bluuebunny/crossref_metadata_2025_split huggingface dataset:
A new viscous instability in weakly ionised protoplanetary discs
Authors: Johansen Anders , Kato Mariko , Sano Takayoshi 
Published: June 2011

AbstractLarge regions of protoplanetary discs are believed to be too weakly ionised to support magnetorotational instabilities, because abundant tiny dust grains soak up free electrons and reduce the conductivity of the gas. At the outer edge of this “dead zone”, the ionisation fraction increases gradually and the resistivity drops until the magnetorotational instability can develop turbulence. We identify a new viscous instability which operates in the semi-turbulent transition region between “dead” and “alive” zones. The strength of the saturated turbulence depends strongly on the local resistivity in this transition region. A slight increase (decrease) in dust density leads to a slight increase (decrease) in resistivity and a slight decrease (increase) in turbulent viscosity. Such spatial variation in the turbulence strength causes a mass pile-up where the turbulence is weak, leading to a run-away process where turbulence is weakened and mass continues to pile up. The final result is the appearance of high-amplitude pressure bumps and deep pressure valleys. Here we present a local linear stability analysis of weakly ionised accretion discs and identify the linear instability responsible for the pressure bumps. A paper in preparation concerns numerical results which confirm and expand the existence of the linear instability.
Weakly imbalanced strong turbulence
Authors: Beresnyak Andrey 
Published: June 2011

AbstractThe theory of strong MHD turbulence with cross-helicity has been a subject of many recent studies. In this paper we focused our attention on low-imbalance limit and performed high-resolution 3D simulations. The results suggest that in this limit both w+=v+b and w−=v−b are cascaded strongly. The model for imbalance based on so-called “dynamic alignment” strongly contradicts numerical evidence.
Similarity of Jupiter and RRATs
Authors: Gezer Ilknur , Pekünlü E. Rennan 
Published: June 2011

AbstractIn the present investigation, radial diffusion of equatorially trapped electrons in the magnetospheres of Jupiter and Rotating Radio Transients (RRATs) are examined and compared. It is assumed that electrons lose energy through synchrotron radiation and the wave-particle interaction. The phase space density of the electrons, which go through gradB drift in Jupiter's and RRATs magnetospheres and thus resonate with the plasma waves, changes and this change predicted by the model seems to be consistent with the Pioneer 10 and Pioneer 11 data for Jupiter's case and a similar result obtained for RRATs.
On the development of a spherical hybrid model - Lessons and applications
Authors: Dyadechkin S. , Kallio E. , Jarvinen R. , Janhunen P. , Semenov V. S. , Biernat H. K. 
Published: June 2011

AbstractWe are developing a spherical hybrid model to study how the solar wind interacts with the solar system bodies. In this brief status report we introduce some lessons from the spherical grid development and illustrate the usage of the new model by showing a preliminary test run.
Development of the PFO–CFO hypothesis of solar system formation: Why do the celestial objects have different isotopic ratios for some chemical elements?
Authors: Kadyshevich E. A. , Ostrovskii V. E. 
Published: June 2011

AbstractThe Solar System formation PFO–CFO hypothesis is developed in the direction of creation of a phenomenological model focused on solution of a number of paradoxes and answering to a number of mysterious questions under the same cover. For explanation of the events and processes that occurred over the period from the middle ages of the pre-solar star to the Solar System formation, original approaches are applied.
A note on using thermally driven solar wind models in MHD space weather simulations
Authors: Pomoell Jens , Vainio Rami 
Published: June 2011

AbstractOne of the challenges in constructing global magnetohydrodynamic (MHD) models of the inner heliosphere for, e.g., space weather forecasting purposes, is to correctly capture the acceleration and expansion of the solar wind. In many current models, the solar wind is driven by varying the polytropic index so that a desired heating is obtained. While such schemes can yield solar wind properties consistent with observations, they are not problem-free. In this work, we demonstrate by performing MHD simulations that altering the polytropic index affects the properties of propagating shocks significantly, which in turn affect the predicted space weather conditions. Thus, driving the solar wind with such a mechanism should be used with care in simulations where correctly capturing the shock physics is essential. As a remedy, we present a simple heating function formulation by which the polytropic wind can be used while still modeling the shock physics correctly.
On radiation-zone dynamos
Authors: Rüdiger Günther , Gellert Marcus , Arlt Rainer 
Published: June 2011

AbstractIt is shown that the magnetic current-driven (‘kink-type’) instability produces flow and field patterns with helicity and even with α-effect but only if the magnetic background field possesses non-vanishing current helicity B⋅ curl B by itself. Fields with positive large-scale current helicity lead to negative small-scale kinetic helicity. The resulting α-effect is positive. These results are very strict for cylindric setups without z-dependence of the background fields. The sign rules also hold for the more complicated cases in spheres where the toroidal fields are the result of the action of differential rotation (induced from fossil poloidal fields) at least for the case that the global rotation is switched off after the onset of the instability.
Emergence of intermittent structures and reconnection in MHD turbulence
Authors: Greco Antonella , Servidio Sergio , Matthaeus William H. , Dmitruk Pablo 
Published: June 2011

AbstractIn recent analyses of numerical simulation and solar wind dataset, the idea that the magnetic discontinuities may be related to intermittent structures that appear spontaneously in MHD turbulence has been explored in details. These studies are consistent with the hypothesis that discontinuity events founds in the solar wind might be of local origin as well, i.e. a by-product of the turbulent evolution of magnetic fluctuations.Using simulations of 2D MHD turbulence, we are exploring a possible link between tangential discontinuities and magnetic reconnection. The goal is to develop numerical algorithms that may be useful for solar wind applications.
Coupled Alfvén and kink oscillations in an inhomogeneous corona
Authors: Pascoe David J. , Wright Andrew N. , De Moortel Ineke 
Published: June 2011

AbstractWe perform 3D numerical simulations of footpoint-driven transverse waves propagating in a low β plasma. The presence of inhomogeneities in the density profile leads to the coupling of the driven kink mode to Alfvén modes by resonant absorption. The decay of the propagating kink wave as energy is transferred to the local Alfvén mode is in good agreement with a modified interpretation of the analytical expression derived for standing kink modes. This coupling may account for the damping of transverse velocity perturbation waves which have recently been observed to be ubiquitous in the solar corona.
Weak turbulence theory of dispersive waves in the solar corona
Authors: Spanier Felix , Vainio Rami 
Published: June 2011

AbstractThe interaction of plasma waves plays a crucial role in the dynamics of weakly turbulent plasmas. So far the interaction of non-dispersive waves has been studied. In this paper the theory is extended to dispersive waves. It is well known that dispersive waves may be found in the solar corona, where they contribute to the heating of the corona. Here the possible interactions in the solar corona are described and the interaction rates are derived in the framework of Hall MHD.
The effect of plume/interplume lanes on ion-cyclotron resonance heating
Authors: Doğan Suzan , Pekünlü E. Rennan 
Published: June 2011

AbstractThe effect of the Plume/Interplume Lane (PIPL) structure of the solar North Polar Coronal Hole (NPCH) on the ion-cyclotron resonance (ICR) process is investigated. The ICR process in the interplume lanes is much more effective than in the plumes, agreeing with the observations which show the source of fast solar wind is interplume lanes.
Small-scale flux emergence events observed by Sunrise/IMaX
Authors: Guglielmino S. L. , Pillet V. Martínez , del Toro Iniesta J. C. , Rubio L. R. Bellot , Zuccarello F. , Solanki S. K. ,   the Sunrise/IMaX team
Published: June 2011

AbstractThanks to the unprecedented combination of high spatial resolution (0″.2) and high temporal cadence (33 s) spectropolarimetric measurements, the IMaX magnetograph aboard the Sunrise balloon-borne telescope is revealing new insights about the plasma dynamics of the all-pervasive small-scale flux concentrations in the quiet Sun. We present the result of a case study concerning the appearance of a bipole, with a size of about 4″ and a flux content of 5 × 1017 Mx, with strong signal of horizontal fields during the emergence. We analyze the data set using the SIR inversion code and obtain indications about the three-dimensional shape of the bipole and its evolution with time.
An hybrid neuro-wavelet approach for long-term prediction of solar wind
Authors: Napoli Christian , Bonanno Francesco , Capizzi Giacomo 
Published: June 2011

AbstractNowadays the interest for space weather and solar wind forecasting is increasing to become a main relevance problem especially for telecommunication industry, military, and for scientific research. At present the goal for weather forecasting reach the ultimate high ground of the cosmos where the environment can affect the technological instrumentation. Some interests then rise about the correct prediction of space events, like ionized turbulence in the ionosphere or impacts from the energetic particles in the Van Allen belts, then of the intensity and features of the solar wind and magnetospheric response. The problem of data prediction can be faced using hybrid computation methods so as wavelet decomposition and recurrent neural networks (RNNs). Wavelet analysis was used in order to reduce the data redundancies so obtaining representation which can express their intrinsic structure. The main advantage of the wavelet use is the ability to pack the energy of a signal, and in turn the relevant carried informations, in few significant uncoupled coefficients. Neural networks (NNs) are a promising technique to exploit the complexity of non-linear data correlation. To obtain a correct prediction of solar wind an RNN was designed starting on the data series. As reported in literature, because of the temporal memory of the data an Adaptative Amplitude Real Time Recurrent Learning algorithm was used for a full connected RNN with temporal delays. The inputs for the RNN were given by the set of coefficients coming from the biorthogonal wavelet decomposition of the solar wind velocity time series. The experimental data were collected during the NASA mission WIND. It is a spin stabilized spacecraft launched in 1994 in a halo orbit around the L1 point. The data are provided by the SWE, a subsystem of the main craft designed to measure the flux of thermal protons and positive ions.
A shell model for turbulent dynamos
Authors: Nigro G. , Perrone D. , Veltri P. 
Published: June 2011

AbstractA self-consistent nonlinear dynamo model is presented. The nonlinear behavior of the plasma at small scale is described by using a MHD shell model for fields fluctuations; this allow us to study the dynamo problem in a large parameter regime which characterizes the dynamo phenomenon in many natural systems and which is beyond the power of supercomputers at today. The model is able to reproduce dynamical situations in which the system can undergo transactions to different dynamo regimes. In one of these the large-scale magnetic field jumps between two states reproducing the magnetic polarity reversals. From the analysis of long time series of reversals we infer results about the statistics of persistence times, revealing the presence of hidden long-time correlations in the chaotic dynamo process.
Energetic particle acceleration and transport by Alfvén/acoustic waves in tokamak-like Solar flares
Authors: Obergaulinger Martin , García-Muñoz Manuel 
Published: June 2011

AbstractAlfven/acoustic waves are ubiquitous in astrophysical as well as in laboratory plasmas. Their interplay with energetic ions is of crucial importance to understanding the energy and particle exchange in astrophysical plasmas as well as to obtaining a viable energy source in magnetically confined fusion devices. In magnetically confined fusion plasmas, an experimental phase-space characterisation of convective and diffusive energetic particle losses induced by Alfven/acoustic waves allows for a better understanding of the underlying physics. The relevance of these results in the problem of the anomalous heating of the solar corona is checked by MHD simulations of Tokamak-like Solar flare tubes.
Hybrid Vlasov simulations for alpha particles heating in the solar wind
Authors: Perrone Denise , Valentini Francesco , Veltri Pierluigi 
Published: June 2011

AbstractHeating and acceleration of heavy ions in the solar wind and corona represent a long-standing theoretical problem in space physics and are distinct experimental signatures of kinetic processes occurring in collisionless plasmas. To address this problem, we propose the use of a low-noise hybrid-Vlasov code in four dimensional phase space (1D in physical space and 3D in velocity space) configuration. We trigger a turbulent cascade injecting the energy at large wavelengths and analyze the role of kinetic effects along the development of the energy spectra. Following the evolution of both proton and α distribution functions shows that both the ion species significantly depart from the maxwellian equilibrium, with the appearance of beams of accelerated particles in the direction parallel to the background magnetic field.
Energetic Solar Electrons – Whistler Bootstrap, Magnetic Knots and Small-scale Reconnection
Authors: Roth Ilan 
Published: June 2011

AbstractThe (near) relativistic electrons, emanating from the solar corona in long-lasting, gradual events, are generally observed at 1 AU as delayed vs the less energetic, type-III beams. The observations are consistent with the delayed electrons being energized along the stretched post-CME coronal field lines, when the tail of an anisotropic seed population, which is injected in conjunction to the observed radioheliograph bursts, interacts with the self-excited whistler waves (bootstrap mechanism). These bursts indicate efficient processes where suprathermal seed electrons are injected as a result of magnetic reconnection at the marginally stable coronal configuration left behind the emerging CME. The dependence of the bootstrap mechanism on the electron injection raises the general question of the MHD description and its deviation over the small electron skin-depth scale. The similarity between MHD and knot theories allows one to characterize any turbulent magnetic configuration through topological invariants, while deviation over electron skin-depth scale, characterized by the generalized vorticity, which is enhanced due to density inhomogeneity, creates the conditions for the potential injection sites.
Solar flares: observations vs simulations
Authors: Rubio da Costa Fatima , Zuccarello Francesca , Labrosse Nicolas , Fletcher Lyndsay , Prosecký Tomáš , Kašparová Jana 
Published: June 2011

AbstractIn order to study the properties of faint, moderate and bright flares, we simulate the conditions of the solar atmosphere using a radiative hydrodynamic model (Abbett & Hawley, 1999). A constant beam of non-thermal electrons is injected at the apex of a 1D coronal loop and heating from thermal soft X-ray emission is included. We compare the results with some observational data in Ly-α (using TRACE 1216 and 1600 Å data and estimating the “pure” Ly-α emission) and in Hα (data taken with a Multichannel Flare Spectrograph, at the Ondrejov Observatory).
Magnetic helicity evolution inside a hexagonal convective cell
Authors: Smyrli Aimilia , Mackay Duncan , Zuccarello Francesca 
Published: June 2011

AbstractMagnetic helicity has received considerable attention in the area of fluid dynamics. Recently, this quantity is attracting the interest of solar physicists and much research has been carried out related to magnetic helicity generation and transport through different solar layers, starting from the interior and the convection zone, towards the photosphere, the corona and finally into the heliosphere. Taking into account the global importance of supergranular cells in convection theories, we study the motion of magnetic features into such a geometrical element simplified as hexagonal cell and we analyse the results in terms of the accumulated magnetic helicity. We compute the emergence of a bipole inside the hexagonal cell and its motion from the centre of the cell towards its sides and its vertices, where the magnetic elements are considered to be sinking down. Multiple bipoles are also considered and phenomena such as cancellation, coalescence and fragmentation are also investigated. We find that the most important process for the accumulation of magnetic helicity is the shear motion between the polarities. The magnetic helicity accumulation changes its trend when one polarity reaches the side of the hexagon, and later the vertex. It has zero value when there is no shear motion inside the hexagonal cell, and it is constant when there is no shear between the two polarities during their motion along the cell sides.
Superdiffusive and ballistic propagation of protons in solar energetic particle events
Authors: Trotta Enrico M. , Zimbardo Gaetano 
Published: June 2011

AbstractIn this work we show that protons can exhibit both superdiffusive and ballistic propagation, at variance with standard diffusion. We carry out an analysis of impulsive solar energetic particle (SEP) events, for which the observed time profile of energetic particle fluxes represent the propagator of the corresponding transport equation. We show that in the case of superdiffusive or ballistic transport the propagator in the time asymptotic regime has a power law form, and that a fit of the observed time profiles allows to determine the transport regime. Using data obtained from ACE and SoHO spacecraft, two proton and electron events, which exhibit both superdiffusive and ballistic transport, will be shown. The finding of these anomalous regimes implies that no finite mean free path can be defined.
Plasma processes in pulsar magnetospheres
Authors: Melrose D. B. 
Published: June 2011

AbstractIt is pointed out that the standard model for pulsar electrodynamics is based on a false premise, related to neglecting the displacement current, and the associated need for current screening. Wave dispersion in the standard model is reviewed, and its relation to the interpretation of pulsar radio emission and its polarization is discussed. Inclusion of the displacement current results in large-amplitude oscillations; some of the implications of these oscillations on the interpretation of the radio emission are discussed.
Theory of quasi-stationary kinetic dynamos in magnetized accretion discs
Authors: Cremaschini Claudio , Miller John C. , Tessarotto Massimo 
Published: June 2011

AbstractMagnetic fields are a distinctive feature of accretion disc plasmas around compact objects (i.e., black holes and neutron stars) and they play a decisive role in their dynamical evolution. A fundamental theoretical question related with this concerns investigation of the so-called gravitational MHD dynamo effect, responsible for the self-generation of magnetic fields in these systems. Experimental observations and theoretical models, based on fluid MHD descriptions of various types support the conjecture that accretion discs should be characterized by coherent and slowly time-varying magnetic fields with both poloidal and toroidal components. However, the precise origin of these magnetic structures and their interaction with the disc plasmas is currently unclear. The aim of this paper is to address this problem in the context of kinetic theory. The starting point is the investigation of a general class of Vlasov-Maxwell kinetic equilibria for axi-symmetric collisionless magnetized plasmas characterized by temperature anisotropy and mainly toroidal flow velocity. Retaining finite Larmor-radius effects in the calculation of the fluid fields, we show how these configurations are capable of sustaining both toroidal and poloidal current densities. As a result, we suggest the possible existence of a kinetic dynamo effect, which can generate a stationary toroidal magnetic field in the disc even without any net radial accretion flow. The results presented may have important implications for equilibrium solutions and stability analysis of accretion disc dynamics.
Stationary and axisymmetric configurations of compact stars with extremely strong and highly localized magnetic fields
Authors: Fujisawa Kotaro , Yoshida Shin'ichiro , Eriguchi Yoshiharu 
Published: June 2011

AbstractUsing a new formulation to compute structures of stationary and axisymmetric magnetized barotropic stars in Newtonian gravity, we have succeeded in obtaining numerically exact models of stars with extremely high interior magnetic fields. In this formulation, there appear four arbitrary functions of the magnetic flux function from the integrability conditions among the basic equations. Since in our new formulation these arbitrary functions appear in the expression of the current density, configurations with different current distributions can be specified by choosing the forms of the arbitrary functions.By choosing appropriate forms for the four arbitrary functions, we have solved many kinds of equilibrium configurations both with poloidal and toroidal magnetic fields. Among them, by choosing special form for the toroidal current density, we have been able to obtain magnetized stars which have extremely strong poloidal magnetic fields deep inside the core region near the symmetric axis. By adopting the appropriate model parameters for the neutron stars, the magnetic fields could be 1014 ~ 1015 G on the surfaces and be about 1017 G in the deep interior regions. For other model parameters appropriate for white dwarfs, the magnetic fields could be around 107 ~ 108 G (surface regions) and 109 ~ 1010 G (core regions). It is remarkable that the regions with very strong interior magnetic fields are confined to a very narrow region around the symmetric axis in the central part of the stars. The issues of stability of these configurations and of evolutionary paths to reach such configurations need to be investigated in the future work.