Author name code: vlahos ADS astronomy entries on 2022-09-14 author:"Vlahos, Loukas" ------------------------------------------------------------------------ Title: Magnetic field spectral evolution in the inner heliosphere Authors: Sioulas, Nikos; Huang, Zesen; Shi, Chen; Velli, Marco; Tenerani, Anna; Vlahos, Loukas; Bowen, Trevor A.; Bale, Stuart D.; Bonnell, J. W.; Harvey, P. R.; Larson, Davin; Pulupa, arc; Livi, Roberto; Woodham, L. D.; Horbury, T. S.; Stevens, Michael L.; Dudok de Wit, T.; MacDowall, R. J.; Malaspina, David M.; Goetz, K.; Huang, Jia; Kasper, Justin; Owen, Christopher J.; Maksimović, Milan; Louarn, P.; Fedorov, A. Bibcode: 2022arXiv220902451S Altcode: The radial evolution of the magnetic field fluctuations spectral index and its dependence on plasma parameters is investigated using a merged Parker Solar Probe ($PSP$) and Solar Orbiter ($SO$) dataset covering heliocentric distances between $0.06 ~ \lesssim R ~\lesssim 1$ au. The spectrum is studied as a function of scale, normalized to the radially dependent ion inertial scale $d_{i}$. In the vicinity of the Sun, the magnetic spectrum inertial range is limited with a power law exponent $\alpha_{B}$ consistent with the Iroshnikov-Kraichman phenomenology of Alfvénic turbulence, $\alpha_{B} = -3/2$, independent of plasma parameters. The inertial range of turbulence grows with distance from the Sun, progressively extending to larger spatial scales, while at the same time steepening towards a Kolomogorov scaling, with a mean value of $\alpha_{B} =-5/3$. Highly alfvénic intervals seem to retain their near-Sun scaling and only show a minor steepening with distance. In contrast, intervals, where turbulence is characterized by large magnetic energy excess and no dominance of outwardly propagating Alfvénic fluctuations, appear to have spectra that steepen significantly with distance from the Sun, resulting in slightly anomalously steep inertial range slopes at $1~au$. Though generically slower solar wind streams exhibit steeper spectra, the correlation can be attributed to the underlying positive correlation between solar wind speed and alfvénicity, i.e. to the relatively rare occurrence of highly Alfvénic slow wind. Title: Magnetic Field Intermittency in the Solar Wind: Parker Solar Probe and SolO Observations Ranging from the Alfvén Region up to 1 AU Authors: Sioulas, Nikos; Huang, Zesen; Velli, Marco; Chhiber, Rohit; Cuesta, Manuel E.; Shi, Chen; Matthaeus, William H.; Bandyopadhyay, Riddhi; Vlahos, Loukas; Bowen, Trevor A.; Qudsi, Ramiz A.; Bale, Stuart D.; Owen, Christopher J.; Louarn, P.; Fedorov, A.; Maksimović, Milan; Stevens, Michael L.; Case, Anthony; Kasper, Justin; Larson, Davin; Pulupa, Marc; Livi, Roberto Bibcode: 2022ApJ...934..143S Altcode: 2022arXiv220600871S Parker Solar Probe (PSP) and SolO data are utilized to investigate magnetic field intermittency in the solar wind (SW). Small-scale intermittency (20-100 d 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., partial variance of increments; PVI) are deemed more appropriate for examining the evolution of the bulk of coherent structures (CSs), PVI ≥ 3. Using PVI, we observe a scale-dependent evolution in the fraction of the data set occupied by CSs, f PVI≥3. Specifically, regardless of the SW speed, a subtle increase is found in f PVI≥3 for ℓ = 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≥6 and higher kurtosis maxima, no statistical differences are observed for f PVI≥3. Highly Alfvénic intervals display lower levels of intermittency. The anisotropy with respect to the angle between the magnetic field and SW flow, ΘVB is investigated. Intermittency is weaker at ΘVB ≍ 0° 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. Title: Statistical Analysis of Intermittency and its Association with Proton Heating in the Near-Sun Environment Authors: Sioulas, Nikos; Velli, Marco; Chhiber, Rohit; Vlahos, Loukas; Matthaeus, William H.; Bandyopadhyay, Riddhi; Cuesta, Manuel E.; Shi, Chen; Bowen, Trevor A.; Qudsi, Ramiz A.; Stevens, Michael L.; Bale, Stuart D. Bibcode: 2022ApJ...927..140S Altcode: 2022arXiv220110067S We use data from the first six encounters of the 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 ≥ 1. We show that, on average, such events constitute ≍19% of the data set, 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 ≥ 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 ≲ PVI ≲ 6 play a 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 . Title: Particle heating and acceleration by reconnecting and nonreconnecting current sheets Authors: Sioulas, Nikos; Isliker, Heinz; Vlahos, Loukas Bibcode: 2022A&A...657A...8S Altcode: 2021arXiv210708314S 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 nonreconnecting current sheets (CS). In particular, we implemented a Monte Carlo simulation to analyze the effects of the fractality and we study how the synergy of energization at reconnecting CSs and at nonreconnecting 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 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 nonreconnecting 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. Title: Statistical analysis of intermittent structures and their implications on heating during the first six PSP encounters. Authors: Sioulas, Nikos; Velli, Marco; Matthaeus, William; Vlahos, Loukas; Qudsi, Ramiz; Chhiber, Rohit; Bandyopadhyay, Riddhi; Bowen, Trevor; Stevens, Michael; Bale, Stuart Bibcode: 2021AGUFMSH35C2098S Altcode: We use high-resolution Parker Solar Probe data from the first six encounters to study the statistical properties of intermittent, coherent structures and investigate the physical connections between magnetic field intermittency and observable consequences such as solar wind dissipation and plasma heating. More specifically, the Partial Variance of Increments (PVI) method is employed to estimate the fraction of coherent structures in our dataset. We find that coherent structures constitute ~2.5 % of the entire dataset, roughly one-tenth of the value reported in the near-earth environment, indicating in-situ formation of intermittent magnetic field structures developed by the non-linear turbulent cascade. We move on to analyze waiting time distributions of identified events by imposing thresholds on the PVI time series. We show that the shape of the waiting time distribution strongly depends on the resolution of the magnetic field time series and the time-lag used to estimate the PVI time series. We proceed to analyze the contribution of coherent structures to the heating of the Solar Wind (SW). We find a positive correlation between proton temperature and PVI, indicating that proton heating is localized in the vicinity and strongly correlated with intermittent structures. More precisely, the strongest discontinuities in the magnetic field are associated with the most abrupt changes in proton temperature . Still, due to the scarcity of such events, their relative contribution to the dissipation of energy in the solar wind is minor. We propose that smaller scale, more frequent, magnetic field variations of PVI events in the range 2 < PVI< 6, determine the global solar wind temperature. Finally, our results indicate that due to the low density of coherent structures in the young solar wind environment, intermittent heating is not as pronounced as in the outer part of the heliosphere. Title: Are Nanoflares Responsible for Coronal Heating? Authors: Vlahos, Loukas; Isliker, Heinz; Sioulas, Nikos Bibcode: 2021arXiv210801722V Altcode: 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? Title: Stochastic Turbulent Acceleration in a Fractal Environment Authors: Sioulas, Nikos; Isliker, Heinz; Vlahos, Loukas Bibcode: 2020ApJ...895L..14S Altcode: 2020arXiv200502668S 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. 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 (∼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 easily pulls 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. Title: Superdiffusive stochastic Fermi acceleration in space and energy Authors: Sioulas, N.; Isliker, H.; Vlahos, L.; Koumtzis, A.; Pisokas, Th Bibcode: 2020MNRAS.491.3860S Altcode: 2019MNRAS.tmp.2837S; 2019arXiv191107973S We analyse 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 (δB/B ≈ 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 <(Δr)2 > inside the stochastic Fermi accelerator is superdiffusive, < (Δ r)^2> ∼ t^{ar}, with ar ∼ 1.2-1.6, for the high-energy electrons with kinetic energy (W) larger than 1 MeV, and it is normal (ar = 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 (λsc). The smaller λsc is, the faster the electrons and ions escape from the acceleration volume. (3) The mean displacement in energy < Δ W> ∼ t^{aW} is strongly enhanced inside the acceleration volume (aW = 1.5-2.5) for the high-energy particles compared to the thermal low-energy particles (aW = 0.4), I.e. high-energy particles undergo an enhanced systematic gain in energy. (4) The mean-square displacement in energy <W2 > is superdiffusive for the high-energy particles and normal for the low-energy, heated particles. Title: Particle Acceleration and Heating in Regions of Magnetic Flux Emergence Authors: Isliker, H.; Archontis, V.; Vlahos, L. Bibcode: 2019ApJ...882...57I Altcode: 2019arXiv190704296I 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 timescales. A fraction of the energized particles escapes from the acceleration volume, with a super-hot component with a temperature close to 150 MK, and with a power-law high-energy tail with an 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. Title: Introduction to the physics of solar eruptions and their space weather impact Authors: Archontis, Vasilis; Vlahos, Loukas Bibcode: 2019RSPTA.37790152A Altcode: 2019arXiv190508361A 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'. Title: Sources of solar energetic particles Authors: Vlahos, Loukas; Anastasiadis, Anastasios; Papaioannou, Athanasios; Kouloumvakos, Athanasios; Isliker, Heinz Bibcode: 2019RSPTA.37780095V Altcode: 2019arXiv190308200V Solar energetic particles 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 coronal mass ejections, CMEs) inside the solar corona. For more than 40 years, the main two-dimensional 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 magnetohydrodynamic simulations that support 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.

This article is part of the theme issue `Solar eruptions and their space weather impact'. Title: Particle acceleration and heating in a turbulent solar corona Authors: Vlahos, Loukas; Isliker, Heinz Bibcode: 2019PPCF...61a4020V Altcode: 2018arXiv180807136V 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. Title: Statistical Analysis of Individual Solar Active Regions Authors: Kromyda, Garyfallia; Vlahos, Loukas Bibcode: 2018IAUS..335....3K Altcode: In the last decades, numerous observational and computational studies have shown that the global flare distribution is a power-law with a slope less than 2. In these studies, active regions are treated as statistically indistinguishable. To test this, we identify and separately analyze the flares produced by ten individual active regions (2006-2016). In five regions, we find a single power-law distribution, with a slope of a < 2. In the other five, we find a broken double power-law distribution, with slopes a1 < 2 and a2 > 2. Title: Diffusive shock acceleration and turbulent reconnection Authors: Garrel, Christian; Vlahos, Loukas; Isliker, Heinz; Pisokas, Theophilos Bibcode: 2018MNRAS.478.2976G Altcode: 2018arXiv180505143G; 2018MNRAS.tmp.1205G Diffusive shock acceleration (DSA) cannot efficiently accelerate particles without the presence of self-consistently generated or pre-existing strong turbulence (δB/B∼ 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 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 analyse 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. Title: Synergy of Stochastic and Systematic Energization of Plasmas during Turbulent Reconnection Authors: Pisokas, Theophilos; Vlahos, Loukas; Isliker, Heinz Bibcode: 2018ApJ...852...64P Altcode: 2017arXiv171203517P The important characteristic of turbulent reconnection is that it combines large-scale magnetic disturbances (δ B/B∼ 1) with randomly distributed unstable current sheets (UCSs). Many well-known nonlinear 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 large-scale environment that combines large-amplitude disturbances propagating with sub-Alfvénic speed with UCSs. The magnetic disturbances interact stochastically (second-order Fermi) with the charged particles and play a crucial role in the heating of the particles, while the UCSs 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 nonthermal particles, the power-law index, and the length of 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 superhot particle population. Title: Particle Acceleration and Fractional Transport in Turbulent Reconnection Authors: Isliker, Heinz; Pisokas, Theophilos; Vlahos, Loukas; Anastasiadis, Anastasios Bibcode: 2017ApJ...849...35I Altcode: 2017arXiv170908269I We consider a large-scale environment of turbulent reconnection that is fragmented into a number of randomly distributed unstable current sheets (UCSs), and we statistically analyze the acceleration of particles within this environment. We address two important cases of acceleration mechanisms when particles interact with the UCS: (a) electric field acceleration and (b) acceleration by 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 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, while the distributions of the energy increments exhibit power-law tails. We then use the fractional transport equation (FTE) derived by Isliker et al., 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 one to judge the appropriateness of either an FTE or a classical FP equation as a transport model. Title: Stochastic Fermi Energization of Coronal Plasma during Explosive Magnetic Energy Release Authors: Pisokas, Theophilos; Vlahos, Loukas; Isliker, Heinz; Tsiolis, Vassilis; Anastasiadis, Anastasios Bibcode: 2017ApJ...835..214P Altcode: 2016arXiv161204246P 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 setup proposed initially by Fermi. These scatterers are formed by the explosive magnetic energy release and propagate with the Alfvén speed along the irregular magnetic fields. They are large-scale local fluctuations (δB/B ≈ 1) randomly distributed inside the unstable magnetic topology and will here be called Alfvénic 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 evolves 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 equation numerically, we can recover the energy distribution of the particles. We have shown that the stochastic Fermi energization of mildly relativistic and relativistic plasma can heat and accelerate the tail of the ambient particle distribution as predicted by Parker & Tidman and Ramaty. The temperature of the hot plasma and the tail of the energetic particles depend on the mean free path (λsc) of the particles between the scatterers inside the energization volume. Title: Limits of applicability of the quasilinear approximation to the electrostatic wave-plasma interaction Authors: Zacharegkas, Georgios; Isliker, Heinz; Vlahos, Loukas Bibcode: 2016PhPl...23k2119Z Altcode: 2016arXiv161103792Z 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, that is, in both regimes, where QLT is valid and where it clearly breaks down. Title: An observationally-driven kinetic approach to coronal heating Authors: Moraitis, K.; Toutountzi, A.; Isliker, H.; Georgoulis, M.; Vlahos, L.; Chintzoglou, G. Bibcode: 2016A&A...596A..56M Altcode: 2016arXiv160307129M; 2016arXiv160307129T
Aims: 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.
Methods: We applied a nonlinear force-free field extrapolation above an observed photospheric magnetogram of NOAA active region (AR) 11 158. We then determined 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 inferred the properties of the fractally distributed electric fields in these volumes. Next, we monitored the evolution of 105 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, or 725 km, much coarser than the particles' collisional mean free path in the low corona (0.1-1 km).
Results: 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.
Conclusions: Fractally distributed, nanoflare-triggening fragmented UCS in the active-region corona can heat electrons and ions with minor enhancements of the local resistivity. This statistical result is independent from the nature of the extrapolation and the spatial resolution of the modeled active-region corona. This finding should be coupled with a complete plasma treatment to determine whether a quasi-steady temperature similar to that of the ambient corona can be maintained, either via a kinetic or via a hybrid, kinetic and fluid, plasma treatment. The finding can also be extended to the quiet solar corona, provided that the currently undetected nanoflares are frequent enough to account for the lower (compared to active regions) energy losses in this case. Title: Contributions of the Cherenkov Telescope Array (CTA) to the 6th International Symposium on High-Energy Gamma-Ray Astronomy (Gamma 2016) Authors: CTA Consortium, The; :; Abchiche, A.; Abeysekara, U.; Abril, Ó.; Acero, F.; Acharya, B. S.; Adams, C.; Agnetta, G.; Aharonian, F.; Akhperjanian, A.; Albert, A.; Alcubierre, M.; Alfaro, J.; Alfaro, R.; Allafort, A. J.; Aloisio, R.; Amans, J. -P.; Amato, E.; Ambrogi, L.; Ambrosi, G.; Ambrosio, M.; Anderson, J.; Anduze, M.; Angüner, E. O.; Antolini, E.; Antonelli, L. A.; Antonucci, M.; Antonuccio, V.; Antoranz, P.; Aramo, C.; Aravantinos, A.; Araya, M.; Arcaro, C.; Arezki, B.; Argan, A.; Armstrong, T.; Arqueros, F.; Arrabito, L.; Arrieta, M.; Asano, K.; Ashley, M.; Aubert, P.; Singh, C. B.; Babic, A.; Backes, M.; Bais, A.; Bajtlik, S.; Balazs, C.; Balbo, M.; Balis, D.; Balkowski, C.; Ballester, O.; Ballet, J.; Balzer, A.; Bamba, A.; Bandiera, R.; Barber, A.; Barbier, C.; Barcelo, M.; Barkov, M.; Barnacka, A.; Barres de Almeida, U.; Barrio, J. A.; Basso, S.; Bastieri, D.; Bauer, C.; Becciani, U.; Becherini, Y.; Becker Tjus, J.; Beckmann, V.; Bednarek, W.; Benbow, W.; Benedico Ventura, D.; Berdugo, J.; Berge, D.; Bernardini, E.; Bernardini, M. G.; Bernhard, S.; Bernlöhr, K.; Bertucci, B.; Besel, M. -A.; Beshley, V.; Bhatt, N.; Bhattacharjee, P.; Bhattacharyya, W.; Bhattachryya, S.; Biasuzzi, B.; Bicknell, G.; Bigongiari, C.; Biland, A.; Bilinsky, A.; Bilnik, W.; Biondo, B.; Bird, R.; Bird, T.; Bissaldi, E.; Bitossi, M.; Blanch, O.; Blasi, P.; Blazek, J.; Bockermann, C.; Boehm, C.; Bogacz, L.; Bogdan, M.; Bohacova, M.; Boisson, C.; Boix, J.; Bolmont, J.; Bonanno, G.; Bonardi, A.; Bonavolontà, C.; Bonifacio, P.; Bonnarel, F.; Bonnoli, G.; Borkowski, J.; Bose, R.; Bosnjak, Z.; Böttcher, M.; Bousquet, J. -J.; Boutonnet, C.; Bouyjou, F.; Bowman, L.; Braiding, C.; Brantseg, T.; Brau-Nogué, S.; Bregeon, J.; Briggs, M.; Brigida, M.; Bringmann, T.; Brisken, W.; Bristow, D.; Britto, R.; Brocato, E.; Bron, S.; Brook, P.; Brooks, W.; Brown, A. 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P.; Finoguenov, A.; Fioretti, V.; Fiorini, M.; Fleischhack, H.; Flores, H.; Florin, D.; Föhr, C.; Fokitis, E.; Fonseca, M. V.; Font, L.; Fontaine, G.; Fontes, B.; Fornasa, M.; Fornasa, M.; Förster, A.; Fortin, P.; Fortson, L.; Fouque, N.; Franckowiak, A.; Franckowiak, A.; Franco, F. J.; Freire Mota Albuquerque, I.; Freixas Coromina, L.; Fresnillo, L.; Fruck, C.; Fuessling, M.; Fugazza, D.; Fujita, Y.; Fukami, S.; Fukazawa, Y.; Fukuda, T.; Fukui, Y.; Funk, S.; Furniss, A.; Gäbele, W.; Gabici, S.; Gadola, A.; Galindo, D.; Gall, D. D.; Gallant, Y.; Galloway, D.; Gallozzi, S.; Galvez, J. A.; Gao, S.; Garcia, A.; Garcia, B.; García Gil, R.; Garcia López, R.; Garczarczyk, M.; Gardiol, D.; Gargano, C.; Gargano, F.; Garozzo, S.; Garrecht, F.; Garrido, L.; Garrido-Ruiz, M.; Gascon, D.; Gaskins, J.; Gaudemard, J.; Gaug, M.; Gaweda, J.; Gebhardt, B.; Gebyehu, M.; Geffroy, N.; Genolini, B.; Gerard, L.; Ghalumyan, A.; Ghedina, A.; Ghislain, P.; Giammaria, P.; Giannakaki, E.; Gianotti, F.; Giarrusso, S.; Giavitto, G.; Giebels, B.; Gieras, T.; Giglietto, N.; Gika, V.; Gimenes, R.; Giomi, M.; Giommi, P.; Giordano, F.; Giovannini, G.; Girardot, P.; Giro, E.; Giroletti, M.; Gironnet, J.; Giuliani, A.; Glicenstein, J. -F.; Gnatyk, R.; Godinovic, N.; Goldoni, P.; Gomez, G.; Gonzalez, M. M.; González, A.; Gora, D.; Gothe, K. S.; Gotz, D.; Goullon, J.; Grabarczyk, T.; Graciani, R.; Graham, J.; Grandi, P.; Granot, J.; Grasseau, G.; Gredig, R.; Green, A. J.; Green, A. M.; Greenshaw, T.; Grenier, I.; Griffiths, S.; Grillo, A.; Grondin, M. -H.; Grube, J.; Grudzinska, M.; Grygorczuk, J.; Guarino, V.; Guberman, D.; Gunji, S.; Gyuk, G.; Hadasch, D.; Hagedorn, A.; Hagge, L.; Hahn, J.; Hakobyan, H.; Hara, S.; Hardcastle, M. J.; Hassan, T.; Hatanaka, K.; Haubold, T.; Haupt, A.; Hayakawa, T.; Hayashida, M.; Heller, M.; Heller, R.; Helo, J. C.; Henault, F.; Henri, G.; Hermann, G.; Hermel, R.; Herrera Llorente, J.; Herrera Llorente, J.; Herrero, A.; Hervet, O.; Hidaka, N.; Hinton, J.; Hirai, W.; Hirotani, K.; Hnatyk, B.; Hoang, J.; Hoffmann, D.; Hofmann, W.; Holch, T.; Holder, J.; Hooper, S.; Horan, D.; Hörandel, J.; Hörbe, M.; Horns, D.; Horvath, P.; Hose, J.; Houles, J.; Hovatta, T.; Hrabovsky, M.; Hrupec, D.; Huet, J. -M.; Huetten, M.; Hughes, G.; Hui, D.; Humensky, T. B.; Hussein, M.; Iacovacci, M.; Ibarra, A.; Ikeno, Y.; Illa, J. 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F.; Torres, M.; Torresi, E.; Toso, G.; Tosti, G.; Totani, T.; Tothill, N.; Toussenel, F.; Tovmassian, G.; Toyama, T.; Travnicek, P.; Trichard, C.; Trifoglio, M.; Troyano Pujadas, I.; Trzeciak, M.; Tsinganos, K.; Tsujimoto, S.; Tsuru, T.; Uchiyama, Y.; Umana, G.; Umetsu, Y.; Upadhya, S. S.; Uslenghi, M.; Vagelli, V.; Vagnetti, F.; Valdes-Galicia, J.; Valentino, M.; Vallania, P.; Valore, L.; van Driel, W.; van Eldik, C.; van Soelen, B.; Vandenbroucke, J.; Vanderwalt, J.; Vasileiadis, G.; Vassiliev, V.; Vázquez, J. R.; Vázquez Acosta, M. L.; Vecchi, M.; Vega, A.; Vegas, I.; Veitch, P.; Venault, P.; Venema, L.; Venter, C.; Vercellone, S.; Vergani, S.; Verma, K.; Verzi, V.; Vettolani, G. P.; Veyssiere, C.; Viana, A.; Viaux, N.; Vicha, J.; Vigorito, C.; Vincent, P.; Vincent, S.; Vink, J.; Vittorini, V.; Vlahakis, N.; Vlahos, L.; Voelk, H.; Voisin, V.; Vollhardt, A.; Volpicelli, A.; von Brand, H.; Vorobiov, S.; Vovk, I.; Vrastil, M.; Vu, L. V.; Vuillaume, T.; Wagner, R.; Wagner, R.; Wagner, S. J.; Wakely, S. P.; Walstra, T.; Walter, R.; Walther, T.; Ward, J. E.; Ward, M.; Warda, K.; Warren, D.; Wassberg, S.; Watson, J. J.; Wawer, P.; Wawrzaszek, R.; Webb, N.; Wegner, P.; Weiner, O.; Weinstein, A.; Wells, R.; Werner, F.; Wetteskind, H.; White, M.; White, R.; Więcek, M.; Wierzcholska, A.; Wiesand, S.; Wijers, R.; Wilcox, P.; Wild, N.; Wilhelm, A.; Wilkinson, M.; Will, M.; Will, M.; Williams, D. A.; Williams, J. T.; Willingale, R.; Wilson, N.; Winde, M.; Winiarski, K.; Winkler, H.; Winter, M.; Wischnewski, R.; Witt, E.; Wojcik, P.; Wolf, D.; Wood, M.; Wörnlein, A.; Wu, E.; Wu, T.; Yadav, K. K.; Yamamoto, H.; Yamamoto, T.; Yamane, N.; Yamazaki, R.; Yanagita, S.; Yang, L.; Yelos, D.; Yoshida, A.; Yoshida, M.; Yoshida, T.; Yoshiike, S.; Yoshikoshi, T.; Yu, P.; Zabalza, V.; Zaborov, D.; Zacharias, M.; Zaharijas, G.; Zajczyk, A.; Zampieri, L.; Zandanel, F.; Zanmar Sanchez, R.; Zaric, D.; Zavrtanik, D.; Zavrtanik, M.; Zdziarski, A.; Zech, A.; Zechlin, H.; Zhao, A.; Zhdanov, V.; Ziegler, A.; Ziemann, J.; Ziętara, K.; Zink, A.; Ziółkowski, J.; Zitelli, V.; Zoli, A.; Zorn, J.; Żychowski, P. Bibcode: 2016arXiv161005151C Altcode: 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. Title: Complexity methods applied to turbulence in plasma astrophysics Authors: Vlahos, L.; Isliker, H. Bibcode: 2016EPJST.225..977V Altcode: 2016arXiv160300394V 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. Using a Cellular Automaton and following the simple rules of Self Organized Criticality (SOC), we were able to reproduce the statistical characteristics of the observed time series of the explosive events, (d) finally, when the AR reaches the turbulently reconnecting state (in the language of the SOC theory this is called SOC state) it is densely populated by UCS which can act as local scatterers (replacing the magnetic clouds in the Fermi scenario) and enhance dramatically the heating and acceleration of charged particles. Title: Particle Acceleration and Heating by Turbulent Reconnection Authors: Vlahos, Loukas; Pisokas, Theophilos; Isliker, Heinz; Tsiolis, Vassilis; Anastasiadis, Anastasios Bibcode: 2016ApJ...827L...3V Altcode: 2016arXiv160405234V 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 stochastic 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. Title: The Major Geoeffective Solar Eruptions of 2012 March 7: Comprehensive Sun-to-Earth Analysis Authors: Patsourakos, S.; Georgoulis, M. K.; Vourlidas, A.; Nindos, A.; Sarris, T.; Anagnostopoulos, G.; Anastasiadis, A.; Chintzoglou, G.; Daglis, I. A.; Gontikakis, C.; Hatzigeorgiu, N.; Iliopoulos, A. C.; Katsavrias, C.; Kouloumvakos, A.; Moraitis, K.; Nieves-Chinchilla, T.; Pavlos, G.; Sarafopoulos, D.; Syntelis, P.; Tsironis, C.; Tziotziou, K.; Vogiatzis, I. I.; Balasis, G.; Georgiou, M.; Karakatsanis, L. P.; Malandraki, O. E.; Papadimitriou, C.; Odstrčil, D.; Pavlos, E. G.; Podlachikova, O.; Sandberg, I.; Turner, D. L.; Xenakis, M. N.; Sarris, E.; Tsinganos, K.; Vlahos, L. Bibcode: 2016ApJ...817...14P Altcode: During the interval 2012 March 7-11 the geospace experienced a barrage of intense space weather phenomena including the second largest geomagnetic storm of solar cycle 24 so far. Significant ultra-low-frequency wave enhancements and relativistic-electron dropouts in the radiation belts, as well as strong energetic-electron injection events in the magnetosphere were observed. These phenomena were ultimately associated with two ultra-fast (>2000 km s-1) coronal mass ejections (CMEs), linked to two X-class flares launched on early 2012 March 7. Given that both powerful events originated from solar active region NOAA 11429 and their onsets were separated by less than an hour, the analysis of the two events and the determination of solar causes and geospace effects are rather challenging. Using satellite data from a flotilla of solar, heliospheric and magnetospheric missions a synergistic Sun-to-Earth study of diverse observational solar, interplanetary and magnetospheric data sets was performed. It was found that only the second CME was Earth-directed. Using a novel method, we estimated its near-Sun magnetic field at 13 R to be in the range [0.01, 0.16] G. Steep radial fall-offs of the near-Sun CME magnetic field are required to match the magnetic fields of the corresponding interplanetary CME (ICME) at 1 AU. Perturbed upstream solar-wind conditions, as resulting from the shock associated with the Earth-directed CME, offer a decent description of its kinematics. The magnetospheric compression caused by the arrival at 1 AU of the shock associated with the ICME was a key factor for radiation-belt dynamics. Title: CTA Contributions to the 34th International Cosmic Ray Conference (ICRC2015) Authors: CTA Consortium, The; :; Abchiche, A.; Abeysekara, U.; Abril, Ó.; Acero, F.; Acharya, B. S.; Actis, M.; Agnetta, G.; Aguilar, J. A.; Aharonian, F.; Akhperjanian, A.; Albert, A.; Alcubierre, M.; Alfaro, R.; Aliu, E.; Allafort, A. J.; Allan, D.; Allekotte, I.; Aloisio, R.; Amans, J. -P.; Amato, E.; Ambrogi, L.; Ambrosi, G.; Ambrosio, M.; Anderson, J.; Anduze, M.; Angüner, E. O.; Antolini, E.; Antonelli, L. A.; Antonucci, M.; Antonuccio, V.; Antoranz, P.; Aramo, C.; Aravantinos, A.; Argan, A.; Armstrong, T.; Arnaldi, H.; Arnold, L.; Arrabito, L.; Arrieta, M.; Arrieta, M.; Asano, K.; Asorey, H. G.; Aune, T.; Singh, C. B.; Babic, A.; Backes, M.; Bais, A.; Bajtlik, S.; Balazs, C.; Balbo, M.; Balis, D.; Balkowski, C.; Ballester, O.; Ballet, J.; Balzer, A.; Bamba, A.; Bandiera, R.; Barber, A.; Barbier, C.; Barceló, M.; Barnacka, A.; Barres de Almeida, U.; Barrio, J. A.; Basso, S.; Bastieri, D.; Bauer, C.; Baushev, A.; Becciani, U.; Becherini, Y.; Becker Tjus, J.; Beckmann, V.; Bednarek, W.; Benbow, W.; Benedico Ventura, D.; Berdugo, J.; Berge, D.; Bernardini, E.; Bernhard, S.; Bernlöhr, K.; Bertucci, B.; Besel, M. -A.; Bhatt, N.; Bhattacharjee, P.; Bhattachryya, S.; Biasuzzi, B.; Bicknell, G.; Bigongiari, C.; Biland, A.; Billotta, S.; Bilnik, W.; Biondo, B.; Bird, T.; Birsin, E.; Bissaldi, E.; Biteau, J.; Bitossi, M.; Blanch Bigas, O.; Blasi, P.; Boehm, C.; Bogacz, L.; Bogdan, M.; Bohacova, M.; Boisson, C.; Boix Gargallo, J.; Bolmont, J.; Bonanno, G.; Bonardi, A.; Bonifacio, P.; Bonnoli, G.; Borkowski, J.; Bose, R.; Bosnjak, Z.; Bottani, A.; Böttcher, M.; Bousquet, J. -J.; Boutonnet, C.; Bouyjou, F.; Braiding, C.; Brandt, L.; Brau-Nogué, S.; Bregeon, J.; Bretz, T.; Briggs, M.; Brigida, M.; Bringmann, T.; Brisken, W.; Brocato, E.; Brook, P.; Brown, A. M.; Brun, P.; Brunetti, G.; Brunetti, L.; Bruno, P.; Bryan, M.; Buanes, T.; Bucciantini, N.; Buchholtz, G.; Buckley, J.; Bugaev, V.; Bühler, R.; Bulgarelli, A.; Bulik, T.; Burton, M.; Burtovoi, A.; Busetto, G.; Buson, S.; Buss, J.; Byrum, K.; Cameron, R.; Camprecios, J.; Canelli, F.; Canestrari, R.; Cantu, S.; Capalbi, M.; Capasso, M.; Capobianco, G.; Caraveo, P.; Cardenzana, J.; Carius, S.; Carlile, C.; Carmona, E.; Carosi, A.; Carosi, R.; Carr, J.; Carroll, M.; Carter, J.; Carton, P. -H.; Caruso, R.; Casandjian, J. -M.; Casanova, S.; Cascone, E.; Casiraghi, M.; Castellina, A.; Catalano, O.; Catalanotti, S.; Cavazzani, S.; Cazaux, S.; Cefalà, M.; Cerchiara, P.; Cereda, M.; Cerruti, M.; Chabanne, E.; Chadwick, P.; Champion, C.; Chaty, S.; Chaves, R.; Cheimets, P.; Chen, A.; Chen, X.; Chernyakova, M.; Chiappetti, L.; Chikawa, M.; Chinn, D.; Chitnis, V. R.; Cho, N.; Christov, A.; Chudoba, J.; Cieślar, M.; Cillis, A.; Ciocci, M. A.; Clay, R.; Cohen-Tanugi, J.; Colafrancesco, S.; Colin, P.; Colombo, E.; Colome, J.; Colonges, S.; Compin, M.; Conforti, V.; Connaughton, V.; Connell, S.; Conrad, J.; Contreras, J. L.; Coppi, P.; Corbel, S.; Coridian, J.; Corona, P.; Corti, D.; Cortina, J.; Cossio, L.; Costa, A.; Costantini, H.; Cotter, G.; Courty, B.; Covino, S.; Covone, G.; Crimi, G.; Criswell, S. J.; Crocker, R.; Croston, J.; Cusumano, G.; Da Vela, P.; Dale, Ø.; D'Ammando, F.; Dang, D.; Daniel, M.; Davids, I.; Dawson, B.; Dazzi, F.; de Aguiar Costa, B.; De Angelis, A.; de Araujo Cardoso, R. F.; De Caprio, V.; De Cesare, G.; De Franco, A.; De Frondat, F.; de Gouveia Dal Pino, E. M.; de la Calle, I.; De La Vega, G. A.; de los Reyes Lopez, R.; De Lotto, B.; De Luca, A.; de Mello Neto, J. R. T.; de Naurois, M.; de Oña Wilhelmi, E.; De Palma, F.; de Souza, V.; Decock, G.; Deil, C.; Del Santo, M.; Delagnes, E.; Deleglise, G.; Delgado, C.; della Volpe, D.; Deloye, P.; Depaola, G.; Detournay, M.; Dettlaff, A.; Di Girolamo, T.; Di Giulio, C.; Di Paola, A.; Di Pierro, F.; Di Sciascio, G.; Díaz, C.; Dick, J.; Dickinson, H.; Diebold, S.; Diez, V.; Digel, S.; Dipold, J.; Disset, G.; Distefano, A.; Djannati-Ataï, A.; Doert, M.; Dohmke, M.; Domainko, W.; Dominik, N.; Dominis Prester, D.; Donat, A.; Donnarumma, I.; Dorner, D.; Doro, M.; Dournaux, J. -L.; Doyle, K.; Drake, G.; Dravins, D.; Drury, L.; Dubus, G.; Dumas, D.; Dumm, J.; Durand, D.; D'Urso, D.; Dwarkadas, V.; Dyks, J.; Dyrda, M.; Ebr, J.; Echaniz, J. C.; Edy, E.; Egberts, K.; Egberts, K.; Eger, P.; Einecke, S.; Eisch, J.; Eisenkolb, F.; Eleftheriadis, C.; Elsässer, D.; Emmanoulopoulos, D.; Engelbrecht, C.; Engelhaupt, D.; Ernenwein, J. -P.; Errando, M.; Eschbach, S.; Etchegoyen, A.; Evans, P.; Fairbairn, M.; Falcone, A.; Fantinel, D.; Farakos, K.; Farnier, C.; Farrell, E.; Farrell, S.; Fasola, G.; Fegan, S.; Feinstein, F.; Ferenc, D.; Fernandez, A.; Fernandez-Alonso, M.; Ferreira, O.; Fesquet, M.; Fetfatzis, P.; Fiasson, A.; Filipčič, A.; Filipovic, M.; Fink, D.; Finley, C.; Finley, J. P.; Finoguenov, A.; Fioretti, V.; Fiorini, M.; Firpo Curcoll, R.; Fleischhack, H.; Flores, H.; Florin, D.; Föhr, C.; Fokitis, E.; Font, L.; Fontaine, G.; Fontes, B.; Forest, F.; Fornasa, M.; Förster, A.; Fortin, P.; Fortson, L.; Fouque, N.; Franckowiak, A.; Franco, F. J.; Frankowski, A.; Frega, N.; Freire Mota Albuquerque, I.; Freixas Coromina, L.; Fresnillo, L.; Fruck, C.; Fuessling, M.; Fugazza, D.; Fujita, Y.; Fukami, S.; Fukazawa, Y.; Fukuda, T.; Fukui, Y.; Funk, S.; Gäbele, W.; Gabici, S.; Gadola, A.; Galante, N.; Gall, D. D.; Gallant, Y.; Galloway, D.; Gallozzi, S.; Gao, S.; Garcia, B.; García Gil, R.; Garcia López, R.; Garczarczyk, M.; Gardiol, D.; Gargano, C.; Gargano, F.; Garozzo, S.; Garrecht, F.; Garrido, D.; Garrido, L.; Gascon, D.; Gaskins, J.; Gaudemard, J.; Gaug, M.; Gaweda, J.; Geffroy, N.; Gérard, L.; Ghalumyan, A.; Ghedina, A.; Ghigo, M.; Ghislain, P.; Giannakaki, E.; Gianotti, F.; Giarrusso, S.; Giavitto, G.; Giebels, B.; Giglietto, N.; Gika, V.; Gimenes, R.; Giomi, M.; Giommi, P.; Giordano, F.; Giovannini, G.; Giro, E.; Giroletti, M.; Giuliani, A.; Glicenstein, J. -F.; Godinovic, N.; Goldoni, P.; Gomez Berisso, M.; Gomez Vargas, G. A.; Gonzalez, M. M.; González, A.; González, F.; González Muñoz, A.; Gothe, K. S.; Gotz, D.; Grabarczyk, T.; Graciani, R.; Grandi, P.; Grañena, F.; Granot, J.; Grasseau, G.; Gredig, R.; Green, A. J.; Green, A. M.; Greenshaw, T.; Grenier, I.; Grillo, A.; Grondin, M. -H.; Grube, J.; Grudzinska, M.; Grygorczuk, J.; Guarino, V.; Guberman, D.; Gunji, S.; Gyuk, G.; Hadasch, D.; Hagedorn, A.; Hahn, J.; Hakansson, N.; Hamer Heras, N.; Hanabata, Y.; Hara, S.; Hardcastle, M. J.; Harris, J.; Hassan, T.; Hatanaka, K.; Haubold, T.; Haupt, A.; Hayakawa, T.; Hayashida, M.; Heller, M.; Heller, R.; Henault, F.; Henri, G.; Hermann, G.; Hermel, R.; Herrera Llorente, J.; Herrero, A.; Hervet, O.; Hidaka, N.; Hinton, J.; Hirai, W.; Hirotani, K.; Hoard, D.; Hoffmann, D.; Hofmann, W.; Hofverberg, P.; Holch, T.; Holder, J.; Hooper, S.; Horan, D.; Hörandel, J. R.; Hormigos, S.; Horns, D.; Hose, J.; Houles, J.; Hovatta, T.; Hrabovsky, M.; Hrupec, D.; Huet, J. -M.; Hütten, M.; Humensky, T. B.; Huovelin, J.; Huppert, J. -F.; Iacovacci, M.; Ibarra, A.; Idźkowski, B.; Ikawa, D.; Illa, J. M.; Impiombato, D.; Incorvaia, S.; Inome, Y.; Inoue, S.; Inoue, T.; Inoue, Y.; Iocco, F.; Ioka, K.; Iori, M.; Ishio, K.; Israel, G. L.; Jablonski, C.; Jacholkowska, A.; Jacquemier, J.; Jamrozy, M.; Janecek, P.; Janiak, M.; Jankowsky, F.; Jean, P.; Jeanney, C.; Jegouzo, I.; Jenke, P.; Jimenez, J. J.; Jingo, M.; Jingo, M.; Jocou, L.; Jogler, T.; Johnson, C. A.; Journet, L.; Juffroy, C.; Jung, I.; Kaaret, P. E.; Kagaya, M.; Kakuwa, J.; Kalekin, O.; Kalkuhl, C.; Kankanyan, R.; Karastergiou, A.; Kärcher, K.; Karczewski, M.; Karkar, S.; Karn, P.; Kasperek, J.; Katagiri, H.; Kataoka, J.; Katarzyński, K.; Katz, U.; Kaufmann, S.; Kawanaka, N.; Kawashima, T.; Kazanas, D.; Kelley-Hoskins, N.; Kellner-Leidel, B.; Kendziorra, E.; Kersten, J.; Khélifi, B.; Kieda, D. B.; Kihm, T.; Kisaka, S.; Kissmann, R.; Klepser, S.; Kluźniak, W.; Knapen, J.; Knapp, J.; Knödlseder, J.; Köck, F.; Kocot, J.; Kodakkadan, A.; Kodani, K.; Kohri, K.; Kojima, T.; Kokkotas, K.; Kolitzus, D.; Komin, N.; Kominis, I.; Konno, Y.; Kosack, K.; Koss, G.; Koul, R.; Kowal, G.; Koyama, S.; Kozioł, J.; Kraus, M.; Krause, J.; Krause, M.; Krawzcynski, H.; Krennrich, F.; Kretzschmann, A.; Kruger, P.; Kubo, H.; Kudryavtsev, V.; Kukec Mezek, G.; Kushida, J.; Kuznetsov, A.; La Barbera, A.; La Palombara, N.; La Parola, V.; La Rosa, G.; Laffon, H.; Lagadec, T.; Lahmann, R.; Lalik, K.; Lamanna, G.; Landriu, D.; Landt, H.; Lang, R. G.; Languignon, D.; Lapington, J.; Laporte, P.; Latovski, N.; Law-Green, D.; Le Fèvre, J. -P.; Le Flour, T.; Le Sidaner, P.; Lee, S. -H.; Lee, W. H.; Leffhalm, K.; Leich, H.; Leigui de Oliveira, M. A.; Lelas, D.; Lemière, A.; Lemoine-Goumard, M.; Lenain, J. -P.; Leonard, R.; Leoni, R.; Lessio, L.; Leto, G.; Leveque, A.; Lieunard, B.; Limon, M.; Lindemann, R.; Lindfors, E.; Liolios, A.; Lipniacka, A.; Lockart, H.; Lohse, T.; Loiseau, D.; Łokas, E.; Lombardi, S.; Longo, F.; Longo, G.; Lopatin, A.; Lopez, M.; López-Coto, R.; López-Oramas, A.; Loreggia, D.; Louge, T.; Louis, F.; Lu, C. -C.; Lucarelli, F.; Lucchesi, D.; Lüdecke, H.; Luque-Escamilla, P. L.; Luz, O.; Lyard, E.; Maccarone, M. C.; Maccarone, T. J.; Mach, E.; Madejski, G. M.; Madonna, A.; Mahabir, M.; Maier, G.; Majumdar, P.; Makariev, M.; Malaguti, G.; Malaspina, G.; Mallot, A. K.; Maltezos, S.; Mancilla, A.; Mandat, D.; Maneva, G.; Manigot, P.; Mankushiyil, N.; Mannheim, K.; Maragos, N.; Marano, D.; Marchegiani, P.; Marcomini, J. A.; Marcowith, A.; Mariotti, M.; Marisaldi, M.; Markoff, S.; Marszałek, A.; Martens, C.; Martí, J.; Martin, J. -M.; Martin, P.; Martínez, G.; Martínez, M.; Martínez, O.; Marx, R.; Massimino, P.; Mastichiadis, A.; Mastroianni, S.; Mastropietro, M.; Masuda, S.; Matsumoto, H.; Matsuoka, S.; Mattiazzo, S.; Maurin, G.; Maxted, N.; Maya, J.; Mayer, M.; Mazin, D.; Mazureau, E.; Mazziotta, M. N.; Mc Comb, L.; McCann, A.; McCubbin, N.; McHardy, I.; McKay, R.; McKinney, K.; Meagher, K.; Medina, C.; Mehrez, F.; Melioli, C.; Melkumyan, D.; Melo, D.; Melse, T.; Mereghetti, S.; Mertsch, P.; Meyer, M.; Meyrelles, J. L., jr; Miccichè, A.; Michałowski, J.; Micolon, P.; Mientjes, P.; Mignot, S.; Mihailidis, A.; Mineo, T.; Minuti, M.; Mirabal, N.; Mirabel, F.; Miranda, J. M.; Mirzoyan, R.; Mistò, A.; Mitchell, A.; Mizuno, T.; Moderski, R.; Mognet, I.; Mohammed, M.; Moharana, R.; Molinari, E.; Monmarthe, E.; Monnier, G.; Montaruli, T.; Monte, C.; Monteiro, I.; Moore, P.; Moralejo Olaizola, A.; Morello, C.; Moretti, E.; Mori, K.; Morlino, G.; Morselli, A.; Mottez, F.; Moudden, Y.; Moulin, E.; Mrusek, I.; Mueller, S.; Mukherjee, R.; Munar-Adrover, P.; Mundell, C.; Muraishi, H.; Murase, K.; Muronga, A.; Murphy, A.; Nagataki, S.; Nagayoshi, T.; Nagesh, B. K.; Naito, T.; Nakajima, D.; Nakamori, T.; Nakayama, K.; Naumann, D.; Nayman, P.; Nellen, L.; Nemmen, R.; Neronov, A.; Neustroev, V.; Neyroud, N.; Nguyen, T.; Nicastro, L.; Nicolau-Kukliński, J.; Niederwanger, F.; Niedźwiecki, A.; Niemiec, J.; Nieto, D.; Nievas, M.; Nikolaidis, A.; Nishijima, K.; Nishikawa, K. -I.; Noda, K.; Nogues, L.; Nolan, S.; Northrop, R.; Nosek, D.; Nozka, L.; Nunio, F.; Oakes, L.; O'Brien, P.; Occhipinti, G.; O'Faolain de Bhroithe, A.; Ogino, M.; Ohira, Y.; Ohishi, M.; Ohm, S.; Ohoka, H.; Okumura, A.; Olive, J. -F.; Olszowski, D.; Ong, R. A.; Ono, S.; Orienti, M.; Orito, R.; Orlati, A.; Orlati, A.; Osborne, J.; Ostrowski, M.; Otero, L. A.; Ottaway, D.; Otte, N.; Oya, I.; Ozieblo, A.; Padovani, M.; Pagano, I.; Paiano, S.; Paizis, A.; Palacio, J.; Palatka, M.; Pallotta, J.; Panagiotidis, K.; Panazol, J. -L.; Paneque, D.; Panter, M.; Panzera, M. R.; Paoletti, R.; Paolillo, M.; Papayannis, A.; Papyan, G.; Paravac, A.; Paredes, J. M.; Pareschi, G.; Park, N.; Parsons, D.; Paśko, P.; Pavy, S.; Arribas, M. Paz; Pech, M.; Peck, A.; Pedaletti, G.; Peet, S.; Pelassa, V.; Pelat, D.; Peres, C.; Perez, M. d. C.; Perri, L.; Persic, M.; Petrashyk, A.; Petrucci, P. -O.; Peyaud, B.; Pfeifer, M.; Pfeiffer, G.; Piano, G.; Pichel, A.; Pieloth, D.; Pierbattista, M.; Pierre, E.; Pinto de Pinho, F.; García, C. Pio; Piret, Y.; Pita, S.; Planes, A.; Platino, M.; Platos, Ł.; Platzer, R.; Podkladkin, S.; Pogosyan, L.; Pohl, M.; Poinsignon, P.; Ponz, J. D.; Porcelli, A.; Potter, W.; Poulios, S.; Poutanen, J.; Prandini, E.; Prast, J.; Preece, R.; Profeti, F.; Prokhorov, D.; Prokoph, H.; Prouza, M.; Proyetti, M.; Pruchniewicz, R.; Pueschel, E.; Pühlhofer, G.; Puljak, I.; Punch, M.; Pyzioł, R.; Queiroz, F.; Quel, E. J.; Quinn, J.; Quirrenbach, A.; Racero, E.; Räck, T.; Rafalski, J.; Rafighi, I.; Rainò, S.; Rajda, P. J.; Rameez, M.; Rando, R.; Rannot, R. C.; Rataj, M.; Rateau, S.; Ravel, T.; Ravignani, D.; Razzaque, S.; Reardon, P.; Reimann, O.; Reimer, A.; Reimer, O.; Reitberger, K.; Renaud, M.; Renner, S.; Reposeur, T.; Rettig, R.; Reville, B.; Rhode, W.; Ribeiro, D.; Ribó, M.; Richards, G.; Richer, M. G.; Rico, J.; Ridky, J.; Rieger, F.; Ringegni, P.; Ristori, P. R.; Rivière, A.; Rivoire, S.; Roache, E.; Rodeghiero, G.; Rodriguez, J.; Rodriguez Fernandez, G.; Rodríguez Vázquez, J. J.; Rogers, T.; Rojas, G.; Romano, P.; Romay Rodriguez, M. P.; Romeo, G.; Romero, G. E.; Roncadelli, M.; Rose, J.; Rosen, S.; Rosier Lees, S.; Ross, D.; Rossiter, P.; Rouaix, G.; Rousselle, J.; Rovero, A. C.; Rowell, G.; Roy, F.; Royer, S.; Różańska, A.; Rudak, B.; Rugliancich, A.; Rulten, C.; Rupiński, M.; Russo, F.; Rutkowski, K.; Saavedra, O.; Sabatini, S.; Sacco, B.; Saemann, E. O.; Saggion, A.; Saha, L.; Sahakian, V.; Saito, K.; Saito, T.; Sakaki, N.; Salega, M.; Salek, D.; Salgado, J.; Salini, A.; Sanchez, D.; Sanchez, F.; Sanchez-Conde, M.; Sandaker, H.; Sandoval, A.; Sangiorgi, P.; Sanguillon, M.; Sano, H.; Santander, M.; Santangelo, A.; Santos, E. M.; Santos-Lima, R.; Sanuy, A.; Sapozhnikov, L.; Sarkar, S.; Satalecka, K.; Savalle, R.; Sawada, M.; Sayède, F.; Schafer, J.; Schanne, S.; Schanz, T.; Schioppa, E. J.; Schlenstedt, S.; Schlickeiser, R.; Schmidt, T.; Schmoll, J.; Schneider, M.; Schovanek, P.; Schubert, A.; Schultz, C.; Schultze, J.; Schulz, A.; Schulz, S.; Schure, K.; Schussler, F.; Schwab, T.; Schwanke, U.; Schwarz, J.; Schweizer, T.; Schwemmer, S.; Schwendicke, U.; Schwerdt, C.; Segreto, A.; Seiradakis, J. -H.; Sembroski, G. H.; Semikoz, D.; Serre, N.; Servillat, M.; Seweryn, K.; Shafi, N.; Sharma, M.; Shayduk, M.; Shellard, R. C.; Shibata, T.; Shiningayamwe Pandeni, K.; Shukla, A.; Shum, E.; Sidoli, L.; Sidz, M.; Sieiro, J.; Siejkowski, H.; Silk, J.; Sillanpää, A.; Simone, D.; Singh, B. B.; Sinha, A.; Sironi, G.; Sitarek, J.; Sizun, P.; Slyusar, V.; Smith, A.; Smith, J.; Sobczyńska, D.; Sol, H.; Sottile, G.; Sowiński, M.; Spanier, F.; Spengler, G.; Spiga, D.; Stadler, R.; Stahl, O.; Stamatescu, V.; Stamerra, A.; Stanič, S.; Starling, R.; Stawarz, Ł.; Steenkamp, R.; Stefanik, S.; Stegmann, C.; Steiner, S.; Stella, C.; Stergioulas, N.; Sternberger, R.; Sterzel, M.; Stevenson, B.; Stinzing, F.; Stodulska, M.; Stodulski, M.; Stolarczyk, T.; Straumann, U.; Strazzeri, E.; Stringhetti, L.; Strzys, M.; Stuik, R.; Sulanke, K. -H.; Supanitsky, A. D.; Suric, T.; Sushch, I.; Sutcliffe, P.; Sykes, J.; Szanecki, M.; Szepieniec, T.; Szwarnog, P.; Tacchini, A.; Tachihara, K.; Tagliaferri, G.; Tajima, H.; Takahashi, H.; Takahashi, K.; Takahashi, M.; Takalo, L.; Takami, H.; Talbot, G.; Tammi, J.; Tanaka, M.; Tanaka, S.; Tanaka, T.; Tanaka, Y.; Tanci, C.; Tarantino, E.; Tavani, M.; Tavecchio, F.; Tavernet, J. -P.; Tayabaly, K.; Tejedor, L. A.; Telezhinsky, I.; Temme, F.; Temnikov, P.; Tenzer, C.; Terada, Y.; Terrier, R.; Tescaro, D.; Teshima, M.; Testa, V.; Tezier, D.; Thayer, J.; Thomas, V.; Thornhill, J.; Thuermann, D.; Tibaldo, L.; Tibolla, O.; Tiengo, A.; Tijsseling, G.; Timpanaro, M. C.; Tluczykont, M.; Todero Peixoto, C. J.; Tokanai, F.; Tokarz, M.; Toma, K.; Toma, K.; Tomastik, J.; Tomono, Y.; Tonachini, A.; Tonev, D.; Torii, K.; Tornikoski, M.; Torres, D. F.; Torres, M.; Torresi, E.; Toscano, S.; Toso, G.; Tosti, G.; Totani, T.; Tothill, N.; Toussenel, F.; Tovmassian, G.; Townsley, C.; Toyama, T.; Travnicek, P.; Trifoglio, M.; Troyano Pujadas, I.; Troyano Pujadas, I.; Trzeciak, M.; Tsinganos, K.; Tsubone, Y.; Tsuchiya, Y.; Tsujimoto, S.; Tsuru, T.; Uchiyama, Y.; Umana, G.; Umetsu, Y.; Underwood, C.; Upadhya, S. S.; Uslenghi, M.; Vagnetti, F.; Valdes-Galicia, J.; Vallania, P.; Vallejo, G.; Valore, L.; van Driel, W.; van Eldik, C.; van Soelen, B.; Vandenbroucke, J.; Vanderwalt, J.; Vasileiadis, G.; Vassiliev, V.; Vázquez Acosta, M. L.; Vecchi, M.; Vegas, I.; Veitch, P.; Venema, L.; Venter, C.; Vercellone, S.; Vergani, S.; Verma, K.; Verzi, V.; Vettolani, G. P.; Viana, A.; Vicha, J.; Videla, M.; Vigorito, C.; Vincent, P.; Vincent, S.; Vink, J.; Vittorini, V.; Vlahakis, N.; Vlahos, L.; Voelk, H.; Vogler, P.; Voisin, V.; Vollhardt, A.; Volpicelli, A.; Vorobiov, S.; Vovk, I.; Vu, L. V.; Wagner, R.; Wagner, R. M.; Wagner, R. G.; Wagner, S. J.; Wakely, S. P.; Walter, R.; Walther, T.; Ward, J. E.; Ward, M.; Warda, K.; Warwick, R.; Wassberg, S.; Watson, J.; Wawer, P.; Wawrzaszek, R.; Webb, N.; Wegner, P.; Weinstein, A.; Weitzel, Q.; Wells, R.; Werner, F.; Werner, M.; Wetteskind, H.; White, M.; White, R.; Więcek, M.; Wierzcholska, A.; Wiesand, S.; Wijers, R.; Wild, N.; Wilhelm, A.; Wilkinson, M.; Will, M.; Williams, D. A.; Williams, J. T.; Willingale, R.; Winde, M.; Winiarski, K.; Winkler, H.; Wischnewski, R.; Wojcik, P.; Wolf, D.; Wood, M.; Wörnlein, A.; Wu, E.; Wu, T.; Yadav, K. K.; Yamamoto, H.; Yamamoto, T.; Yamazaki, R.; Yanagita, S.; Yang, L.; Yebras, J. M.; Yelos, D.; Yeung, W.; Yoshida, A.; Yoshida, T.; Yoshiike, S.; Yoshikoshi, T.; Yu, P.; Zabalza, V.; Zabalza, V.; Zacharias, M.; Zaharijas, G.; Zajczyk, A.; Zampieri, L.; Zandanel, F.; Zanin, R.; Zanmar Sanchez, R.; Zavrtanik, D.; Zavrtanik, M.; Zdziarski, A.; Zech, A.; Zechlin, H.; Zhao, A.; Ziegler, A.; Ziemann, J.; Ziętara, K.; Ziółkowski, J.; Zitelli, V.; Zoli, A.; Zurbach, C.; Żychowski, P. Bibcode: 2015arXiv150805894C Altcode: List of contributions from the CTA Consortium presented at the 34th International Cosmic Ray Conference, 30 July - 6 August 2015, The Hague, The Netherlands. Title: Particle acceleration in regions of magnetic flux emergence: a statistical approach using test-particle- and MHD-simulations Authors: Vlahos, Loukas; Archontis, Vasilis; Isliker, Heinz Bibcode: 2014cosp...40E3539V Altcode: We consider 3D nonlinear MHD simulations of an emerging flux tube, from the convection zone into the corona, focusing on the coronal part of the simulations. We first analyze the statistical nature and spatial structure of the electric field, calculating histograms and making use of iso-contour visualizations. Then test-particle simulations are performed for electrons, in order to study heating and acceleration phenomena, as well as to determine HXR emission. This study is done by comparatively exploring quiet, turbulent explosive, and mildly explosive phases of the MHD simulations. Also, the importance of collisional and relativistic effects is assessed, and the role of the integration time is investigated. Particular aim of this project is to verify the quasi- linear assumptions made in standard transport models, and to identify possible transport effects that cannot be captured with the latter. In order to determine the relation of our results to Fermi acceleration and Fokker-Planck modeling, we determine the standard transport coefficients. After all, we find that the electric field of the MHD simulations must be downscaled in order to prevent an un-physically high degree of acceleration, and the value chosen for the scale factor strongly affects the results. In different MHD time-instances we find heating to take place, and acceleration that depends on the level of MHD turbulence. Also, acceleration appears to be a transient phenomenon, there is a kind of saturation effect, and the parallel dynamics clearly dominate the energetics. The HXR spectra are not yet really compatible with observations, we have though to further explore the scaling of the electric field and the integration times used. Title: Particle acceleration in solar active regions being in the state of self-organized criticality. Authors: Vlahos, Loukas Bibcode: 2014cosp...40E3540V Altcode: We review the recent observational results on flare initiation and particle acceleration in solar active regions. Elaborating a statistical approach to describe the spatiotemporally intermittent electric field structures formed inside a flaring solar active region, we investigate the efficiency of such structures in accelerating charged particles (electrons and protons). The large-scale magnetic configuration in the solar atmosphere responds to the strong turbulent flows that convey perturbations across the active region by initiating avalanche-type processes. The resulting unstable structures correspond to small-scale dissipation regions hosting strong electric fields. Previous research on particle acceleration in strongly turbulent plasmas provides a general framework for addressing such a problem. This framework combines various electromagnetic field configurations obtained by magnetohydrodynamical (MHD) or cellular automata (CA) simulations, or by employing a statistical description of the field’s strength and configuration with test particle simulations. We work on data-driven 3D magnetic field extrapolations, based on a self-organized criticality models (SOC). A relativistic test-particle simulation traces each particle’s guiding center within these configurations. Using the simulated particle-energy distributions we test our results against observations, in the framework of the collisional thick target model (CTTM) of solar hard X-ray (HXR) emission and compare our results with the current observations. Title: A statistical study of current-sheet formation above solar active regions based on selforganized criticality Authors: Dimitropoulou, M.; Isliker, H.; Vlahos, L.; Georgoulis, M.; Anastasiadis, A.; Toutountzi, A. Bibcode: 2013hell.conf...16D Altcode: We treat flaring solar active regions as physical systems having reached the self-organized critical state. Their evolving magnetic configurations in the low corona may satisfy an instability criterion, related to the excession of a specific threshold in the curl of the magnetic field. This imposed instability criterion implies an almost zero resistivity everywhere in the solar corona, except in regions where magnetic-field discontinuities and. hence, local currents, reach the critical value. In these areas, current-driven instabilities enhance the resistivity by many orders of magnitude forming structures which efficiently accelerate charged particles. Simulating the formation of such structures (thought of as current sheets) via a refined SOC cellular-automaton model provides interesting information regarding their statistical properties. It is shown that the current density in such unstable regions follows power-law scaling. Furthermore, the size distribution of the produced current sheets is best fitted by power laws, whereas their formation probability is investigated against the photospheric magnetic configuration (e.g. Polarity Inversion Lines, Plage). The average fractal dimension of the produced current sheets is deduced depending on the selected critical threshold. The above-mentioned statistical description of intermittent electric field structures can be used by collisional relativistic test particle simulations, aiming to interpret particle acceleration in flaring active regions and in strongly turbulent media in astrophysical plasmas. The above work is supported by the Hellenic National Space Weather Research Network (HNSWRN) via the THALIS Programme. Title: A Data-Driven, Integrated Flare Model Based on Self-Organized Criticality Authors: Dimitropoulou, M.; Isliker, H.; Vlahos, L.; Georgoulis, M. Bibcode: 2013hell.conf....7D Altcode: We interpret solar flares as events originating in solar active regions having reached the self-organized critical state, by alternatively using two versions of an "integrated flare model" - one static and one dynamic. In both versions the initial conditions are derived from observations aiming to investigate whether well-known scaling laws observed in the distribution functions of characteristic flare parameters are reproduced after the self-organized critical state has been reached. In the static model, we first apply a nonlinear force-free extrapolation that reconstructs the three-dimensional magnetic fields from two-dimensional vector magnetograms. We then locate magnetic discontinuities exceeding a threshold in the Laplacian of the magnetic field. These discontinuities are relaxed in local diffusion events, implemented in the form of cellular-automaton evolution rules. Subsequent loading and relaxation steps lead the system to self-organized criticality, after which the statistical properties of the simulated events are examined. In the dynamic version we deploy an enhanced driving mechanism, which utilizes the observed evolution of active regions, making use of sequential vector magnetograms. We first apply the static cellular automaton model to consecutive solar vector magnetograms until the self-organized critical state is reached. We then evolve the magnetic field inbetween these processed snapshots through spline interpolation, acting as a natural driver in the dynamic model. The identification of magnetically unstable sites as well as their relaxation follow the same rules as in the static model after each interpolation step. Subsequent interpolation/driving and relaxation steps cover all transitions until the end of the sequence. Physical requirements, such as the divergence-free condition for the magnetic field vector, are approximately satisfied in both versions of the model. We obtain robust power laws in the distribution functions of the modelled flaring events with scaling indices in good agreement with observations. We therefore conclude that well-known statistical properties of flares are reproduced after active regions reach self-organized criticality. The significant enhancement in both the static and the dynamic integrated flare models is that they initiate the simulation from observations, thus facilitating energy calculation in physical units. Especially in the dynamic version of the model, the driving of the system is based on observed, evolving vector magnetograms, allowing for the separation between MHD and kinetic timescales through the assignment of distinct MHD timestamps to each interpolation step. Title: Particle Acceleration in a Statistically Modeled Solar Active-Region Corona Authors: Toutounzi, A.; Vlahos, L.; Isliker, H.; Dimitropoulou, M.; Anastasiadis, A.; Georgoulis, M. Bibcode: 2013hell.conf....8T Altcode: Elaborating a statistical approach to describe the spatiotemporally intermittent electric field structures formed inside a flaring solar active region, we investigate the efficiency of such structures in accelerating charged particles (electrons). The large-scale magnetic configuration in the solar atmosphere responds to the strong turbulent flows that convey perturbations across the active region by initiating avalanche-type processes. The resulting unstable structures correspond to small-scale dissipation regions hosting strong electric fields. Previous research on particle acceleration in strongly turbulent plasmas provides a general framework for addressing such a problem. This framework combines various electromagnetic field configurations obtained by magnetohydrodynamical (MHD) or cellular automata (CA) simulations, or by employing a statistical description of the field's strength and configuration with test particle simulations. Our objective is to complement previous work done on the subject. As in previous efforts, a set of three probability distribution functions describes our ad-hoc electromagnetic field configurations. In addition, we work on data-driven 3D magnetic field extrapolations. A collisional relativistic test-particle simulation traces each particle's guiding center within these configurations. We also find that an interplay between different electron populations (thermal/non-thermal, ambient/injected) in our simulations may also address, via a re-acceleration mechanism, the so called `number problem'. Using the simulated particle-energy distributions at different heights of the cylinder we test our results against observations, in the framework of the collisional thick target model (CTTM) of solar hard X-ray (HXR) emission. The above work is supported by the Hellenic National Space Weather Research Network (HNSWRN) via the THALIS Programme. Title: Sun-to-Earth Analysis of a Major Geoeffective Solar Eruption within the Framework of the Authors: Patsourakos, S.; Vlahos, L.; Georgoulis, M.; Tziotziou, K.; Nindos, A.; Podladchikova, O.; Vourlidas, A.; Anastasiadis, A.; Sandberg, I.; Tsinganos, K.; Daglis, I.; Hillaris, A.; Preka-Papadema, P.; Sarris, M.; Sarris, T. Bibcode: 2013hell.conf...10P Altcode: Transient expulsions of gigantic clouds of solar coronal plasma into the interplanetary space in the form of Coronal Mass Ejections (CMEs) and sudden, intense flashes of electromagnetic radiation, solar flares, are well-established drivers of the variable Space Weather. Given the innate, intricate links and connections between the solar drivers and their geomagnetic effects, synergistic efforts assembling all pieces of the puzzle along the Sun-Earth line are required to advance our understanding of the physics of Space Weather. This is precisely the focal point of the Hellenic National Space Weather Research Network (HNSWRN) under the THALIS Programme. Within the HNSWRN framework, we present here the first results from a coordinated multi-instrument case study of a major solar eruption (X5.4 and X1.3 flares associated with two ultra-fast (>2000 km/s) CMEs) which were launched early on 7 March 2012 and triggered an intense geomagnetic storm (min Dst =-147 nT) approximately two days afterwards. Several elements of the associated phenomena, such as the flare and CME, EUV wave, WL shock, proton and electron event, interplanetary type II radio burst, ICME and magnetic cloud and their spatiotemporal relationships and connections are studied all way from Sun to Earth. To this end, we make use of satellite data from a flotilla of solar, heliospheric and magnetospheric missions and monitors (e.g., SDO, STEREO, WIND, ACE, Herschel, Planck and INTEGRAL). We also present our first steps toward formulating a cohesive physical scenario to explain the string of the observables and to assess the various physical mechanisms than enabled and gave rise to the significant geoeffectiveness of the eruption. Title: CTA contributions to the 33rd International Cosmic Ray Conference (ICRC2013) Authors: CTA Consortium, The; :; Abril, O.; Acharya, B. S.; Actis, M.; Agnetta, G.; Aguilar, J. A.; Aharonian, F.; Ajello, M.; Akhperjanian, A.; Alcubierre, M.; Aleksic, J.; Alfaro, R.; Aliu, E.; Allafort, A. J.; Allan, D.; Allekotte, I.; Aloisio, R.; Amato, E.; Ambrosi, G.; Ambrosio, M.; Anderson, J.; Angüner, E. O.; Antonelli, L. A.; Antonuccio, V.; Antonucci, M.; Antoranz, P.; Aravantinos, A.; Argan, A.; Arlen, T.; Aramo, C.; Armstrong, T.; Arnaldi, H.; Arrabito, L.; Asano, K.; Ashton, T.; Asorey, H. G.; Aune, T.; Awane, Y.; Baba, H.; Babic, A.; Baby, N.; Bähr, J.; Bais, A.; Baixeras, C.; Bajtlik, S.; Balbo, M.; Balis, D.; Balkowski, C.; Ballet, J.; Bamba, A.; Bandiera, R.; Barber, A.; Barbier, C.; Barceló, M.; Barnacka, A.; Barnstedt, J.; Barres de Almeida, U.; Barrio, J. A.; Basili, A.; Basso, S.; Bastieri, D.; Bauer, C.; Baushev, A.; Becciani, U.; Becerra, J.; Becerra, J.; Becherini, Y.; Bechtol, K. C.; Becker Tjus, J.; Beckmann, V.; Bednarek, W.; Behera, B.; Belluso, M.; Benbow, W.; Berdugo, J.; Berge, D.; Berger, K.; Bernard, F.; Bernardino, T.; Bernlöhr, K.; Bertucci, B.; Bhat, N.; Bhattacharyya, S.; Biasuzzi, B.; Bigongiari, C.; Biland, A.; Billotta, S.; Bird, T.; Birsin, E.; Bissaldi, E.; Biteau, J.; Bitossi, M.; Blake, S.; Blanch Bigas, O.; Blasi, P.; Bobkov, A.; Boccone, V.; Böttcher, M.; Bogacz, L.; Bogart, J.; Bogdan, M.; Boisson, C.; Boix Gargallo, J.; Bolmont, J.; Bonanno, G.; Bonardi, A.; Bonev, T.; Bonifacio, P.; Bonnoli, G.; Bordas, P.; Borgland, A.; Borkowski, J.; Bose, R.; Botner, O.; Bottani, A.; Bouchet, L.; Bourgeat, M.; Boutonnet, C.; Bouvier, A.; Brau-Nogué, S.; Braun, I.; Bretz, T.; Briggs, M.; Brigida, M.; Bringmann, T.; Britto, R.; Brook, P.; Brun, P.; Brunetti, L.; Bruno, P.; Bucciantini, N.; Buanes, T.; Buckley, J.; Bühler, R.; Bugaev, V.; Bulgarelli, A.; Bulik, T.; Busetto, G.; Buson, S.; Byrum, K.; Cailles, M.; Cameron, R.; Camprecios, J.; Canestrari, R.; Cantu, S.; Capalbi, M.; Caraveo, P.; Carmona, E.; Carosi, A.; Carosi, R.; Carr, J.; Carter, J.; Carton, P. -H.; Caruso, R.; Casanova, S.; Cascone, E.; Casiraghi, M.; Castellina, A.; Catalano, O.; Cavazzani, S.; Cazaux, S.; Cerchiara, P.; Cerruti, M.; Chabanne, E.; Chadwick, P.; Champion, C.; Chaves, R.; Cheimets, P.; Chen, A.; Chiang, J.; Chiappetti, L.; Chikawa, M.; Chitnis, V. R.; Chollet, F.; Christof, A.; Chudoba, J.; Cieślar, M.; Cillis, A.; Cilmo, M.; Codino, A.; Cohen-Tanugi, J.; Colafrancesco, S.; Colin, P.; Colome, J.; Colonges, S.; Compin, M.; Conconi, P.; Conforti, V.; Connaughton, V.; Conrad, J.; Contreras, J. L.; Coppi, P.; Coridian, J.; Corona, P.; Corti, D.; Cortina, J.; Cossio, L.; Costa, A.; Costantini, H.; Cotter, G.; Courty, B.; Couturier, S.; Covino, S.; Crimi, G.; Criswell, S. J.; Croston, J.; Cusumano, G.; Dafonseca, M.; Dale, O.; Daniel, M.; Darling, J.; Davids, I.; Dazzi, F.; de Angelis, A.; De Caprio, V.; De Frondat, F.; de Gouveia Dal Pino, E. M.; de la Calle, I.; De La Vega, G. A.; de los Reyes Lopez, R.; de Lotto, B.; De Luca, A.; de Naurois, M.; de Oliveira, Y.; de Oña Wilhelmi, E.; de Palma, F.; de Souza, V.; Decerprit, G.; Decock, G.; Deil, C.; Delagnes, E.; Deleglise, G.; Delgado, C.; della Volpe, D.; Demange, P.; Depaola, G.; Dettlaff, A.; Di Girolamo, T.; Di Giulio, C.; Di Paola, A.; Di Pierro, F.; di Sciascio, G.; Díaz, C.; Dick, J.; Dickherber, R.; Dickinson, H.; Diez-Blanco, V.; Digel, S.; Dimitrov, D.; Disset, G.; Djannati-Ataï, A.; Doert, M.; Dohmke, M.; Domainko, W.; Dominis Prester, D.; Donat, A.; Dorner, D.; Doro, M.; Dournaux, J. -L.; Drake, G.; Dravins, D.; Drury, L.; Dubois, F.; Dubois, R.; Dubus, G.; Dufour, C.; Dumas, D.; Dumm, J.; Durand, D.; Dwarkadas, V.; Dyks, J.; Dyrda, M.; Ebr, J.; Edy, E.; Egberts, K.; Eger, P.; Einecke, S.; Eleftheriadis, C.; Elles, S.; Emmanoulopoulos, D.; Engelhaupt, D.; Enomoto, R.; Ernenwein, J. -P.; Errando, M.; Etchegoyen, A.; Evans, P. A.; Falcone, A.; Faltenbacher, A.; Fantinel, D.; Farakos, K.; Farnier, C.; Farrell, E.; Fasola, G.; Favill, B. W.; Fede, E.; Federici, S.; Fegan, S.; Feinstein, F.; Ferenc, D.; Ferrando, P.; Fesquet, M.; Fetfatzis, P.; Fiasson, A.; Fillin-Martino, E.; Fink, D.; Finley, C.; Finley, J. P.; Fiorini, M.; Firpo Curcoll, R.; Flandrini, E.; Fleischhack, H.; Flores, H.; Florin, D.; Focke, W.; Föhr, C.; Fokitis, E.; Font, L.; Fontaine, G.; Fornasa, M.; Förster, A.; Fortson, L.; Fouque, N.; Franckowiak, A.; Franco, F. J.; Frankowski, A.; Fransson, C.; Fraser, G. W.; Frei, R.; Fresnillo, L.; Fruck, C.; Fugazza, D.; Fujita, Y.; Fukazawa, Y.; Fukui, Y.; Funk, S.; Gäbele, W.; Gabici, S.; Gabriele, R.; Gadola, A.; Galante, N.; Gall, D.; Gallant, Y.; Gámez-García, J.; Garczarczyk, M.; García, B.; Garcia López, R.; Gardiol, D.; Gargano, F.; Garrido, D.; Garrido, L.; Gascon, D.; Gaug, M.; Gaweda, J.; Gebremedhin, L.; Geffroy, N.; Gerard, L.; Ghedina, A.; Ghigo, M.; Ghislain, P.; Giannakaki, E.; Gianotti, F.; Giarrusso, S.; Giavitto, G.; Giebels, B.; Giglietto, N.; Gika, V.; Giomi, M.; Giommi, P.; Giordano, F.; Girard, N.; Giro, E.; Giuliani, A.; Glanzman, T.; Glicenstein, J. -F.; Godinovic, N.; Golev, V.; Gomez Berisso, M.; Gómez-Ortega, J.; Gonzalez, M. M.; González, A.; González, F.; González Muñoz, A.; Gothe, K. S.; Grabarczyk, T.; Gougerot, M.; Graciani, R.; Grandi, P.; Grañena, F.; Granot, J.; Grasseau, G.; Gredig, R.; Green, A.; Greenshaw, T.; Grégoire, T.; Grillo, A.; Grimm, O.; Grondin, M. -H.; Grube, J.; Grudzinska, M.; Gruev, V.; Grünewald, S.; Grygorczuk, J.; Guarino, V.; Gunji, S.; Gyuk, G.; Hadasch, D.; Hagedorn, A.; Hagiwara, R.; Hahn, J.; Hakansson, N.; Hallgren, A.; Hamer Heras, N.; Hara, S.; Hardcastle, M. J.; Harezlak, D.; Harris, J.; Hassan, T.; Hatanaka, K.; Haubold, T.; Haupt, A.; Hayakawa, T.; Hayashida, M.; Heller, R.; Henault, F.; Henri, G.; Hermann, G.; Hermel, R.; Herrero, A.; Hervet, O.; Hidaka, N.; Hinton, J. A.; Hirotani, K.; Hoffmann, D.; Hofmann, W.; Hofverberg, P.; Holder, J.; Hörandel, J. R.; Horns, D.; Horville, D.; Houles, J.; Hrabovsky, M.; Hrupec, D.; Huan, H.; Huber, B.; Huet, J. -M.; Hughes, G.; Humensky, T. B.; Huovelin, J.; Huppert, J. -F.; Ibarra, A.; Ikawa, D.; Illa, J. 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M.; Madhavan, A.; Mahabir, M.; Maier, G.; Majumdar, P.; Malaguti, G.; Malaspina, G.; Maltezos, S.; Manalaysay, A.; Mancilla, A.; Mandat, D.; Maneva, G.; Mangano, A.; Manigot, P.; Mannheim, K.; Manthos, I.; Maragos, N.; Marcowith, A.; Mariotti, M.; Marisaldi, M.; Markoff, S.; Marszałek, A.; Martens, C.; Martí, J.; Martin, J. -M.; Martin, P.; Martínez, G.; Martínez, F.; Martínez, M.; Massaro, F.; Masserot, A.; Mastichiadis, A.; Mathieu, A.; Matsumoto, H.; Mattana, F.; Mattiazzo, S.; Maurer, A.; Maurin, G.; Maxfield, S.; Maya, J.; Mazin, D.; Mc Comb, L.; McCann, A.; McCubbin, N.; McHardy, I.; McKay, R.; Meagher, K.; Medina, C.; Melioli, C.; Melkumyan, D.; Melo, D.; Mereghetti, S.; Mertsch, P.; Meucci, M.; Meyer, M.; Michałowski, J.; Micolon, P.; Mihailidis, A.; Mineo, T.; Minuti, M.; Mirabal, N.; Mirabel, F.; Miranda, J. M.; Mirzoyan, R.; Mistò, A.; Mizuno, T.; Moal, B.; Moderski, R.; Mognet, I.; Molinari, E.; Molinaro, M.; Montaruli, T.; Monte, C.; Monteiro, I.; Moore, P.; Moralejo Olaizola, A.; Mordalska, M.; Morello, C.; Mori, K.; Morlino, G.; Morselli, A.; Mottez, F.; Moudden, Y.; Moulin, E.; Mrusek, I.; Mukherjee, R.; Munar-Adrover, P.; Muraishi, H.; Murase, K.; StJ. Murphy, A.; Nagataki, S.; Naito, T.; Nakajima, D.; Nakamori, T.; Nakayama, K.; Naumann, C.; Naumann, D.; Naumann-Godo, M.; Nayman, P.; Nedbal, D.; Neise, D.; Nellen, L.; Neronov, A.; Neustroev, V.; Neyroud, N.; Nicastro, L.; Nicolau-Kukliński, J.; Niedźwiecki, A.; Niemiec, J.; Nieto, D.; Nikolaidis, A.; Nishijima, K.; Nishikawa, K. -I.; Noda, K.; Nolan, S.; Northrop, R.; Nosek, D.; Nowak, N.; Nozato, A.; Oakes, L.; O'Brien, P. T.; Ohira, Y.; Ohishi, M.; Ohm, S.; Ohoka, H.; Okuda, T.; Okumura, A.; Olive, J. -F.; Ong, R. A.; Orito, R.; Orr, M.; Osborne, J. P.; Ostrowski, M.; Otero, L. 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M.; Sanuy, A.; Sapozhnikov, L.; Sarkar, S.; Sartore, N.; Sasaki, H.; Satalecka, K.; Sawada, M.; Scalzotto, V.; Scapin, V.; Scarcioffolo, M.; Schafer, J.; Schanz, T.; Schlenstedt, S.; Schlickeiser, R.; Schmidt, T.; Schmoll, J.; Schovanek, P.; Schroedter, M.; Schubert, A.; Schultz, C.; Schultze, J.; Schulz, A.; Schure, K.; Schussler, F.; Schwab, T.; Schwanke, U.; Schwarz, J.; Schwarzburg, S.; Schweizer, T.; Schwemmer, S.; Schwendicke, U.; Schwerdt, C.; Segreto, A.; Seiradakis, J. -H.; Sembroski, G. H.; Servillat, M.; Seweryn, K.; Sharma, M.; Shayduk, M.; Shellard, R. C.; Shi, J.; Shibata, T.; Shibuya, A.; Shore, S.; Shum, E.; Sideras-Haddad, E.; Sidoli, L.; Sidz, M.; Sieiro, J.; Sikora, M.; Silk, J.; Sillanpää, A.; Singh, B. B.; Sironi, G.; Sitarek, J.; Skole, C.; Smareglia, R.; Smith, A.; Smith, D.; Smith, J.; Smith, N.; Sobczyńska, D.; Sol, H.; Sottile, G.; Sowiński, M.; Spanier, F.; Spiga, D.; Spyrou, S.; Stamatescu, V.; Stamerra, A.; Starling, R. L. C.; Stawarz, Ł.; Steenkamp, R.; Stegmann, C.; Steiner, S.; Stella, C.; Stergioulas, N.; Sternberger, R.; Sterzel, M.; Stinzing, F.; Stodulski, M.; Stolarczyk, Th.; Straumann, U.; Strazzeri, E.; Stringhetti, L.; Suarez, A.; Suchenek, M.; Sugawara, R.; Sulanke, K. -H.; Sun, S.; Supanitsky, A. D.; Suric, T.; Sutcliffe, P.; Sykes, J. M.; Szanecki, M.; Szepieniec, T.; Szostek, A.; Tagliaferri, G.; Tajima, H.; Takahashi, H.; Takahashi, K.; Takalo, L.; Takami, H.; Talbot, G.; Tammi, J.; Tanaka, M.; Tanaka, S.; Tasan, J.; Tavani, M.; Tavernet, J. -P.; Tejedor, L. A.; Telezhinsky, I.; Temnikov, P.; Tenzer, C.; Terada, Y.; Terrier, R.; Teshima, M.; Testa, V.; Tezier, D.; Thayer, J.; Thuermann, D.; Tibaldo, L.; Tibaldo, L.; Tibolla, O.; Tiengo, A.; Timpanaro, M. C.; Tluczykont, M.; Todero Peixoto, C. J.; Tokanai, F.; Tokarz, M.; Toma, K.; Tonachini, A.; Torii, K.; Tornikoski, M.; Torres, D. F.; Torres, M.; Toscano, S.; Toso, G.; Tosti, G.; Totani, T.; Toussenel, F.; Tovmassian, G.; Travnicek, P.; Treves, A.; Trifoglio, M.; Troyano, I.; Tsinganos, K.; Ueno, H.; Umana, G.; Umehara, K.; Upadhya, S. S.; Usher, T.; Uslenghi, M.; Vagnetti, F.; Valdes-Galicia, J. F.; Vallania, P.; Vallejo, G.; van Driel, W.; van Eldik, C.; Vandenbrouke, J.; Vanderwalt, J.; Vankov, H.; Vasileiadis, G.; Vassiliev, V.; Veberic, D.; Vegas, I.; Vercellone, S.; Vergani, S.; Verzi, V.; Vettolani, G. P.; Veyssière, C.; Vialle, J. P.; Viana, A.; Videla, M.; Vigorito, C.; Vincent, P.; Vincent, S.; Vink, J.; Vlahakis, N.; Vlahos, L.; Vogler, P.; Voisin, V.; Vollhardt, A.; von Gunten, H. -P.; Vorobiov, S.; Vuerli, C.; Waegebaert, V.; Wagner, R.; Wagner, R. G.; Wagner, S.; Wakely, S. P.; Walter, R.; Walther, T.; Warda, K.; Warwick, R. S.; Wawer, P.; Wawrzaszek, R.; Webb, N.; Wegner, P.; Weinstein, A.; Weitzel, Q.; Welsing, R.; Werner, M.; Wetteskind, H.; White, R. J.; Wierzcholska, A.; Wiesand, S.; Wilhelm, A.; Wilkinson, M. I.; Williams, D. A.; Willingale, R.; Winde, M.; Winiarski, K.; Wischnewski, R.; Wiśniewski, Ł.; Wojcik, P.; Wood, M.; Wörnlein, A.; Xiong, Q.; Yadav, K. K.; Yamamoto, H.; Yamamoto, T.; Yamazaki, R.; Yanagita, S.; Yebras, J. M.; Yelos, D.; Yoshida, A.; Yoshida, T.; Yoshikoshi, T.; Yu, P.; Zabalza, V.; Zacharias, M.; Zajczyk, A.; Zampieri, L.; Zanin, R.; Zdziarski, A.; Zech, A.; Zhao, A.; Zhou, X.; Zietara, K.; Ziolkowski, J.; Ziółkowski, P.; Zitelli, V.; Zurbach, C.; Zychowski, P. Bibcode: 2013arXiv1307.2232C Altcode: Compilation of CTA contributions to the proceedings of the 33rd International Cosmic Ray Conference (ICRC2013), which took place in 2-9 July, 2013, in Rio de Janeiro, Brazil Title: Dynamic data-driven integrated flare model based on self-organized criticality Authors: Dimitropoulou, M.; Isliker, H.; Vlahos, L.; Georgoulis, M. K. Bibcode: 2013A&A...553A..65D Altcode: Context. We interpret solar flares as events originating in active regions that have reached the self-organized critical state. We describe them with a dynamic integrated flare model whose initial conditions and driving mechanism are derived from observations.
Aims: We investigate whether well-known scaling laws observed in the distribution functions of characteristic flare parameters are reproduced after the self-organized critical state has been reached.
Methods: To investigate whether the distribution functions of total energy, peak energy, and event duration follow the expected scaling laws, we first applied the previously reported static cellular automaton model to a time series of seven solar vector magnetograms of the NOAA active region 8210 recorded by the Imaging Vector Magnetograph on May 1 1998 between 18:59 UT and 23:16 UT until the self-organized critical state was reached. We then evolved the magnetic field between these processed snapshots through spline interpolation, mimicking a natural driver in our dynamic model. We identified magnetic discontinuities that exceeded a threshold in the Laplacian of the magnetic field after each interpolation step. These discontinuities were relaxed in local diffusion events, implemented in the form of cellular automaton evolution rules. Subsequent interpolation and relaxation steps covered all transitions until the end of the processed magnetograms' sequence. We additionally advanced each magnetic configuration that has reached the self-organized critical state (SOC configuration) by the static model until 50 more flares were triggered, applied the dynamic model again to the new sequence, and repeated the same process sufficiently often to generate adequate statistics. Physical requirements, such as the divergence-free condition for the magnetic field, were approximately imposed.
Results: We obtain robust power laws in the distribution functions of the modeled flaring events with scaling indices that agree well with observations. Peak and total flare energy obey single power laws with indices -1.65 ± 0.11 and -1.47 ± 0.13, while the flare duration is best fitted with a double power law (-2.15 ± 0.15 and -3.60 ± 0.09 for the flatter and steeper parts, respectively).
Conclusions: We conclude that well-known statistical properties of flares are reproduced after active regions reach the state of self-organized criticality. A significant enhancement of our refined cellular automaton model is that it initiates and further drives the simulation from observed evolving vector magnetograms, thus facilitating energy calculation in physical units, while a separation between MHD and kinetic timescales is possible by assigning distinct MHD timestamps to each interpolation step. Title: Introducing the CTA concept Authors: Acharya, B. S.; Actis, M.; Aghajani, T.; Agnetta, G.; Aguilar, J.; Aharonian, F.; Ajello, M.; Akhperjanian, A.; Alcubierre, M.; Aleksić, J.; Alfaro, R.; Aliu, E.; Allafort, A. J.; Allan, D.; Allekotte, I.; Amato, E.; Anderson, J.; Angüner, E. O.; Antonelli, L. A.; Antoranz, P.; Aravantinos, A.; Arlen, T.; Armstrong, T.; Arnaldi, H.; Arrabito, L.; Asano, K.; Ashton, T.; Asorey, H. G.; Awane, Y.; Baba, H.; Babic, A.; Baby, N.; Bähr, J.; Bais, A.; Baixeras, C.; Bajtlik, S.; Balbo, M.; Balis, D.; Balkowski, C.; Bamba, A.; Bandiera, R.; Barber, A.; Barbier, C.; Barceló, M.; Barnacka, A.; Barnstedt, J.; Barres de Almeida, U.; Barrio, J. A.; Basili, A.; Basso, S.; Bastieri, D.; Bauer, C.; Baushev, A.; Becerra, J.; Becherini, Y.; Bechtol, K. C.; Becker Tjus, J.; Beckmann, V.; Bednarek, W.; Behera, B.; Belluso, M.; Benbow, W.; Berdugo, J.; Berger, K.; Bernard, F.; Bernardino, T.; Bernlöhr, K.; Bhat, N.; Bhattacharyya, S.; Bigongiari, C.; Biland, A.; Billotta, S.; Bird, T.; Birsin, E.; Bissaldi, E.; Biteau, J.; Bitossi, M.; Blake, S.; Blanch Bigas, O.; Blasi, P.; Bobkov, A.; Boccone, V.; Boettcher, M.; Bogacz, L.; Bogart, J.; Bogdan, M.; Boisson, C.; Boix Gargallo, J.; Bolmont, J.; Bonanno, G.; Bonardi, A.; Bonev, T.; Bonifacio, P.; Bonnoli, G.; Bordas, P.; Borgland, A.; Borkowski, J.; Bose, R.; Botner, O.; Bottani, A.; Bouchet, L.; Bourgeat, M.; Boutonnet, C.; Bouvier, A.; Brau-Nogué, S.; Braun, I.; Bretz, T.; Briggs, M.; Bringmann, T.; Brook, P.; Brun, P.; Brunetti, L.; Buanes, T.; Buckley, J.; Buehler, R.; Bugaev, V.; Bulgarelli, A.; Bulik, T.; Busetto, G.; Buson, S.; Byrum, K.; Cailles, M.; Cameron, R.; Camprecios, J.; Canestrari, R.; Cantu, S.; Capalbi, M.; Caraveo, P.; Carmona, E.; Carosi, A.; Carr, J.; Carton, P. -H.; Casanova, S.; Casiraghi, M.; Catalano, O.; Cavazzani, S.; Cazaux, S.; Cerruti, M.; Chabanne, E.; Chadwick, P.; Champion, C.; Chen, A.; Chiang, J.; Chiappetti, L.; Chikawa, M.; Chitnis, V. R.; Chollet, F.; Chudoba, J.; Cieślar, M.; Cillis, A.; Cohen-Tanugi, J.; Colafrancesco, S.; Colin, P.; Colome, J.; Colonges, S.; Compin, M.; Conconi, P.; Conforti, V.; Connaughton, V.; Conrad, J.; Contreras, J. L.; Coppi, P.; Corona, P.; Corti, D.; Cortina, J.; Cossio, L.; Costantini, H.; Cotter, G.; Courty, B.; Couturier, S.; Covino, S.; Crimi, G.; Criswell, S. J.; Croston, J.; Cusumano, G.; Dafonseca, M.; Dale, O.; Daniel, M.; Darling, J.; Davids, I.; Dazzi, F.; De Angelis, A.; De Caprio, V.; De Frondat, F.; de Gouveia Dal Pino, E. M.; de la Calle, I.; De La Vega, G. A.; de los Reyes Lopez, R.; De Lotto, B.; De Luca, A.; de Mello Neto, J. R. T.; de Naurois, M.; de Oliveira, Y.; de Oña Wilhelmi, E.; de Souza, V.; Decerprit, G.; Decock, G.; Deil, C.; Delagnes, E.; Deleglise, G.; Delgado, C.; Della Volpe, D.; Demange, P.; Depaola, G.; Dettlaff, A.; Di Paola, A.; Di Pierro, F.; Díaz, C.; Dick, J.; Dickherber, R.; Dickinson, H.; Diez-Blanco, V.; Digel, S.; Dimitrov, D.; Disset, G.; Djannati-Ataï, A.; Doert, M.; Dohmke, M.; Domainko, W.; Dominis Prester, D.; Donat, A.; Dorner, D.; Doro, M.; Dournaux, J. -L.; Drake, G.; Dravins, D.; Drury, L.; Dubois, F.; Dubois, R.; Dubus, G.; Dufour, C.; Dumas, D.; Dumm, J.; Durand, D.; Dyks, J.; Dyrda, M.; Ebr, J.; Edy, E.; Egberts, K.; Eger, P.; Einecke, S.; Eleftheriadis, C.; Elles, S.; Emmanoulopoulos, D.; Engelhaupt, D.; Enomoto, R.; Ernenwein, J. -P.; Errando, M.; Etchegoyen, A.; Evans, P.; Falcone, A.; Fantinel, D.; Farakos, K.; Farnier, C.; Fasola, G.; Favill, B.; Fede, E.; Federici, S.; Fegan, S.; Feinstein, F.; Ferenc, D.; Ferrando, P.; Fesquet, M.; Fiasson, A.; Fillin-Martino, E.; Fink, D.; Finley, C.; Finley, J. P.; Fiorini, M.; Firpo Curcoll, R.; Flores, H.; Florin, D.; Focke, W.; Föhr, C.; Fokitis, E.; Font, L.; Fontaine, G.; Fornasa, M.; Förster, A.; Fortson, L.; Fouque, N.; Franckowiak, A.; Fransson, C.; Fraser, G.; Frei, R.; Albuquerque, I. F. M.; Fresnillo, L.; Fruck, C.; Fujita, Y.; Fukazawa, Y.; Fukui, Y.; Funk, S.; Gäbele, W.; Gabici, S.; Gabriele, R.; Gadola, A.; Galante, N.; Gall, D.; Gallant, Y.; Gámez-García, J.; García, B.; Garcia López, R.; Gardiol, D.; Garrido, D.; Garrido, L.; Gascon, D.; Gaug, M.; Gaweda, J.; Gebremedhin, L.; Geffroy, N.; Gerard, L.; Ghedina, A.; Ghigo, M.; Giannakaki, E.; Gianotti, F.; Giarrusso, S.; Giavitto, G.; Giebels, B.; Gika, V.; Giommi, P.; Girard, N.; Giro, E.; Giuliani, A.; Glanzman, T.; Glicenstein, J. -F.; Godinovic, N.; Golev, V.; Gomez Berisso, M.; Gómez-Ortega, J.; Gonzalez, M. M.; González, A.; González, F.; González Muñoz, A.; Gothe, K. S.; Gougerot, M.; Graciani, R.; Grandi, P.; Grañena, F.; Granot, J.; Grasseau, G.; Gredig, R.; Green, A.; Greenshaw, T.; Grégoire, T.; Grimm, O.; Grube, J.; Grudzinska, M.; Gruev, V.; Grünewald, S.; Grygorczuk, J.; Guarino, V.; Gunji, S.; Gyuk, G.; Hadasch, D.; Hagiwara, R.; Hahn, J.; Hakansson, N.; Hallgren, A.; Hamer Heras, N.; Hara, S.; Hardcastle, M. J.; Harris, J.; Hassan, T.; Hatanaka, K.; Haubold, T.; Haupt, A.; Hayakawa, T.; Hayashida, M.; Heller, R.; Henault, F.; Henri, G.; Hermann, G.; Hermel, R.; Herrero, A.; Hidaka, N.; Hinton, J.; Hoffmann, D.; Hofmann, W.; Hofverberg, P.; Holder, J.; Horns, D.; Horville, D.; Houles, J.; Hrabovsky, M.; Hrupec, D.; Huan, H.; Huber, B.; Huet, J. -M.; Hughes, G.; Humensky, T. B.; Huovelin, J.; Ibarra, A.; Illa, J. M.; Impiombato, D.; Incorvaia, S.; Inoue, S.; Inoue, Y.; Ioka, K.; Ismailova, E.; Jablonski, C.; Jacholkowska, A.; Jamrozy, M.; Janiak, M.; Jean, P.; Jeanney, C.; Jimenez, J. J.; Jogler, T.; Johnson, T.; Journet, L.; Juffroy, C.; Jung, I.; Kaaret, P.; Kabuki, S.; Kagaya, M.; Kakuwa, J.; Kalkuhl, C.; Kankanyan, R.; Karastergiou, A.; Kärcher, K.; Karczewski, M.; Karkar, S.; Kasperek, J.; Kastana, D.; Katagiri, H.; Kataoka, J.; Katarzyński, K.; Katz, U.; Kawanaka, N.; Kellner-Leidel, B.; Kelly, H.; Kendziorra, E.; Khélifi, B.; Kieda, D. B.; Kifune, T.; Kihm, T.; Kishimoto, T.; Kitamoto, K.; Kluźniak, W.; Knapic, C.; Knapp, J.; Knödlseder, J.; Köck, F.; Kocot, J.; Kodani, K.; Köhne, J. -H.; Kohri, K.; Kokkotas, K.; Kolitzus, D.; Komin, N.; Kominis, I.; Konno, Y.; Köppel, H.; Korohoda, P.; Kosack, K.; Koss, G.; Kossakowski, R.; Kostka, P.; Koul, R.; Kowal, G.; Koyama, S.; Kozioł, J.; Krähenbühl, T.; Krause, J.; Krawzcynski, H.; Krennrich, F.; Krepps, A.; Kretzschmann, A.; Krobot, R.; Krueger, P.; Kubo, H.; Kudryavtsev, V. A.; Kushida, J.; Kuznetsov, A.; La Barbera, A.; La Palombara, N.; La Parola, V.; La Rosa, G.; Lacombe, K.; Lamanna, G.; Lande, J.; Languignon, D.; Lapington, J.; Laporte, P.; Lavalley, C.; Le Flour, T.; Le Padellec, A.; Lee, S. -H.; Lee, W. H.; Leigui de Oliveira, M. A.; Lelas, D.; Lenain, J. -P.; Leopold, D. J.; Lerch, T.; Lessio, L.; Lieunard, B.; Lindfors, E.; Liolios, A.; Lipniacka, A.; Lockart, H.; Lohse, T.; Lombardi, S.; Lopatin, A.; Lopez, M.; López-Coto, R.; López-Oramas, A.; Lorca, A.; Lorenz, E.; Lubinski, P.; Lucarelli, F.; Lüdecke, H.; Ludwin, J.; Luque-Escamilla, P. L.; Lustermann, W.; Luz, O.; Lyard, E.; Maccarone, M. C.; Maccarone, T. J.; Madejski, G. M.; Madhavan, A.; Mahabir, M.; Maier, G.; Majumdar, P.; Malaguti, G.; Maltezos, S.; Manalaysay, A.; Mancilla, A.; Mandat, D.; Maneva, G.; Mangano, A.; Manigot, P.; Mannheim, K.; Manthos, I.; Maragos, N.; Marcowith, A.; Mariotti, M.; Marisaldi, M.; Markoff, S.; Marszałek, A.; Martens, C.; Martí, J.; Martin, J. -M.; Martin, P.; Martínez, G.; Martínez, F.; Martínez, M.; Masserot, A.; Mastichiadis, A.; Mathieu, A.; Matsumoto, H.; Mattana, F.; Mattiazzo, S.; Maurin, G.; Maxfield, S.; Maya, J.; Mazin, D.; Mc Comb, L.; McCubbin, N.; McHardy, I.; McKay, R.; Medina, C.; Melioli, C.; Melkumyan, D.; Mereghetti, S.; Mertsch, P.; Meucci, M.; Michałowski, J.; Micolon, P.; Mihailidis, A.; Mineo, T.; Minuti, M.; Mirabal, N.; Mirabel, F.; Miranda, J. M.; Mirzoyan, R.; Mizuno, T.; Moal, B.; Moderski, R.; Mognet, I.; Molinari, E.; Molinaro, M.; Montaruli, T.; Monteiro, I.; Moore, P.; Moralejo Olaizola, A.; Mordalska, M.; Morello, C.; Mori, K.; Mottez, F.; Moudden, Y.; Moulin, E.; Mrusek, I.; Mukherjee, R.; Munar-Adrover, P.; Muraishi, H.; Murase, K.; Murphy, A.; Nagataki, S.; Naito, T.; Nakajima, D.; Nakamori, T.; Nakayama, K.; Naumann, C.; Naumann, D.; Naumann-Godo, M.; Nayman, P.; Nedbal, D.; Neise, D.; Nellen, L.; Neustroev, V.; Neyroud, N.; Nicastro, L.; Nicolau-Kukliński, J.; Niedźwiecki, A.; Niemiec, J.; Nieto, D.; Nikolaidis, A.; Nishijima, K.; Nolan, S.; Northrop, R.; Nosek, D.; Nowak, N.; Nozato, A.; O'Brien, P.; Ohira, Y.; Ohishi, M.; Ohm, S.; Ohoka, H.; Okuda, T.; Okumura, A.; Olive, J. -F.; Ong, R. A.; Orito, R.; Orr, M.; Osborne, J.; Ostrowski, M.; Otero, L. A.; Otte, N.; Ovcharov, E.; Oya, I.; Ozieblo, A.; Padilla, L.; Paiano, S.; Paillot, D.; Paizis, A.; Palanque, S.; Palatka, M.; Pallota, J.; Panagiotidis, K.; Panazol, J. -L.; Paneque, D.; Panter, M.; Paoletti, R.; Papayannis, A.; Papyan, G.; Paredes, J. M.; Pareschi, G.; Parks, G.; Parraud, J. -M.; Parsons, D.; Paz Arribas, M.; Pech, M.; Pedaletti, G.; Pelassa, V.; Pelat, D.; Perez, M. d. C.; Persic, M.; Petrucci, P. -O.; Peyaud, B.; Pichel, A.; Pita, S.; Pizzolato, F.; Platos, Ł.; Platzer, R.; Pogosyan, L.; Pohl, M.; Pojmanski, G.; Ponz, J. D.; Potter, W.; Poutanen, J.; Prandini, E.; Prast, J.; Preece, R.; Profeti, F.; Prokoph, H.; Prouza, M.; Proyetti, M.; Puerto-Gimenez, I.; Pühlhofer, G.; Puljak, I.; Punch, M.; Pyzioł, R.; Quel, E. J.; Quinn, J.; Quirrenbach, A.; Racero, E.; Rajda, P. J.; Ramon, P.; Rando, R.; Rannot, R. C.; Rataj, M.; Raue, M.; Reardon, P.; Reimann, O.; Reimer, A.; Reimer, O.; Reitberger, K.; Renaud, M.; Renner, S.; Reville, B.; Rhode, W.; Ribó, M.; Ribordy, M.; Richer, M. G.; Rico, J.; Ridky, J.; Rieger, F.; Ringegni, P.; Ripken, J.; Ristori, P. R.; Riviére, A.; Rivoire, S.; Rob, L.; Roeser, U.; Rohlfs, R.; Rojas, G.; Romano, P.; Romaszkan, W.; Romero, G. E.; Rosen, S.; Rosier Lees, S.; Ross, D.; Rouaix, G.; Rousselle, J.; Rousselle, S.; Rovero, A. C.; Roy, F.; Royer, S.; Rudak, B.; Rulten, C.; Rupiński, M.; Russo, F.; Ryde, F.; Sacco, B.; Saemann, E. O.; Saggion, A.; Sahakian, V.; Saito, K.; Saito, T.; Saito, Y.; Sakaki, N.; Sakonaka, R.; Salini, A.; Sanchez, F.; Sanchez-Conde, M.; Sandoval, A.; Sandaker, H.; Sant'Ambrogio, E.; Santangelo, A.; Santos, E. M.; Sanuy, A.; Sapozhnikov, L.; Sarkar, S.; Sartore, N.; Sasaki, H.; Satalecka, K.; Sawada, M.; Scalzotto, V.; Scapin, V.; Scarcioffolo, M.; Schafer, J.; Schanz, T.; Schlenstedt, S.; Schlickeiser, R.; Schmidt, T.; Schmoll, J.; Schovanek, P.; Schroedter, M.; Schultz, C.; Schultze, J.; Schulz, A.; Schure, K.; Schwab, T.; Schwanke, U.; Schwarz, J.; Schwarzburg, S.; Schweizer, T.; Schwemmer, S.; Segreto, A.; Seiradakis, J. -H.; Sembroski, G. H.; Seweryn, K.; Sharma, M.; Shayduk, M.; Shellard, R. C.; Shi, J.; Shibata, T.; Shibuya, A.; Shum, E.; Sidoli, L.; Sidz, M.; Sieiro, J.; Sikora, M.; Silk, J.; Sillanpää, A.; Singh, B. B.; Sitarek, J.; Skole, C.; Smareglia, R.; Smith, A.; Smith, D.; Smith, J.; Smith, N.; Sobczyńska, D.; Sol, H.; Sottile, G.; Sowiński, M.; Spanier, F.; Spiga, D.; Spyrou, S.; Stamatescu, V.; Stamerra, A.; Starling, R.; Stawarz, Ł.; Steenkamp, R.; Stegmann, C.; Steiner, S.; Stergioulas, N.; Sternberger, R.; Sterzel, M.; Stinzing, F.; Stodulski, M.; Straumann, U.; Strazzeri, E.; Stringhetti, L.; Suarez, A.; Suchenek, M.; Sugawara, R.; Sulanke, K. -H.; Sun, S.; Supanitsky, A. D.; Suric, T.; Sutcliffe, P.; Sykes, J.; Szanecki, M.; Szepieniec, T.; Szostek, A.; Tagliaferri, G.; Tajima, H.; Takahashi, H.; Takahashi, K.; Takalo, L.; Takami, H.; Talbot, G.; Tammi, J.; Tanaka, M.; Tanaka, S.; Tasan, J.; Tavani, M.; Tavernet, J. -P.; Tejedor, L. A.; Telezhinsky, I.; Temnikov, P.; Tenzer, C.; Terada, Y.; Terrier, R.; Teshima, M.; Testa, V.; Tezier, D.; Thuermann, D.; Tibaldo, L.; Tibolla, O.; Tiengo, A.; Tluczykont, M.; Todero Peixoto, C. J.; Tokanai, F.; Tokarz, M.; Toma, K.; Torii, K.; Tornikoski, M.; Torres, D. F.; Torres, M.; Tosti, G.; Totani, T.; Toussenel, F.; Tovmassian, G.; Travnicek, P.; Trifoglio, M.; Troyano, I.; Tsinganos, K.; Ueno, H.; Umehara, K.; Upadhya, S. S.; Usher, T.; Uslenghi, M.; Valdes-Galicia, J. F.; Vallania, P.; Vallejo, G.; van Driel, W.; van Eldik, C.; Vandenbrouke, J.; Vanderwalt, J.; Vankov, H.; Vasileiadis, G.; Vassiliev, V.; Veberic, D.; Vegas, I.; Vercellone, S.; Vergani, S.; Veyssiére, C.; Vialle, J. P.; Viana, A.; Videla, M.; Vincent, P.; Vincent, S.; Vink, J.; Vlahakis, N.; Vlahos, L.; Vogler, P.; Vollhardt, A.; von Gunten, H. -P.; Vorobiov, S.; Vuerli, C.; Waegebaert, V.; Wagner, R.; Wagner, R. G.; Wagner, S.; Wakely, S. P.; Walter, R.; Walther, T.; Warda, K.; Warwick, R.; Wawer, P.; Wawrzaszek, R.; Webb, N.; Wegner, P.; Weinstein, A.; Weitzel, Q.; Welsing, R.; Werner, M.; Wetteskind, H.; White, R.; Wierzcholska, A.; Wiesand, S.; Wilkinson, M.; Williams, D. A.; Willingale, R.; Winiarski, K.; Wischnewski, R.; Wiśniewski, Ł.; Wood, M.; Wörnlein, A.; Xiong, Q.; Yadav, K. K.; Yamamoto, H.; Yamamoto, T.; Yamazaki, R.; Yanagita, S.; Yebras, J. M.; Yelos, D.; Yoshida, A.; Yoshida, T.; Yoshikoshi, T.; Zabalza, V.; Zacharias, M.; Zajczyk, A.; Zanin, R.; Zdziarski, A.; Zech, A.; Zhao, A.; Zhou, X.; Ziętara, K.; Ziolkowski, J.; Ziółkowski, P.; Zitelli, V.; Zurbach, C.; Żychowski, P.; CTA Consortium Bibcode: 2013APh....43....3A Altcode: 2013APh....43....3C The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project. Title: Current Fragmentation and Particle Acceleration in Solar Flares Authors: Cargill, P. J.; Vlahos, L.; Baumann, G.; Drake, J. F.; Nordlund, Å. Bibcode: 2013pacp.book..223C Altcode: No abstract at ADS Title: Turbulence, Magnetic Reconnection in Turbulent Fluids and Energetic Particle Acceleration Authors: Lazarian, A.; Vlahos, L.; Kowal, G.; Yan, H.; Beresnyak, A.; de Gouveia Dal Pino, E. M. Bibcode: 2013pacp.book..557L Altcode: No abstract at ADS Title: Current Fragmentation and Particle Acceleration in Solar Flares Authors: Cargill, P. J.; Vlahos, L.; Baumann, G.; Drake, J. F.; Nordlund, Å. Bibcode: 2012SSRv..173..223C Altcode: 2012SSRv..tmp...36C Particle acceleration in solar flares remains an outstanding problem in plasma physics and space science. While the observed particle energies and timescales can perhaps be understood in terms of acceleration at a simple current sheet or turbulence site, the vast number of accelerated particles, and the fraction of flare energy in them, defies any simple explanation. The nature of energy storage and dissipation in the global coronal magnetic field is essential for understanding flare acceleration. Scenarios where the coronal field is stressed by complex photospheric motions lead to the formation of multiple current sheets, rather than the single monolithic current sheet proposed by some. The currents sheets in turn can fragment into multiple, smaller dissipation sites. MHD, kinetic and cellular automata models are used to demonstrate this feature. Particle acceleration in this environment thus involves interaction with many distributed accelerators. A series of examples demonstrate how acceleration works in such an environment. As required, acceleration is fast, and relativistic energies are readily attained. It is also shown that accelerated particles do indeed interact with multiple acceleration sites. Test particle models also demonstrate that a large number of particles can be accelerated, with a significant fraction of the flare energy associated with them. However, in the absence of feedback, and with limited numerical resolution, these results need to be viewed with caution. Particle in cell models can incorporate feedback and in one scenario suggest that acceleration can be limited by the energetic particles reaching the condition for firehose marginal stability. Contemporary issues such as footpoint particle acceleration are also discussed. It is also noted that the idea of a "standard flare model" is ill-conceived when the entire distribution of flare energies is considered. Title: Turbulence, Magnetic Reconnection in Turbulent Fluids and Energetic Particle Acceleration Authors: Lazarian, A.; Vlahos, L.; Kowal, G.; Yan, H.; Beresnyak, A.; de Gouveia Dal Pino, E. M. Bibcode: 2012SSRv..173..557L Altcode: 2012SSRv..tmp...90L; 2012SSRv..tmp...86L; 2012arXiv1211.0008L Turbulence is ubiquitous in astrophysics. It radically changes many astrophysical phenomena, in particular, the propagation and acceleration of cosmic rays. We present the modern understanding of compressible magnetohydrodynamic (MHD) turbulence, in particular its decomposition into Alfvén, slow and fast modes, discuss the density structure of turbulent subsonic and supersonic media, as well as other relevant regimes of astrophysical turbulence. All this information is essential for understanding the energetic particle acceleration that we discuss further in the review. For instance, we show how fast and slow modes accelerate energetic particles through the second order Fermi acceleration, while density fluctuations generate magnetic fields in pre-shock regions enabling the first order Fermi acceleration of high energy cosmic rays. Very importantly, however, the first order Fermi cosmic ray acceleration is also possible in sites of magnetic reconnection. In the presence of turbulence this reconnection gets fast and we present numerical evidence supporting the predictions of the Lazarian and Vishniac (Astrophys. J. 517:700-718, 1999) model of fast reconnection. The efficiency of this process suggests that magnetic reconnection can release substantial amounts of energy in short periods of time. As the particle tracing numerical simulations show that the particles can be efficiently accelerated during the reconnection, we argue that the process of magnetic reconnection may be much more important for particle acceleration than it is currently accepted. In particular, we discuss the acceleration arising from reconnection as a possible origin of the anomalous cosmic rays measured by Voyagers as well as the origin cosmic ray excess in the direction of Heliotail. Title: Design concepts for the Cherenkov Telescope Array CTA: an advanced facility for ground-based high-energy gamma-ray astronomy Authors: Actis, M.; Agnetta, G.; Aharonian, F.; Akhperjanian, A.; Aleksić, J.; Aliu, E.; Allan, D.; Allekotte, I.; Antico, F.; Antonelli, L. A.; Antoranz, P.; Aravantinos, A.; Arlen, T.; Arnaldi, H.; Artmann, S.; Asano, K.; Asorey, H.; Bähr, J.; Bais, A.; Baixeras, C.; Bajtlik, S.; Balis, D.; Bamba, A.; Barbier, C.; Barceló, M.; Barnacka, A.; Barnstedt, J.; Barres de Almeida, U.; Barrio, J. A.; Basso, S.; Bastieri, D.; Bauer, C.; Becerra, J.; Becherini, Y.; Bechtol, K.; Becker, J.; Beckmann, V.; Bednarek, W.; Behera, B.; Beilicke, M.; Belluso, M.; Benallou, M.; Benbow, W.; Berdugo, J.; Berger, K.; Bernardino, T.; Bernlöhr, K.; Biland, A.; Billotta, S.; Bird, T.; Birsin, E.; Bissaldi, E.; Blake, S.; Blanch, O.; Bobkov, A. A.; Bogacz, L.; Bogdan, M.; Boisson, C.; Boix, J.; Bolmont, J.; Bonanno, G.; Bonardi, A.; Bonev, T.; Borkowski, J.; Botner, O.; Bottani, A.; Bourgeat, M.; Boutonnet, C.; Bouvier, A.; Brau-Nogué, S.; Braun, I.; Bretz, T.; Briggs, M. S.; Brun, P.; Brunetti, L.; Buckley, J. H.; Bugaev, V.; Bühler, R.; Bulik, T.; Busetto, G.; Buson, S.; Byrum, K.; Cailles, M.; Cameron, R.; Canestrari, R.; Cantu, S.; Carmona, E.; Carosi, A.; Carr, J.; Carton, P. H.; Casiraghi, M.; Castarede, H.; Catalano, O.; Cavazzani, S.; Cazaux, S.; Cerruti, B.; Cerruti, M.; Chadwick, P. M.; Chiang, J.; Chikawa, M.; Cieślar, M.; Ciesielska, M.; Cillis, A.; Clerc, C.; Colin, P.; Colomé, J.; Compin, M.; Conconi, P.; Connaughton, V.; Conrad, J.; Contreras, J. L.; Coppi, P.; Corlier, M.; Corona, P.; Corpace, O.; Corti, D.; Cortina, J.; Costantini, H.; Cotter, G.; Courty, B.; Couturier, S.; Covino, S.; Croston, J.; Cusumano, G.; Daniel, M. K.; Dazzi, F.; de Angelis, A.; de Cea Del Pozo, E.; de Gouveia Dal Pino, E. M.; de Jager, O.; de La Calle Pérez, I.; de La Vega, G.; de Lotto, B.; de Naurois, M.; de Oña Wilhelmi, E.; de Souza, V.; Decerprit, B.; Deil, C.; Delagnes, E.; Deleglise, G.; Delgado, C.; Dettlaff, T.; di Paolo, A.; di Pierro, F.; Díaz, C.; Dick, J.; Dickinson, H.; Digel, S. W.; Dimitrov, D.; Disset, G.; Djannati-Ataï, A.; Doert, M.; Domainko, W.; Dorner, D.; Doro, M.; Dournaux, J. -L.; Dravins, D.; Drury, L.; Dubois, F.; Dubois, R.; Dubus, G.; Dufour, C.; Durand, D.; Dyks, J.; Dyrda, M.; Edy, E.; Egberts, K.; Eleftheriadis, C.; Elles, S.; Emmanoulopoulos, D.; Enomoto, R.; Ernenwein, J. -P.; Errando, M.; Etchegoyen, A.; Falcone, A. D.; Farakos, K.; Farnier, C.; Federici, S.; Feinstein, F.; Ferenc, D.; Fillin-Martino, E.; Fink, D.; Finley, C.; Finley, J. P.; Firpo, R.; Florin, D.; Föhr, C.; Fokitis, E.; Font, Ll.; Fontaine, G.; Fontana, A.; Förster, A.; Fortson, L.; Fouque, N.; Fransson, C.; Fraser, G. W.; Fresnillo, L.; Fruck, C.; Fujita, Y.; Fukazawa, Y.; Funk, S.; Gäbele, W.; Gabici, S.; Gadola, A.; Galante, N.; Gallant, Y.; García, B.; García López, R. J.; Garrido, D.; Garrido, L.; Gascón, D.; Gasq, C.; Gaug, M.; Gaweda, J.; Geffroy, N.; Ghag, C.; Ghedina, A.; Ghigo, M.; Gianakaki, E.; Giarrusso, S.; Giavitto, G.; Giebels, B.; Giro, E.; Giubilato, P.; Glanzman, T.; Glicenstein, J. -F.; Gochna, M.; Golev, V.; Gómez Berisso, M.; González, A.; González, F.; Grañena, F.; Graciani, R.; Granot, J.; Gredig, R.; Green, A.; Greenshaw, T.; Grimm, O.; Grube, J.; Grudzińska, M.; Grygorczuk, J.; Guarino, V.; Guglielmi, L.; Guilloux, F.; Gunji, S.; Gyuk, G.; Hadasch, D.; Haefner, D.; Hagiwara, R.; Hahn, J.; Hallgren, A.; Hara, S.; Hardcastle, M. J.; Hassan, T.; Haubold, T.; Hauser, M.; Hayashida, M.; Heller, R.; Henri, G.; Hermann, G.; Herrero, A.; Hinton, J. A.; Hoffmann, D.; Hofmann, W.; Hofverberg, P.; Horns, D.; Hrupec, D.; Huan, H.; Huber, B.; Huet, J. -M.; Hughes, G.; Hultquist, K.; Humensky, T. B.; Huppert, J. -F.; Ibarra, A.; Illa, J. M.; Ingjald, J.; Inoue, Y.; Inoue, S.; Ioka, K.; Jablonski, C.; Jacholkowska, A.; Janiak, M.; Jean, P.; Jensen, H.; Jogler, T.; Jung, I.; Kaaret, P.; Kabuki, S.; Kakuwa, J.; Kalkuhl, C.; Kankanyan, R.; Kapala, M.; Karastergiou, A.; Karczewski, M.; Karkar, S.; Karlsson, N.; Kasperek, J.; Katagiri, H.; Katarzyński, K.; Kawanaka, N.; Kȩdziora, B.; Kendziorra, E.; Khélifi, B.; Kieda, D.; Kifune, T.; Kihm, T.; Klepser, S.; Kluźniak, W.; Knapp, J.; Knappy, A. R.; Kneiske, T.; Knödlseder, J.; Köck, F.; Kodani, K.; Kohri, K.; Kokkotas, K.; Komin, N.; Konopelko, A.; Kosack, K.; Kossakowski, R.; Kostka, P.; Kotuła, J.; Kowal, G.; Kozioł, J.; Krähenbühl, T.; Krause, J.; Krawczynski, H.; Krennrich, F.; Kretzschmann, A.; Kubo, H.; Kudryavtsev, V. A.; Kushida, J.; La Barbera, N.; La Parola, V.; La Rosa, G.; López, A.; Lamanna, G.; Laporte, P.; Lavalley, C.; Le Flour, T.; Le Padellec, A.; Lenain, J. -P.; Lessio, L.; Lieunard, B.; Lindfors, E.; Liolios, A.; Lohse, T.; Lombardi, S.; Lopatin, A.; Lorenz, E.; Lubiński, P.; Luz, O.; Lyard, E.; Maccarone, M. C.; Maccarone, T.; Maier, G.; Majumdar, P.; Maltezos, S.; Małkiewicz, P.; Mañá, C.; Manalaysay, A.; Maneva, G.; Mangano, A.; Manigot, P.; Marín, J.; Mariotti, M.; Markoff, S.; Martínez, G.; Martínez, M.; Mastichiadis, A.; Matsumoto, H.; Mattiazzo, S.; Mazin, D.; McComb, T. J. L.; McCubbin, N.; McHardy, I.; Medina, C.; Melkumyan, D.; Mendes, A.; Mertsch, P.; Meucci, M.; Michałowski, J.; Micolon, P.; Mineo, T.; Mirabal, N.; Mirabel, F.; Miranda, J. M.; Mirzoyan, R.; Mizuno, T.; Moal, B.; Moderski, R.; Molinari, E.; Monteiro, I.; Moralejo, A.; Morello, C.; Mori, K.; Motta, G.; Mottez, F.; Moulin, E.; Mukherjee, R.; Munar, P.; Muraishi, H.; Murase, K.; Murphy, A. Stj.; Nagataki, S.; Naito, T.; Nakamori, T.; Nakayama, K.; Naumann, C.; Naumann, D.; Nayman, P.; Nedbal, D.; Niedźwiecki, A.; Niemiec, J.; Nikolaidis, A.; Nishijima, K.; Nolan, S. J.; Nowak, N.; O'Brien, P. T.; Ochoa, I.; Ohira, Y.; Ohishi, M.; Ohka, H.; Okumura, A.; Olivetto, C.; Ong, R. A.; Orito, R.; Orr, M.; Osborne, J. P.; Ostrowski, M.; Otero, L.; Otte, A. N.; Ovcharov, E.; Oya, I.; Oziȩbło, A.; Paiano, S.; Pallota, J.; Panazol, J. L.; Paneque, D.; Panter, M.; Paoletti, R.; Papyan, G.; Paredes, J. M.; Pareschi, G.; Parsons, R. D.; Paz Arribas, M.; Pedaletti, G.; Pepato, A.; Persic, M.; Petrucci, P. O.; Peyaud, B.; Piechocki, W.; Pita, S.; Pivato, G.; Płatos, Ł.; Platzer, R.; Pogosyan, L.; Pohl, M.; Pojmański, G.; Ponz, J. D.; Potter, W.; Prandini, E.; Preece, R.; Prokoph, H.; Pühlhofer, G.; Punch, M.; Quel, E.; Quirrenbach, A.; Rajda, P.; Rando, R.; Rataj, M.; Raue, M.; Reimann, C.; Reimann, O.; Reimer, A.; Reimer, O.; Renaud, M.; Renner, S.; Reymond, J. -M.; Rhode, W.; Ribó, M.; Ribordy, M.; Rico, J.; Rieger, F.; Ringegni, P.; Ripken, J.; Ristori, P.; Rivoire, S.; Rob, L.; Rodriguez, S.; Roeser, U.; Romano, P.; Romero, G. E.; Rosier-Lees, S.; Rovero, A. C.; Roy, F.; Royer, S.; Rudak, B.; Rulten, C. B.; Ruppel, J.; Russo, F.; Ryde, F.; Sacco, B.; Saggion, A.; Sahakian, V.; Saito, K.; Saito, T.; Sakaki, N.; Salazar, E.; Salini, A.; Sánchez, F.; Sánchez Conde, M. Á.; Santangelo, A.; Santos, E. M.; Sanuy, A.; Sapozhnikov, L.; Sarkar, S.; Scalzotto, V.; Scapin, V.; Scarcioffolo, M.; Schanz, T.; Schlenstedt, S.; Schlickeiser, R.; Schmidt, T.; Schmoll, J.; Schroedter, M.; Schultz, C.; Schultze, J.; Schulz, A.; Schwanke, U.; Schwarzburg, S.; Schweizer, T.; Seiradakis, J.; Selmane, S.; Seweryn, K.; Shayduk, M.; Shellard, R. C.; Shibata, T.; Sikora, M.; Silk, J.; Sillanpää, A.; Sitarek, J.; Skole, C.; Smith, N.; Sobczyńska, D.; Sofo Haro, M.; Sol, H.; Spanier, F.; Spiga, D.; Spyrou, S.; Stamatescu, V.; Stamerra, A.; Starling, R. L. C.; Stawarz, Ł.; Steenkamp, R.; Stegmann, C.; Steiner, S.; Stergioulas, N.; Sternberger, R.; Stinzing, F.; Stodulski, M.; Straumann, U.; Suárez, A.; Suchenek, M.; Sugawara, R.; Sulanke, K. H.; Sun, S.; Supanitsky, A. D.; Sutcliffe, P.; Szanecki, M.; Szepieniec, T.; Szostek, A.; Szymkowiak, A.; Tagliaferri, G.; Tajima, H.; Takahashi, H.; Takahashi, K.; Takalo, L.; Takami, H.; Talbot, R. G.; Tam, P. H.; Tanaka, M.; Tanimori, T.; Tavani, M.; Tavernet, J. -P.; Tchernin, C.; Tejedor, L. A.; Telezhinsky, I.; Temnikov, P.; Tenzer, C.; Terada, Y.; Terrier, R.; Teshima, M.; Testa, V.; Tibaldo, L.; Tibolla, O.; Tluczykont, M.; Todero Peixoto, C. J.; Tokanai, F.; Tokarz, M.; Toma, K.; Torres, D. F.; Tosti, G.; Totani, T.; Toussenel, F.; Vallania, P.; Vallejo, G.; van der Walt, J.; van Eldik, C.; Vandenbroucke, J.; Vankov, H.; Vasileiadis, G.; Vassiliev, V. V.; Vegas, I.; Venter, L.; Vercellone, S.; Veyssiere, C.; Vialle, J. P.; Videla, M.; Vincent, P.; Vink, J.; Vlahakis, N.; Vlahos, L.; Vogler, P.; Vollhardt, A.; Volpe, F.; von Gunten, H. P.; Vorobiov, S.; Wagner, S.; Wagner, R. M.; Wagner, B.; Wakely, S. P.; Walter, P.; Walter, R.; Warwick, R.; Wawer, P.; Wawrzaszek, R.; Webb, N.; Wegner, P.; Weinstein, A.; Weitzel, Q.; Welsing, R.; Wetteskind, H.; White, R.; Wierzcholska, A.; Wilkinson, M. I.; Williams, D. A.; Winde, M.; Wischnewski, R.; Wiśniewski, Ł.; Wolczko, A.; Wood, M.; Xiong, Q.; Yamamoto, T.; Yamaoka, K.; Yamazaki, R.; Yanagita, S.; Yoffo, B.; Yonetani, M.; Yoshida, A.; Yoshida, T.; Yoshikoshi, T.; Zabalza, V.; Zagdański, A.; Zajczyk, A.; Zdziarski, A.; Zech, A.; Ziȩtara, K.; Ziółkowski, P.; Zitelli, V.; Zychowski, P. Bibcode: 2011ExA....32..193A Altcode: 2011ExA...tmp..121A; 2010arXiv1008.3703C Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA. Title: Recent Advances in Understanding Particle Acceleration Processes in Solar Flares Authors: Zharkova, V. V.; Arzner, K.; Benz, A. O.; Browning, P.; Dauphin, C.; Emslie, A. G.; Fletcher, L.; Kontar, E. P.; Mann, G.; Onofri, M.; Petrosian, V.; Turkmani, R.; Vilmer, N.; Vlahos, L. Bibcode: 2011SSRv..159..357Z Altcode: 2011SSRv..tmp..156Z; 2011SSRv..tmp..249Z; 2011SSRv..tmp..232Z; 2011arXiv1110.2359Z; 2011SSRv..tmp..278Z We review basic theoretical concepts in particle acceleration, with particular emphasis on processes likely to occur in regions of magnetic reconnection. Several new developments are discussed, including detailed studies of reconnection in three-dimensional magnetic field configurations (e.g., current sheets, collapsing traps, separatrix regions) and stochastic acceleration in a turbulent environment. Fluid, test-particle, and particle-in-cell approaches are used and results compared. While these studies show considerable promise in accounting for the various observational manifestations of solar flares, they are limited by a number of factors, mostly relating to available computational power. Not the least of these issues is the need to explicitly incorporate the electrodynamic feedback of the accelerated particles themselves on the environment in which they are accelerated. A brief prognosis for future advancement is offered. Title: Simulating flaring events in complex active regions driven by observed magnetograms Authors: Dimitropoulou, M.; Isliker, H.; Vlahos, L.; Georgoulis, M. K. Bibcode: 2011A&A...529A.101D Altcode: 2011arXiv1102.2352D Context. We interpret solar flares as events originating in active regions that have reached the self organized critical state, by using a refined cellular automaton model with initial conditions derived from observations.
Aims: We investigate whether the system, with its imposed physical elements, reaches a self organized critical state and whether well-known statistical properties of flares, such as scaling laws observed in the distribution functions of characteristic parameters, are reproduced after this state has been reached.
Methods: To investigate whether the distribution functions of total energy, peak energy and event duration follow the expected scaling laws, we first applied a nonlinear force-free extrapolation that reconstructs the three-dimensional magnetic fields from two-dimensional vector magnetograms. We then locate magnetic discontinuities exceeding a threshold in the Laplacian of the magnetic field. These discontinuities are relaxed in local diffusion events, implemented in the form of cellular automaton evolution rules. Subsequent loading and relaxation steps lead the system to self organized criticality, after which the statistical properties of the simulated events are examined. Physical requirements, such as the divergence-free condition for the magnetic field vector, are approximately imposed on all elements of the model.
Results: Our results show that self organized criticality is indeed reached when applying specific loading and relaxation rules. Power-law indices obtained from the distribution functions of the modeled flaring events are in good agreement with observations. Single power laws (peak and total flare energy) are obtained, as are power laws with exponential cutoff and double power laws (flare duration). The results are also compared with observational X-ray data from the GOES satellite for our active-region sample.
Conclusions: We conclude that well-known statistical properties of flares are reproduced after the system has reached self organized criticality. A significant enhancement of our refined cellular automaton model is that it commences the simulation from observed vector magnetograms, thus facilitating energy calculation in physical units. The model described in this study remains consistent with fundamental physical requirements, and imposes physically meaningful driving and redistribution rules. Title: Simulating Flaring Events via an Intelligent Cellular Automata Mechanism Authors: Dimitropoulou, M.; Vlahos, L.; Isliker, H.; Georgoulis, M. Bibcode: 2010ASPC..424...28D Altcode: We simulate flaring events through a Cellular Automaton (CA) model, in which, for the first time, we use observed vector magnetograms as initial conditions. After non-linear force free extrapolation of the magnetic field from the vector magnetograms, we identify magnetic discontinuities, using two alternative criteria: (1) the average magnetic field gradient, or (2) the normalized magnetic field curl (i.e. the current). Magnetic discontinuities are identified at the grid-sites where the magnetic field gradient or curl exceeds a specified threshold. We then relax the magnetic discontinuities according to the rules of Lu and Hamilton (1991) or Lu et al. (1993), i.e. we redistribute the magnetic field locally so that the discontinuities disappear. In order to simulate the flaring events, we consider several alternative scenarios with regard to: (1) The threshold above which magnetic discontinuities are identified (applying low, high, and height-dependent threshold values); (2) The driving process that occasionally causes new discontinuities (at randomly chosen grid sites, magnetic field increments are added that are perpendicular (or may-be also parallel) to the existing magnetic field). We address the question whether the coronal active region magnetic fields can indeed be considered to be in the state of self-organized criticality (SOC). Title: Local re-acceleration and a modified thick target model of solar flare electrons Authors: Brown, J. C.; Turkmani, R.; Kontar, E. P.; MacKinnon, A. L.; Vlahos, L. Bibcode: 2009A&A...508..993B Altcode: 2009arXiv0909.4243B Context: The collisional thick target model (CTTM) of solar hard X-ray (HXR) bursts has become an almost “standard model” of flare impulsive phase energy transport and radiation. However, it faces various problems in the light of recent data, particularly the high electron beam density and anisotropy it involves.
Aims: We consider how photon yield per electron can be increased, and hence fast electron beam intensity requirements reduced, by local re-acceleration of fast electrons throughout the HXR source itself, after injection.
Methods: We show parametrically that, if net re-acceleration rates due to e.g. waves or local current sheet electric (E) fields are a significant fraction of collisional loss rates, electron lifetimes, and hence the net radiative HXR output per electron can be substantially increased over the CTTM values. In this local re-acceleration thick target model (LRTTM) fast electron number requirements and anisotropy are thus reduced. One specific possible scenario involving such re-acceleration is discussed, viz, a current sheet cascade (CSC) in a randomly stressed magnetic loop.
Results: Combined MHD and test particle simulations show that local E fields in CSCs can efficiently accelerate electrons in the corona and and re-accelerate them after injection into the chromosphere. In this HXR source scenario, rapid synchronisation and variability of impulsive footpoint emissions can still occur since primary electron acceleration is in the high Alfvén speed corona with fast re-acceleration in chromospheric CSCs. It is also consistent with the energy-dependent time-of-flight delays in HXR features.
Conclusions: Including electron re-acceleration in the HXR source allows an LRTTM modification of the CTTM in which beam density and anisotropy are much reduced, and alleviates theoretical problems with the CTTM, while making it more compatible with radio and interplanetary electron numbers. The LRTTM is, however, different in some respects such as spatial distribution of atmospheric heating by fast electrons. Title: The correlation of fractal structures in the photospheric and the coronal magnetic field Authors: Dimitropoulou, M.; Georgoulis, M.; Isliker, H.; Vlahos, L.; Anastasiadis, A.; Strintzi, D.; Moussas, X. Bibcode: 2009A&A...505.1245D Altcode: 2009arXiv0908.3950D Context: This work examines the relation between the fractal properties of the photospheric magnetic patterns and those of the coronal magnetic fields in solar active regions.
Aims: We investigate whether there is any correlation between the fractal dimensions of the photospheric structures and the magnetic discontinuities formed in the corona.
Methods: To investigate the connection between the photospheric and coronal complexity, we used a nonlinear force-free extrapolation method that reconstructs the 3d magnetic fields using 2d observed vector magnetograms as boundary conditions. We then located the magnetic discontinuities, which are considered as spatial proxies of reconnection-related instabilities. These discontinuities form well-defined volumes, called here unstable volumes. We calculated the fractal dimensions of these unstable volumes and compared them to the fractal dimensions of the boundary vector magnetograms.
Results: Our results show no correlation between the fractal dimensions of the observed 2d photospheric structures and the extrapolated unstable volumes in the corona, when nonlinear force-free extrapolation is used. This result is independent of efforts to (1) bring the photospheric magnetic fields closer to a nonlinear force-free equilibrium and (2) omit the lower part of the modeled magnetic field volume that is almost completely filled by unstable volumes. A significant correlation between the fractal dimensions of the photospheric and coronal magnetic features is only observed at the zero level (lower limit) of approximation of a current-free (potential) magnetic field extrapolation.
Conclusions: We conclude that the complicated transition from photospheric non-force-free fields to coronal force-free ones hampers any direct correlation between the fractal dimensions of the 2d photospheric patterns and their 3d counterparts in the corona at the nonlinear force-free limit, which can be considered as a second level of approximation in this study. Correspondingly, in the zero and first levels of approximation, namely, the potential and linear force-free extrapolation, respectively, we reveal a significant correlation between the fractal dimensions of the photospheric and coronal structures, which can be attributed to the lack of electric currents or to their purely field-aligned orientation. Title: The Solar Flare: A Strongly Turbulent Particle Accelerator Authors: Vlahos, L.; Krucker, S.; Cargill, P. Bibcode: 2009LNP...778..157V Altcode: The topics of explosive magnetic energy release on a large scale (a solar flare) and particle acceleration during such an event are rarely discussed together in the same article. Many discussions of magnetohydrodynamic (MHD) mod- eling of solar flares and/or CMEs have appeared (see [143] and references therein) and usually address large-scale destabilization of the coronal mag- netic field. Particle acceleration in solar flares has also been discussed exten- sively [74, 164, 116, 166, 87, 168, 95, 122, 35] with the main emphasis being on the actual mechanisms for acceleration (e.g., shocks, turbulence, DC electric fields) rather than the global magnetic context in which the acceleration takes place. Title: Solar and Stellar Active Regions Authors: Vlahos, Loukas Bibcode: 2009ASSP....8..423V Altcode: 2009chas.book..423V No abstract at ADS Title: Turbulence in Space Plasmas by Peter Cargill, Loukas Vlahos. Lecture Notes in Physics, Vol. 778. Berlin: Springer, 2009. Authors: Cargill, Peter; Vlahos, Loukas Bibcode: 2009LNP...778.....C Altcode: No abstract at ADS Title: Hard X-ray emission from the solar corona Authors: Krucker, S.; Battaglia, M.; Cargill, P. J.; Fletcher, L.; Hudson, H. S.; MacKinnon, A. L.; Masuda, S.; Sui, L.; Tomczak, M.; Veronig, A. L.; Vlahos, L.; White, S. M. Bibcode: 2008A&ARv..16..155K Altcode: 2008A&ARv.tmp....8K This review surveys hard X-ray emissions of non-thermal electrons in the solar corona. These electrons originate in flares and flare-related processes. Hard X-ray emission is the most direct diagnostic of electron presence in the corona, and such observations provide quantitative determinations of the total energy in the non-thermal electrons. The most intense flare emissions are generally observed from the chromosphere at footpoints of magnetic loops. Over the years, however, many observations of hard X-ray and even γ-ray emission directly from the corona have also been reported. These coronal sources are of particular interest as they occur closest to where the electron acceleration is thought to occur. Prior to the actual direct imaging observations, disk occultation was usually required to study coronal sources, resulting in limited physical information. Now RHESSI has given us a systematic view of coronal sources that combines high spatial and spectral resolution with broad energy coverage and high sensitivity. Despite the low density and hence low bremsstrahlung efficiency of the corona, we now detect coronal hard X-ray emissions from sources in all phases of solar flares. Because the physical conditions in such sources may differ substantially from those of the usual “footpoint” emission regions, we take the opportunity to revisit the physics of hard X-radiation and relevant theories of particle acceleration. Title: Imprints of cosmic strings on the cosmological gravitational wave background Authors: Kleidis, K.; Papadopoulos, D. B.; Verdaguer, E.; Vlahos, L. Bibcode: 2008PhRvD..78b4027K Altcode: 2008arXiv0806.2999K The equation which governs the temporal evolution of a gravitational wave (GW) in curved space-time can be treated as the Schrödinger equation for a particle moving in the presence of an effective potential. When GWs propagate in an expanding universe with constant effective potential, there is a critical value (kc) of the comoving wave number which discriminates the metric perturbations into oscillating (k>kc) and nonoscillating (k<kc) modes. As a consequence, if the nonoscillatory modes are outside the horizon they do not freeze out. The effective potential is reduced to a nonvanishing constant in a cosmological model which is driven by a two-component fluid, consisting of radiation (dominant) and cosmic strings (subdominant). It is known that the cosmological evolution gradually results in the scaling of a cosmic-string network and, therefore, after some time (Δτ) the Universe becomes radiation dominated. The evolution of the nonoscillatory GW modes during Δτ (while they were outside the horizon), results in the distortion of the GW power spectrum from what it is anticipated in a pure radiation model, at present-time frequencies in the range 10-16Hz<f≲105Hz. Title: Solar and Stellar Active Regions:Cosmic laboratories for the study of Complexity Authors: Vlahos, Loukas Bibcode: 2008arXiv0803.3158V Altcode: Solar active regions are driven dissipative dynamical systems. The turbulent convection zone forces new magnetic flux tubes to rise above the photosphere and shuffles the magnetic fields which are already above the photosphere. The driven 3D active region responds to the driver with the formation of Thin Current Sheets in all scales and releases impulsively energy, when special thresholds are met, on the form of nano-, micro-, flares and large scale coronal mass ejections. It has been documented that active regions form self similar structures with area Probability Distribution Functions (PDF's) following power laws and with fractal dimensions ranging from $1.2-1.7$. The energy release on the other hand follows a specific energy distribution law $f(E_T)\sim E_T^{-a}$, where $a \sim 1.6-1.8$ and $E_T$ is the total energy released. A possible explanation for the statistical properties of the magnetogrms and the energy release by the active region is that the magnetic field formation follows rules analogous to \textbf{percolating models}, and the 3D magnetic fields above the photosphere reach a \textbf{Self Organized Critical (SOC) state}. The implications of these findings on the acceleration of energetic particles during impulsive phenomena will briefly be outlined. Title: Dynamo Effects in Magnetized Ideal Plasma Cosmologies Authors: Kleidis, Kostas; Kuiroukidis, Apostolos; Papadopoulos, Demetrios; Vlahos, Loukas Bibcode: 2008IJMPA..23.1697K Altcode: 2007arXiv0712.4239K The excitation of cosmological perturbations in an anisotropic cosmological model and in the presence of a homogeneous magnetic field has been studied, using the ideal magnetohydrodynamic (MHD) equations. In this case, the system of partial differential equations which governs the evolution of the magnetized cosmological perturbations can be solved analytically. Our results verify that fast-magnetosonic modes propagating normal to the magnetic field, are excited. But, what is most important, is that, at late times, the magnetic-induction contrast (δB/B) grows, resulting in the enhancement of the ambient magnetic field. This process can be particularly favored by condensations, formed within the plasma fluid due to gravitational instabilities. Title: Gravitomagnetic Instabilities in Anisotropically Expanding Fluids Authors: Kleidis, Kostas; Kuiroukidis, Apostolos; Papadopoulos, Demetrios B.; Vlahos, Loukas Bibcode: 2008IJMPA..23.4467K Altcode: 2008arXiv0806.4362K Gravitational instabilities in a magnetized Friedman-Robertson-Walker (FRW) universe, in which the magnetic field was assumed to be too weak to destroy the isotropy of the model, are known and have been studied in the past. Accordingly, it became evident that the external magnetic field disfavors the perturbations' growth, suppressing the corresponding rate by an amount proportional to its strength. However, the spatial isotropy of the FRW universe is not compatible with the presence of large-scale magnetic fields. Therefore, in this paper we use the general-relativistic version of the (linearized) perturbed magnetohydrodynamic equations with and without resistivity, to discuss a generalized Jeans criterion and the potential formation of density condensations within a class of homogeneous and anisotropically expanding, self-gravitating, magnetized fluids in curved space-time. We find that, for a wide variety of anisotropic cosmological models, gravitomagnetic instabilities can lead to subhorizontal, magnetized condensations. In the nonresistive case, the power spectrum of the unstable cosmological perturbations suggests that most of the power is concentrated on large scales (small k), very close to the horizon. On the other hand, in a resistive medium, the critical wave-numbers so obtained, exhibit a delicate dependence on resistivity, resulting in the reduction of the corresponding Jeans lengths to smaller scales (well bellow the horizon) than the nonresistive ones, while increasing the range of cosmological models which admit such an instability. Title: Magnetohydrodynamics and Plasma Cosmology Authors: Kleidis, Kostas; Kuiroukidis, Apostolos; Papadopoulos, Demetrios; Vlahos, Loukas Bibcode: 2007IJTP...46.2283K Altcode: 2005gr.qc....12131K; 2007IJTP..tmp..132K; 2007IJTP..tmp..208K We study the linear magnetohydrodynamic (MHD) equations, both in the Newtonian and the general-relativistic limit, as regards a viscous magnetized fluid of finite conductivity and discuss instability criteria. In addition, we explore the excitation of cosmological perturbations in anisotropic spacetimes, in the presence of an ambient magnetic field. Acoustic, electromagnetic (e/m) and fast-magnetosonic modes, propagating normal to the magnetic field, can be excited, resulting in several implications of cosmological significance. Title: Summary of Joint Discussion 1 Authors: Vlahos, Loukas Bibcode: 2007HiA....14..104V Altcode: We review the main ideas discussed during the meeting and propose methods for a new generation of space accelerators Title: Excitation of MHD waves in magnetized anisotropic cosmologies Authors: Kuiroukidis, A.; Kleidis, K.; Papadopoulos, D. B.; Vlahos, L. Bibcode: 2007A&A...471..409K Altcode: 2007arXiv0705.2194K The excitation of cosmological perturbations in an anisotropic cosmological model and in the presence of a homogeneous magnetic field was studied, using the resistive magnetohydrodynamic (MHD) equations. We have shown that fast-magnetosonic modes, propagating normal to the magnetic field, grow exponentially and saturate at high values, due to the resistivity. We also demonstrate that Jeans-like instabilities can be enhanced inside a resistive fluid and that the formation of condensations influence the growing magnetosonic waves. Title: High energy particle transport in stochastic magnetic fields in the solar corona Authors: Gkioulidou, M.; Zimbardo, G.; Pommois, P.; Veltri, P.; Vlahos, L. Bibcode: 2007A&A...462.1113G Altcode: Aims:We study energetic particle transport in the solar corona in the presence of magnetic fluctuations by analyzing the motion of protons injected at the center of a model coronal loop.
Methods: We set up a numerical realization of magnetic turbulence, in which the magnetic fluctuations are represented by a Fourier expansion with random phases. We perform test particle simulations by varying the turbulence correlation length λ, the turbulence level, and the proton energy. Coulomb collisions are neglected.
Results: For large λ, the ratio ρ/λ (with ρ the Larmor radius) is small, and the magnetic moment is conserved. In this case, a fraction of the injected protons, which grow with the fluctuation level, are trapped at the top of the magnetic loop, near the injection region, by magnetic mirroring due to the magnetic fluctuations. The rest of the protons propagate freely along B, corresponding to nearly ballistic transport. Decreasing λ, that is, increasing the ratio ρ/λ, the magnetic moment is no longer well conserved, and pitch angle diffusion progressively sets in. Pitch angle diffusion leads to a decrease in the trapped population and progressively changes proton transport from ballistic to superdiffusive, and finally, for small λ, to diffusive.
Conclusions: .Particle mirroring by magnetic turbulence makes for compact trapping regions. The particle dynamics inside the magnetic loop is non-Gaussian and the statistical description of transport properties requires the use of such ideas as the Lévy random walk. Title: Magnetic Complexity, Fragmentation, Particle Acceleration and Radio Emission from the Sun Authors: Vlahos, Loukas Bibcode: 2007LNP...725...15V Altcode: The most popular flare model used to explain the energy release, particle acceleration and radio emission is based on the following assumptions: (1) The formation of a current sheet above a magnetic loop, (2) The stochastic acceleration of particles in the current sheet at the helmet of the loop, (3) the transport and trapping of particles inside the flaring loop. We review the observational consequences of the above model and try to generalize by putting forward a new suggestion, namely assuming that a complex active region driven by the photospheric motions forms naturally a large number of stochastic current sheets that accelerate particles, which in turn can be trapped or move along complex field line structures. The emphasis will be placed on the efficiency and the observational tests of the different models proposed for a flare. Title: Particle Acceleration in a Three-Dimensional Model of Reconnecting Coronal Magnetic Fields Authors: Cargill, Peter J.; Vlahos, Loukas; Turkmani, Rim; Galsgaard, Klaus; Isliker, Heinz Bibcode: 2007sdeh.book..249C Altcode: No abstract at ADS Title: Interaction of gravitational waves with strongly magnetized plasmas Authors: Isliker, Heinz; Sandberg, Ingmar; Vlahos, Loukas Bibcode: 2006PhRvD..74j4009I Altcode: 2006astro.ph..3828I We study the interaction of a gravitational wave (GW) with a plasma that is strongly magnetized. The GW is considered a small disturbance, and the plasma is modeled by the general relativistic analogue of the induction equation of ideal MHD and the single fluid equations. The equations are specified to two different cases, first to Cartesian coordinates and a constant background magnetic fields, and second to spherical coordinates together with a background magnetic field that decays with the inverse radial distance. The equations are derived without neglecting any of the nonlinear interaction terms, and the nonlinear equations are integrated numerically. We find that for strong magnetic fields of the order of 1015G the GW excites electromagnetic plasma waves very close to the magnetosonic mode. The magnetic and electric field oscillations have very high amplitude, and a large amount of energy is absorbed from the GW by the electromagnetic oscillations, of the order of 1023erg/cm3 in the case presented here, which, when assuming a relatively small volume in a star’s magnetosphere as an interaction region, can yield a total energy of at least 1041erg and may be up to 1043erg. The absorbed energy is proportional to B02, with B0 the background magnetic field. The energizing of the plasma takes place on fast time scales of the order of milliseconds. Our results imply that the GW-plasma interaction is an efficient and important mechanism in magnetar atmospheres, most prominently close to the star, and, under very favorable conditions though, it might even be the primary energizing mechanism behind giant flares. Title: Gyrokinetic electron acceleration in the force-free corona with anomalous resistivity Authors: Arzner, K.; Vlahos, L. Bibcode: 2006A&A...454..957A Altcode: 2006astro.ph..4161A Aims.We numerically explore electron acceleration and coronal heating by dissipative electric fields.
Methods: .Electrons are traced in linear force-free magnetic fields extrapolated from SOHO/MDI magnetograms, endowed with anomalous resistivity (η) in localized dissipation regions where the magnetic twist ∇ × hat{{b}} exceeds a given threshold. Associated with η > 0 is a parallel electric field {E} = η {j} that can accelerate runaway electrons. In order to gain observational predictions, we inject electrons inside the dissipation regions and follow them for several seconds in real time.
Results: .Precipitating electrons that leave the simulation system at height z = 0 are associated with hard X rays, and electrons that escape at height z ∼ 3× 104 km are associated with normal-drifting type IIIs at the local plasma frequency. A third, trapped population is related to gyrosynchrotron emission. Time profiles and spectra of all three emissions are calculated, and their dependence on the geometric model parameters and on η is explored. It is found that precipitation generally precedes escape by fractions of a second and that the electrons perform many visits to the dissipation regions before leaving the simulation system. The electrons impacting z = 0 reach higher energies than the escaping ones, and non-Maxwellian tails are observed at energies above the largest potential drop across a single dissipation region. Impact maps at z = 0 show the tendency of the electrons to arrive at the borders of sunspots of one polarity.
Conclusions: .Although the magnetograms used here belong to non-flaring times, so that the simulations refer to nanoflares and "quiescent" coronal heating, it is conjectured that the same process, on a larger scale, is responsible for solar flares. Title: High Energy Particle Transport in Stochastic Magnetic Fields in Solar Coronal Loops Authors: Gkioulidou, M.; Zimbardo, G.; Pammois, P.; Veltri, P.; Vlahos, L. Bibcode: 2006ESASP.617E.152G Altcode: 2006soho...17E.152G No abstract at ADS Title: Particle Acceleration in a Three-Dimensional Model of Reconnecting Coronal Magnetic Fields Authors: Cargill, Peter J.; Vlahos, Loukas; Turkmani, Rim; Galsgaard, Klaus; Isliker, Heinz Bibcode: 2006SSRv..124..249C Altcode: 2006SSRv..tmp..111C Particle acceleration in large-scale turbulent coronal magnetic fields is considered. Using test particle calculations, it is shown that both cellular automata and three dimensional MHD models lead to the production of relativistic particles on sub-second timescales with power law distribution functions. In distinction with the monolithic current sheet models for solar flares, particles gain energy by multiple interactions with many current sheets. Difficulties that need to be addressed, such as feedback between particle acceleration and MHD, are discussed. Title: Particle acceleration in stochastic current sheets in stressed coronal active regions Authors: Turkmani, R.; Cargill, P. J.; Galsgaard, K.; Vlahos, L.; Isliker, H. Bibcode: 2006A&A...449..749T Altcode: Aims.To perform numerical experiments of particle acceleration in the complex magnetic and electric field environment of the stressed solar corona.Methods.The magnetic and electric fields are obtained from a 3-D MHD experiment that resembles a coronal loop with photospheric regions at both footpoints. Photospheric footpoint motion leads to the formation of a hierarchy of stochastic current sheets. Particles (protons and electrons) are traced within these current sheets starting from a thermal distribution using a relativistic test particle code.Results.In the corona the particles are subject to acceleration as well as deceleration, and a considerable portion of them leave the domain having received a net energy gain. Particles are accelerated to high energies in a very short time (both species can reach energies up to 100 GeV within 5 × 10-2 s for electrons and 5 × 10-1 s for protons). The final energy distribution shows that while one quarter of the particles retain their thermal distribution, the rest have been accelerated, forming a two-part power law. Accelerated particles are either trapped within electric field regions of opposite polarities, or escape the domain mainly through the footpoints. The particle dynamics are followed in detail and it is shown how this dynamic affects the time evolution of the system and the energy distribution. The scaling of these results with time and length scale is examined and the Bremstrahlung signature of X-ray photons resulting from escaping particles hitting the chromosphere is calculated and found to have a main power law part with an index γ = - 1.8, steeper than observed. Possible resolutions of this discrepency are discussed. Title: Acceleration of low energy charged particles by gravitational waves Authors: Voyatzis, G.; Vlahos, L.; Ichtiaroglou, S.; Papadopoulos, D. Bibcode: 2006PhLA..352..261V Altcode: 2005astro.ph.12192V The acceleration of charged particles in the presence of a magnetic field and gravitational waves is under consideration. It is shown that the weak gravitational waves can cause the acceleration of low energy particles under appropriate conditions. Such conditions may be satisfied close to the source of the gravitational waves if the magnetized plasma is in a turbulent state. Title: Stochastic Acceleration in Turbulent Electric Fields Generated by 3D Reconnection Authors: Onofri, Marco; Isliker, Heinz; Vlahos, Loukas Bibcode: 2006PhRvL..96o1102O Altcode: 2006astro.ph..4192O Electron and proton acceleration in three-dimensional electric and magnetic fields is studied through test particle simulations. The fields are obtained by a three-dimensional magnetohydrodynamic simulation of magnetic reconnection in slab geometry. The nonlinear evolution of the system is characterized by the growth of many unstable modes and the initial current sheet is fragmented with formation of small scale structures. We inject at random points inside the evolving current sheet a Maxwellian distribution of particles. In a relatively short time (less than a millisecond) the particles develop a power-law tail. The acceleration is extremely efficient and the electrons absorb a large percentage of the available energy in a small fraction of the characteristic time of the MHD simulation, suggesting that resistive MHD codes are unable to represent the full extent of particle acceleration. Title: The Effect of Coherent Structures on Stochastic Acceleration in MHD Turbulence Authors: Arzner, Kaspar; Knaepen, Bernard; Carati, Daniele; Denewet, Nicolas; Vlahos, Loukas Bibcode: 2006ApJ...637..322A Altcode: 2005astro.ph..9717A We investigate the influence of coherent structures on particle acceleration in the strongly turbulent solar corona. By randomizing the Fourier phases of a pseudospectral simulation of isotropic magnetohydrodynamic (MHD) turbulence (Re~300) and tracing collisionless test protons in both the exact-MHD and phase-randomized fields, it is found that the phase correlations enhance the acceleration efficiency during the first adiabatic stage of the acceleration process. The underlying physical mechanism is identified as the dynamical MHD alignment of the magnetic field with the electric current, which favors parallel (resistive) electric fields responsible for initial injection. Conversely, the alignment of the magnetic field with the bulk velocity weakens the acceleration by convective electric fields -uXb at a nonadiabatic stage of the acceleration process. We point out that nonphysical parallel electric fields in random-phase turbulence proxies lead to artificial acceleration and that the dynamical MHD alignment can be taken into account on the level of the joint two-point function of the magnetic and electric fields and is therefore amenable to Fokker-Planck descriptions of stochastic acceleration. Title: a Model of Quiet Time Particle Acceleration in Interplanetary Space Authors: Lepreti, F.; Isliker, H.; Vlahos, L.; Petraki, K. Bibcode: 2005ESASP.600E.130L Altcode: 2005ESPM...11..130L; 2005dysu.confE.130L No abstract at ADS Title: AFS dynamics in a short-lived active region Authors: Zuccarello, F.; Battiato, V.; Contarino, L.; Romano, P.; Spadaro, D.; Vlahos, L. Bibcode: 2005A&A...442..661Z Altcode: In the framework of the study on active region emergence, we report the results obtained from the analysis of the short-lived (7 days) active region NOAA 10407. The data used were acquired during an observational campaign carried out with the THEMIS telescope in IPM mode in July 2003, coordinated with other ground- and space-based instruments (INAF-OACT, DOT, BBSO, MDI/SOHO, EIT/SOHO, TRACE). We determined the morphological and magnetic evolution of NOAA 10407, as well as the velocity fields associated with its magnetic structures. Within the limits imposed by the spatial and temporal resolution of the images analyzed, the first evidence of the active region formation is initially observed in the transition region and lower corona, and later on (i.e. after about 7 h) in the inner layers, as found in a previous analysis concerning a long-lived, recurrent active region. The results also indicate that the AFS formed in the active region shows typical upward motion at the AFS's tops and downward motion at the footpoints. The velocity values relevant to the upward motions decrease over the evolution of the region, similarly to the case of the recurrent active region, while we notice an increasing trend in the downflow velocity during the early phases of the time interval analyzed by THEMIS. On the other hand, the AFS preceding legs show a higher downflow than the following ones, a result in contrast with that found in the long-lived active region. The chromospheric area overhanging the sunspot umbra shows an upward motion of ∼ 2 km s-1, while that above the pores shows a downward motion of ~4 km s-1. Title: Galaxy Formation and Cosmic-Ray Acceleration in a Magnetized Universe Authors: Vlahos, Loukas; Tsagas, Christos G.; Papadopoulos, Demetrios Bibcode: 2005ApJ...629L...9V Altcode: 2005astro.ph..6742V We study the linear magnetohydrodynamic behavior of a Newtonian cosmology with a viscous magnetized fluid of finite conductivity and generalize the Jeans instability criterion. The presence of the field favors the anisotropic collapse of the fluid, which in turn leads to further magnetic amplification and to enhanced current-sheet formation in the plane normal to the ambient magnetic field. When the currents exceed a certain threshold, the resulting electrostatic turbulence can dramatically amplify the resistivity of the medium (anomalous resistivity). This could trigger strong electric fields and subsequently the acceleration of ultra-high-energy cosmic rays during the formation of protogalactic structures. Title: Quiet time particle acceleration in interplanetary space Authors: Lepreti, F.; Isliker, H.; Petraki, K.; Vlahos, L. Bibcode: 2005A&A...432.1049L Altcode: 2004astro.ph.12214L We propose a model for the acceleration of charged particles in interplanetary space that appear during quiet time periods, that is, not associated with solar activity events like intense flares or coronal mass ejections. The interaction of charged particles with modeled turbulent electromagnetic fields, which mimic the fields observed in the interplanetary medium, is studied. The turbulence is modeled by means of a dynamical system, the Gledzer-Ohkitani-Yamada (GOY) shell model, which describes the gross features of the Navier-Stokes equations. The GOY model is used to build a 3D velocity field, which in turn is used to numerically solve the ideal magneto-hydrodynamic (MHD) induction equation, while the electric field is calculated from the ideal Ohm's law. Particle acceleration in such an environment is investigated by test particle simulations, and the resulting energy distributions are discussed and compared to observations of suprathermal electrons and ions during quiet periods in interplanetary space. Title: Statistical Properties of Dissipative MHD Accelerators Authors: Arzner, Kaspar; Vlahos, Loukas; Knaepen, Bernard; Denewet, Nicolas Bibcode: 2005astro.ph..3147A Altcode: We use exact orbit integration to investigate particle acceleration in a Gauss field proxy of magnetohydrodynamic (MHD) turbulence. Regions where the electric current exceeds a critical threshold are declared to be `dissipative' and endowed with super-Dreicer electric field ${\bf E}_\Omega = \eta {\bf j}$. In this environment, test particles (electrons) are traced and their acceleration to relativistic energies is studied. As a main result we find that acceleration mostly takes place within the dissipation regions, and that the momentum increments have heavy (non-Gaussian) tails, while the waiting times between the dissipation regions are approximately exponentially distributed with intensity proportional to the particle velocity. No correlation between the momentum increment and the momentum itself is found. Our numerical results suggest an acceleration scenario with ballistic transport between independent `black box' accelerators. Title: Particle Acceleration in Stressed Coronal Magnetic Fields Authors: Turkmani, R.; Vlahos, L.; Galsgaard, K.; Cargill, P. J.; Isliker, H. Bibcode: 2005ApJ...620L..59T Altcode: This Letter presents an analysis of particle acceleration in a model of the complex magnetic field environment in the flaring solar corona. A slender flux tube, initially in hydrodynamic equilibrium, is stressed by random photospheric motions. A three-dimensional MHD code is used to follow the stochastic development of transient current sheets. These processes generate a highly fragmented electric field, through which particles are tracked using a relativistic test particle code. It is shown that both ions and electrons are accelerated readily to relativistic energies in times of order 10-2 s for electrons and 10-1 s for protons forming power-law distributions in energy. Title: Electron acceleration and radiation in evolving complex active regions Authors: Anastasiadis, A.; Gontikakis, C.; Vilmer, N.; Vlahos, L. Bibcode: 2004A&A...422..323A Altcode: We present a model for the acceleration and radiation of solar energetic particles (electrons) in evolving complex active regions. The spatio - temporal evolution of active regions is calculated using a cellular automaton model, based on self-organized criticality. The acceleration of electrons is due to the presence of randomly placed, localized electric fields produced by the energy release process, simulated by the cellular automaton model. We calculate the resulting kinetic energy distributions of the particles and their emitted X-ray radiation spectra using the thick target approximation, and we perform a parametric study with respect to number of electric fields present and thermal temperature of the injected distribution. Finally, comparing our results with the existing observations, we find that they are in a good agreement with the observed X-ray spectra in the energy range 100-1000 keV. Title: On the distribution of magnetic energy storage in solar active regions Authors: Fragos, T.; Rantsiou, E.; Vlahos, L. Bibcode: 2004A&A...420..719F Altcode: A two-dimensional probabilistic Cellular Automaton is used to model the appearance of active regions at the solar surface. We assume that two main competing processes control the magnetic field evolution at the solar surface (1) the magnetic field is locally enhanced by the flux emergence and/or the coalescence of emerged magnetic flux and (2) it is diminished by flux cancellation or diffusion. The flux emergence follows a basic percolation rule; it is more probable at the points were magnetic flux already exists. The magnetic field is also enhanced when magnetic fields of the same polarity collide. The flux cancellation is due either to the gradual diffusion of the magnetic field, when it is isolated, or to the partial release of energy when opposite magnetic field lines collide. The percolation model proposed in this article is capable of reproducing the statistical properties of the evolving active regions. The evolving simulated magnetograms, derived from our model, are used to estimate the 3-D magnetic fields above the photosphere using constant α force-free extrapolation techniques. Based on the above analysis we are able to estimate a variety of observed statistical characteristics, e.g. the size and flux distribution of the magnetic fields at the solar surface, the fractal dimension of the magnetic structures formed at the photosphere, the energy release frequency distribution, the waiting time distribution of the sporadic energy releases and the statistical properties of the steep horizontal magnetic field gradients in the extrapolated coronal magnetic field. Our main conclusion is that the photospheric driver plays a crucial role in the observed flare statistics, and the solar magnetograms, when interpreted properly, carry important statistical information for the solar coronal activity (coronal heating, flares, CME etc.). Title: Particle Acceleration in an Evolving Network of Unstable Current Sheets Authors: Vlahos, Loukas; Isliker, Heinz; Lepreti, Fabio Bibcode: 2004ApJ...608..540V Altcode: 2004astro.ph..2645V We study the acceleration of electrons and protons interacting with localized, multiple, small-scale dissipation regions inside an evolving, turbulent active region. The dissipation regions are unstable current sheets (UCSs), and in their ensemble they form a complex, fractal, evolving network of acceleration centers. Acceleration and energy dissipation are thus assumed to be fragmented. A large-scale magnetic topology provides the connectivity between the UCSs and in this way determines the degree of possible multiple acceleration. The particles travel along the magnetic field freely without losing or gaining energy until they reach a UCS. In a UCS, a variety of acceleration mechanisms are active, with the end result that the particles depart with a new momentum. The stochastic acceleration process is represented in the form of continuous-time random walk, which allows one to estimate the evolution of the energy distribution of the particles. It is found that under certain conditions, electrons are heated and accelerated to energies above 1 MeV in much less than 1 s. Hard X-ray and microwave spectra are calculated from the electrons' energy distributions, and they are found to be compatible with the observations. Ions (protons) are also heated and accelerated, reaching energies up to 10 MeV almost simultaneously with the electrons. The diffusion of the particles inside the active region is extremely fast (anomalous superdiffusion). Although our approach does not provide insight into the details of the specific acceleration mechanisms involved, its benefits are that it relates acceleration to the energy release, it well describes the stochastic nature of the acceleration process, and it can incorporate the flaring large-scale magnetic topology, potentially even its temporal evolution. Title: Particle Acceleration in Multiple Dissipation Regions Authors: Arzner, Kaspar; Vlahos, Loukas Bibcode: 2004ApJ...605L..69A Altcode: 2004astro.ph..2605A The sharp magnetic discontinuities that naturally appear in solar magnetic flux tubes driven by turbulent photospheric motions are associated with intense currents. Parker proposed that these currents can become unstable to a variety of microscopic processes, with the net result of dramatically enhanced resistivity and heating (nano-flares). The electric fields associated with such ``hot spots'' are also expected to enhance particle acceleration. We test this hypothesis by exact relativistic orbit simulations in strong random phase magnetohydrodynamic turbulence that is forming localized super-Dreicer Ohm electric fields (102<=EΩ/ED<=105) occurring in 2%-15% of the volume. It is found that these fields indeed yield a large amplification of acceleration of electrons and ions and can effectively overcome the injection problem. We suggest in this article that nanoflare heating will be associated with sporadic particle acceleration. Title: On the Self-Similarity of Unstable Magnetic Discontinuities in Solar Active Regions Authors: Vlahos, Loukas; Georgoulis, Manolis K. Bibcode: 2004ApJ...603L..61V Altcode: We investigate the statistical properties of possible magnetic discontinuities in two solar active regions over the course of several hours. We use linear force-free extrapolations to calculate the three-dimensional magnetic structure in the active regions. Magnetic discontinuities are identified using various selection criteria. Independently of the selection criterion, we identify large numbers of magnetic discontinuities whose free magnetic energies and volumes obey well-formed power-law distribution functions. The power-law indices for the free energies are in the range [-1.6, -1.35], in remarkable agreement with the power-law indices found in the occurrence frequencies of solar flare energies. This agreement and the strong self-similarity of the volumes that are likely to host flares suggest that the observed statistics of flares may be the natural outcome of a preexisting spatial self-organization accompanying the energy fragmentation in solar active regions. We propose a dynamical picture of flare triggering consistent with recent observations by reconciling our results with the concepts of percolation theory and self-organized criticality. These concepts rely on self-organization, which is expected from the fully turbulent state of the magnetic fields in the solar atmosphere. Title: Impulsive Electron Acceleration by Gravitational Waves Authors: Vlahos, Loukas; Voyatzis, George; Papadopoulos, Demetrios Bibcode: 2004ApJ...604..297V Altcode: 2003astro.ph.12151V We investigate the nonlinear interaction of a strong gravitational wave with the plasma during the collapse of a massive magnetized star and subsequent formation of a black hole or during the merging of neutron star binaries (the central engine). We found that under certain conditions this coupling may result in an efficient energy-space diffusion of particles. We suggest that the atmosphere created around the central engine is filled with three-dimensional magnetic neutral sheets (magnetic nulls). We demonstrate that the passage of strong pulses of gravitational waves through the magnetic neutral sheets accelerates electrons to very high energies. Superposition of many such short-lived accelerators, embedded inside a turbulent plasma, may be the source of the observed impulsive short-lived bursts. We conclude that in several astrophysical events, gravitational pulses may accelerate the tail of the ambient plasma to very high energies and become the driver for many types of astrophysical bursts. Title: Energetic Particle Acceleration and Radiation in Evolving Complex Active Regions Authors: Anastasiadis, A.; Gontikakis, C.; Vilmer, N.; Vlahos, L. Bibcode: 2004hell.conf...71A Altcode: No abstract at ADS Title: Are Gamma Ray Bursts Driven by Gravitational Waves? Authors: Vlahos, L.; Voyatzis, G.; Papadopoulos, D. Bibcode: 2004BaltA..13..324V Altcode: 2004OAst...13..324V We investigate the non-linear interaction of a strong gravitational wave with the plasma during the collapse of a massive magnetized star to form a black hole, or during the merging of neutron star binaries. We find that under certain conditions this coupling may result in an efficient energy space diffusion of particles. We suggest that the atmosphere created around the central engine is filled with 3-D magnetic neutral sheets (magnetic nulls). The passage of strong pulses of gravitational waves through the magnetic neutral sheets accelerates electrons to very high energies. We conclude that in several astrophysical events the gravitational pulses may accelerate the tail of the ambient plasma to very high energies and become the driver for many types of astrophysical bursts. Title: Energy Release and Particle Acceleration in Complex Active Regions Authors: Vlahos, L. Bibcode: 2004cosp...35.1994V Altcode: 2004cosp.meet.1994V Photospheric fluid flows and magnetic fields, magnetic energy release and particle acceleration have been treated so far as separate issues. In the widely used model the impulsive energy release (flares) is related to the 3D reconnection in current sheet(s), which are adjusted to simple magnetic topologies (magnetic loops). MHD turbulence generated from the reconnecting sheet fill the adjusted loop(s) and accelerate particles. We will review the strong and weak points of this scenario and attempt to generalized it, for complex and possibly more realistic magnetic topologies generated by the extrapolation of observed photospheric magnetic fields. Title: Particle acceleration and radiation in an evolving active region based on a Cellular Automaton (CA) model Authors: Anastasiadis, A.; Gontikakis, C.; Vilmer, N.; Vlahos, L. Bibcode: 2002ESASP.506..265A Altcode: 2002svco.conf..265A; 2002ESPM...10..265A We present a model for the acceleration and radiation of solar energetic particles (electrons) in an evolving active region. The spatio-temporal evolution of the active region is calculated using a Cellular Automaton (CA) model for the energy release process. The acceleration of particles is due to the presence of randomly placed, localized electric fields. We calculate the resulting kinetic energy distributions of the particles and the emitted radiation by performing a parametric study with respect to the trapping time of the injected distribution. Title: The extended cellular automaton (X-CA) model for lolar flares Authors: Isliker, H.; Anastasiadis, A.; Vlahos, L. Bibcode: 2002ESASP.506..641I Altcode: 2002svco.conf..641I; 2002ESPM...10..641I We have developed a new type of cellular automaton (CA) model, the extended CA (X-CA), for the study of solar flares. The X-CA model is consistent with the MHD approach, in contrast to the previously proposed CA models (classical CAs), it consistently yields all the relevant physical variables, and it successfully reproduces the observed distributions of total energy, peak flux, and duration of solar flares. We present and discuss the relevant plasma processes and set-up which are implemented by the X-CA model in the framework of a commonly accepted solar flare scenario. Title: Acceleration and radiation model of solar energetic particles in an evolving active region Authors: Anastasiadis, A.; Gontikakis, C.; Vilmer, N.; Vlahos, L. Bibcode: 2002ESASP.505..337A Altcode: 2002solm.conf..337A; 2002IAUCo.188..337A We present a model for the acceleration and radiation of solar energetic particles (electrons) in an evolving active region. The spatio-temporal evolution of the active region is calculated using a Cellular Automaton (CA) model for the energy release process. The acceleration of particles is due to the presence of randomly placed, localized electric fields. We calculate the resulting kinetic energy distributions of the particles by performing a parametric study with respect to the trapping time of the injected distribution. Our results show a power law or a power law with an exponential tail behavior for the resulting kinetic energy distribution, depending on the maximum trapping time of the injected particles in the acceleration volume. Finally we calculate the emitted radiation spectrum from the resulting energy distributions. Title: Statistical properties of the evolution of solar magnetic fields Authors: Vlahos, Loukas Bibcode: 2002ESASP.505..105V Altcode: 2002solm.conf..105V; 2002IAUCo.188..105V We are making an attempt to associate the formation and evolution of active regions with the heating and flaring of the plasmas in the solar atmosphere above the active region. The working hypothesis in our review is that "active regions are open dynamical systems away from equilibrium, driven by the turbulent convection zone". New magnetic loops emerge from the convection zone and are subject to surface diffusion (random motion), which leads to cancellation of magnetic energy when they collide with magnetic structures of opposite polarity. In the course of their evolution from birth to disappearance, active regions reach quickly the stage of Self-Organized Criticality (SOC), heat the corona by driving nano-flares, micro-flares and flares. We believe that the statistical properties of the active regions at the photosphere (size distribution, fractal dimension etc.) are correlated with the statistical properties of all aspects of solar activity (Ellerman Bombs, Bright Points, flares, particle acceleration, etc.). Surprisingly, all these phenomena share one common characteristic, they are self-similar and their statistical behaviour follows well defined power laws. This last point reinforces our belief that the solar atmosphere is coupled, through the magnetic field, with the convection zone, which drives all the observed activity at the photosphere, chromosphere, corona and interplanetary space. Title: Statistical Properties of the Energy Release in Emerging and Evolving Active Regions Authors: Vlahos, Loukas; Fragos, Tassos; Isliker, Heinz; Georgoulis, Manolis Bibcode: 2002ApJ...575L..87V Altcode: 2002astro.ph..7340V The formation and evolution of active regions are inherently complex phenomena. Magnetic fields generated at the base of the convection zone follow a chaotic evolution before reaching the solar surface. In this article, we use a two-dimensional probabilistic cellular automaton to model the statistical properties of the magnetic patterns formed on the solar surface and to estimate the magnetic energy released in the interaction of opposite polarities. We assume that newly emerged magnetic flux tubes stimulate the emergence of new magnetic flux in their neighborhood. The flux tubes move randomly on the surface of the Sun, and they cancel and release their magnetic energy when they collide with magnetic flux of opposite polarity, or diffuse into the ``empty'' photosphere. We assume that cancellation of magnetic flux in collisions causes ``flares'' and determine the released energy as the difference in the square of the magnetic field flux (E~B2). The statistics of the simulated flares follow a power-law distribution in energy, f(E)~E-a, where a=2.2+/-0.1. The size distribution function of the simulated active regions exhibits a power-law behavior with index k~1.93+/-0.08, and the fractal dimension of the magnetized areas on the simulated solar surface is close to DF~1.42+/-0.12. Both quantities, DF and k, are inside the range of the observed values. Title: Waves and instabilities in an anisotropic universe Authors: Papadopoulos, D.; Vlahos, L.; Esposito, F. P. Bibcode: 2002A&A...382....1P Altcode: 2001astro.ph.10455P The excitation of low frequency plasma waves in an expanding anisotropic cosmological model that contains a magnetic field frozen into the matter and pointing in the longitudinal direction is discussed. Using the exact equations governing finite-amplitude wave propagation in hydromagnetic media within the framework of the general theory of relativity, we show that a spectrum of magnetized sound waves will be excited and form large-scale ``damped oscillations'' in the expanding universe. The characteristic frequency of the excited waves is slightly shifted away from the sound frequency and the shift depends on the strength of the primordial magnetic field. This magnetic field dependent shift may have an effect on the acoustic peaks of the CMB. Title: MHD consistent cellular automata (CA) models. II. Applications to solar flares Authors: Isliker, H.; Anastasiadis, A.; Vlahos, L. Bibcode: 2001A&A...377.1068I Altcode: 2001astro.ph..8365I In Isliker et al. (\cite{Isliker00b}), an extended cellular automaton (X-CA) model for solar flares was introduced. In this model, the interpretation of the model's grid-variable is specified, and the magnetic field, the current, and an approximation to the electric field are yielded, all in a way that is consistent with Maxwell's and the MHD equations. The model also reproduces the observed distributions of total energy, peak-flux, and durations. Here, we reveal which relevant plasma physical processes are implemented by the X-CA model and in what form, and what global physical set-up is assumed by this model when it is in its natural state (self-organized criticality, SOC). The basic results are: (1) On large-scales, all variables show characteristic quasi-symmetries: the current has everywhere a preferential direction, the magnetic field exhibits a quasi-cylindrical symmetry. (2) The global magnetic topology forms either (i) closed magnetic field lines around and along a more or less straight neutral line for the model in its standard form, or (ii) an arcade of field lines above the bottom plane and centered along a neutral line, if the model is slightly modified. (3) In case of the magnetic topology (ii), loading can be interpreted as if there were a plasma which flows predominantly upwards, whereas in case of the magnetic topology (i), as if there were a plasma flow expanding from the neutral line. (4) The small-scale physics in the bursting phase represent localized diffusive processes, which are triggered when a quantity which is an approximately linear function of the current exceeds a threshold. (5) The interplay of loading and bursting in the X-CA model can be interpreted as follows: the local diffusivity usually has a value which is effectively zero, and it turns locally to an anomalous value if the mentioned threshold is exceeded, whereby diffusion dominates the quiet evolution (loading), until the critical quantity falls below the threshold again. (6) Flares (avalanches) are accompanied by the appearance of localized, intense electric fields. A typical example of the spatio-temporal evolution of the electric field during a flare is presented. (7) In a variant on the X-CA model, the magnitude of the current is used directly in the instability criterion, instead of the approximately linear function of it. First results indicate that the SOC state persists and is only slightly modified: distributions of the released energy are still power-laws with slopes comparable to the ones of the non-modified X-CA model, and the large scale structures, a characteristic of the SOC state, remain unchanged. (8) The current-dissipation during flares is spatially fragmented into a large number of dissipative current-surfaces of varying sizes, which are spread over a considerably large volume, and which do not exhibit any kind of simple spatial organization as a whole. These current-surfaces do not grow in the course of time, they are very short-lived, but they multiply, giving rise to new dissipative current-surfaces which are spread further around. They show thus a highly dynamic temporal evolution.\ It follows that the X-CA model represents an implementation of the flare scenario of Parker (\cite{Parker93}) in a rather complete way, comprising aspects from small scale physics to the global physical set-up, making though some characteristic simplifications which are unavoidable in the frame-work of a CA. Title: Fast magnetosonic waves driven by gravitational waves Authors: Papadopoulos, D.; Stergioulas, N.; Vlahos, L.; Kuijpers, J. Bibcode: 2001A&A...377..701P Altcode: 2001astro.ph..7043P The propagation of a gravitational wave (GW) through a magnetized plasma is considered. In particular, we study the excitation of fast magnetosonic waves (MSW) by a gravitational wave, using the linearized general-relativistic hydromagnetic equations. We derive the dispersion relation for the plasma, treating the gravitational wave as a perturbation in a Minkowski background space-time. We show that the presence of gravitational waves will drive magnetosonic waves in the plasma and discuss the potential astrophysical implications. Title: The study of solar flares with the extended cellular automaton (X-CA) model Authors: Isliker, H.; Anastasiadis, A.; Vlahos, L. Bibcode: 2001hell.confE..39I Altcode: No abstract at ADS Title: A cellular automaton model for the magnetic activity in accretion discs Authors: Pavlidou, V.; Kuijpers, J.; Vlahos, L.; Isliker, H. Bibcode: 2001A&A...372..326P Altcode: In this paper we attempt, for the first time, to simulate the magnetic activity of an accretion disc using a probabilistic cellular automaton model. Our model is based on three free parameters, the probabilities of spontaneous and stimulated generation of magnetic flux above the surface of the disc (S0, and, respectively, P), and the probability of diffusive disappearance of flux below the surface (D). The model describes a changing collection of flux tubes which stick out of the disc and are anchored inside the disc at their foot-points. Magnetic flux tubes transfer angular momentum outwards at a rate which is analytically estimated for each single loop. Our model monitors the dynamic evolution of both the distribution of magnetic loops and the mass transfer which results from angular momentum transport due to this distribution. The energy release due to magnetic flaring is also recorded as a function of time and exhibits temporal fluctuations with power spectra that depend on the assumed emission-profile of single flaring loops: (i) for instantaneous emission, the power-spectra are flat at low frequencies and turn over at high frequencies to a power-law with index -0.3; (ii) for emission-profiles in the form of one-sided exponentials, the power-spectra exhibit clear power-law behaviour with index -1.7. Fluctuations with a power law index between -1 and -1.7 are observed in many systems undergoing accretion. We found that our approach allows steady accretion in a disc by the action of coronal magnetic flux tubes alone. If we express the effective viscosity caused by coronal loops in the usual Shakura-Sunyaev alpha parameter of viscosity, we find values which are in good agreement with observed values. Title: MHD consistent cellular automata (CA) models. I. Basic features Authors: Isliker, H.; Anastasiadis, A.; Vlahos, L. Bibcode: 2000A&A...363.1134I Altcode: 2001astro.ph..6111I A set-up is introduced which can be superimposed onto the existing solar flare cellular automata (CA) models, and which specifies the interpretation of the model's variables. It extends the CA models, yielding the magnetic field, the current, and an approximation to the electric field, in a way that is consistent with Maxwell's and the MHD equations. Applications to several solar flare CA models during their natural state (self-organized criticality (SOC)) show, among others, that (1) the magnetic field exhibits characteristic large-scale organization over the entire modeled volume; (2) the magnitude of the current seems spatially dis-organized, with no obvious tendency towards large-scale structures or even local organization; (3) bursts occur at sites with increased current, and after a burst the current is relaxed; (4) by estimating the energy released in individual bursts with the use of the current as Ohmic dissipation, it turns out that the power-law distributions of the released energy persist. The CA models, extended with the set-up, can thus be considered as models for energy-release through current-dissipation. The concepts of power-law loading and anisotropic events (bursts) in CA models are generalized to 3-D vector-field models, and their effect on the magnetic field topology is demonstrated. Title: Particle-Acceleration and Radiation in the Turbulent Flow of a Jet Authors: Manolakou, Konstantina; Anastasiadis, Anastasios; Vlahos, Loukas Bibcode: 1999ptep.proc..333M Altcode: We present a numerical model for electron acceleration and radiation inside the body of an extragalactic jet. We model the jet environment as a turbulent medium generating non-linear structures (eddies and/or shocks) through a cascading process. These structures act like in-situ accelerators for the electrons that are initially injected from the central engine. Two types of acceleration processes are considered: second order Fermi-acceleration and shock-drift acceleration, depending on the velocity of the turbulent eddies encountered. We study the modulation of the energy distribution of electrons in such an environment, by incorporating synchrotron radiation losses in the time intervals between successive interactions of the particles with the turbulent structures. By performing a parametric study with respect to the level of turbulent activity and the time intervals between interactions, we calculate the temporal evolution of the cut-off frequency of the synchrotron radiation spectrum of the particles and discuss our results in connection with recent observations. Title: Flare Physics in Accretion Discs Authors: Pavlidou, V.; Kuijpers, J.; Vlahos, L.; Isliker, H. Bibcode: 1999ESASP.448..859P Altcode: 1999mfsp.conf..859P; 1999ESPM....9..859P No abstract at ADS Title: Particle-acceleration and radiation in the turbulent flow of a jet Authors: Manolakou, K.; Anastasiadis, A.; Vlahos, L. Bibcode: 1999A&A...345..653M Altcode: We present a numerical model for electron acceleration and radiation inside the body of an extragalactic jet. We model the jet environment as a turbulent medium generating non-linear structures (eddies and/or shocks) through a cascading process. These structures act like in-situ accelerators for the electrons that are initially injected from the central engine. Two types of acceleration processes are considered: second order Fermi-acceleration and shock-drift acceleration, depending on the velocity of the turbulent eddies encountered. We study the modulation of the energy distribution of electrons in such an environment, by incorporating synchrotron radiation losses in the time intervals between successive interactions of the particles with the turbulent structures. By performing a parametric study with respect to the level of turbulent activity and the time intervals between interactions, we calculate the temporal evolution of the cut-off frequency of the synchrotron radiation spectrum of the particles and discuss our results in connection with recent observations. Title: Derivation of Solar Flare Cellular Automata Models from a Subset of the Magnetohydrodynamic Equations Authors: Vassiliadis, D.; Anastasiadis, A.; Georgoulis, M.; Vlahos, L. Bibcode: 1998ApJ...509L..53V Altcode: Cellular automata (CA) models account for the power-law distributions found for solar flare hard X-ray observations, but their physics has been unclear. We examine four of these models and show that their criteria and magnetic field distribution rules can be derived by discretizing the MHD diffusion equation as obtained from a simplified Ohm's law. Identifying the discrete MHD with the CA models leads to an expression for the resistivity as a function of the current on the flux tube boundary, as may be expected from current-driven instabilities. Anisotropic CA models correspond to a nonlinear resistivity η(J), while isotropic ones are associated with hyperresistivity η(▽2J). The discrete equations satisfy the necessary conditions for self-organized criticality (Lu): there is local conservation of a field (magnetic flux), while the nonlinear resistivity provides a rapid dissipation and relaxation mechanism. The approach justifies many features of the CA models that were originally based on intuition. Title: Variability of the occurrence frequency of solar flares and the statistical flare Authors: Georgoulis, Manolis K.; Vlahos, Loukas Bibcode: 1998A&A...336..721G Altcode: Self-Organised Criticality (SOC), embedded in cellular automata models, has been so far viewed as an attractive phenomenological approach for studying the statistical behaviour of flaring activity in solar active regions. Well-known statistical properties of flares, like the robust scaling laws seen in the distribution functions of characteristic parameters of the events, as well as correlations linking those parameters, are successfully reproduced by SOC models. Recent observations, however, challenge the flexibility of SOC, as they reveal a variation of the flaring power-law indices over short-time activity periods. The initial SOC models, based on a small-amplitude, constant external driver and isotropic instability criteria, appear inefficient to predict variable power-law indices. In this paper we introduce a SOC-type numerical model, with a number of modifications of the original SOC concept. We show that scaling laws and correlations between the events' characteristic parameters survive under the action of a highly variable driver. A variable driver initiates a variability in the resulting power-law indices. We reproduce qualitatively and quantitatively the statistics of flaring activity during the 154-day periodicity. Moreover, small-scale, anisotropic instability criteria imply the existence of a soft population of events, with statistical properties analogous to those attributed to the hypothetical nanoflares. We show that numerous small-scale events could be the dominant energy release mechanism in cases of quiescent coronal activity. Title: A stochastic model for solar type III bursts Authors: Isliker, H.; Vlahos, L.; Benz, A. O.; Raoult, A. Bibcode: 1998A&A...336..371I Altcode: A stochastic model for type III bursts is introduced, discussed, and compared to observations. The active region is assumed to be inhomogeneous, with a large number of emerging magnetic fibers. At their bases, random energy release events take place, in the course of which electrons are accelerated, travel along the fibers and eventually undergo the bump-on-tail instability. In the non-linear regime, the formed Langmuir waves induce strong turbulence in the ambient plasma, with secondary electrostatic waves appearing. Wave-wave scattering finally leads to the emission of transverse electro-magnetic waves at the fundamental and the harmonic of the local plasma-frequency. The superposition of the emissions from all the fibers yields a model spectrogram for type III bursts (flux as a function of frequency and time). Peak-flux distributions of the model are compared to the ones of five observations of type III bursts. It turns out that, in a statistical sense, the model is largely compatible with the observations: the majority of the observations can be considered generated by a process which corresponds with the presented model. The details of the different sub-processes constituting the model play no decisive role concerning the statistical properties of the generated spectrograms, to describe them approximately by randomizing the unknown elements is sufficient. Therewith, the correspondence of the model with the data is not unique. Likewise, intrinsic shortness of observed type III events does not allow a strict enough discrimination between different possible sub-processes of the model through statistical tests. With that, the conclusion is that the observations are compatible with a model which assumes (i) a randomly structured active region, (ii) a flare-particle acceleration-process which is fragmented into a large number of sub-processes, (iii) a distribution of the accelerated particles which is a random fraction of the ambient density and of power-law form with random index, and (iv) the fragmentary acceleration events to occur randomly in time, i.e. the temporal structure of type III events to be random, without any correlations between the individual bursts. Title: Solar flare cellular automata interpreted as discretized MHD equations Authors: Isliker, H.; Anastasiadis, A.; Vassiliadis, D.; Vlahos, L. Bibcode: 1998A&A...335.1085I Altcode: We show that the Cellular Automaton (CA) model for Solar flares of Lu and Hamilton (1991) can be understood as the solution to a particular partial differential equation (PDE), which describes diffusion in a localized region in space if a certain instability threshold is met, together with a slowly acting source term. This equation is then compared to the induction equation of MHD, the equation which governs the energy release process in solar flares. The similarities and differences are discussed. We make some suggestions how improved Cellular Automaton models might be constructed on the basis of MHD, and how physical units can be introduced in the existing respective Cellular Automaton models. The introduced formalism of recovering equations from Cellular Automata models is rather general and can be applied to other situations as well. Title: Competition Model for the Formation and Evolution of Active Regions Authors: Mylonas, N.; Vlahos, L.; Kluiving, R. Bibcode: 1998ASPC..155...19M Altcode: 1998sasp.conf...19M No abstract at ADS Title: Formation of Active Regions: Observations and Theory (Invited review) Authors: Vlahos, L. Bibcode: 1998ASPC..155....3V Altcode: 1998sasp.conf....3V No abstract at ADS Title: Fundamental-Harmonic emission in a fibrous jet Authors: Delouis, J. M.; Raoult, A.; Vlahos, L. Bibcode: 1998cee..workE..34D Altcode: We propose a type III emission model in the middle corona including both fundamental and harmonic radiation from a fibrous medium. We state that emitted fundamental and harmonic radiation have the intrinsic polarization corresponding to coronal field conditions, i.e.,close to 100% for fundamental radiation and close to 0% for harmonic radiation. The polarisation ratio of the resulting radiation is then due to the proportion of fundamental emission over total emission. This allows a new interpretation for spacial and spectral polarization properties of type III emission. It also gives a new explanation for Fundamental-Harmonic pairs of type III bursts. The Fundamental component and the Harmonic component appear as a mix of fundamental and harmonic radiation in different proportions. In this frame, most of abnomalous observational properties of Fondamental-Harmonic pairs (altitudes, frequency ratio, polarization) find a straightforward explanation. The model fits the polarisation diagrams established by the statistical study of (Dulk and Suzuki, 1990). This model is also in agrement with recent interplanetary stereo observations (Hoang et al.,1998) which show that type III radiation is emitted at both fundamental and harmonic modes. Title: Electron Acceleration by Random DC Electric Fields Authors: Anastasiadis, Anastasios; Vlahos, Loukas; Georgoulis, Manolis K. Bibcode: 1997ApJ...489..367A Altcode: We present a global model for the acceleration of electrons in the framework of the statistical flare model of Vlahos et al. In this model, solar flares are the result of an internal self-organized critical (SOC) process in a complex, evolving, and highly inhomogeneous active region. The acceleration of electrons is due to localized DC electric fields closely related to the energy-release process in the active region. Our numerical results for the kinetic energy distribution of accelerated electrons show a power-law or an exponential-law behavior, depending on the maximum trapping time of the energetic particles inside the acceleration volume. Title: Variability of the Occurrence Frequency of Solar Flares and the Statistical Flare Authors: Georgoulis, M. K.; Vlahos, L. Bibcode: 1997jena.confE..39G Altcode: Self-Organized Criticality (SOC) embedded in cellular automata models has been so far acknowledged as an adequate qualitative way of studying the statistical behaviour of flaring activity in solar active regions. These models are able of producing robust power laws featuring the frequency distributions of the events obtained, which are closely consistent with observations of flares, as well as much steeper power-law cut-offs, which may indicate the existence of the presently unobserved nanoflares. SOC models are based on the substantial concept that active regions are driven dissipative nonlinear dynamical systems. The role of the external driver is attributed to the feedback of magnetic flux that is injected to the system through the photospheric boundary and to the random shuffling of the footpoints of coronal loops taking place on the upper photosphere. In previous numerical studies, the driver used was an infinitesimal perturbation acting on localized magnetic topologies due to which avalanche-type instabilities were triggered. Furthermore, the resulting power-law indices were unique. Recent observations, however, have shown that the scaling indices of flares' frequency distributions are not kept constant during certain phases of the solar activity, such as the 154-day periodicity. To tackle this problem we investigate the role of the driver used, by introducing a highly variable driving mechanism. We show that the variability of the driver induces a respective variability in the resulting power-law indices. The variability of the indices can be well represented by a linear dependence between them and the driver's scaling index for both "nanoflaring" and "flaring" activity. Furthermore, a first attempt of connecting the driver with certain statistical properties of active regions is introduced. The results stand closely in favor of the observations of flaring activity during the 154-day periodicity. Title: Solar and Heliospheric Plasma Physics Authors: Simnett, George M.; Alissandrakis, Constantine E.; Vlahos, Loukas Bibcode: 1997LNP...489.....S Altcode: 1997shpp.conf.....S This volume brings together theoretical ideas on the plasma physics of both hot and dense plasmas in the solar atmosphere and similar physics applied to the tenuous and cooler plasmas found in the heliosphere. It is complemented by recent observations. Helioseismology covers the solar interior and the neutrino problem. Solar and stellar activity cycles are addressed. The dynamics of magnetic flux tubes in the solar atmosphere and material flows through the chromosphere into the upper atmosphere are comprehensively reviewed. Energy release processes and the production of energetic particles are important to understanding events in the solar atmosphere and to the dynamics of the tenuous heliosphere. A glimpse of the future is offered by concluding chapters on new ground-based and space instrumentation. Title: Energy Release in the Solar Corona Authors: Bastian, Timothy S.; Vlahos, Loukas Bibcode: 1997LNP...483...68B Altcode: 1997cprs.conf...68B Energy release in the solar corona drives a wide variety of phenomena, including flares, filament/prominence eruptions, coronal mass ejections, solar particle events, as well as coronal heating and the solar wind. The basic physics of these phenomena and their relationship to each other remains a vigorous area of inquiry. The Working Group on Energy Release at Mont Evray directed its attention to recent observational and theoretical developments relevant to flares and coronal heating. Particular attention was given to the "fragmentation" of energy release in solar flares and its interpretation; to the statistics of the flare phenomenon and whether they can be understood in terms of "driven dissipative systems"; to quasisteady energy release and the problem of coronal heating; and to recent observations of flares and related phenomena. Title: Coronal Heating by Nanoflares and the Variability of the Occurence Frequency in Solar Flares Authors: Georgoulis, Manolis K.; Vlahos, Loukas Bibcode: 1996ApJ...469L.135G Altcode: It has been proposed that flares in the solar corona may well be a result of an internal self-organized critical (SOC) process in active regions. We have developed a cellular automaton SOC model that simulates flaring activity extending over an active sub-flaring background. In the resulting frequency distributions we obtain two distinct power laws. That of the weaker events is shorter and much steeper (power law with index ~=-3.26) than that of the intermediate and large events (power law with index ~=-1.73). The flatter power law is in close agreement with observations of flares. Weaker events are responsible for ~=90% of the total magnetic energy released, indicating a possible connection of nanoflares with coronal heating. Moreover, certain mechanisms cause the variability of the resulting indices and may provide answers to the problem of the variability of flares' occurrence frequency during the solar cycle. Title: Cellular automaton models of solar flare occurence. Authors: MacKinnon, A. L.; MacPherson, K. P.; Vlahos, L. Bibcode: 1996A&A...310L...9M Altcode: We describe a class of stochastic cellular automaton models for the occurence of the solar flares. "Flaring elements" of a deliberately unspecified nature are supposed to interact. A 1-D version of the model, capable of analytical description, conflicts in detail with the observations but indicates the quantitative success of such a picture. Minimal a priori assumptions are involved. The qualitative importance of such a model lies in the arbitrary means of communication between flaring elements. Title: Competetion Model for the Evolution of solar active Regions Authors: Mylonas, N.; Kluiving, R.; Vlahos, L.; Gergoulis, M. Bibcode: 1996hell.conf...46M Altcode: No abstract at ADS Title: Are Flares the Result of a self-organisation Process in active Regions? Authors: Georgoulis, M. K.; Vlahos, L.; Kluiving, R. Bibcode: 1996hell.conf...52G Altcode: No abstract at ADS Title: The Acceleration and Transport of energetic Particles inside an evolving active Region Authors: Anastasiadis, A.; Vlahos, L. Bibcode: 1996hell.conf...59A Altcode: No abstract at ADS Title: Global Models for the Active Sun and the Statistics of Flares Authors: Vlahos, L. Bibcode: 1996ASPC...93..355V Altcode: 1996ress.conf..355V No abstract at ADS Title: Stochastic particle Acceleration in a strongly turbulent Flow of a Jet Authors: Manolakou, C.; Anastasiadis, A.; Vlahos, L. Bibcode: 1996hell.conf..445M Altcode: No abstract at ADS Title: The statistical flare. Authors: Vlahos, L.; Georgoulis, M.; Kluiving, R.; Paschos, P. Bibcode: 1995A&A...299..897V Altcode: Solar and stellar flares are interpreted so far as an instability of a large scale magnetic neutral sheet. In this article, however, we assume that the active region is highly inhomogeneous: a large number of magnetic loops are simultaneously present interacting and randomly forming discontinuities in many independent points in space. These magnetic discontinuities release energy and force weaker discontinuities in their neighbourhood to release energy as well. This complex dynamical system releases constantly energy in the form of small and large scale explosions. Clustering of many discontinuities in the same area has the effect of larger scale explosions (flares). This type of flare with spatiotemporal fragmentation and clustering in small and large scale structures will be called here the statistical flare. The statistical flare is simulated using avalanche models originally introduced by Bak et al. (1988). Avalanche models applied so far to solar flares (Lu & Hamilton 1991) were isotropic (the field was distributed equally to the closest neighbours of an unstable point). These models simulate relatively large events (microflares and flares). Here we introduce a more refined isotropic avalanche model as well as an anisotropic avalanche model (energy is distributed only among the unstable point and those neighbours that develop gradients higher than a critical value). The anisotropic model simulates better the smaller events (nanoflares): in contrast to the well-known results of the isotropic model (a power law with index ~-1.8 in the peak-luminosity distribution), the anisotropic model produces a much steeper power law with index ~-3.5. Finally, we introduce a mixed model (a combination of isotropic and anisotropic models) which gives rise to two distinct power-law regions in the peak-luminosity distribution, one with index ~-3.5 accounting for the small events, and one with index ~-1.8 accounting for large events. This last model therefore explains coronal heating as well as flaring. The three models introduced in this paper show length-scale invariant behaviour. Model-dependent memory effects are detected in the peak-luminosity time series produced by these models. Title: Beam fragmentation and type III bursts. Authors: Vlahos, L.; Raoult, A. Bibcode: 1995A&A...296..844V Altcode: We re-investigate the problem of electron beam propagation and the formation of type III bursts, assuming that the corona is inhomogeneous and the magnetic field is split in small independent fibers. We start by injecting in each fiber a beam with random characteristics (beam density, beam energy, etc.) and follow its non linear evolution during propagation. We add the contribution of N independent beams in each frequency and form the expected spectrum. We discuss the fine structure of type III bursts, the decimetric spikes associated with type III bursts and the formation of groups of type III bursts. Title: Particle acceleration in the heliosphere Authors: Vlahos, L. Bibcode: 1995HiA....10..307V Altcode: No abstract at ADS Title: Acceleration and Radiation from a Complex Active Region Authors: Vlahos, L. Bibcode: 1995LNP...444..115V Altcode: 1995cmer.conf..115V Active regions are treated in this review as a "paradigm" of a complex dynamical system. Active regions are formed by magnetic fibers escaping from the turbulent convection zone. Random movements of the feet of the fibers in the photosphere and emergence of new magnetic flux from the convection zone are responsible for the formation of neutral sheets and magnetic discontinuities inside the active region, which are the sites for magnetic dissipation. A simple model, based on the scenario of self organised criticality, reproduces many of the known flare characteristics. The same model is used to provide a large number of nanoflares, which can heat the corona. We also show that this complex, inhomogeneous active region, is an efficient accelerator and reproduce the observed dm spikes and type III bursts. Title: Particle Acceleration in an Evolving Active Region by an Ensemble of Shock Waves Authors: Anastasiadis, Anastasios; Vlahos, Loukas Bibcode: 1994ApJ...428..819A Altcode: We present a model for the acceleration and transport of energetic particles (electrons and ions) inside an active region. We propose that the agent for the acceleration is an ensemble of oblique shock waves, generated inside an evolving and constantly changing active region. The acceleration is based on the 'shock drift' mechanism, using the adiabatic treatment. The high-energy particles are losing energy via Coulomb collisions and radiation. We calculate the energy distribution of the particles, their acceleration time, and their maximum energy as a function of the number of shock waves present. The high-energy particles are transported inside a chaotic magnetic field and are reflected randomly when they meet a magnetic mirror point. Finally we compare our numerical results with the observations. Title: Foreword Authors: van den Oord, Bert; Kuijpers, Jan; Kuperus, Max; Benz, A. O.; Brown, J. C.; Einaudi, G.; Kuperus, M.; Raadu, M. A.; Trottet, G.; van den Oord, G. H. J.; Vlahos, L.; Zheleznyakov, V. V.; Wijburg, Marion; Fletcher, Lyndsay; Volwerk, Martin Bibcode: 1994SSRv...68D..17V Altcode: No abstract at ADS Title: Theory of fragmented energy release in the Sun Authors: Vlahos, Loukas Bibcode: 1994SSRv...68...39V Altcode: The magnetic energy released inside an active region is closely related to its formation and evolution. Following the evolution of a collection of flux tubes inside the convection zone and above the photosphere we can show that many nonlinear structures (current sheets, shock waves, double layers etc.) are formed. We propose in this review that coronal heating, flares and particle acceleration are due to the interaction of the plasma with these nonlinear structures. Approaching active regions as a driven complex dynamical system we can show that several coherent ensembles of the nonlinear structures will appear spontaneously. The statistical analysis of these structures is a major problem in solar physics. We can also show that many observed large scale structures are the result of the convolution of non-observable fragmentation in the energy release process. Title: Galactic Dynamics and N-Body Simulations Authors: Contopoulos, G.; Spyrou, N. K.; Vlahos, L. Bibcode: 1994LNP...433.....C Altcode: 1994gdnb.conf.....C This book provides an in-depth coverage of modern research on dynamical systems. The first part discusses stellar dynamics, integrable systems, the transition to chaos and instabilities in stellar dynamics as well as the dynamics of spiral galaxies. Models are given and compared with observations. The second part is devoted to the direct method of N-body simulations, to gas dynamics simulations and to galaxy formation. Special care is taken to give to a pedagogical presentation of the material which makes this a unique text well suited for graduate courses in astrophysics. Title: Filamentation of magnetic structures and particle acceleration in solar and stellar flares Authors: Vlahos, Loukas Bibcode: 1993AdSpR..13i.161V Altcode: 1993AdSpR..13..161V We review models for the evolution and energy release of active region magnetic fields. We propose that turbulent photospheric motions will break the active region magnetic field in small fibers (current carrying magnetic filaments) and we address the possibility that this type of magnetic topology may become the host of thousands of neutral sheets. We have subsequently analyzed the acceleration and transport of energetic particles in such an environment. Standard techniques such as stochastic or shock acceleration, E-field acceleration or coherent acceleration are reviewed on the basis of a ``statistical flare'' model, where many explosions are organised (self-organised when they reach a critical stage or externally driven by emerging flux or large scale photospheric flows around filaments) to create a flare (micro-flare or large flare). Title: Particle acceleration by multiple shocks at the hot spots of extragalatic radio sources Authors: Anastasiadis, A.; Vlahos, L. Bibcode: 1993A&A...275..427A Altcode: We present a model for the acceleration of energetic electrons by an ensemble of oblique shock waves at the hot spots of extended extragalactic radio sources. The energization of the electrons is based on the "shock drift" acceleration mechanism, using the adiabatic treatment, and electrons are subject to synchrotron losses as they travel between the shock fronts. We calculate numerically the energy distribution of electrons and the corresponding intensity of the radiation produced. We compare our results with the analytical solution of the Fokker-Planck equation. Our numerical and analytical results agree well with the existing observations. Title: Transport Phenomena in Solar and Stellar Active Regions Authors: Vlahos, L. Bibcode: 1993sdtp.conf..235V Altcode: No abstract at ADS Title: High Energy Emission from Normal Stars Authors: Vlahos, L. Bibcode: 1993LNP...418..129V Altcode: 1993ghea.conf..129V Introduction Energy Flow and Particle Acceleration Basic Concepts Spontaneous Emission Bremsstrahlung Emission Cyclotron and Synchrotron Emission Collective Plasma Emission Plasma Radiation Electron Cyclotron Maser Instability Observations Hard X-Ray Emission Radio Observations Summary and Conclusions References Title: On the efficiency of electron cyclotron maser instability in solar flares Authors: MacKinnon, A.; Vlahos, L.; Vilmer, N. Bibcode: 1992A&A...256..613M Altcode: We describe a simple model, designed to estimate the efficiency of the electron cyclotron maser (ECM) instability inside a magnetic trap. The emphasis in this model is on the energy loss associated with the formation of the loss cone distribution, rather than on the microscopic plasma physics. We find that the net radiation efficiency is low. Repeated mirroring does not greatly enhance the intrinsic efficiency, because of the energy lost from the corona in precipitating electrons. In consequence, absorption of ECM radiation is unlikely to be an important factor in understanding soft X-ray observations. Title: Particle acceleration inside a 'gas' of shock waves Authors: Anastasiadis, A.; Vlahos, L. Bibcode: 1991A&A...245..271A Altcode: A process by which shock waves are formed in jet flows or during supernovae explosions is presented. Particular attention is given to the short time scale evolution of N-shock wave-particle interaction. It is found that most of the energy released by the 'thermal' explosions will return to thermal energy via shock-shock interactions. Title: An injection model for type III/V bursts in solar flares Authors: Raoult, A.; Mangeney, A.; Vlahos, L. Bibcode: 1990A&A...233..229R Altcode: A possible interpretation of the injection of electrons and the correlation of hard X-ray and type III/V radio emission is presented. A possible model is also presented of the nonlinear excitation of ion sound fluctations from type III beams and the streaming of low-energy superthermal electrons inside a turbulent plasma. It is suggested that the high-energy electron stream, which generates type III bursts and the spiky X-ray component, excites intense plasma waves which nonlinearly drive ion density fluctuations. These fluctuations enhance the collision frequency of the streaming low-energy particles and force them to form a positive slope. This allows the 30 keV hot plasma to excite plasma waves in the same frequency range as type III bursts and to generate type V emission. Title: Energetic Particles in Solar Flares: Observations Modeling and Acceleration Processes (Extended Abstract) Authors: Trottet, G.; Vlahos, L. Bibcode: 1990PDHO....7..184T Altcode: 1990dysu.conf..184T; 1990ESPM....6..184T No abstract at ADS Title: Particle Acceleration in Solar Flares Authors: Vlahos, Loukas Bibcode: 1989SoPh..121..431V Altcode: 1989IAUCo.104..431V Particle acceleration during solar flares is a complex process where the main `actors' (Direct (D.C.) or turbulent electric fields) are hidden from us. It is easy to construct a successful particle accelertion model if we are allowed to impose on the flaring region arbitrary conditions (e.g., strength and scale length of the D.C. or turbulent electric fields), but then we have not solved the acceleration problem; we have simply re-defined it. We outline in this review three recent observations which indicate that the following physical processes may happen during solar flares: (1) Release of energy in a large number of microflares; (2) short time-scales; (3) small length scales; and (4) coherent radiation and acceleration sources. We propose that these new findings force us to reformulate the acceleration process inside a flaring active region assuming that a large number of reconnection sites will burst almost simultaneously. All the well-known acceleration mechanisms (electric fields, turbulent fields, shock waves, etc.) reviewed briefly here, can be used in a statistical model where each particle is gaining energy through its interaction with many small reconnection sites. Title: Particle acceleration. Authors: Vlahos, L.; Machado, M. E.; Ramaty, R.; Murphy, R. J.; Alissandrakis, C.; Bai, T.; Batchelor, D.; Benz, A. O.; Chupp, E.; Ellison, D.; Evenson, P.; Forrest, D. J.; Holman, G.; Kane, S. R.; Kaufmann, P.; Kundu, M. R.; Lin, R. P.; MacKinnon, A.; Nakajima, H.; Pesses, M.; Pick, M.; Ryan, J.; Schwartz, R. A.; Smith, D. F.; Trottet, G.; Tsuneta, S.; van Hoven, G. Bibcode: 1989epos.conf..127V Altcode: Contents: 1. Introduction. 2. Phenomena associated with mildly-relativistic electrons. 3. Phenomena associated with ions and relativistic electrons in solar flares. 4. Theoretical studies of particle acceleration. 5. Achievements - outstanding questions. Title: Particle acceleration and plasma heating at collisionless shocks in solar flares. Authors: Cargill, P. J.; Vlahos, L. Bibcode: 1989sasf.confP.325C Altcode: 1989IAUCo.104P.325C; 1988sasf.conf..325C Recent observations suggest that the energy release in solar flares may occur in many small bursts. If these bursts give rise to plasma heating, a large number of collisionless shocks will be generated. These shocks can individually heat plasma and accelerate particles, but the interaction of particles with many shocks as well as of shocks with each other can give rise to further heating and acceleration. Title: Dynamics of sub-relativistic electron beams in magnetic traps - A model for solar N-bursts Authors: Hillaris, A.; Alissandrakis, C. E.; Vlahos, L. Bibcode: 1988A&A...195..301H Altcode: The dynamic evolution of mildly relativistic electrons (10 - 100 keV) injected into a model magnetic trap is studied numerically, using the drift approximation. Wave-particle interactions are neglected, since the beam plasma system is shown to be non-linearly decoupled in the range of parameters used in this study. The results from the simulation are used to interpret certain observational characteristics of the N-bursts observed by the Nançay radio spectrograph. N-bursts are believed to be the first direct radio evidence for mirror effects in solar magnetic loops. Title: Narrow-bandwidth radiation processes in space and astrophysical plasmas: a review. Authors: Vlahos, L. Bibcode: 1988imdk.conf..411V Altcode: Energetic electrons "injected" or accelerated inside a dipole magnetic field radiate broad band electromagnetic radiation due to their fast gyration around the magnetic field (synchrotron emission). It is well known that trapped particles form two classes of unstable velocity distributions inside the magnetic trap: (1) a beam velocity distribution and (2) a loss cone velocity distribution. The author reviews the plasma radiation signatures from these two velocity distributions inside strongly magnetised plasmas and shows that a highly polarised narrow bandwidth signal should be expected from these magnetic traps. Such narrow bandwidth emission is observed in all known magnetic traps in space and astrophysical plasmas (e.g. Earth, Jupiter, solar and stellar atmospheres, pulsars, etc). Title: Collisionless shock formation and the prompt acceleration of solar flare ions Authors: Cargill, P. J.; Goodrich, C. C.; Vlahos, L. Bibcode: 1988A&A...189..254C Altcode: The formation mechanisms of collisionless shocks in solar flare plasmas are investigated. The priamry flare energy release is assumed to arise in the coronal portion of a flare loop as many small regions or 'hot spots' where the plasma beta locally exceeds unity. One dimensional hybrid numerical simulations show that the expansion of these 'hot spots' in a direction either perpendicular or oblique to the ambient magnetic field gives rise to collisionless shocks in a few Omega(i), where Omega(i) is the local ion cyclotron frequency. For solar parameters, this is less than 1 second. The local shocks are then subsequently able to accelerate particles to 10 MeV in less than 1 second by a combined drift-diffusive process. The formation mechanism may also give rise to energetic ions of 100 keV in the shock vicinity. The presence of these energetic ions is due either to ion heating or ion beam instabilities and they may act as a seed population for further acceleration. The prompt acceleration of ions inferred from the Gamma Ray Spectrometer on the Solar Maximum Mission can thus be explained by this mechanism. Title: Evolution of the Axial Electron Cyclotron Maser Instability, with Applications to Solar Microwave Spikes Authors: Vlahos, Loukas; Sprangle, Phillip Bibcode: 1987ApJ...322..463V Altcode: The nonlinear evolution of cyclotron radiation from streaming and gyrating electrons in an external magnetic field is analyzed. The nonlinear dynamics of both the fields and the particles are treated fully relativistically and self-consistently. The model includes a background plasma and electrostatic effects. The analytical and numerical results show that a substantial portion of the beam particle energy can be converted to electromagnetic wave energy at frequencies far above the electron cyclotron frequency. In general, the excited radiation can propagate parallel to the magnetic field and, hence, escape gyrothermal absorption at higher cyclotron harmonics. The high-frequency Doppler-shifted cyclotron instability can have saturation efficiencies far higher than those associated with well-known instabilities of the electron cyclotron maser type. Although the analysis is general, the possibility of using this model to explain the intense radio emission observed from the sun is explored in detail. Title: Electron Cyclotron Harmonic Wave Acceleration Authors: Karimabadi, H.; Menyuk, C. R.; Sprangle, P.; Vlahos, L. Bibcode: 1987ApJ...316..462K Altcode: A nonlinear analysis of particle acceleration in a finite bandwidth, obliquely propagating electromagnetic cyclotron wave is presented. It has been suggested by Sprangle and Vlahos in 1983 that the narrow bandwidth cyclotron radiation emitted by the unstable electron distribution inside a flaring solar loop can accelerate electrons outside the loop by the interaction of a monochromatic wave propagating along the ambient magnetic field with the ambient electrons. It is shown here that electrons gyrating and streaming along a uniform, static magnetic field can be accelerated by interacting with the fundamental or second harmonic of a monochromatic, obliquely propagating cyclotron wave. It is also shown that the acceleration is virtually unchanged when a wave with finite bandwidth is considered. This acceleration mechanism can explain the observed high-energy electrons in type III bursts. Title: Electron Cyclotron Maser Emission from Solar Flares Authors: Vlahos, Loukas Bibcode: 1987SoPh..111..155V Altcode: Energetic electrons, with energies 10-100 keV, accelerated during the impulsive phase of solar flares, sometimes encounter increasing magnetic fields as they stream towards the chromosphere. A consequence of the conservation of their magnetic moment is that the electrons with large initial pitch angle will be reflected at different heights from the atmosphere. Energetic electrons reflected below the transition zone will lose most of their energy to collisions and will never return to the corona. Thus, electrons reflected above the transition zone form a loss-cone velocity distribution which can be unstable to Electron Cyclotron Maser (ECM). The interaction of quasi-perpendicular shocks with the ambient coronal plasma will form a `ring' or `hollow beam' velocity distribution upstream of the shock. `Ring' velocity distributions are also unstable to the ECM instability. A review of the recent results on the theory of ECM will be presented. We will focus our discussion on the questions: (a) What are the characteristics of the linear growth rate of the ECM during solar flares? (b) How does the ECM saturate and what is its efficiency? (c) How does the ECM generated radiation modify the flare environment? Finally we will review the outstanding questions in the theory of ECM and we will relate the theoretical predictions to current observations. Title: Phenomena Associated with Ions and Relativistic Electrons Authors: Vlahos, L.; Machado, M. E.; Ramaty, R.; Murphy, R. J.; Allisandrakis, C.; Bai, T.; Batchelor, D.; Benz, A. O.; Chupp, E.; Ellison, D.; Evenson, P.; Forrest, D. J.; Holman, G.; Kane, S. R.; Kaufmann, P.; Kundu, M. R.; Lin, R. P.; MacKinnon, A.; Nakajima, H.; Pesses, M.; Pick, M.; Ryan, J.; Schwartz, R. A.; Smith, D. F.; Trottet, G.; Tsuneta, S.; van Hoven, G. Bibcode: 1986epos.conf.2.30V Altcode: 1986epos.confB..30V No abstract at ADS Title: Modeling of ion acceleration through drift and diffusion at interplanetary shocks Authors: Decker, R. B.; Vlahos, L. Bibcode: 1986JGR....9113349D Altcode: We describe a test particle simulation designed to study energetic charged particle acceleration at oblique fast-mode shocks when magnetic fluctuations exist upstream and downstream of the shock. The technique consists of integrating along exact particle orbits in a system where the angle θ1 between the shock normal and mean upstream magnetic field, the level of magnetic fluctuations, and the energy of injected particles can assume a range of values. This allows us to study time-dependent shock acceleration under conditions not amenable to analytical techniques. To illustrate the capability of the numerical mode, we consider proton acceleration under conditions appropriate for interplanetary shocks near 1 AU, including large-amplitude transverse magentic fluctuations derived from power spectra of both ambient and shock associated MHD waves.

With all other parameters held fixed, protons injected at θ1=0° and θ1=60° shocks are accelerated from 10 keV to ~100 keV and ~1 MeV, repectively, within 300 gyroperiods by a combination of the shock drift and first-order Fermi processes. The energy spectrum downstream of the 60° shock can be fit with two power laws, with spectral exponent γ=1.7 from 10 keV to ~80 keV and γ=2.6 from ~ 80 keV to 800 keV. Results are also presented for the situation where the variance and upstream extent of the shock-associated waves at the 60° shock are reduced relative to the values used at the 0° shock. Title: Mechanisms for Particle Accleration in Flares Authors: Vlahos, L.; Machado, M. E.; Ramaty, R.; Murphy, R. J.; Allisandrakis, C.; Bai, T.; Batchelor, D.; Benz, A. O.; Chupp, E.; Ellison, D.; Evenson, P.; Forrest, D. J.; Holman, G.; Kane, S. R.; Kaufmann, P.; Kundu, M. R.; Lin, R. P.; MacKinnon, A.; Nakajima, H.; Pesses, M.; Pick, M.; Ryan, J.; Schwartz, R. A.; Smith, D. F.; Trottet, G.; Tsuneta, S.; van Hoven, G. Bibcode: 1986epos.conf.2.42V Altcode: 1986epos.confB..42V No abstract at ADS Title: Phenomena Associated with Mildly Relativistic Electrons Authors: Vlahos, L.; Machado, M. E.; Ramaty, R.; Murphy, R. J.; Allisandrakis, C.; Bai, T.; Batchelor, D.; Benz, A. O.; Chupp, E.; Ellison, D.; Evenson, P.; Forrest, D. J.; Holman, G.; Kane, S. R.; Kaufmann, P.; Kundu, M. R.; Lin, R. P.; MacKinnon, A.; Nakajima, H.; Pesses, M.; Pick, M.; Ryan, J.; Schwartz, R. A.; Smith, D. F.; Trottet, G.; Tsuneta, S.; van Hoven, G. Bibcode: 1986epos.conf..2.2V Altcode: 1986epos.confB...2V No abstract at ADS Title: Numerical Studies of Particle Acceleration at Turbulent, Oblique Shocks with an Application to Prompt Ion Acceleration during Solar Flares Authors: Decker, R. B.; Vlahos, L. Bibcode: 1986ApJ...306..710D Altcode: In this paper we address the problem of charged particle acceleration at oblique, fast-mode collisionless MHD shock waves when magnetic turbulence exists in the regions upstream and downstream of the shock. Specifically, we consider how the acceleration rate depends upon the angle θ1 between the shock normal and the mean upstream magnetic field. To handle the general situation where θ1, the turbulence level, the shock strength, and the energy of injected particles can assume a range of values, we perform fully relativistic, test particle simulations that involve integrating along particle phase space orbits in the shock turbulence system. As an application of the numerical code, we study proton acceleration at shocks under conditions appropriate to the lower solar corona to simulate prompt ion acceleration during solar flares. Particles undergo shock acceleration through a combination of the shock drift and first-order Fermi processes. For protons injected at 100 keV and left in the system for 500 gyroperiods (∼ 7 ms in a 50 G magnetic field) we obtain the following results: (1) the percentage of protons accelerated above 10 MeV within 7 ms increases with increasing θ1, from 0% at θ1 = 0° to a maximum of 9% at θ1= 60° (2) the case θ1 = 75° produces the largest, most rapid energy gains, with ∼1% of the protons accelerated above 50 MeV; (3) for 45° < θ1 < 75° , a separate proton population with energies between 100 keV and 10 MeV is produced during a superfast acceleration phase lasting only ∼ 10 gyroperiods (∼ 100 μs) after injection; (4) we compare the peak energy reached at O = 0 and energy spectrum produced at θ1 = 75° with predictions from theoretical models, and find reasonable agreement, although discrepancies do exist. We discuss the implications of the numerical results as they pertain to time constraints and collisional loss processes during shock acceleration in the solar corona. Title: Timescales for Shock Formation in Collisionless Plasmas Authors: Cargill, P. J.; Goodrich, C. C.; Vlahos, L. Bibcode: 1986BAAS...18..710C Altcode: No abstract at ADS Title: Nonlinear analysis of a relativistic beam-plasma cyclotron instability Authors: Sprangle, P.; Vlahos, L. Bibcode: 1986PhRvA..33.1261S Altcode: A self-consistent set of nonlinear and relativistic wave-particle equations are derived for a magnetized beam-plasma system interacting with electromagnetic cyclotron waves. In particular, the high-frequency cyclotron mode interacting with a streaming and gyrating electron beam within a background plasma is considered in some detail. This interaction mode may possibly find application as a high-power source of coherent short-wavelength radiation for laboratory devices. The background plasma, although passive, plays a central role in this mechanism by modifying the dielectric properties in which the magnetized electron beam propagates. For a particular choice of the transverse beam velocity (i.e., the speed of light divided by the relativistic mass factor), the interaction frequency equals the nonrelativistic electron cyclotron frequency times the relativistic mass factor. For this choice of transverse beam velocity the detrimental effects of a longitudinal beam velocity spread is virtually removed. Power conversion efficiencies in excess of 18 percent are both analytically calculated and obtained through numerical simulations of the wave-particle equations. The quality of the electron beam, degree of energy and pitch angle spread, and its effect on the beam-plasma cyclotron instability is studied. Title: Particle acceleration. Authors: Vlahos, L.; Machado, M. E.; Ramaty, R.; Murphy, R. J.; Allisandrakis, C.; Bai, T.; Batchelor, D.; Benz, A. O.; Chupp, E.; Ellison, D.; Evenson, P.; Forrest, D. J.; Holman, G.; Kane, S. R.; Kaufmann, P.; Kundu, M. R.; Lin, R. P.; MacKinnon, A.; Nakajima, H.; Pesses, M.; Pick, M.; Ryan, J.; Schwartz, R. A.; Smith, D. F.; Trottet, G.; Tsuneta, S.; van Hoven, G. Bibcode: 1986NASCP2439....2V Altcode: Contents: 1. Introduction. 2. Phenomena associated with mildly-relativistic electrons: soft and hard X-ray source structure, location and development, microwave source structure, location and development, time structures and time delays in radio and hard X-rays, microwave rich flares, decimetric - metric observations and comparison with X-ray observations, discussion of models for X-ray and microwave emission. 3. Phenomena associated with ions and relativistic electrons: gamma-ray observations, neutron observations, implications of gamma-ray and neutron observations, interplanetary charged-particle observations, acceleration mechanisms. 4. Mechanisms for particle acceleration in flares: particle acceleration in reconnecting magnetic fields, electron acceleration along the magnetic field with sub-Dreicer electric fields, lower hybrid waves, Fermi acceleration and MHD turbulence, shock acceleration, acceleration of electrons by intense radio waves, preferential acceleration of heavy ions. 5. Achievements - outstanding questions. Title: Theoretical studies on rapid fluctuations in solar flares. Authors: Vlahos, Loukas Bibcode: 1986NASCP2449..455V Altcode: 1986rfsf.nasa..455V Rapid fluctuations in the emission of solar bursts may have many different origins. In two separate studies (Vlahos, Sharma, and Papadopulos, 1983, see Abstr. 34.073.209; Vlahos and Rowland, 1984, see Abstr. 38.073.055) the conditions for rapid fluctuations in solar flare driven emissions are analysed. Title: Energetic Ion Acceleration at Collisionless Shocks Authors: Decker, R. B.; Vlahos, L. Bibcode: 1985ICRC....4..166D Altcode: 1985ICRC...19d.166D An example is presented from a test particle simulation designed to study ion acceleration at oblique turbulent shocks. For conditions appropriate at interplanetary shocks near 1 AU, it is found that a shock with thetaB n = 60 deg is capable of producing an energy spectrum extending from 10 keV to approx. 1 MeV in approx 1 hour. In this case total energy gains result primarily from several separate episodes of shock drift acceleration, each of which occurs when particles are scattered back to the shock by magnetic fluctuations in the shock vicinity. Title: Prompt acceleration of ions by oblique turbulent shocks in solar flares. Authors: Decker, R. B.; Vlahos, L. Bibcode: 1985ICRC....4...10D Altcode: 1985ICRC...19d..10D Solar flares often accelerate ions and electrons to relativistic energies. The details of the acceleration process are not well understood, but until recently the main trend was to divide the acceleration process into two phases. During the first phase elctrons and ions are heated and accelerated up to several hundreds of keV simultaneously with the energy release. These mildly relativistic electrons interact with the ambient plasma and magnetic fields and generate hard X-ray and radio radiation. The second phase, usually delayed from the first by several minutes, is responsible for accelerating ions and electrons to relativistic energies. Relativistic electrons and ions interact with the solar atmosphere or escape from the Sun and generate gamma ray continuum, gamma ray line emission, neutron emission or are detected in space by spacecraft. In several flares the second phase is coincident with the start of a type 2 radio burst that is believed to be the signature of a shock wave. Observations from the Solar Maximum Mission spacecraft have shown, for the first time, that several flares accelerate particles to all energies nearly simultaneously. These results posed a new theoretical problem: How fast are shocks and magnetohydrodynamic turbulence formed and how quickly can they accelerate ions to 50 MeV in the lower corona? This problem is discussed. Title: Electron cyclotron maser instability in the solar corona - The role of superthermal tails Authors: Vlahos, L.; Sharma, R. R. Bibcode: 1985ApJ...290..347V Altcode: The effect of a superthermal component of electrons on the loss-cone-driven electron cyclotron maser instability is analyzed. It is found that for a superthermal tail with temperature about 10 KeV, the first harmonic (X- and O-mode) is suppressed for n(t)/n(r) of about 1 (n/t/ and n/r/ are the densities of superthermal tail and loss-cone electrons) and the second harmonic (X- and O-modes) is suppressed for n(t)/n(r) less than about 0.1. A qualitative discussion on the formation of superthermal tails is presented and it is suggested that superthermal tails play an important role on the observed or available power, at microwave frequencies, from the electron cyclotron maser instability in the solar corona. Title: Return currents in solar flares - Collisionless effects Authors: Rowland, H. L.; Vlahos, L. Bibcode: 1985A&A...142..219R Altcode: If the primary, precipitating electrons in a solar flare are unstable to beam plasma interactions, it is shown that strong Langmuir turbulence can seriously modify the way in which a return current is carried by the background plasma. In particular, the return (or reverse) current will not be carried by the bulk of the electrons, but by a small number of high velocity electrons. For beam/plasma densities >10-3, this can reduce the effects of collisions on the return current. For higher density beams where the return current could be unstable to current driven instabilities, the effects of strong turbulence anomalous resistivity is shown to prevent the appearance of such instabilities. Again in this regime, how the return current is carried is determined by the beam generated strong turbulence. Title: The coalescence instability in solar flares Authors: Tajima, T.; Brunel, F.; Sakai, J. -I.; Vlahos, L.; Kundu, M. R. Bibcode: 1985IAUS..107..197T Altcode: The nonlinear coalescence instability of current carrying solar loops can explain many of the characteristics of the solar flares such as their impulsive nature, heating and high energy particle acceleration, amplitude oscillations of electromagnetic and emission as well as the characteristics of two-dimensional microwave images obtained during a flare. The plasma compressibility leads to the explosive phase of loop coalescence and its overshoot results in amplitude oscillations in temperatures by adiabatic compression and decompression. It is noted that the presence of strong electric fields and super-Alfvenic flows during the course of the instability play an important role in the production of nonthermal particles. A qualitative explanation on the physical processes taking place during the nonlinear stages of the instability is given. Title: Shock drift acceleration in the presence of waves Authors: Decker, R. B.; Vlahos, L. Bibcode: 1985JGR....90...47D Altcode: Charged particle acceleration via the shock drift mechanism at quasi-perpendicular shocks has generally been analyzed by assuming uniform, time-independent conditions at and near the shock. We present results from a model designed to study how the shock drift mechanism is modified when wave activity is included in the shock's upstream and downstream vicinities. The technique involves numerically following test particle trajectories in the wave-shock system for predefined wave fields. In order to compare these results with those obtained in the scatter-free (i.e., nonwave) case, we restricted particles to a single shock encounter, which is here defined as the period during which the particle remains within a gyrodiameter of the shock. As a particular example, we injected ensembles of ions into a system consisting of a quasi-perpendicular shock moving through the interplanetary spectrum of ambient Alfvén waves. As compared with a single encounter in the scatter-free limit, the inclusion of waves (1) increases particle transmission through the shock, (2) produces broader energy distributions for reflected and transmitted particles, with high-energy tails at energies several times the maximum energy obtained in the scatter-free case and (3) reduces anisotropies, particularly of reflected particles, but does not eliminate them. Also, for the range of energies studied, it was found that the approximate invariance of the magnetic moment for particle interactions with quasi-perpendicular shocks is no longer valid when waves are present. Title: Collisionless effects on beam-return current systems in solar flares Authors: Vlahos, L.; Rowland, H. L. Bibcode: 1985IAUS..107..521V Altcode: A theoretical study of the beam-return current system (BRCS) in solar flares shows that the precipitating electrons modify the way in which the return current (RC) is carried by the background plasma. In particular it is found that the RC is not carried by the bulk of the electrons but by a small number of high-velocity electrons. For beam/plasma densities exceeding approximately 0.001, this can reduce the effects of collisions and heating by the RC. For higher-density beams, where the RC could be unstable to current-driven instabilities, the effects of strong turbulence anomalous resistivity prevent the appearance of such instabilities. The main conclusion is that the BRCS is interconnected, and that the beam-generated strong turbulence determines how the RC is carried. Title: Electron precipitation in solar flares - Collisionless effects Authors: Vlahos, L.; Rowland, H. L. Bibcode: 1984A&A...139..263V Altcode: A large fraction of the electrons which are accelerated during the impulsive phase of solar flares stream towards the chromosphere and are unstable to the growth of plasma waves. The linear and non-linear evolution of plasma waves as a function of time is analyzed with the use of a set of rate equations that follow in time the nonlinearly coupled system of plasma waves - ion fluctuations. As an outcome of the fast transfer of wave energy from the beam to the ambient plasma, non-thermal electron tails are formed which can stabilize the anomalous Doppler resonance instability that is responsible for the pitch angle scattering of the beam electrons. The authors estimate the non-collisional losses of the precipitating electrons and discuss the observational implications of their results. Title: Predictions of lithium interactions with earth's bow shock in the presence of wave activity Authors: Decker, R. B.; Lui, A. T. Y.; Vlahos, L. Bibcode: 1984JGR....89.7331D Altcode: The results of a test-particle simulation studying the movement of a lithium tracer ion injected upstream of the bow shock are reported. Wave activity consists of parallel and antiparallel propagating Alfven waves characterized by a frequency power spectrum within a frequency or range of amplitudes defined separately in the upstream and downstream regions. The results show that even a moderate level of wave activity can substantially change the results obtained in the absence of waves. Among the effects observed are: (1) increased ion transmission; (2) both the average energy gain and spread about the average are increased for transmitted and reflected particles; (3) the average final pitch angle for transmitted particles tends to 90 deg, and the spread of reflected particles is reduced; and (4) the spatial dispersion of the ions on the bow shock after a single encounter is increased. Title: Comparative study of the loss cone-driven instabilities in the low solar corona Authors: Sharma, R. R.; Vlahos, L. Bibcode: 1984ApJ...280..405S Altcode: A comparative study of the loss cone-driven instabilities in the low solar corona is undertaken. The instabilities considered are the electron cyclotron maser, the whistler, and the electrostatic upper hybrid. It is shown that the first-harmonic extraordinary mode of the electron cyclotron maser instability is the fastest growing mode for strong magnetized plasma (the ratio of plasma frequency to cyclotron frequency being less than 0.35). For values of the ratio between 0.35 and 1.0, the first-harmonic ordinary mode of the electron cyclotron maser instability dominates the emission. For ratio values greater than 1.0, no direct electromagnetic radiation is expected since other instabilities, which do not escape directly, saturate the electron cyclotron maser (the whistler or the electrostatic upper hybrid waves). It is also shown that the second-harmonic electron cyclotron maser emission never grows to an appreciable level. Thus, it is suggested that the electron cyclotron maser instability can be the explanation for the escape of the first harmonic from a flaring loop. Title: Dynamic Development of Run-Away Tails in Flares Authors: Moghaddam-Taaheri, E.; Vlahos, L.; Rowland, H. L.; Papadopoulos, K. Bibcode: 1984BAAS...16..536M Altcode: No abstract at ADS Title: Particle Acceleration in Flares Authors: Vlahos, L. Bibcode: 1984BAAS...16..511V Altcode: No abstract at ADS Title: Electron Cyclotron Maser Instability: The Role of Superthermal Tails Authors: Sharma, R. R.; Vlahos, L. Bibcode: 1984BAAS...16..536S Altcode: No abstract at ADS Title: Collisionless Effects on Beam/Return Current Systems in Flares Authors: Rowland, H. L.; Vlahos, L. Bibcode: 1984BAAS...16R.544R Altcode: No abstract at ADS Title: Flare Induced Stimulated Electron Cyclotron Radiation Authors: Sprangle, P.; Vlahos, L.; Papadopoulos, K. Bibcode: 1984BAAS...16R.524S Altcode: No abstract at ADS Title: Particle acceleration. Authors: Rosner, R.; Chupp, E. L.; Gloeckler, G.; Gorney, D. J.; Krimigis, S. M.; Mok, Y.; Ramaty, R.; Swift, D. W.; Vlahos, L.; Zweibel, E. G. Bibcode: 1984NASRP1120....2R Altcode: Contents: 1. Introduction. 2. Phenomenology: Solar flares. Acceleration processes in the interplanetary medium. Magnetospheric and ionospheric observations. Particle acceleration outside the solar system. 3. Theoretical particle acceleration mechanisms: Adiabatic compression, magnetic pumping, and diffusion. Acceleration in direct electric fields. Stochastic acceleration. Shock particle acceleration. Coherent wave acceleration. Injection. 4. The remaining questions. Title: Stochastic three-wave interaction in flaring solar loops Authors: Vlahos, L.; Sharma, R. R.; Papadopoulos, K. Bibcode: 1983ApJ...275..374V Altcode: A model is proposed for the dynamic structure of high-frequency microwave bursts. The dynamic component is attributed to beams of precipitating electrons which generate electrostatic waves in the upper hybrid branch. Coherent upconversion of the electrostatic waves to electromagnetic waves produces an intrinsically stochastic emission component which is superposed on the gyrosynchrotron continuum generated by stably trapped electron fluxes. The role of the density and temperature of the ambient plasma in the wave growth and the transition of the three wave upconversion to stochastic, despite the stationarity of the energy source, are discussed in detail. The model appears to reproduce the observational features for reasonable parameters of the solar flare plasma. Title: Electron cyclotron wave acceleration outside a flaring loop Authors: Sprangle, P.; Vlahos, L. Bibcode: 1983ApJ...273L..95S Altcode: A model for the secondary acceleration of electrons outside a flaring loop is proposed. The results suggest that the narrow bandwidth radiation emitted by the unstable electron distribution inside a flaring loop can become the driver for secondary electron acceleration outside the loop. It is shown that a system of electrons gyrating about and streaming along an adiabatically spatially varying, static magnetic field can be efficiently accelerated to high energies by an electromagnetic wave propagating along and polarized transverse to the static magnetic field. The predictions from our model appear to be in general agreement with existing observations. Title: Solar microwave bursts — A review Authors: Kundu, M. R.; Vlahos, L. Bibcode: 1982SSRv...32..405K Altcode: We review the observational and theoretical results on the physics of microwave bursts that occur in the solar atmosphere. We particularly emphasize the advances made in burst physics over the last few years with the great improvement in spatial and time resolution especially with instruments like the NRAO three element interferometer, Westerbork Synthesis Radio Telescope and more recently the Very Large Array (VLA). We review the observations on pre-flare build-up of an active region at centimeter wavelengths. In particular we discuss the observations that in addition to the active region undergoing brightness and polarization changes on time scales of the order of an hour before a flare, there can be a change of the sense of polarization of a component of the relevant active region situated at the same location as the flare, implying the emergence of a flux of reverse polarity at coronal levels. The intensity distribution of cm-λ bursts is similar to that of soft X-ray and hard X-ray bursts. Indeed, it appears that the flaring behavior of the Sun at cm wavelengths is similar to that of some other cosmic transients such as flare stars and X-ray bursters. We discuss three distinct phases in the evolution of cm bursts, namely, impulsive phase, post-burst phase, and gradual rise and fall. The radiation mechanism for the impulsive phase of the microwave burst is gyrosynchrotron emission from mildly relativistic electrons that are accelerated near the energy release site and spiral in the strong magnetic field in the low corona. The details of the velocity distribution function of the energetic electrons and its time evolution are not known. We review the spectral characteristics for two kinds of velocity distribution, e.g., Maxwellian and Maxwellian with a power law tail for the energetic electrons. In the post-burst phase the energetic electrons are gradually thermalized. The thermal plasma released in the energy release region as well as the expanded parts of the overheated upper chromosphere may alter the emission mechanism. Thus, in the post-burst phase, depending on the average density and temperature of the thermal plasma, the emission mechanism may change from gyrosynchrotron to collisional bremsstrahlung from a thermal plasma. The gradual rise and fall (GFR) burst represents the heating of a flare plasma to temperatures of the order of 106 K, in association with a flare or an X-ray transient following a filament disruption. We discuss the flux density spectra of centimeter bursts. The great majority of the bursts have a single spectral maximum, commonly around 6 cm-λ The U-shaped signature sometimes found in cm-dcm burst spectrum of large bursts is believed to a be a reflection of only the fact that there are two different sources of burst radiation, one for cm-λ and the other for dcm-λ, with different electron energy distributions and different magnetic fields. Observations of fine structures with temporal resolutionof 10 100 ms in the intensity profiles of cm-λ bursts are described. The existence of such fine time structures imply brightness temperatures in burst sources of order 1015 K; their interpretation in terms of gyrosynchrotron measuring or the coherent interaction of upper hybrid waves excited by percipitating electron beams in a flaring loop is discussed. High spatial resolution observations (a few seconds of arc to ∼ 1″ arc) are discussed, with special reference to the one- and two-dimensional maps of cm burst sources. The dominance of one sense of circular polarization in some weak 6 cm bursts and its interpretation in terms of energetic electrons confined in an asymmetric magnetic loop is discussed. Two-dimensional snapshot maps obtained with the VLA show that multi-peak impulsive 6 cm burst phase radiation originates from several arcades of loops and that the burst source often occupies a substantial portion of the flaring loop, and is not confined strictly to the top of the loop. This phenomenon is interpreted in terms of the trapping of energetic electrons due to anomalous doppler resonance instability and the characteristic scale length of the magnetic field variation along the loop. The VLA observations also indicate that the onset of the impulsive phase of a 6 cm burst can be associated with the appearance of a new system of loops. The presence of two loop systems with opposite polarities or a quadrupole field configuration is reminiscent of flare models in which a current sheet develops in the interface between two closed loops. We provide an extensive review of the emission and absorption processes in thermal and non-thermal velocity distributions. Unlike the thermal plasma where absorption and emission are inter-related through Kirchoff's law, the radiation emitted from a small population of non-thermal electrons can be reabsorbed from the same electrons (self-absorption) or from the background (thermal) electrons through gyro-resonance absorption, and free-free absorption. We also suggest that the non-thermal electrons can be unstable and these instabilities can be the source of very high brightness temperature, fine structure (∼ 10 ms) pulsations. Finally in the last part of this review we present several microwave burst models-the magnetic trap model, the two-component model, thermal model and the flaring loop model and give a critical discussion of the strength and weakness of these models. Title: A Model for the Simultaneous Release of Hard X-ray and Type III Bursts Authors: Vlahos, L.; Sprangle, P. Bibcode: 1982BAAS...14..874V Altcode: No abstract at ADS Title: The importance of plasma effects on electron-cyclotron maser-emission from flaring loops Authors: Sharma, R. R.; Vlahos, L.; Papadopoulos, K. Bibcode: 1982A&A...112..377S Altcode: Electron cyclotron maser instability has been suggested as the cause of the observed short (10-20 msec), intense (an approximate brightness temperature of 10 to the 15th K) and up to 100% polarized microwave solar emission. It is shown that plasma effects and thermal cyclotron damping, ignored in previous theories, play an important role in controlling the frequency range of the emission. The radio emission is suppressed for ratios of the plasma frequency to the cyclotron frequency smaller than 0.4. An examination of the cyclotron damping, reveals that the maser action is suppressed unless a large fraction (i.e., over 10%) of the accelerated electrons participates in the emission process. Title: Electron acceleration and radiation signatures in loop coronal transients Authors: Vlahos, L.; Gergely, T. E.; Papadopoulos, K. Bibcode: 1982ApJ...258..812V Altcode: It is proposed that in loop coronal transients an erupting loop moves away from the solar surface, with a velocity exceeding the local Alfven speed, pushing against the overlying magnetic fields and driving a shock in the front of the moving part of the loop. Lower hybrid waves are excited at the shock front and propagate radially toward the center of the loop with phase velocity along the magnetic field that exceeds the thermal velocity. The lower hybrid waves stochastically accelerate the tail of the electron distribution inside the loop. The manner in which the accelerated electrons are trapped in the moving loop are discussed, and their radiation signature is estimated. It is suggested that plasma radiation can explain the power observed in stationary and moving type IV bursts. Title: Radio imaging of solar flares using the very large array - New insights into flare process Authors: Kundu, M. R.; Schmahl, E. J.; Velusamy, T.; Vlahos, L. Bibcode: 1982A&A...108..188K Altcode: An interpretation of VLA observations of microwave bursts is presented in an attempt to distinguish between certain models of flares. The VLA observations provide information about the pre-flare magnetic field topology and the existence of mildly relativistic electrons accelerated during flares. Examples are shown of changes in magnetic field topology in the hour before flares. In one case, new bipolar loops appear to emerge, which is an essential component of the model developed by Heyvaerts et al. (1977). In another case, a quadrupole structure, suggestive of two juxtaposed bipolar loops, appears to trigger the flare. Because of the observed diversity of magnetic field topologies in microwave bursts, it is believed that the magnetic energy must be dissipated in more than one way. The VLA observations are clearly providing means for sorting out the diverse flare models. Title: Limitation the upconversion of ion sound to langmuir turbulence. Authors: Vlahos, L.; Papadopoulos, K. Bibcode: 1982ApJ...252L..75V Altcode: The weak turbulence theory of Tsytovich, Stenflo and Wilhelmsson (1981) for evaluation of the nonlinear transfer of ion acoustic waves to Langmuir waves is shown to be limited in its region of validity to the level of ion acoustic waves. It is also demonstrated that, in applying the upconversion of ion sound to Langmuir waves for electron acceleration, nonlinear scattering should be self-consistently included, with a suppression of the upconversion process resulting. The impossibility of accelerating electrons by such a process for any reasonable physical system is thereby reaffirmed. Title: Radiation signatures from a locally energized flaring loop Authors: Emslie, A. G.; Vlahos, L. Bibcode: 1980ApJ...242..359E Altcode: The radiation signatures from a locally energized solar flare loop based on the physical properties of the energy release mechanisms were consistent with hard X-ray, microwave, and EUV observations for plausible source parameters. It was found that a suprathermal tail of high energy electrons is produced by the primary energy release, and that the number of energetic charged particles ejected into the interplanetary medium in the model is consistent with observations. The radiation signature model predicts that the intrinsic polarization of the hard X-ray burst should increase over the photon energy range of 20 to 100 keV. Title: Microwave emission from flaring magnetic loops Authors: Vlahos, L. Bibcode: 1980IAUS...86..173V Altcode: Characteristics of the microwave emission from a flaring loop are discussed under the assumptions that (a) the magnetic energy is released into a small volume compared to the volume of the loop, and the rate at which magnetic energy is transformed into plasma energy is faster than the energy losses from the same volume; and (b) the bulk of the energy released goes into the heating of the plasma and heats primarily the electrons. A mechanism by which mildly relativistic electrons escape from the flaring loop to the upper corona is proposed. Title: On the upconversion of ion-sound to Langmuir turbulence Authors: Vlahos, L.; Papadopoulos, K. Bibcode: 1979ApJ...234L.217V Altcode: Consideration is given to the upconversion of ion sound waves to excited Langmuir waves in solar flares. It is shown that, due to the process of anomalous high-frequency resistivity in which the reabsorption of Langmuir waves in the presence of ion waves is enhanced, the upconversion of ion sound waves to Langmuir waves is impossible in the absence of suprathermal particles and when the electron drift velocity is less than the electron thermal velocity. Title: Collective plasma effects associated with the continuous injection model of solar flare particle streams. Authors: Vlahos, L.; Papadopoulos, K. Bibcode: 1979ApJ...233..717V Altcode: A modified continuous injection model for impulsive solar flares that includes self-consistent plasma nonlinearities based on the concept of marginal stability is presented. A quasi-stationary state is established, composed of a hot truncated electron Maxwellian distribution confined by acoustic turbulence on the top of the loop and energetic electron beams precipitating in the chromosphere. It is shown that the radiation properties of the model are in accordance with observations. Title: An interpretation of the polarization structure of microwave bursts. Authors: Kundu, M. R.; Vlahos, L. Bibcode: 1979ApJ...232..595K Altcode: High-spatial-resolution (a few seconds of arc) observations of microwave bursts have demonstrated that only the impulsive phase of the burst is polarized; one observes only one polarity in the burst source if it is weak (Alissandrakis and Kundu) and both polarities if it is intense (Enome et al.). These results are interpreted in terms of an asymmetrical bipolar field structure of the loop in which the energetic electrons responsible for the radiation are contained. The role of unequal field strengths at the feet of the loop on the number of electrons trapped and their pitch angle distribution are discussed in a specific model. Computations of the polarized intensity originating from each foot of the loop seem to be consistent with the observations at present available. Title: Plasma properties and radiation signatures of current driven active region loops on the Sun Authors: Vlahos, L. Bibcode: 1979PhDT.........2V Altcode: The average plasma density in the coronal part of a loop is higher than in its surroundings but the ratio of the plasma kinetic pressure to magnetic pressure is less than one. These loop-like magnetic field lines may act as the energy transport lines from the internal layers of the Sun's photosphere to the chromosphere and the corona where the energy is stored and suddenly released or slowly dissipated. The photospheric disturbances which are associated with steady currents along the potential magnetic field lines were studied in order to identify the radiation signatures of the current density and its profile as well as the size of the loop in which the current flows. Three basic phenomena in which the current supposedly plays an important role are examined: the sudden release of energy in small loop-like structures (flares); the heating of loops of different sizes by anomalous current dissipation; and the sudden erosion of large scale loops (coronal transients). It is concluded that (1) the current driven from the fluid motions in the photosphere can, under certain conditions, trigger the tearing mode instability in a finite volume at the top of a small loop which may result in a sudden local heating and a transient electron acceleration; (2) the heating of a loop by 'anomalous' dissipation of currents is not feasible if the currents are driven from the photosphere and (3) it appears that under certain conditions the current flowing along a very large loop can force a loop upwards. Title: Plasma Properties and Radiation Signatures of Current Driven Active Region Loops on the Sun. Authors: Vlahos, Loukas Bibcode: 1979PhDT........33V Altcode: Our present understanding of the various phenomena taking place in the Sun's atmosphere has radically changed over the past few years. Recent observations, primarily from Skylab, have confirmed the existence of small scale structures which in turn have put severe constraints on the homogeneous plain parallel models of the Sun's atmosphere traditionally used to analyze the observations. It is now generally accepted that the small scale magnetic fields are responsible for these structures. Generally speaking, three types of regions can be identified on the Sun depending on the dominant magnetic field structure, i.e. the coronal holes, where the magnetic field lines are diffuse, irregular and weak; and the active regions, which are characterized by strong closed magnetic field structures. We usually identify as loops the magnetic field structures connecting opposite magnetic polarities in the photosphere. The average plasma density in the coronal part of the loop is higher than in its surroundings but the ratio of the plasma kinetic pressure to magnetic pressure is less than one. These loop-like magnetic field lines may act as the energy transport lines from the internal layers of the Sun's photosphere to the chromosphere and the corona where the energy is stored and suddenly released or slowly dissipated. We focus our studies at the photospheric disturbances which are associated with steady currents along the potential magnetic field lines. The current density and its profile as well as the size of the loop in which the current flows have important radiation signatures which we have tried to identify in our study. We examine three basic phenomena in which the current supposedly plays an important role: The sudden release of energy in small loop-like structures (flares), the heating of loops of different sizes by anomalous current dissipation and the sudden eruption of large scale loops (coronal transients). Our main conclusions are the following:. (a) The current driven from the fluid motions in the photosphere can, under certain conditions, trigger the tearing mode instability in a finite volume at the top of a small loop (length L > 10('9) cm). This may result in a sudden local heating and a transient electron acceleration. We studied the dynamical evolution of this "explosion" and analyzed its hard x-ray, EUV microwave and meter wavelength signatures for the impulsive phase (e.g. t < 1-10 sec). We have also investigated the microwave and soft x-ray emission of a typical flaring loop in the post-burst phase. (b) The heating of a loop by "anomalous" dissipation of currents is not feasible if the currents are driven from the photosphere. We showed that low frequency current -driven instabilities cannot be excited in large steady loops (L > 10('10) cm). Further, we showed that if the current-driven low frequency instabilities are excited, several radiation signatures should be associated with the loop that is heated. (c) It appears that under certain conditions the current flowing along a very large loop (L > 10('11) cm) can force a loop upwards. We examined the interaction of the erupting loops with the surrounding magnetic field. We found that the loop could act as a piston driving and forming a magnetic shock at the front edge of the loop, if the expansion velocity is larger than the local Alfven velocity. Electrons are accelerated in the wake of the shock (e.g. in the interior of the loop) and subsequently trapped. We studied the dynamic evolution of the accelerated electrons and their radio signatures. We found that many of the observed radio bursts associated with coronal transients can easily be explained by this model.