Latitudinal density dependence of magnetic field lines inferred from Polar plasma wave data

Using observations of the electron density ne based on measurement of the upper hybrid resonance frequency by the Polar spacecraft Plasma Wave Instrument, we have examined the radial density dependence along field lines in the outer plasmasphere and the near plasma trough. Our technique depends on the fact that Polar crosses particular L values at two different points with different radial distance R. Sampled L values range from 2.5 to 6.3. In our plasmaspheric data set (ne ≥ 100 cm−3) we find that, on average, ne is flat along field lines from the equator up to the latitudes sampled by Polar (R ≳ 2 RE). In the plasma trough data set (ne < 100 cm−3) there is, on average, a mild radial dependence ne ∝ R−1.7.