains an open problem in solar physics. Recent hydrodynamical models attempt an investigation by placing swarms of 'nanoflares' at random sites and times in modeled one-dimensional coronal loops. We investigate the problem in three dimensions, using extrapolated coronal magnetic fields of observed solar active regions. We apply a nonlinear force-free field extrapolation above an observed photospheric magnetogram of NOAA active region (AR) 11158. We then determine the locations, energy contents, and volumes of 'unstable' areas, namely areas prone to releasing magnetic energy due to locally accumulated electric current density. Statistical distributions of these volumes and their fractal dimension are inferred, investigating also their dependence on spatial resolution. Further adopting a simple resistivity model, we infer the properties of the fractally distributed electric fields in these volumes. Next, we monitor the evolution of 10^5 particles (electrons and ions) obeying an initial Maxwellian distribution with a temperature of 10 eV, by following their trajectories and energization when subjected to the resulting electric fields. For computational convenience, the length element of the magnetic-field extrapolation is 1 arcsec, much coarser than the particles collisional mean free path in the low corona. The presence of collisions traps the bulk of the plasma around the unstable volumes, or current sheets (UCS), with only a tail of the distribution gaining substantial energy. Assuming that the distance between UCS is similar to the collisional mean free path we find that the low active-region corona is heated to 100-200 eV, corresponding to temperatures exceeding 2 MK, within tens of seconds for electrons and thousands of seconds for ions. Fractally distributed, nanoflare-triggening fragmented UCS ..., Comment: accepted by A&A

In this review many of the well known tools for the analysis of Complex systems are used in order to study the global coupling of the turbulent convection zone with the solar atmosphere where the magnetic energy is dissipated explosively. Several well documented observations are not easy to interpret with the use of Magnetohydrodynamic (MHD) and/or Kinetic numerical codes. Such observations are: (1) The size distribution of the Active Regions (AR) on the solar surface, (2) The fractal and multi fractal characteristics of the observed magnetograms, (3) The Self-Organised characteristics of the explosive magnetic energy release and (4) the very efficient acceleration of particles during the flaring periods in the solar corona. We review briefly the work published the last twenty five years on the above issues and propose solutions by using methods borrowed from the analysis of complex systems. The scenario which emerged is as follows: (a) The fully developed turbulence in the convection zone generates and transports magnetic flux tubes to the solar surface. Using probabilistic percolation models we were able to reproduce the size distribution and the fractal properties of the emerged and randomly moving magnetic flux tubes. (b) Using a Non Linear Force Free (NLFF) magnetic extrapolation numerical code we can explore how the emerged magnetic flux tubes interact nonlinearly and form thin and Unstable Current Sheets (UCS) inside the coronal part of the AR. (c) The fragmentation of the UCS and the redistribution of the magnetic field locally, when the local current exceeds a Critical threshold, is a key process which drives avalanches and forms coherent structures. This local reorganization of the magnetic field, enhances the energy dissipation and influences the global evolution of the complex magnetic topology., Comment: 23 pages, 12 Figures. Submitted to the European Physical Journal (2016)