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, Ala ska, we present high-resolution observations of structure in auroral HF wav es 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, na rrowband emissions occuring below the local plasma frequency during times o f intense HF wave emission. We have termed these emissions "HF bands" due t o their appearance in spectrogram images. These emissions are probably iden tical to the "spike" emissions identified by previous observers using lower time resolution data from the AUREOL/ARCAD3 satellite which showed a narro w peak spectra below the local plasma frequency. HF bands often occur when the local plasma density is varying and are associated with regions of inte nse Langmuir wave generation. We investigate the hypothesis that the HF ban ds are created when a Langmuir wave propagates from a low-density region in to a higher density region. The wave moves onto the whistler mode branch an d propagates as an HF band. Theoretical calculations of propagation times o f whistler mode waves support this hypothesis.