Statistical studies of plasma structuring in the auroral ionosphere by the Swarm satellites

Abstract

This study uses over two years of 16 Hz density measurements, 50 Hz magnetic field data and ROTI data from the Swarm mission to perform long term statistics of plasma structuring in the polar ionosphere. The timeframe covers more than two years near the 24th solar cycle peak. We additionally use three years of data obtained from a timeframe close to solar minimum for discussion. We present power spectral densities (PSD) of electron density irregularities and magnetic field for one-minute intervals. These PSD have been characterized by the probability of a slope steepening, and by integrating the power deposited within frequency intervals corresponding to kilometer scales. For the electron density, we observe seasonal dependencies for both the integrated power and slope characteristics. While the dual slope probability, especially within the polar cap, varies with solar EUV-radiation, the integrated power is strongest around the equinoxes. Additionally, while we found similar results for the slope probability for both hemispheres, the integrated power exhibits strong hemispheric asymmetries with stronger enhancements within local summer in the southern hemisphere. The ROTI data shows a similar seasonal variability as the density PSD integrated power, in both seasonal dependency and interhemispheric variability. However, for the ROTI data the strongest fluctuations were found within the nightside auroral oval and the cusp. For the PSD of the magnetic field data, we obtain the strongest enhancements within the cusp for all seasons and all hemispheres. The fluctuations may indicate an increase in Alfvénic energy associated with a downward Poynting flux.