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Inner magnetospheric electron temperature and spacecraft potential estimated from concurrent Polar upper hybrid frequency and relative potential measurements
Author(s) -
Boardsen S. A.,
Adrian M. L.,
Pfaff R.,
Menietti J. D.
Publication year - 2014
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2014ja019852
Subject(s) - electron , physics , electron density , atomic physics , electric field , spacecraft , ionosphere , magnetosphere , plasma , electron temperature , polar , spacecraft charging , geophysics , quantum mechanics , astronomy
Direct measurement of low < 1 eV electron temperature is difficult to make in the Earth's inner magnetosphere for electron densities ( N e ) < 3 × 10 2 cm −3 . We compute these quantities by solving current balance equations in low‐density regions. Concurrent measurements from the Polar spacecraft of the relative potential ( V S − V P ), between the spacecraft body and the electric field probe, and the electron density ( N e ), derived from upper hybrid frequency ( f UHR ), were used in the current balance equations to solve for the electron temperature ( T e ), V s , and V p . Where V P is the probe potential and V S is the spacecraft potential relative to the nearby plasma. The assumption that the bulk plasma electrons are Maxwellian is used in the computations. Our data set covered 1.5 years of measurements when f UHR was detectable ( L < 10). The following “averaged” T e versus L relation for 3 < L < 5 was obtained: T e = 0.58 + 0.49 ( L − 3) eV. This expression is in reasonable agreement with extrapolations of ionospheric T e measurements by Akebono at lower altitudes. However, the solution is sensitive to the photoemission coefficients, substituting those of Scudder et al. (2000) with those of Escoubet et al. (1997), the T e curve shifted upward by ~1 eV. Also, the solution is sensitive to measurement error of V S − V P , applying a voltage shift of ±0.1 and ±0.2 V to V S − V P , the relative median error for our data set was computed to be 0.27 and 1.04, respectively. We believe that our T e values computed outside the plasmasphere are unrealistically low. We conclude that this method shows promise inside the plasmasphere but should be used with caution. We also quantified the N e versus V S − V P relationship. The running median N e versus V S − V P curve shows no significant variation over the 1.5 year period of the data set, suggesting that the photoemission coefficients did not change significantly over this time span. The Scudder et al. (2000) N e model, based on only one Polar orbit, is in reasonable agreement (within a factor of 2) with our results.