Waves in the innermost open boundary layer formed by dayside magnetopause reconnection

We present two Time History of Events and Macroscale Interactions during Substorms observations of whistler mode and electrostatic wave events in the innermost open boundary layer (IOBL), formed by dayside magnetopause reconnection. The IOBL is identified by high-speed electrons from the magnetosheath on the magnetospheric side of the ion outflow from the reconnection site. Quasi-parallel whistler mode waves propagating toward the reconnection region are observed, along with a partial shortage of magnetospheric electrons moving away from the reconnection region. Calculation of wave linear growth rates shows that the waves can be excited by the perpendicular electron temperature anisotropy that develops due to the partial shortage of field-aligned magnetospheric electrons. Electrostatic waves close to the lower hybrid resonance frequency are observed in the IOBL in the second event, which occurred during the main phase of a magnetospheric storm. Magnetospheric electrons are almost completely lost in the event, except at pitch angles close to 90 degrees, yet whistler mode waves are not observed. An electron beam from the magnetosheath and counterstreaming cold electrons originating from the plasmaspheric plume are observed in association with the electrostatic waves. Growth rate calculations show that the waves are likely to be ion acoustic waves excited via couplings between the flowing cold electrons and background cold ions. We suggest that differences in solar wind conditions and magnetic reconnection characteristics may control the shapes of the electron velocity distribution functions and the resulting plasma wave properties in the IOBL.