Novel Ion Bernstein Waves Observed in the Polar Cap on the Endurance Sounding Rocket

Abstract The Endurance sounding rocket, launched in May 2022 at an altitude of 768 km, provided the first observations of plasma waves in the sunlit polar cap on a sounding rocket. Despite the quiet geomagnetic conditions required for the primary mission science, Endurance observed a variety of interesting electric field and plasma density waves. This work presents observations of novel ion Bernstein waves ordered by the proton cyclotron frequency with unusual polarization characteristics. These waves are most prominent at harmonics n > 6 up to the lower hybrid frequency and from 250 to 600 km altitude, but also as weak modulations of very low frequency hiss at higher frequencies. At low altitudes near the F‐region density peak, their temporal profile strongly follows that of the lower hybrid frequency, indicating that the plasma density, in addition to the magnetic field, controls wave properties. While these waves share similarities with ion Bernstein waves previously reported in the auroral region, they exhibit left‐hand elliptical polarization rather than the expected linear polarization. In addition, a previously unreported extension of certain harmonic bands to altitudes <250 km is observed, with a particularly strong enhancement near 125 km altitude coinciding with an increase in E‐region plasma density. We present observations and analysis of the ion Bernstein waves and suggest that they may represent signatures of ion upflow.