Bifurcated cusp ion signatures: Evidence for re‐reconnection?

Toroidal Imaging Mass‐Angle Spectrograph (TIMAS) ion composition measurements during a pass of the POLAR spacecraft through the Earth's magnetospheric cusp show a bifurcated ion signature. High energy (several keV/e) solar wind ions are observed simultaneously with a low energy (up to several hundred eV/e) component. The high energy component exhibits an energy‐latitude dispersion consistent with magnetic reconnection at the dayside magnetopause and velocity filtering in the cusp. The low energy component exhibits no such dispersion. Simultaneous observations of the solar wind ion composition from the WIND spacecraft indicate that the high and low energy components in the cusp have a higher He 2+ /H + density ratio than that in the solar wind. Additional solar wind data show that the IMF was southward and the solar wind plasma and magnetic field were relatively steady. These POLAR and WIND observations are interpreted as evidence for continuous re‐reconnection of magnetospheric field lines that thread the high latitude cusp.