Observation of ion‐cyclotron turbulence at small values of magnetic‐field‐aligned current

Fluctuations in plasma density are observed in the ion‐cyclotron frequency range in a Q‐machine plasma column at values of magnetic‐field‐aligned current significantly below the critical value for current‐driven electrostatic ion‐cyclotron (CDEIC) waves. These fluctuations arise from the inhomogeneous energy‐density driven (IEDD) instability, are relatively broadband, and can have large amplitude compared to CDEIC fluctuations. These results impact the interpretation of ion heating and acceleration in the ionosphere, especially at low‐altitude where magnetic‐ field‐aligned current densities estimated to be large enough to be supercritical to current‐driven or beam‐driven electrostatic ion‐cyclotron waves are rarely detected.