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We apply a novel pre-flare tracking of sunspot groups towards improving the estimation of flare onset time by focusing on the evolution of the 3D magnetic field construction of AR 11429. The 3D magnetic structure is based on potential field extrapolation encompassing a vertical range from the photosphere through the chromosphere and transition region into the low corona. The basis of our proxy measure of activity prediction is the so-called weighted horizontal gradient of magnetic field (WG_M) defined between spots of opposite polarities close to the polarity inversion line of an active region. The temporal variation of the distance of the barycenter of the opposite polarities is also found to possess potentially important diagnostic information about the flare onset time estimation as function of height similar to its counterpart introduced initially in an application at the photosphere only in Korsos et al. (2015). We apply the photospheric pre-flare behavioural patterns of sunspot groups to the evolution of their associated 3D-constructed AR 11429 as function of height. We found that at a certain height in the lower solar atmosphere the onset time may be estimated much earlier than at the photosphere or at any other heights. Therefore, we present a tool and recipe that may potentially identify the optimum height for flare prognostic in the solar atmosphere allowing to improve our flare prediction capability and capacity.

The presence of magneto-acoustic waves in magnetic structures in the solar atmosphere is well-documented. Applying the technique of solar magneto-seismology (SMS) allows us to infer the background properties of these structures. Here, we aim to identify properties of the observed magneto-acoustic waves and study the background properties of magnetic structures within the lower solar atmosphere. Using the Dutch Open Telescope (DOT) and Rapid Oscillations in the Solar Atmosphere (ROSA) instruments, we captured two series of high-resolution intensity images with short cadence of two isolated magnetic pores. Combining wavelet analysis and empirical mode decomposition (EMD), we determined characteristic periods within the cross-sectional (i.e., area) and intensity time series. Then, by applying the theory of linear magnetohydrodynamics (MHD), we identified the mode of these oscillations within the MHD framework. Several oscillations have been detected within these two magnetic pores. Their periods range from 3 to 20 minutes. Combining wavelet analysis and EMD enables us to confidently find the phase difference between the area and intensity oscillations. From these observed features, we concluded that the detected oscillations can be classified as slow sausage MHD waves. Further, we determined several key properties of these oscillations such as the radial velocity perturbation, magnetic field perturbation and vertical wavenumber using solar magnetoseismology. The estimated range of the related wavenumbers reveals that these oscillations are trapped within these magnetic structures. Our results suggest that the detected oscillations are standing harmonics, and, this allows us to estimate the expansion factor of the waveguides by employing SMS. The calculated expansion factor ranges from 4-12., Comment: 10 pages, 4 figures. Accepted into ApJ

We consider a recursive iterative algorithm for identification of parameters of the Preisach model, one of the most commonly used models of hysteretic input-output relationships. The classical identification algorithm due to Mayergoyz defines explicitly a series of test inputs that allow one to find parameters of the Preisach model with any desired precision provided that (a) such input time series can be implemented and applied; and, (b) the corresponding output data can be accurately measured and recorded. Recursive iterative identification schemes suitable for a number of engineering applications have been recently proposed as an alternative to the classical algorithm. These recursive schemes do not use any input design but rather rely on an input-output data stream resulting from random fluctuations of the input variable. Furthermore, only recent values of the input-output data streams are available for the scheme at any time instant. In this work, we prove exponential convergence of such algorithms, estimate explicitly the convergence rate, and explore which properties of the stochastic input and the algorithm affect the guaranteed convergence rate., Comment: 19 pages, 3 figures

A localized perturbation of a magnetic flux tube produces a pair of wave trains that propagate in opposite directions along the tube. These wave packets disperse as they propagate, where the extent of dispersion depends on the physical properties of the magnetic structure, on the length of the initial excitation, and on its nature (e.g., transverse or axisymmetric). In Oliver et al. (2014) we considered a transverse initial perturbation, whereas the temporal evolution of an axisymmetric one is examined here. In both papers we use a method based on Fourier integrals to solve the initial value problem. Previous studies on wave propagation in magnetic wave guides have emphasized that the wave train dispersion is influenced by the particular dependence of the group velocity on the longitudinal wavenumber. Here we also find that long initial perturbations result in low amplitude wave packets and that large values of the magnetic tube to environment density ratio yield longer wave trains. To test the detectability of propagating transverse or axisymmetric wave packets in magnetic tubes of the solar atmosphere (e.g., coronal loops, spicules, or prominence threads) a forward modelling of the perturbations must be carried out. This is left for a future work.

We show that the electron thermal conductivity may strongly affect the heliosheath plasma flow and the global pattern of the solar wind (SW) interaction with the local interstellar medium (LISM). In particular, it leads to strong reduction of the inner heliosheath thickness that makes possible to explain (qualitatively) why Voyager 1 (V1) has crossed the heliopause at unexpectedly small heliocentric distance of 122 AU. To estimate the effect of thermal conductivity we consider a limiting case when thermal conduction is very effective. To do that we assume the plasma flow in the entire heliosphere is nearly isothermal. Due to this effect, the heliospheric distance of the termination shock has increased by about 15 AU in V1 direction compared to the adiabatic case with gamma = 5/3. The heliospheric distance of the heliopause has decreased by about 27 AU. As a result, the thickness of the inner heliosheath in the model has decreased by about 42 AU and become equal to 32 AU., Comment: 7 pages, 2 figures, accepted for publication in Astrophys. Journal Letters

Magnetic Rayleigh-Taylor (MRT) instabilities may play a relevant role in many astrophysical problems. In this work the effect of magnetic shear on the growth rate of the MRT instability is investigated. The eigenmodes of an interface and a slab model under the presence of gravity are analytically calculated assuming that the orientation of the magnetic field changes in the equilibrium, i.e., there is magnetic shear. We solve the linearised magnetohydrodynamic (MHD) equations in the incompressible regime.We find that the growth rate is bounded under the presence of magnetic shear. We have derived simple analytical expressions for the maximum growth rate, corresponding to the most unstable mode of the system. These expressions provide the explicit dependence of the growth rate on the various equilibrium parameters. For small angles the growth time is linearly proportional to the shear angle, and in this regime the single interface problem and the slab problem tend to the same result. On the contrary, in the limit of large angles and for the interface problem the growth time is essentially independent of the shear angle. In this regime we have also been able to calculate an approximate expression for the growth time for the slab configuration. Magnetic shear can have a strong effect on the growth rates of the instability. As an application of the results found in this paper we have indirectly determined the shear angle in solar prominence threads using their lifetimes and the estimation of the Alfv\'en speed of the structure.

The dispersion of small amplitude, impulsively excited wave trains propagating along a magnetic flux tube is investigated. The initial disturbance is a localized transverse displacement of the tube that excites a fast kink wave packet. The spatial and temporal evolution of the perturbed variables (density, plasma displacement, velocity, ...) is given by an analytical expression containing an integral that is computed numerically. We find that the dispersion of fast kink wave trains is more important for shorter initial disturbances (i.e. more concentrated in the longitudinal direction) and for larger density ratios (i.e. for larger contrasts of the tube density with respect to the environment density). This type of excitation generates a wave train whose signature at a fixed position along a coronal loop is a short event (duration ~ 20 s) in which the velocity and density oscillate very rapidly with typical periods of the order of a few seconds. The oscillatory period is not constant but gradually declines during the course of this event. Peak values of the velocity are of the order of 10 km/s and are accompanied by maximum density variations of the order of 10-15% the unperturbed loop density.

The details of the mechanism(s) responsible for the observed heating and dynamics of the solar atmosphere still remain a mystery. Magnetohydrodynamic (MHD) waves are thought to play a vital role in this process. Although it has been shown that incompressible waves are ubiquitous in off-limb solar atmospheric observations their energy cannot be readily dissipated. We provide here, for the first time, on-disk observation and identification of concurrent MHD wave modes, both compressible and incompressible, in the solar chromosphere. The observed ubiquity and estimated energy flux associated with the detected MHD waves suggest the chromosphere is a vast reservoir of wave energy with the potential to meet chromospheric and coronal heating requirements. We are also able to propose an upper bound on the flux of the observed wave energy that is able to reach the corona based on observational constraints, which has important implications for the suggested mechanism(s) for quiescent coronal heating., Comment: Published in Nature Communications, 3, 1315 (2012)

We present a rational solution for a mixed nonlinear Schr\"odinger (MNLS) equation. This solution has two free parameters $a$ and $b$ representing the contributions of self-steepening and self phase-modulation (SPM) of an associated physical system. It describes five soliton states: a paired bright-bright soliton, single soliton, a paired bright-grey soliton, a paired bright-black soliton, and a rogue wave state. We show that the transition among these five states is induced by self-steepening and SPM through tuning the values of $a$ and $b$. This is a unique and potentially fundamentally important phenomenon in a physical system described by the MNLS equation., Comment: 4 pages, 6 figures

Transverse oscillations of thin threads in solar prominences are frequently reported in high-resolution observations. Two typical features of the observations are that the oscillations are damped in time and that simultaneous mass flows along the threads are detected. Flows cause the dense threads to move along the prominence magnetic structure while the threads are oscillating. The oscillations have been interpreted in terms of standing magnetohydrodynamic (MHD) kink waves of the magnetic flux tubes which support the threads. The damping is most likely due to resonant absorption caused by plasma inhomogeneity. The technique of seismology uses the observations combined with MHD wave theory to estimate prominence physical parameters. This paper presents a theoretical study of the joint effect of flow and resonant absorption on the amplitude of standing kink waves in prominence threads. We find that flow and resonant absorption can either be competing effects on the amplitude or both can contribute to damp the oscillations depending on the instantaneous position of the thread within the prominence magnetic structure. The amplitude profile deviates from the classic exponential profile of resonantly damped kink waves in static flux tubes. Flow also introduces a progressive shift of the oscillation period compared to the static case, although this effect is in general of minor importance. We test the robustness of seismological estimates by using synthetic data aiming to mimic real observations. The effect of the thread flow can significantly affect the estimation of the transverse inhomogeneity length scale. The presence of random background noise adds uncertainty to this estimation. Caution needs to be paid to the seismological estimates that do not take the influence of flow into account., Comment: Accepted for publication in A&A

We study non-axisymmetric oscillations of a straight magnetic tube with an elliptic cross-section and density varying along the tube. The governing equations for kink and fluting modes in the thin tube approximation are derived. We found that there are two kink modes, polarised along the large and small axes of the elliptic cross-section. We have shown that the ratio of frequencies of the first overtone and fundamental harmonic is the same for both kink modes and independent of the ratio of the ellipse axes. On the basis of this result we concluded that the estimates of the atmospheric scale height obtained using simultaneous observations of the fundamental harmonic and first overtone of the coronal loop kink oscillations are independent of the ellipticity of the loop cross-section.

In the approximation of linear dissipative magnetohydrodynamics (MHD) it can be shown that driven MHD waves in magnetic plasmas with high Reynolds number exhibit a near resonant behaviour if the frequency of the wave becomes equal to the local Alfven (or slow) frequency of a magnetic surface. This near resonant behaviour is confined to a thin region, known as the dissipative layer, which embraces the resonant magnetic surface. Although driven MHD waves have small dimensionless amplitude far away from the resonant surface, this near-resonant behaviour in the dissipative layer may cause a breakdown of linear theory. Our aim is to study the nonlinear effects in the Alfven dissipative layer. In the present paper, the method of simplified matched asymptotic expansions developed for nonlinear slow resonant waves is used to describe nonlinear effects inside the Alfven dissipative layer. The nonlinear corrections to resonant waves in the Alfven dissipative layer are derived and it is proved that at the Alfven resonance (with isotropic/anisotropic dissipation) wave dynamics can be described by the linear theory with great accuracy., Comment: A&A accepted for publication

Corrected analytical solutions describing the enhanced phase mixing of Alfven waves propagating in divergent stratified coronal structures are presented. These show that the enhanced phase mixing mechanism can dissipate Alfven waves at heights less than half that is predicted by the previous analytical solutions. The enhanced phase mixing of 0.1 Hz harmonic Alfven waves propagating in strongly divergent, H_b=5 Mm, stratified coronal structures, H_rho=50 Mm, can fulfill 100% of an active region heating requirement, by generating viscous heating fluxes of F_H~2100 J /m^2 /s. The Alfven waves in this configuration are fully dissipated within 20 Mm, which is six times lower than would occur as a result of standard phase mixing in uniform magnetic fields. This results in the heating scale height, s_H, being lowered by a factor of six, to less than half of an active regions density scale height. Using the corrected analytical solutions it was found that, for a given wave frequency, the generation of a heating scale height of s_H<=50 Mm, by enhanced phase mixing in strongly divergent magnetic fields, requires a shear viscosity eight orders of magnitude lower, than required by standard phase mixing in uniform magnetic fields. It was also found that the enhanced phase mixing of observable, 0.01 rads /s Alfven waves, in strongly divergent magnetic fields, H_b=5 Mm, can generate heating scale heights within a density scale height, H_rho=50 Mm, using classical Braginskii viscosity. It is therefore not necessary to invoke anomalous viscosity in corona, if phase mixing takes place in strongly divergent magnetic fields. This study shows that the importance of enhanced phase mixing as a mechanism for dissipating Alfven waves in the solar corona (a stratified and divergent medium), has been seriously underestimated., Comment: Accepted by A&A - 24th September 2007

We consider the stability of a compressible shear flow separating two streams of different speeds and temperatures. The velocity and temperature profiles in this mixing layer are hyperbolic tangents. The normal mode analysis of the flow stability reduces to an eigenvalue problem for the pressure perturbation. We briefly describe the numerical method that we used to solve this problem. Then, we introduce the notions of the absolute and convective instabilities and examine the effects of Mach number, and the velocity and temperature ratios of each stream on the transition between convective and absolute instabilities. Finally, we discuss the implication of the results presented in this paper for the heliopause stability., Comment: 5 pages, 6 figures, accepted by Astronomical Notes (Astron. Nachrichten)

In the core of a canonical spinning magnetized neutron star(NS) a nearly uniform superfluid neutron vortex-array interacts strongly with a twisted array of magnetic flux-tubes threading the core's superconducting protons. One consequence is that changes in NS-spin alter both arrays and also the magnetic field distribution on the surface of the surrounding crust. Among predicted consequences for very young spinning-down NSs are "spin-down indices" increasing from 2 to 3, and a family of (Crab-like) spin-period "glitches" with permanent fractional jumps in spin-down torque 10E5 times greater than those in NS-spin. For older NSs, average spin-down indices increase to around 5, and an additional (Vela-like) family of giant glitches develops. NS spin-up to millesecond pulsars results in a high abundance of orthogonal and aligned rotators, and anomolously small polar cap areas. Observations do not conflict with these expectations. An epoch of NS magnetic field evolution between the onset of proton superconductivity (approx. yr) and neutron superfluidity (approx. 10E3 yrs ?) may be important for large surface magnetic field changes and needs further study. Observations generally considered evidence for NS precession seem to need reconsideration., Comment: 9 Pages, 21 Figures, Latex, he_symp.cls. To appear in the proceedings of the 363rd Heraeus Seminar on Neutron Stars and Pulsars, held in Bad Honnef, May 14-19 2006

We propose a model for rotating current patterns within radiopulsar polar cap accelerators which has observational consequences that mimic those which have been attributed to neutron star free precession. The model is a simple extension of a canonical one for the origin of the "drifting subpulses" often observed within the pulse envelope of radiopulsars. The new model's current pattern rotation period (with respect to the neutron star) is estimated to be of order a year. Associated with that rotation are small oscillations in spin-down torque, pulse arrival time, and radiobeam direction with this same period. These have estimated magnitudes which support a reinterpretation of free precession "observations" which could resolve the severe problem of obtaining anywhere near the otherwise required precession parameters with canonical neutron star models.

Because of the quantum fluid properties of a neutron star core's neutrons and protons, its magnetic field is expected to be coupled strongly to its spin. This predicts a simple evolution of the surface-field of such stars as they spin down or, less commonly, are spun up. Consequences and comparisons with observations are given for properties of solitary spinning down pulsars, including their glitches and spin-down ages, X-ray pulsars, and the formation and pulse characteristics of Millisecond Pulsars. For none of these is there a present conflict between model predictions and what has been observed., Comment: 17 pages, 19 figures

Low energy X-ray emission (0.1-10 keV) from all six millisecond radio pulsars (MSPs) for which such emission has been reported support a proposed pulsar magnetic field evolution previously compared only to radiopulse data: old, very strongly spun-up neutron stars become mainly orthogonal rotators (magnetic dipole moment perpendicular to stellar spin) or aligned rotators. The neutron star properties which lead to such evolution are reviewed. Special consideration is given to agreement between predictions and observed X-ray emission for the aligned MSP candidate PSR J0437-4715., Comment: 7 pages, 4 eps figures ; minor changes and few typos corrected

A young neutron star with large spin-down power is expected to be closely surrounded by an e+/- pair plasma maintained by the conversion of gamma-rays associated with the star's polar-cap and/or outer-gap accelerators. Cyclotron-resonance scattering by the e- and e+ within several radii of such neutron stars prevents direct observations of thermal X-rays from the stellar surface. Estimates are presented for the parameters of the Planck-like X-radiation which ultimately diffuses out through this region. Comparisons with observations, especially of apparent blackbody emission areas as a function of neutron star age, support the proposition that we are learning about a neutron star's magnetosphere rather than about its surface from observations of young neutron star thermal X-rays., Comment: 10 pages, 5 figures, in 4th AGILE Science Workshop, "X-ray and Gamma-ray Astrophysics of Galactic Sources," 2003 June 11-13, Rome

Modeling the dynamics of the quantum fluids within a spinning-down neutron star gives a description consistent with observed pulsar magnetic field evolution and spin-period "glitches." The long-standing problem of large predicted excesses in spin-down sustained pulsar heating from such models now see ms resolvable. However, the origin of some pulsar spin-period and pulse-shape modulations which have been interpreted as manifestations of very long period (~year) stellar precession is a crucial challenge to canonical neutron star models., Comment: 10 pages, 4 figures, Invited talk at van Paradijs memorial conference

One of a family previously proposed ``central engines'' for cosmic gamma-ray burst sources (Klu\'zniak & Ruderman 1998) is considered in some detail. A steadily accreting $10^6$ Gauss magnetic white dwarf should ultimately collapse to a strongly differentially rotating, millisecond-rotation-period neutron star for a wide range of steady accretion rates and initial masses if the accreting white dwarf has an evolved O-Ne-Mg composition. A similar neutron star could also result from an initial C-O white dwarf but only for more constrained accretion rates. Because the collapsing white dwarf begins as a $\gamma=4/3$ polytrope, the final neutron star's spin-rate increases strongly with cylindrical radius. A stable wind-up of the neutron star's poloidal magnetic field then produces buoyant magnetic toroids which grow, break loose, rise, and partly penetrate the neutron star surface to form a transient, $B\approx 10^{17}$ G millisecond spin-period pulsar with a powerful pulsar wind (Usov 1992). This pulsar wind emission is then rapidly suppressed by the surface shear motion from the strong stellar differential rotation. This wind-up and transient pulsar formation can occur at other times on different cylinders and/or repeat on the same one, with (re-)wind up and surface penetration time scales hugely longer han the neutron star's millisecond spin period. In this way, differential rotation both opens and closes the doors which allow neutron star spin-energy to be emitted in powerful bursts of pulsar wind. Predictions of this model compare favorably to needed central engine properties of gamma-ray burst sources (total energy, duration, sub-burst fluctuations and time scales, variability among burst events, and baryon loading)., Comment: 27 pages (including 2 figures), LaTeX (AASTeX), submitted to the Astrophysical Journal

GRBs are thought to arise in relativistic blast-wave shocks at distances of 10 to 1000 AU from the point where the explosive energy is initially released. To account for the observed duration and variability of the gamma-ray emission in most GRBs, a central engine powering the shocks must remain active for several seconds to many minutes but must strongly fluctuate in its output on much shorter timescales. We show how a neutron star differentially rotating at millisecond periods (DROMP) could be such an engine. A magnetized DROMP would repeatedly wind up toroidal magnetic fields to about 10**17 G and only release the corresponding magnetic energy, when each buoyant magnetic field torus floats up to, and breaks through, the stellar surface. The resulting rapid sub-bursts, separated by relatively quiescent phases, repeat until the kinetic energy of differential rotation is exhausted by these events. Calculated values of the energy released and of the various timescales are in agreement with observations of GRBs. The baryon loading in each sub-burst may also be consistent with theoretical requirements for a blast wave capable of giving the X-ray, optical and radio afterglows recently observed from cosmological distances. DROMPs could be created in several kinds of astrophysical events; some of these would be expected to occur at about the observed GRB rate. The requisite differential rotation could be imparted to neutron stars as they are born or at the end of their existence: some DROMPs may be created close to star forming regions while others may arise far from galaxies., Comment: 6 pages, 1 figure

A model is proposed for the observed combination of power-law and thermal emission of keV X-rays from rotationally powered pulsars. For gamma-ray pulsars with accelerators very many stellar radii above the neutron star surface, 100 MeV curvature gamma-rays from $e^{-}$ or $e^{+}$ flowing starward out of such accelerators are converted to electron-positron pairs on closed field lines all around the star. These pairs strongly affect X-ray emission from near the star in two ways. (1) The pairs are a source of synchrotron emission immediately following their creation in regions where $B \sim 10^{10}$ G. This emission, in the photon energy range 0.1 keV $\lesssim E_{X} \lesssim$ 5 MeV, has a power-law spectrum with energy index 0.5 and X-ray luminosity that depends on the backflow current, and is typically $\sim 10^{33}$ \lum. (2) The pairs ultimately form a cyclotron resonance ``blanket'' surrounding the star except for two holes along the open field line bundles which pass through it. In such a blanket the gravitational pull on electron-positron pairs toward the star is balanced by the hugely amplified push of outflowing surface emitted X-rays wherever cyclotron resonance occurs. Because of it the neutron star is surrounded by a leaky ``hohlraum'' of hot blackbody radiation with two small holes, which prevents direct X-ray observation of a heated polar cap of a gamma-ray pulsar. Weakly spin-modulated radiation from the blanket together with more strongly spin-modulated radiation from the holes through it would then dominate observed low energy (0.1--10 keV) emission. For non-gamma-ray pulsars, in which no such accelerators with their accompanying extreme relativistic backflow toward the star are ..., Comment: 34 pages, 7 figures