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 curre nt, J(hv), as a function of the spacecraft's (SIC) "floating potential," De lta Phi(S/C). The photoemission current function is found to be time indepe ndent using nearly 10 months of GGS-Polar data from April 1996 to March 199 7 including 1.6 million separate spectra from the Hydra (hot plasma) and EF I (electric field) instruments. The photoemission current density leaving t he 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 de termined from the ratio of sunlit to spacecraft areas and the plasma curren t density, J(RC), collected by the spacecraft. For the Polar spacecraft thi s ratio of areas is not constant in time, and the observed return current v oltage relation is time dependent. After correction for the orbitally induc ed time-dependent ratio of areas, the photoemission curve reported above is obtained and is essentially time independent. After correcting for the dif ferent 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 interrupt ed, the photoemission-voltage relationship is essential for the assignment of the ambient kinetic energy of the detected particle fluxes. We demonstra te a method using the plasma data and the statistical return current relati onship that recovers the floating potential of the spacecraft with a typica l precision that is the larger of 0.5 V and 0.1 Delta Phi(S/C). We also dem onstrate that the ion and electron densities determined by numerical integr ation over distributions corrected with opposite energy shifts implied by t he potential enhances their routine agreement, a further check on the absol ute precision of the potential inferences. The "systematic" departures of 4 0% of the data from the statistically defined J(hv)(Delta Phi(S/C)) curve r eported above are time varying and organized in radius and with L shell. Th ese data are inferred to be the signature of missing ambient plasma current s to the spacecraft that are not directly detected by the Hydra instrumenta tion (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 Geophysi cal Research, 1999).