Large amplitude electrostatic proton plasma frequency waves in the
magnetospheric separatrix and outflow regions during magnetic
We report Magnetospheric Multiscale observations of large amplitude,
parallel, electrostatic, proton plasma frequency waves on the
magnetospheric side of the reconnecting magnetopause. The waves are
often found in the magnetospheric separatrix region and in the outflow
close to the magnetospheric ion edge. Statistical results from five
months of data show that these waves are closely tied to the presence of
cold (typically tens of eV) ions, found for 91% of waves in the
separatrix region, and that plasma properties are consistent with ion
acoustic wavegrowth. We analyze one wave event in detail, concluding
that the wave is ion acoustic. We provide a simple explanation for the
mechanisms leading to the development of the ion acoustic instability.
These waves can be important for separatrix dynamics, by heating the
cold ion component and providing a mechanism to damp the kinetic Alfvén
waves propagating away from the reconnection site.