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The Freja Cold Plasma Analyzer (CPA) is a hemispherical electrostatic analyzer which forms 2-D, energy/arrival angle images of low-energy (< 200 eV) particle distribution functions. In addition to its 2-D imaging capability, the CPA is unique in that its sensor head is displaced from the spacecraft on a 2 m boom which allows control of the probe-to-plasma potential, thereby compensating for variations in spacecraft potential. Furthermore, the detector biases can be set to measure either positively or negatively charged particle species. This study emphasizes temperature measurements of the core electron population (∼ 1 eV) which are difficult to make since, among other reasons, they are affected by spacecraft charging and are susceptible to contamination from photoelectrons and finite gyroradius effects. We demonstrate that the CPA sensor does detect the core population by showing that probe-to-plasma potential changes of only a fraction of 1 V cause large changes in the electron flux measured at the detector anode. The relation between detector current and sensor bias comprises a modified Langmuir curve, which in contrast to standard Langmuir measurements results from a single particle species only (electrons) and from particles which are restricted in energy (0-70 eV) and in arrival angle to within ±4° of the detector plane. The modified Langmuir curve can be compared to a standard Langmuir curve formed by measuring current onto the external skin of the sensor, which contains contributions from both electrons and ions at all energies and over a wide angular acceptance. Somewhat surprisingly, the two methods agree fairly well in the sunlit portion of a sample orbit, i.e. in the presence of a background photoelectron population, but differ by up to a factor of two in the dark part of the orbit. |
