Electron density in the magnetosphere

Abstract

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 MLT values three times. In a previous study, we modeled the electron density dependence along field lines as n e = n e0 ( R max / R ) α , where n e0 is the equatorial electron density, R max ≈ LR E is the maximum geocentric radius R to any point on the field line, and α = α model = 8.0 − 3.0 log 10 n e 0 + 0.28(log 10 n e 0 ) 2 − 0.43( R max / R E ), for all categories of plasma (plasmasphere and plasmatrough). (In the formula for α model , n e0 is expressed in cm −3 .) Here, we illustrate the field line dependence using several example events. We show that the plasmapause is much more evident on the large radius portion of the orbit and that at R ∼ 2 R E the electron density tends to level out at large R max to a constant value ∼100 cm −3 . We also present an example of plasmaspheric plasma extending out to at least L ∼ 9 on the dawnside during particularly calm geomagnetic conditions (as indicated by low Kp). Then we present the average equatorial profiles of n e0 versus R max for plasmasphere and plasmatrough. Our average plasmasphere profile is found to have values intermediate between those based on the models of Carpenter and Anderson and Sheeley et al. The plasmatrough equatorial density n e0 scales with respect to R max like R max −3.4 , but in the region for which our plasmatrough data is most reliable ( L ≤ 6), it is well fit by the R max −4.0 scaling of Sheeley et al. or the R max −4.5 scaling of Carpenter and Anderson. We present a simple interpretation for the field line dependence of the density. For large n e0 , such as occurs in the plasmasphere, α is close to zero on average (implying that n e is roughly constant along field lines). When n e0 decreases, so does n e at R = 2 R E , but the value there does not decrease much below 100 cm −3 . (It is unclear if this value is an absolute lower density limit because most often the upper hybrid resonance emission disappears at R ∼ 2 R E because f p / f ce < 1, where f p is the plasma frequency and f ce is the electron cyclotron frequency.) Finally, we examined the dependence of α and the density at the equator and at R ∼ 2 R E on the average 〈Kp〉 (Kp averaged with a 3‐day timescale). There is no clear dependence of the average α − α model on 〈Kp〉 or on MLT. In the plasmasphere, n e0 decreases with respect to increasing 〈Kp〉.