Acoustic/shock wave heating in the gravitationally stratified partially ionized plasmas: the two-fluid effects

<p> The chromospheric heating problem is a long-standing intriguing topic of solar physics, and the acoustic wave/shock wave heating in the chromospheric plasma has been investigated in the last several decades. It has been confirmed that acoustic waves, and the shock waves induced by the steepening acoustic waves in the gravitationally stratified chromospheric plasma, are able to transport energy to the chromosphere, but the energy supplied in this way is not necessarily sufficient for heating the chromosphere. Here, we further investigate the acoustic/shock wave heating process while taking into account the two-fluid effects.</p><p> As the plasma in the chromosphere is weakly or partially ionized,  neutrals play an important role in wave propagation in this region. Therefore,  a two-fluid computational model treating neutrals and charged particles (electrons and ions) as two separate fluids is used for modelling the acoustic/shock wave propagation in idealised partially ionized plasmas, while taking into account the ion-neutral collisions, ionization and recombination. We have thus investigated  the collisional and reactive interactions between separated ions and neutrals, as well as the resulting effects in the acoustic/shock wave propagation and damping. In the numerical simulations, both the initial hydrostatic equilibrium and chemical equilibrium are taken into account to provide different density profiles for comparison.</p><p>We have found that the shock heating in the partially ionized plasmas strongly depends on the ionization fraction. In particular, the relatively smaller ionization fraction resulting from the initial chemical equilibrium significantly enhances the shock wave heating, which dominates the overall heating effect according to an approximated quantitative comparison. Moreover, the decoupling between ions and neutrals is also enhanced while considering ionization and recombination, resulting in stronger collisional heating.</p>

Partial Stellar Explosions - Ejected Mass and Minimal Energy

Many massive stars appear to undergo enhanced mass loss during late stages of their evolution. In some cases, the ejected mass likely originates from non-terminal explosive outbursts, rather than continuous winds. Here we study the dependence of the ejecta mass, mej, on the energy budget E of an explosion deep within the star, using both analytical arguments and numerical hydrodynamics simulations. Focusing on polytropic stellar models, we find that for explosion energies smaller than the stellar binding energy, the ejected mass scales as $m_{\rm ej} \propto E^{\varepsilon _{m}}$, where ϵm = 2.4 − 3.0 depending on the polytropic index. The loss of energy due to shock breakout emission near the stellar edge leads to the existence of a minimal mass-shedding explosion energy, corresponding to a minimal ejecta mass. For a wide range of progenitors, from Wolf-Rayet stars to red supergiants, we find a similar limiting energy of $E_{\rm min} \approx 10^{46}-10^{47} \rm \, erg$, almost independent of the stellar radius. The corresponding minimal ejecta mass varies considerably across different progenitors, ranging from $\sim \! 10^{-8} \, \rm M_\odot$ in compact stars, up to $\sim \! 10^{-2} \, \rm M_\odot$ in red supergiants. We discuss implications of our results for pre-supernova outbursts driven by wave heating, and complications caused by the non-constant opacity and adiabatic index of realistic stars.

A study of detection of internal defects in CFRP using different modes of planar laser

The detection for internal defects of the practical aviation carbon fiber reinforced polymer (CFRP) using different modes of laser was discussed in this paper. The results show that the effect of the laser with long pulsed wave heating mode is the best, and then sawtooth wave mode is followed by sine wave mode. Besides, the valid image processing method plays a significant role in improving the effect of differential laser infrared thermography. Compared with the detection results of thermal lamp thermography, the effect of differential laser thermography is better. There are two main advantages of using a planar laser to heat the aviation carbon fiber composites: (1) the detection speed is so fast, which only takes a few seconds to complete the whole detection process; (2) laser method is more flexible than thermal lamp thermography because of the range detected can be adjusted according to the aim of experiments.

Effect of coronal loop structure on wave heating through phase mixing

Context. The mechanism(s) behind coronal heating still elude(s) direct observation and modelling of viable theoretical processes and the subsequent effect on coronal structures is one of the key tools available to assess possible heating mechanisms. Wave heating via the phase mixing of magnetohydrodynamic (MHD) transverse waves has been proposed as a possible way to convert magnetic energy into thermal energy, but MHD models increasingly suggest this is not an efficient enough mechanism. Aims. We modelled heating by phase mixing transverse MHD waves in various configurations in order to investigate whether certain circumstances can enhance the heating sufficiently to sustain the million degree solar corona and to assess the impact of the propagation and phase mixing of transverse MHD waves on the structure of the boundary shell of coronal loops. Methods. We used 3D MHD simulations of a pre-existing density enhancement in a magnetised medium and a boundary driver to trigger the propagation of transverse waves with the same power spectrum as measured by the Coronal Multi-Channel Polarimeter. We consider different density structures, boundary conditions at the non-drive footpoint, characteristics of the driver, and different forms of magnetic resistivity. Results. We find that different initial density structures significantly affect the evolution of the boundary shell and that some driver configurations can enhance the heating generated from the dissipation of the MHD waves. In particular, drivers coherent on a larger spatial scale and higher dissipation coefficients can generate significant heating, although it is still insufficient to balance the radiative losses in this setup. Conclusions. We conclude that while phase mixing of transverse MHD waves is unlikely to sustain the thermal structure of the corona, there are configurations that allow for an enhanced efficiency of this mechanism. We provide possible signatures to identify the presence of such configurations, such as the location of where the heating is deposited along the coronal loop.