Magnetospheric electron density model inferred from Polar plasma wave data

Observations of the electron density n e based on measurement of the upper hybrid resonance frequency by the Polar spacecraft Plasma Wave Instrument (PWI) are available for March 1996 to September 1997, during which time the Polar orbit sampled all magnetic local time (MLT) values three times. Using the entire data set, we bin the n e values with respect to maximum field line radius LR E and radius R to generate an average density model for 3.5 ≤ L ≤ 7.8 and 3 R E ≤ R ≤ LR E . While our method does not assume any particular functional form for the density dependence along field lines, we find that the dependence can be roughly described by the power law form n e = n e0 ( LR E / R ) α with α having on average a value 0.2–0.9 in the plasmasphere (inner L region, 3.5 ≤ L ≤ 5.5), and a value 1.6–2.1 in the plasmatrough (outer L region, L ≥ 7). For instance, α = 0.8 ± 1.2 at L = 4.4 and α = 2.1 ± 1.4 at L = 7 (where the errors noted are overestimates). This result is similar to a previous result based on the Polar PWI data but using a different method. Alternately, the scale length for field line variation of density L α ≡ ( LR E )/ = 5.5 ± 4.1 R E across the entire region 4.4 ≤ L ≤ 7.8 (where the error noted is an overestimate). The data is also separated according to MLT, and it is shown that in the dawn sector the density is depleted and the radial dependence is steeper (larger α) than for other MLT sectors. A similar result holds when the averaged Kp value is large (>2).