Electron density in the magnetosphere

Abstract

Observations of the electron density ne 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 ne = ne0(Rmax/R)α, where ne0 is the equatorial electron density, Rmax ≈ LRE is the maximum geocentric radius R to any point on the field line, and α = αmodel = 8.0 − 3.0 log10ne0 + 0.28(log10ne0)2 − 0.43(Rmax/RE), for all categories of plasma (plasmasphere and plasmatrough). (In the formula for αmodel, ne0 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 RE the electron density tends to level out at large Rmax 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 ne0 versus Rmax 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 ne0 scales with respect to Rmax like Rmax−3.4, but in the region for which our plasmatrough data is most reliable (L ≤ 6), it is well fit by the Rmax−4.0 scaling of Sheeley et al. or the Rmax−4.5 scaling of Carpenter and Anderson. We present a simple interpretation for the field line dependence of the density. For large ne0, such as occurs in the plasmasphere, α is close to zero on average (implying that ne is roughly constant along field lines). When ne0 decreases, so does ne at R = 2 RE, 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 RE because fp/fce < 1, where fp is the plasma frequency and fce is the electron cyclotron frequency.) Finally, we examined the dependence of α and the density at the equator and at R ∼ 2 RE on the average 〈Kp〉 (Kp averaged with a 3‐day timescale). There is no clear dependence of the average α − αmodel on 〈Kp〉 or on MLT. In the plasmasphere, ne0 decreases with respect to increasing 〈Kp〉.