We present simultaneous observations of ion distribution functions and plasma waves in the high‐latitude topside ionosphere (500–1000 km) near local midnight during substorm activation. Using a new instrument, the Suprathermal Ion Imager (SII), we are able to explore two‐dimensional ion distribution functions with unprecedented resolution in time (93 s−1) and energy, focusing on the lowest‐energy core (<1 eV) population and suprathermal extensions thereof. The GEODESIC sounding rocket flew through regions containing broadband extremely low frequency (BB ELF) waves, large‐amplitude Alfvén waves, and lower‐hybrid solitary structures (LHSS), all of which have been shown previously to correlate with ion heating. However, GEODESIC detected heating only in association with LHSS. Ion distributions in and near LHSS showed acceleration in the direction transverse to (TAI) with temperatures as high as 5–10 eV, appearing in regions 65 ± 25 m across. TAI were centered on much smaller structures (∼20 m across) of depleted plasma density and enhanced perpendicular electric fields from 0.1 to 10 kHz, consistent with observations from previous rocket flights. Also in agreement with previous findings, we find two distinct ion populations inside LHSS. The dominant population is composed of unheated, isotropic “core” Maxwellian ions having temperatures comparable to the surrounding ambient plasma, while roughly 10% of the plasma density corresponds to the TAI at pitch angles of 90 ± 5°. We argue that the TAI associated with LHSS is consistent with bulk heating of the core ions, and we describe two scenarios that can lead to the observed two‐temperature distributions. The BB ELF wave emissions were correlated with LHSS heating at short scales and anticorrelated with auroral electron precipitation at large scales. The GEODESIC instruments observed no large‐scale ion heating in association with BB ELF waves. Similarly, no ion heating was detected in the presence of large‐amplitude, short perpendicular wavelength Alfvén waves.
