Dense plasma and Kelvin-Helmholtz waves at Earth\textquoterights dayside magnetopause

Spacecraft observations of boundary waves at the dayside terrestrial magnetopause and their ground-based signatures are presented. Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft measured boundary waves at the magnetopause while ground-based HF radar measured corresponding signatures in the ionosphere indicating a large-scale response and tailward propagating waves. The properties of the oscillations are consistent with linear phase Kelvin-Helmholtz waves along the magnetopause boundary. During this time period multiple THEMIS spacecraft also measured a plasmaspheric plume contacting the local magnetopause and mass loading the boundary. Previous work has demonstrated that increasing the density at the magnetopause can lower the efficiency of reconnection. Extending this further, present observations suggest that a plume can modulate instability processes such as the Kelvin-Helmholtz instability and allow them to form closer to the subsolar point along the magnetopause than without a plume. The current THEMIS observations from 21 September 2010 are consistent with a theory which predicts that increasing the density at the boundary will lower the Kelvin-Helmholtz threshold and allow waves to form for a lower velocity shear.