7,817 research outputs found
Testing predictors of eruptivity using parametric flux emergence simulations
Solar flares and coronal mass ejections (CMEs) are among the most energetic
events in the solar system, impacting the near-Earth environment. Flare
productivity is empirically known to be correlated with the size and complexity
of active regions. Several indicators, based on magnetic-field data from active
regions, have been tested for flare forecasting in recent years. None of these
indicators, or combinations thereof, have yet demonstrated an unambiguous
eruption or flare criterion. Furthermore, numerical simulations have been only
barely used to test the predictability of these parameters. In this context, we
used the 3D parametric MHD numerical simulations of the self-consistent
formation of the flux emergence of a twisted flux tube, inducing the formation
of stable and unstable magnetic flux ropes of Leake (2013, 2014). We use these
numerical simulations to investigate the eruptive signatures observable in
various magnetic scalar parameters and provide highlights on data analysis
processing. Time series of 2D photospheric-like magnetograms are used from
parametric simulations of stable and unstable flux emergence, to compute a list
of about 100 different indicators. This list includes parameters previously
used for operational forecasting, physical parameters used for the first time,
as well as new quantities specifically developed for this purpose. Our results
indicate that only parameters measuring the total non-potentiality of active
regions associated with magnetic inversion line properties, such as the
Falconer parameters , , and , as well as the
new current integral and length parameters, present a
significant ability to distinguish the eruptive cases of the model from the
non-eruptive cases, possibly indicating that they are promising flare and
eruption predictors.Comment: 46 pages, 16 figures, accepted for publication in Space Weather and
Space Climate on June, 8t
Long-time properties of MHD turbulence and the role of symmetries
We investigate long-time properties of three-dimensional MHD turbulence in
the absence of forcing and examine in particular the role played by the
quadratic invariants of the system and by the symmetries of the initial
configurations. We observe that, when sufficient accuracy is used, initial
conditions with a high degree of symmetries, as in the absence of helicity, do
not travel through parameter space over time whereas by perturbing these
solutions either explicitly or implicitly using for example single precision
for long times, the flows depart from their original behavior and can become
either strongly helical, or have a strong alignment between the velocity and
the magnetic field. When the symmetries are broken, the flows evolve towards
different end states, as predicted by statistical arguments for non-dissipative
systems with the addition of an energy minimization principle, as already
analyzed in \cite{stribling_90} for random initial conditions using a moderate
number of Fourier modes. Furthermore, the alignment properties of these flows,
between velocity, vorticity, magnetic potential, induction and current,
correspond to the dominance of two main regimes, one helically dominated and
one in quasi-equipartition of kinetic and magnetic energy. We also contrast the
scaling of the ratio of magnetic energy to kinetic energy as a function of
wavenumber to the ratio of eddy turn-over time to Alfv\'en time as a function
of wavenumber. We find that the former ratio is constant with an approximate
equipartition for scales smaller than the largest scale of the flow whereas the
ratio of time scales increases with increasing wavenumber.Comment: 14 pages, 6 figure
Effect of physical aging on the low-frequency vibrational density of states of a glassy polymer
The effects of the physical aging on the vibrational density of states (VDOS)
of a polymeric glass is studied. The VDOS of a poly(methyl methacrylate) glass
at low-energy (<15 meV), was determined from inelastic neutron scattering at
low-temperature for two different physical thermodynamical states. One sample
was annealed during a long time at temperature lower than Tg, and another was
quenched from a temperature higher than Tg. It was found that the VDOS around
the boson peak, relatively to the one at higher energy, decreases with the
annealing at lower temperature than Tg, i.e., with the physical aging.Comment: To be published in Europhys. Let
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