Modified Kelvin-Helmholtz instabilities and resonant flow instabilities in a one-dimensional coronal plume model:: Results for plasma β=0

In this manuscript we study the effect of the velocity shear in the mass flow between the coronal plume structure and the interplume region on the spectrum of magnetohydrodynamic (MHD) waves trapped in the plume structure. To illustrate the concept of resonant flow instability of the trapped modes, we consider both a one-dimensional slab model and a one-dimensional cylindrical model for a coronal plume. A nonuniform intermediate region between the plume and the interplume region is taken into account so that the waves can be subject to resonant absorption. We show how the resonance can lead to instability of the trapped modes and that this resonant instability, which is physically distinct from the nonresonant Kelvin-Helmholtz instability, occurs for velocity shears significantly below the Kelvin-Helmholtz threshold. These resonant flow instabilities could lead to disruption of the coronal plumes and mixing with interplume plasma. The dependence of the critical velocity shear for which the resonant instability occurs on the difference between plume and interplume density is investigated. Our results seem to suggest that resonant flow instability in a pressureless plasma will only appear for rather high density contrasts. However, it is clearly shown that the velocity shear needed for Kelvin-Helmholtz instability to occur is too high for all density contrasts. Hence, Kelvin-Helmholtz instability will not be operative in pressureless coronal plumes.