Seasonal variations along auroral field lines: Measurements from the Polar spacecraft

Measurements from the Polar electric field instrument are used to study large electric fields and the ambient plasma density as a function of altitude (1.8–6.0 R E geocentric). Results from the premidnight sector (1800–2400 MLT) along auroral field lines show roughly a fourfold increase in the occurrence of small scale size, large amplitude electric fields (>100 mV m −1 ) at altitudes from 1.9 to 2.5 R E for dark compared to sunlit ionospheric conditions. Density values inferred from spacecraft potential measurements show these electric fields to be correlated with low plasma densities (0.2 to 3.0 cm −3 ). An increase in the average plasma density from 10 to 60 cm −3 is also observed for sunlit compared to dark conditions for the same altitude range. In addition, the distribution of density measurements from 1.9 to 2.2 R E also show evidence for an increase in cold ionospheric plasma (from ∼30 to 60 cm −3 ) for sunlit compared to dark ionospheric conditions.