Measurements of potential of a cylindrical monopole antenna on a rotating spacecraft

Measurements of antenna potential on the Helios 1 spacecraft are presented and discussed. Normally, the antenna potential varies by about 6 V between positions normal to the sun line and pointing at the sun. The behavior of the potential indicates that Ly α and shorter wavelengths determine the potential; longer wavelengths only create photoelectrons which cannot escape. As expected, there is usually a voltage spike when the antenna is shadowed. We do not measure this voltage. Sometimes this spike is not present, which is puzzling, but which allows us to estimate the potential variations on an antenna which is not shadowed. The spectrum is then roughly (Fourier coefficient) 2 V × (2fo/f)² where fo is the spacecraft spin frequency, and f is an even harmonic of the spin frequency.