Comparison of VLF Wave Activity in the Solar Wind During Solar Maximum and Minimum: Ulysses Observations

Abstract

We have compared observations of VLF waves (0.2 to 448 Hz) made by Ulysses during its second fast latitude scan (near the solar maximum) with the wave observations during the first fast latitude scan in 1995, 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 intensity of the electromagnetic waves for the two solar cycle periods is 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 9 Hz signals, is contaminated by the spin modulation of the electric field due to photoelectrons around the spacecraft, especially when the ambient plasma density is low. This noise increases with increasing solar aspect angle.