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 RE geocentric). Results from the premidnight sector (1800-2400 MLT) along auroral field lines show roughly a fourfold increase in the occurence of small scale size, large amplitude electric fields (>100 mV m-1) at altitudes from 1.9 to 2.5 RE 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 RE also show evidence for an increase in cold ionospheric plasma (from ∼30 to 60 cm-3) for sunlit compared to dark ionospheric conditions.
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