Electron density in the magnetosphere

R.E. Denton, J.D. Menietti, J. Goldstein,  S.L. Young, and R.R. Anderson 


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)^alpha, where ne0 is the
equatorial electron density, Rmax ~ LRE is the maximum geocentric radius R to
any point on the field line, and alpha = alpha(model) = 8.0 - 3.0 log10 ne0 +
0.28(log10 ne0)^2 - 0.43(Rmax/RE), for all categories of plasma (plasmasphere
and plasmatrough). (In the formula for alpha(model), 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,
alpha 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 ~ ne^(1/2) is the
plasma frequency and fce is the electron cyclotron frequency.) Finally, we
examined the dependence of alpha 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 alpha - alpha(model) on <Kp> or on MLT. In the
plasmasphere, ne0 decreases with respect to increasing <Kp>. 

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Journal of Geophysical Research, 109, A09215, doi:10.1029/2003JA010245, 2004.