The ACES‐High rocket, part of the Auroral Current and Electrodynamics Structure (ACES) mission launched from Poker Flat Research Range on 29 January 2009, obtained the first in situ measurements indicative of both of the observational characteristics associated with the ionospheric feedback instability as it flew through an auroral arc and its associated return current region. ACES‐High observed Alfvénic wave structures localized in areas of roughly 10 km near the boundaries of the return current region associated with the discrete auroral arc and increased electron density with a temperature characteristic of a cold ionosphere. This density enhancement is believed to be caused by the excavation of plasma from lower altitudes via the ponderomotive force produced by the ionospheric Alfvén resonator, as shown by Streltsov and Lotko (2008). While this density is lower than expected from simulations and other observations by as much as an order of magnitude, the ratio of the enhancement to the background density is in agreement with predictions. The observations made by ACES‐High agree with the model results by Streltsov and Lotko (2008) but show the localized wave structures only near the boundaries of the return current region and not throughout it. This can be explained by strong small‐scale magnetic field‐aligned currents that are generated by the interaction between the large‐scale downward current and the ionosphere at these boundaries. Finally, a new model, based on that by Streltsov and Marklund (2006), was run with only one downward current region and produced results very similar to the observations seen by ACES‐High.
