Tracking Magnetopause Motion Using Cold Plasmaspheric Ions

We demonstrate that any plasmaspheric/cold ions accelerated in the vicinity of the magnetopause boundary, can proxy the local magnetopause motion over many minutes. The timeseries of this motion capture local structures such as waves on the boundary. We determine cold ion velocities normal to full magnetopause boundary crossings for three events with varying distances to the predicted reconnection X-line, thus, providing a proof-of-concept study demonstrating the potential for using cold ion velocities to track magnetopause motion over a long period of time. Obtaining the time history of the (local) motion of the magnetopause relative to the spacecraft is determined by integrating the bulk (<100 eV for H+) ion velocities normal to the boundary. Timeseries of these tracked cold ion accelerations may be used to investigate boundary layer thicknesses, potential wave structures on the magnetopause, and their evolution beyond the boundary crossing. This method generally tracks magnetopause motion out to distances of similar to 1-2RE away from the spacecraft during quasi-steady space weather conditions.