Combining particle acceleration and coronal heating via data-constrained calculations of nanoflares in coronal loops

We model nanoflare heating of extrapolated active-region coronal loops via the acceleration of electrons and protons in Harris-type current sheets. The kinetic energy of the accelerated particles is estimated using semi-analytical and test-particle-tracing approaches. Vector magnetograms and photospheric Doppler velocity maps of NOAA active region 09114, recorded by the Imaging Vector Magnetograph (IVM), were used for this analysis. A current-free field extrapolation of the active-region corona was first constructed. The corresponding Poynting fluxes at the footpoints of 5000 extrapolated coronal loops were then calculated. Assuming that reconnecting current sheets develop along these loops, we utilized previous results to estimate the kinetic-energy gain of the accelerated particles and we related this energy to nanoflare heating and macroscopic loop characteristics. Kinetic energies of 0.1 to 8 keV (for electrons) and 0.3 to 470 keV (for protons) were found to cause heating rates ranging from $10^{-6}$ to 1 $\mathrm{erg\, s^{-1} cm^{-3}}$. Hydrodynamic simulations show that such heating rates can sustain plasma in coronal conditions inside the loops and generate plasma thermal distributions which are consistent with active region observations. We concluded the analysis by computing the form of X-ray spectra generated by the accelerated electrons using the thick target approach that were found to be in agreement with observed X-ray spectra, thus supporting the plausibility of our nanoflare-heating scenario.Comment: 11 figure

Non-destructive evaluation of cement-based materials from pressure-stimulated electrical emission - Preliminary results

This is the post-print version of the final paper published in Construction and Building Materials. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2010 Elsevier B.V.This paper introduces the possibility of in situ assessment of loading and remaining strength in concrete structures by means of measuring discharge of electric current from loaded specimens. The paper demonstrates that the techniques have been applied to other rock-like materials, but that for the first time they are applied to cement-based materials and a theoretical model is proposed in relation to the appearance of electrical signals during sample loading and up to fracture. A series of laboratory experiments on cement mortar specimens in simple uniaxial compression, and subsequently in bending – hence displaying both tension and compression – are described and show clear correlations between resulting strains and currents measured. Under uniaxial loading there is a well-defined relationship between the pressure-stimulated current (PSC) as a result of a monotonic mechanical loading regime. Similar results are observed in the three-point bending tests where a range of loading regimes is studied, including stepped changes in loading. While currents can be measured at low strains, best results seem to be obtained when strains approach and exceed yield stress values. This technique clearly has immense potential for structural health monitoring of cement-based structures. Both intermittent and continuous monitoring becomes possible, and given an ongoing campaign of monitoring, remaining strength can be estimated

MHD consistent cellular automata (CA) models II. Applications to solar flares

In Isliker et al. (2000b), 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. 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 (SOC). The basic results are: (1) On large-scales, all variables show characteristic quasi-symmetries. (2) The global magnetic topology forms either (i) closed magnetic field lines, or (ii) an arcade of field lines above the bottom plane 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. (5) The local diffusivity usually has a value which is effectively zero, and it turns locally to an anomalous value if a threshold is exceeded, whereby diffusion dominates the quiet evolution (loading). (6) Flares (avalanches) are accompanied by the appearance of localized, intense electric fields. (7) In a variant on the X-CA model, the magnitude of the current is used directly in the instability criterion. First results indicate that the SOC state persists. (8) The current-dissipation during flares is spatially fragmented into a large number of dissipative current-surfaces of varying sizes, which show a highly dynamic temporal evolution.Comment: 13 pages, 12 figures; in press at Astronomy and Astrophysics (2001

A comparative study of a theoretical neural net model with MEG data from epileptic patients and normal individuals

OBJECTIVE: The aim of this study was to compare a theoretical neural net model with MEG data from epileptic patients and normal individuals. METHODS: Our experimental study population included 10 epilepsy sufferers and 10 healthy subjects. The recordings were obtained with a one-channel biomagnetometer SQUID in a magnetically shielded room. RESULTS: Using the method of x(2)-fitting it was found that the MEG amplitudes in epileptic patients and normal subjects had Poisson and Gauss distributions respectively. The Poisson connectivity derived from the theoretical neural model represents the state of epilepsy, whereas the Gauss connectivity represents normal behavior. The MEG data obtained from epileptic areas had higher amplitudes than the MEG from normal regions and were comparable with the theoretical magnetic fields from Poisson and Gauss distributions. Furthermore, the magnetic field derived from the theoretical model had amplitudes in the same order as the recorded MEG from the 20 participants. CONCLUSION: The approximation of the theoretical neural net model with real MEG data provides information about the structure of the brain function in epileptic and normal states encouraging further studies to be conducted

Nonlinear Site Response During the 7 September 1999 Athens, Greece, Earthquake (M\u3csub\u3eW\u3c/sub\u3e 5.6)

The largest available strong-motion recording (PGA=0.35g), least affected by topography, structural response and/or soil-structure interaction, is investigated for possible nonlinear site response during the M, 5.9 Athens earthquake of 7 September 1999. Smoothed horizontal-to-vertical spectral ratios (HVSR) are calculated in subsequent overlapping 3.5-s windows, thus covering a wide range of excitation levels. Mean HVSR curves are computed for a so-called “weak-“ and “strong-“ motion range (mean horizontal ground acceleration in window, MGA\u3c=10.2 cm/s/s and \u3e=20.5 cm/s/s). The two curves have similar shape, with the “strong” curve visibly shifted toward lower frequencies relative to the “weak” one; the dominant site resonance occurs at 4.0 Hz (0.25 s) and 4.7 Hz (0.21 s), respectively. Linear correlation analysis shows that the resonance frequency, f0, and MGA are significantly correlated (t=-0.661). We attribute this behaviour to the degradation of the sediment shear modulus (nonlinearity). Our results, combined with indications that sediment sites in the near-fault area were exposed to ground shaking well above PGA=0.35 g during the earthquake of 7 September 1999, imply that these sites exhibited considerable nonlinear response

Interdiffusion in dilute polymer mixtures. A subtle concentration effect

Dynamic light scattering has been used to investigate the diffusional dynamics in very dilute polystyrene/poly(propylene oxide), PS/PPO, polymer blends. Compared to previous investigations in the field, this system is more suitable for this type of investigation due to the significant refractive index difference between the two components and the fact that the matrix (PPO) dynamics do not interfere with the measurements. The tracer diffusion coefficient of PS thus obtained in the limit of infinite dilution scales as N−0.8±0.04PS with the PS degree of polymerization, i.e., behavior intermediate between the limits of nondraining Zimm and free‐draining Rouse behavior. The effect of the addition of a third component even at tracer concentrations on the diffusion dynamics was investigated both experimentally and theoretically in the framework of the dynamic random phase approximation. Similarities and differences between theory and experiment were found that are rather due to a modification of hydrodynamic interactions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70731/2/JCPSA6-101-4-3222-1.pd