Langmuir Waves Associated with the Kelvin-Helmholtz Instability at Saturn's Magnetopause

Langmuir waves are electrostatic waves generated near the electron plasma frequency by a positive gradient in the electron velocity distribution parallel to the magnetic field. These waves are commonly seen upstream of planetary magnetosheaths in the electron foreshock, where reflection by the bow shock creates the necessary beam of electrons. When the plasma advects through the bow shock, these waves quickly damp out and are generally not observed elsewhere in the magnetosheath. In March 2005, however, the Radio and Plasma Wave Science instrument, on the Cassini spacecraft, observed these waves in the magnetosheath concurrent with Kelvin-Helmholtz-driven motions of the magnetopause. We find that the Langmuir wave occurrence is strongly correlated with the magnetic field orientation. These field orientations of maximum wave activity are similar to the orientation of the magnetopause during the Kelvin-Helmholtz oscillations. We examine the possibility that the Kelvin-Helmholtz waves are perturbing the electron distribution to make the plasma unstable to the growth of Langmuir waves. Such a relationship raises the possibility that Langmuir waves may be used as a remote indicator of the presence of this instability even when the spacecraft is in the magnetosheath but does not encounter the magnetopause.