Electron density in the magnetosphere

Abstract

[1] 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) a , 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 a = a model = 8.0 À 3.0 log 10 n e0 + 0.28(log 10 n e0 ) 2 À 0.43(R max /R E ), for all categories of plasma (plasmasphere and plasmatrough). (In the formula for a model , n e0 is expressed in cm À3 .) 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 À3 . 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 À3.4 , but in the region for which our plasmatrough data is most reliable (L 6), it is well fit by the R max À4.0 scaling of Sheeley et al. or the R max À4.5 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, a 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 À3 . (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 / ffiffiffiffi ffi n e p is the plasma frequency and f ce is the electron cyclotron frequency.) Finally, we examined the dependence of a and the density at the equator and at R $ 2 R E on the average hKpi (Kp averaged with a 3-day timescale). There is no clear dependence of the average a À a model on hKpi or on MLT. In the plasmasphere, n e0 decreases with respect to increasing hKpi.

Eastward sub-auroral ion drifts or ASAID

“…2) ion precipitation originates probably in the ring current and the ASAID is located very close to the outer edge of the plasmasphere but inside. This would mean that the plasmasphere extends to 6-7 R E , which is not surprising because both y and z components of the IMF are small and in such cases the plasmasphere could extend to more than 7 R E (Denton et al, 2004). The 21 September 2003 event occurs at much lower latitudes but one should keep in mind that the geomagnetic activity is higher and the interplanetary conditions are no longer quiet.…”

The interaction of ultra-low-frequency pc3-5 waves with charged particles in Earth’s magnetosphere

“…This involves using pairs of ground magnetometers (at a given magnetic longitude and different latitudes) to compute field line eigenfrequencies by comparing the wave phase and amplitude at each of two locations. Knowledge of cold plasma mass density also enables the Alfven speed to be determined and hence the magnetospheric response to low-frequency ULF disturbances (Denton et al 2004a(Denton et al , b, 2006.…”

“…Cold plasma mass density affects the Alfvén speed and thus the magnetospheric response to ULF waves, while the electron number density controls the highfrequency (VLF and radio wave frequency range) response (Denton et al 2004a(Denton et al , b, 2006. It is unclear how plasma mass density structure near the plasmapause and plasmasphere boundary layer (PBL) affect other properties of ULF waves and field line resonances (Fraser et al 2005).…”

“…The period of successive dispersion signatures is comparable to the ULF wave period of * 100 s, implying that the modulations in flux are the local response of ions to ULF waves rather than escaping ions from Earth's ionosphere. Once these oxygen ions escape into the magnetosphere they will alter the plasma mass density and hence the properties of ULF waves (Denton et al 2004a(Denton et al , b, 2006.…”

Spatial and temporal characteristics of poloidal waves in the terrestrial plasmasphere: a CLUSTER case study

“…The plasma number density along the field line is assumed as a power law (Cummings et al, 1969) with an exponent α=1 typical for the plasmasphere (e.g. Goldstein et al, 2001;Denton et al, 2004):…”