Electromagnetic wave structures within subauroral polarization streams

<jats:p>We report on oscillations in electric (δ<jats:italic>E</jats:italic><jats:sub><jats:italic>Y</jats:italic></jats:sub>) and magnetic (δ<jats:italic>B</jats:italic><jats:sub><jats:italic>Z</jats:italic></jats:sub>) fields and plasma density (δ<jats:italic>N</jats:italic><jats:sub><jats:italic>i</jats:italic></jats:sub>) observed by Defense Meteorological Satellite Program (DMSP) satellites within fast subauroral convection streams in the evening sector during the magnetic storm of 6 November 2001. There are two types of wave phenomena. The first and more common is characterized by electromagnetic and plasma density variations that have the same frequency range of ∼0.15 Hz in the spacecraft frame of reference. The second is characterized by large‐amplitude plasma and field oscillations over a broader range of frequencies ∼0.1 to 0.3 Hz. In this case the perturbation densities and fields appear to have different frequency responses. In this and other magnetic storms, strong waves are associated with the precipitation of ∼30 keV ions. Ratios of δ<jats:italic>E</jats:italic><jats:sub><jats:italic>Y</jats:italic></jats:sub>/δ<jats:italic>B</jats:italic><jats:sub><jats:italic>Z</jats:italic></jats:sub> indicate encounters with mixtures of electromagnetic (in part Alfvénic) and electrostatic modes. Poynting vectors associated with the oscillations can be directed either into or out of the ionosphere. The density perturbations appear to be extended east‐west corrugations in the plasma flow streams with north‐south wavelengths of ∼50 km. The δ<jats:italic>E</jats:italic><jats:sub><jats:italic>Y</jats:italic></jats:sub> and δ<jats:italic>N</jats:italic><jats:sub><jats:italic>i</jats:italic></jats:sub> variations were anticorrelated, as required for current conservation. Our analysis shows that Alfvénic perturbations are consistent with expected effects of irregular potential distribution around ionospheric density irregularities mapped to the magnetosphere. Inertial currents act to generate mesoscale field‐aligned currents carried by Alfvén waves, as was previously discussed with regards to auroral arcs formation. We suggest that δ<jats:italic>N</jats:italic><jats:sub><jats:italic>i</jats:italic></jats:sub> irregularities observed by DMSP satellites in the evening sector began as striated plasma patches in the polar cap that convected to subauroral latitudes.</jats:p>