Low Frequency Oscillation Observed in the Ionosphere Around the Substorm Onsets

Abstract Three substorm events in which Defense Meteorological Satellite Program (DMSP) spacecraft are geomagnetically co‐located with substorm onset locations in IMAGE far ultraviolet auroral images are analyzed. A Hilbert‐Huang Transform is used to decompose the DMSP in the ionosphere to its intrinsic mode functions. A dominant mode near 10 mHz shows that its wave instantaneous energy starts increasing 2–5 min prior to and peaks about the commencement of sudden broadband and monoenergetic electron accelerations and the substorm expansion phase. The auroral beads exhibit azimuthal structuring with , which corresponds to the Doppler shifted kinetic ballooning frequency of 3–20 mHz and which is consistent with the 10 mHz wave. The kinetic ballooning mode can become unstable during the substorm growth phase when the pressure gradient and magnetic field‐line curvature become large. The 10 mHz oscillations thus can provide a signature marking approximately the start of the substorm expansion phase.