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Cluster : PEACE

Issues Affecting Accuracy of PEACE Onboard Moments

Spacecraft Potential

 Effects of non-zero Spacecraft Potential
It is not always possible to produce accurate moments in a straightforward way, even if we get a perfectly calibrated instrument. The key issue is that the actual electron velocity distribution in the plasma could differ from the measured one, for electron energies close to or less than the spacecraft potential. The moments integration onboard uses the measured (sometimes perturbed) distribution. Thus it is necessary to be very careful when using PEACE moments, to verify that the potential is not having a misleading affect. If spacecraft electrons are present in the energy band over which the moments were calculated, they will in effect be combined with the natural plasma electrons, leading to moments which do not properly describe the natural plasma.

above: sketch showing the trajectories of high and low (relative to the potential) energy electrons in purple and green; and low energy electrons of spacecraft origin (e.g. photoelectrons or secondary electrons) in blue, for the case of a positively charged spacecraft. The red radial lines show the boundaries between the fields of view of the 12 PEACE anodes, and also serve as a reference against which to compare the trajectroies of electrons which are being attracted by the positively charged spacecraft.

above: PEACE data from one PEACE detector, from each of two Cluster spacecraft, with ASPOC in action controlling the potential on Salsa (Spacecraft 2) but not on Rumba (Spacecraft 1). ASPOC is active between about 02:30 and 05:15, and its effects can clearly be seen by comparing the two spectrograms. The green region, bounded at its upper edge by a yellow line, and below by red, is a measurement of electrons of spacecraft origin, and the yellow line marks the spacecraft potential. Typically LEEA moments cover the energy range 10 eV to 1 keV, and without ASPOC the energy range clearly (in this example) contains spacecraft electrons as well as natural plasma electrons. Typically HEEA moments cover the energy range 34 eV to 26 keV. In the example shown, spacecraft electrons are seen at energies near or above 34 eV for at least the 45 minutes, and the potential remains intermittently high until about 04:30 after which it stabilises near 10 or 20 Volts. Thus,  HEEA moments may be fairly reliable after 04:30, but are more likely to be affected by variations in potential before then and may even by contaminated by spacecraft electrons in the first hour or so

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By Andrew Fazakerley
Last updated on 14/01/02  by  Andrew Fazakerley

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