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Munoz, V., Asenjo, F. A., Dominguez, M., Lopez, R. A., Valdivia, J. A., Vinas, A., et al. (2014). Large-amplitude electromagnetic waves in magnetized relativistic plasmas with temperature. Nonlinear Process Geophys., 21(1), 217–236.
Abstract: Propagation of large-amplitude waves in plasmas is subject to several sources of nonlinearity due to relativistic effects, either when particle quiver velocities in the wave field are large, or when thermal velocities are large due to relativistic temperatures. Wave propagation in these conditions has been studied for decades, due to its interest in several contexts such as pulsar emission models, laser-plasma interaction, and extragalactic jets. For large-amplitude circularly polarized waves propagating along a constant magnetic field, an exact solution of the fluid equations can be found for relativistic temperatures. Relativistic thermal effects produce: (a) a decrease in the effective plasma frequency (thus, waves in the electromagnetic branch can propagate for lower frequencies than in the cold case); and (b) a decrease in the upper frequency cutoff for the Alfven branch (thus, Alfven waves are confined to a frequency range that is narrower than in the cold case). It is also found that the Alfven speed decreases with temperature, being zero for infinite temperature. We have also studied the same system, but based on the relativistic Vlasov equation, to include thermal effects along the direction of propagation. It turns out that kinetic and fluid results are qualitatively consistent, with several quantitative differences. Regarding the electromagnetic branch, the effective plasma frequency is always larger in the kinetic model. Thus, kinetic effects reduce the transparency of the plasma. As to the Alfven branch, there is a critical, nonzero value of the temperature at which the Alfven speed is zero. For temperatures above this critical value, the Alfven branch is suppressed; however, if the background magnetic field increases, then Alfven waves can propagate for larger temperatures. There are at least two ways in which the above results can be improved. First, nonlinear decays of the electromagnetic wave have been neglected; second, the kinetic treatment considers thermal effects only along the direction of propagation. We have approached the first subject by studying the parametric decays of the exact wave solution found in the context of fluid theory. The dispersion relation of the decays has been solved, showing several resonant and nonresonant instabilities whose dependence on the wave amplitude and plasma temperature has been studied systematically. Regarding the second subject, we are currently performing numerical 1-D particle in cell simulations, a work that is still in progress, although preliminary results are consistent with the analytical ones.
Reszka, P., Cruz, J. J., Valdivia, J., Gonzalez, F., Rivera, J., Carvajal, C., et al. (2020). Ignition delay times of live and dead pinus radiata needles. Fire Saf. J., 112, 7 pp.
Abstract: There are still many open questions related to the fire behavior of live and dead wildland fuels and their senescence process. We have physically and biochemically studied live and dead pinus radiata needles, their aging process, and their fire behavior using a systematic aging procedure which allows to characterize the evolution of the fuel moisture content and the photosynthetic pigments over time, and to determine the period of time after sample collection in which specimens can be considered to be alive. Results show that pine needles stay alive for up to 12 h after collection if they remain attached to the twigs. The influence of senescence on spontaneous ignition was tested on two bench-scale devices, the I-FIT and the SCALA, under discontinuous and continuous configurations, respectively. Live pine needles showed larger critical heat fluxes than dead needles, while dead and re-hydrated samples have increased critical heat fluxes for greater moisture contents. Experimental results were interpreted with thermal models based on a two-phase description of the fuel layer. We established a correlation of the form 1/t(ig)proportional to q(inc)" for both ignition configurations, which is adequate for engineering applications and allows the estimation of effective properties for wildland fuel beds.