Photoemission current-spacecraft voltage relation: Key to routine, quantitative low-energy plasma measurements

Measurements of the floating potential and plasma return current on the GGS-Polar spacecraft are used to determine the equilibrium photoemission current, J(hv), as a function of the spacecraft's (SIC) floating potential, Delta Phi(S/C). The photoemission current function is found to be time independent using nearly 10 months of GGS-Polar data from April 1996 to March 1997 including 1.6 million separate spectra from the Hydra (hot plasma) and EFI (electric field) instruments. The photoemission current density leaving the spacecraft at positive floating potentials Delta Phi(S/C) < 50 V is well fit by a sum of two exponentials of the form: J(ftv)(mu A/m(2)) = A exp(2) (-Delta Phi(S/C)B) + C exp (-Delta Phi(S/C)/D), where best fit estimates A similar or equal to 152 +/- 57 mu A/m(2), B similar or equal to 1.7 +/- 0.2 eV, C similar or equal to 0.86 +/- 0.29 mu A/m(2), and D similar or equal to 9.5 +/- 1.0 eV. In equilibrium this photoemission current density is determined from the ratio of sunlit to spacecraft areas and the plasma current density, J(RC), collected by the spacecraft. For the Polar spacecraft this ratio of areas is not constant in time, and the observed return current voltage relation is time dependent. After correction for the orbitally induced time-dependent ratio of areas, the photoemission curve reported above is obtained and is essentially time independent. After correcting for the different procedures used the present results are illustrated to be consistent with early results with less resolution reported by Pedersen [1995]. When the sensing of the floating potential by EFI on the spacecraft is interrupted, the photoemission-voltage relationship is essential for the assignment of the ambient kinetic energy of the detected particle fluxes. We demonstrate a method using the plasma data and the statistical return current relationship that recovers the floating potential of the spacecraft with a typical precision that is the larger of 0.5 V and 0.1 Delta Phi(S/C). We also demonstrate that the ion and electron densities determined by numerical integration over distributions corrected with opposite energy shifts implied by the potential enhances their routine agreement, a further check on the absolute precision of the potential inferences. The systematic departures of 40% of the data from the statistically defined J(hv)(Delta Phi(S/C)) curve reported above are time varying and organized in radius and with L shell. These data are inferred to be the signature of missing ambient plasma currents to the spacecraft that are not directly detected by the Hydra instrumentation (cf. X. Cao et al., Properties of very cold (T-e similar or equal to 0.1 eV) electrons within the magnetosphere, submitted to Journal of Geophysical Research, 1999).