Magnetoacoustic waves of small amplitude in optically thin quasi-isentropic plasmas

The evolution of quasi-isentropic magnetohydrodynamic waves of small but fi nite amplitude in an optically thin plasma is analyzed. The plasma is assum ed to be initially homogeneous, in thermal equilibrium and with a straight and homogeneous magnetic held frozen in. Depending on the particular form o f the heating/cooling function, the plasma may act as a dissipative or acti ve medium for magnetoacoustic waves, while Alfven waves are not affected di rectly. An evolutionary equation for fast and slow magnetoacoustic waves in the single wave limit has been derived and solved that allows us to analyz e the wave modification by competition of weakly nonlinear and quasi-isentr opic effects. It was shown that the sign of the quasi-isentropic term deter mines the scenario of the evolution, either dissipative or active. In the d issipative case, when the plasma is first-order isentropically stable, the magnetoacoustic waves are damped and the time for shock wave formation is d elayed. However, in the active case when the plasma is isentropically overs table, the wave amplitude grows, the strength of the shock increases, and t he breaking time decreases. The magnitude of the above effects depends on t he angle between the wavevector and the magnetic field. For hot (T > 10(4) K) atomic plasmas with solar abundances either in the interstellar medium o r in the solar atmosphere, as well as for the cold (T < 10(3) K) ISM molecu lar gas, the range of temperatures in which the plasma is isentropically un stable and the corresponding timescale and length scale for wave breaking h ave been found.