PROCEEDINGS of the 23rd International Congress on Acoustics 9 to 13 September 2019 in Aachen, Germany On some properties of magnetoacoustic waves in acoustically active non-adiabatic plasma Dmitrii ZAVERSHINSKII 1,2 ; Nonna MOLEVICH 1,2 ; Igor ZAVERSHINSKII 1 ; 1 Samara National Research University, Russia, 2 Lebedev Physical Institute, Russia ABSTRACT Magnetoacoustic(MA) waves in the plasma with acoustic activity caused by the non-adiabatic processes is under investigation. Acoustic activity of the medium is a consequence of temperature and density dependence of heating and cooling process which take place in the medium. It is shown that non-adiabatic process results in the frequency dependence of group/phase velocity. Effect of frequency dependence is most pronounced near the frequency defined by the inverted heating/cooling time. In the low-/high-frequency limits effect of frequency dependence can be neglected. However, in contrast to high-frequency case where phase/group velocities equal to their value in the equilibrium medium, in the low-frequency limit both velocities are defined by the non-adiabatic processes only. Furthermore, frequency dependence of group velocity in contrast to phase velocity has as an extrema which can be maxima or minima depending on type of dispersion (negative or positive, respectively). This result indicates that some harmonics in non-adiabatic plasma can propagate faster or slower than all others. The expression for group velocity has been obtained under the assumption of weak dispersion/dissipation. Effects caused by the weak but finite dispersion/dissipation on group velocity is analyzed as well. Keywords: acoustic instability, magnetoacoustic wave, non-adiabatic plasma. 1. INTRODUCTION Generally, the total internal energy of a gas or plasma includes mainly the energy of the translational motion of the particles, rotational and vibrational energies of the molecules, the chemical energy. A certain time is required to excite these degrees of freedom and to establish a thermodynamic equilibrium in the medium. This time is known as a relaxation time. Depending on the degree of freedom, this characteristic time can vary significantly. Therefore, the thermodynamic equilibrium tends to be established on certain degrees of freedom faster than on the others. Namely, the relaxation time of the translational degrees of freedom has the shortest value, implying that some initial velocity distribution of particles becomes Maxwellian after even a few elastic collisions. For particles with comparable masses, the time required for the Maxwell distribution to be established is of the same order as the average time between gas kinetic collisions. In the case where different degrees of freedom are present in particles, a typical time for establishing the full thermodynamic equilibrium is determined by the slowest relaxation time. The characteristic times of these slow processes can differ by several orders of magnitude from the translational relaxation time. For instance, in the carbon dioxide at the room temperature the relaxation time of the translational and vibrational degrees of freedom is 10 -10 s and 10 -5 s, respectively (1). Such a huge difference between various relaxation times allows one to analyze each relaxation process independently from the others. In other words, one may reckon that the thermodynamic equilibrium of the degrees of freedom with the shortest relaxation times always exists, while the relaxation of slower processes would generally depend upon the time scales considered. Thus, the time characterizing such a “slow” process allows us to subdivide the whole frequency spectrum into two qualitatively different ranges. Namely, these ranges are where the thermodynamic equilibrium is achieved for the “fast” processes only (at the high-frequency part of the spectrum), and where the full 1 [email protected]2 [email protected][Please note: It is optional to provide the email address(es) of the author(s). Please make sure that your co-authors concur with the mention of their email address in this paper.] 7586
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PROCEEDINGS of the 23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany
On some properties of magnetoacoustic waves in acoustically
active non-adiabatic plasma
Dmitrii ZAVERSHINSKII1,2; Nonna MOLEVICH1,2; Igor ZAVERSHINSKII1;
1 Samara National Research University, Russia,
2 Lebedev Physical Institute, Russia
ABSTRACT
Magnetoacoustic(MA) waves in the plasma with acoustic activity caused by the non-adiabatic processes is
under investigation. Acoustic activity of the medium is a consequence of temperature and density
dependence of heating and cooling process which take place in the medium. It is shown that non-adiabatic
process results in the frequency dependence of group/phase velocity. Effect of frequency dependence is most
pronounced near the frequency defined by the inverted heating/cooling time. In the low-/high-frequency
limits effect of frequency dependence can be neglected. However, in contrast to high-frequency case where
phase/group velocities equal to their value in the equilibrium medium, in the low-frequency limit both
velocities are defined by the non-adiabatic processes only. Furthermore, frequency dependence of group
velocity in contrast to phase velocity has as an extrema which can be maxima or minima depending on type of
dispersion (negative or positive, respectively). This result indicates that some harmonics in non-adiabatic
plasma can propagate faster or slower than all others. The expression for group velocity has been obtained
under the assumption of weak dispersion/dissipation. Effects caused by the weak but finite
dispersion/dissipation on group velocity is analyzed as well.