In low density plasmas, the photo electron current of a sunlit spacecraft will often not be offset by the electron collection through collisions. So in effect the spacecraft will be positively charged relative to the surrounding plasma, ranging from a few volts to several tens of volts depending on plasma temperature, plasma density, solar flux and properties of the spacecraft surface.

For electron spectrometers this can be a problem since the flux of ambient electrons with an energy below the spacecraft potential will be obscured by a large flux of photo electrons being returned by the spacecraft potential. Ion spectrometers will naturally have the opposite effect, that low energy ions cannot reach the instrument.

Solving the sheet equation for the spacecraft and plasma should give information about the density and energy distribution close to the spacecraft. Though it is my understanding that doing this analytically is anything but straightforward. It is however relatively simple to use the spacecraft potential to estimate the density outside the sheet, in the unperturbed plasma. This is a commonly used technique in space plasma physics.

Thanks, Love Alm. Jan

There are seevral papers on the secondary electron from surface of the satellite and sensor.

1.1976 JGR Whillpe E.C.,observation of photoelectrons and secondary electrons reflected from a potential barrier in the visiti of ATS-6

2 1986 JGR  Mullen eat al., SCATHA survey of high -level spacecraft charging in sun light 

3. 1969 JGR Whipple ,Effects of secondary electron emission on electron trap measurements in the magnetosphere and solar wind

1976 JGR Whipple , Theory of teh speherically symmetric photoelectron sheath-A thick sheath approximation  and comaprison with 6 observation of a potential barrier.

Cheers

Oyama