Slow Magnetoacoustic Waves in a Viscous Two-Fluid Stratified Plasma: Local Dispersion and Dissipation Channels

We examine slow magnetoacoustic disturbances in a local, isothermal, gravitationally stratified two-fluid model. The charged and neutral components have separate kinematic viscosities and are coupled by ion–neutral friction. Using a consistent thin-flux-tube equilibrium, we derive the linear velocity equations and retain the complete transformed viscous operator. The ion-like branch has the usual stratification cutoff. For the self-consistent reference state, the cutoff is 2.24 mHz. To first order in the locally constant ion viscosity, the imaginary frequency correction changes sign at k=1/(4Hi), where k is the longitudinal wavenumber and Hi is the charged fluid density scale height. This behavior is a property of the transformed local amplitude and is not interpreted as viscous energy production. The homogeneous neutral equation admits a finite underdamped interval only if a quadratic discriminant condition is satisfied. In the fixed-temperature hydrogen collision model, this condition reduces approximately to νnνni/cSn2<1. Here νn is the neutral kinematic viscosity (in m2 s−1), νni is the neutral-ion collision frequency (in s−1), and cSn is the neutral sound speed, so the ratio is dimensionless. The corresponding spatial scale interval does not satisfy the thin-tube and neutral continuum orderings for resolved chromospheric structures. It is therefore reported as a formal property of the reduced model rather than as a solar prediction. The calculation isolates the dissipation channels but does not model non-local thermodynamic equilibrium (non-LTE) radiative transfer or propagation through a height-dependent chromosphere.