Multipoint Cluster Observations of Kinetic Alfvén Waves, Electron Energization, and O+ Ion Outflow Response in the Mid‐Altitude Cusp Associated With Solar Wind Pressure and/or IMF BZ Variations

Abstract

AbstractWe report the properties of low frequency (LF) electromagnetic fluctuations (≤11 Hz) in relation to input electron and ion outflow response observed by Cluster in the mid‐altitude cusp during high solar wind dynamic pressure PSW and active magnetospheric conditions (Kp = 4+). The multipoint observations reveal the dynamic interplay between the spatial‐temporal properties of the wave and electron inputs and the ion outflow response enabling an assessment of causal connections. The LF waves are identified as ingoing traveling Alfvén waves that become dispersive at the ion gyroradius scale (i.e., kinetic Alfvén waves KAWs). The KAWs are collocated with ingoing field‐aligned electrons and ion outflow at keV energies and below. The KAWs are associated with earthward directed Poynting fluxes and energy densities with peak amplitudes occurring at multiple energy enhancements (“step‐ups”) exhibited in the electrons and ions, attributed to IMF BZ and/or PSW variations. Indicative of a causal connection, KAW Poynting fluxes and energy densities are strongly correlated with precipitating electron energy fluxes and outflowing O+ energies and energy fluxes. These yield mid‐altitude cusp empirical relationships that can be incorporated into or constrain cusp transport models. These results demonstrate the important role played by KAWs in enhancing electron precipitation into the cusp ionosphere and subsequent O+ energization upward along the magnetic field into the mid‐altitude cusp and beyond. The results also suggest the importance of PSW and/or IMF BZ variations in driving/controlling the reconnection source Alfvén wave and electron inputs into the cusp that significantly impact the ion outflow process.