Rocket observations of banded structure in waves near the Langmuir frequency in the auroral ionosphere

Using data from the PHAZE II sounding rocket, launched from Poker Flat, Alaska, we present high‐resolution observations of structure in auroral HF waves at and below the local plasma frequency. These observations were made in the altitude range of 390–945 km where the local plasma frequency is below the electron cyclotron frequency. We observe monochromatic, long‐lived, narrowband emissions occuring below the local plasma frequency during times of intense HF wave emission. We have termed these emissions “HF bands” due to their appearance in spectrogram images. These emissions are probably identical to the “spike” emissions identified by previous observers using lower time resolution data from the AUREOL/ARGAD3 satellite which showed a narrow peak spectra below the local plasma frequency. HF bands often occur when the local plasma density is varying and are associated with regions of intense Langmuir wave generation. We investigate the hypothesis that the HF bands are created when a Langmuir wave propagates from a low‐density region into a higher density region. The wave moves onto the whistler mode branch and propagates as an HF band. Theoretical calculations of propagation times of whistler mode waves support this hypothesis.