VLF wave activity in the solar wind and the photoelectron effect in electric field measurements: Ulysses observations

We present observations of very low frequency (VLF) waves (0.2 to 448 Hz) made by Ulysses during the solar maximum and compare the observations with those obtained when the solar activity was approaching a minimum. The occurrences and properties of the waves are found to be similar during the solar maximum and solar minimum periods for slow and intermediate speed solar wind. The maximum intensities of the electromagnetic waves for the two solar cycle periods are comparable. These similarities suggest that the plasma conditions for the waves' excitation are similar for the slow and intermediate solar wind in both solar maximum and minimum phases. It is also found that the electric field noise detected in the low band channels, which are measuring less than 10 Hz signals, is contaminated by the spin modulation of the electric field caused by the photoelectron cloud around the spacecraft, especially when the ambient plasma density is low. This effect enhances with increasing solar aspect angle.