Interferometric determination of broadband ELF wave phase velocity within a region of transverse auroral ion acceleration

Broadband electric field fluctuations with typical amplitudes of 10–20 mV/m peak‐to‐peak and frequencies from 0 Hz to 3 kHz (BB‐ELF) were observed coincident with a region of ≤200 eV transverse H + acceleration (TAI) near the poleward edge of the pre‐midnight aurora. The coherence and phase velocity of the electric fields were measured using a interferometric antenna array over the frequency range of ≈ 100 Hz to 3 kHz. These electric field fluctuations were found to have the following characteristics: 1) incoherence perpendicular to the geomagnetic field, 2) coherence parallel to the the geomagnetic field, 3) parallel phase velocity (ω/ k ∥ ) of 30–35 km/s upwards, 4) 0 < | k ∥ / k ⟂ | < 0.22. We show that these properties are compatible with the emission being electrostatic H + cyclotron (EHC) waves. We also discuss possible generation mechanisms for the waves, and their relationship to the TAI.