Very low frequency waves in the heliosphere: Ulysses observations

An overall profile of plasma wave activity in a frequency range of 0.2 to 448 Hz in the solar wind is presented using 6 years of Ulysses data which cover a large range of heliographic latitudes (0 degrees to +/- 80 degrees) at distances from 1 to 5 AU from the Sun. The spacecraft has continuously observed wave activity with peak power below the local electron cyclotron frequency f(ce). Four distinct types of fluctuation phenomena have been observed: (1) enhanced electromagnetic fluctuations associated with the interplanetary shocks and heliospheric current sheet crossings and other solar wind turbulence; (2) enhanced electromagnetic fluctuations associated with compression regions of high-speed stream interfaces, which were observed in periods of increasing solar wind velocity; (3) electric fluctuations associated with the expanding solar wind, which were observed in periods of decreasing solar wind velocity; and (4) enhanced electric fluctuations in the high-latitude fast solar wind plasma. The fourth type of wave activity was observed nearly continuously with the relative power observed peaked near the local f(ce). The first three types of waves were observed in the heliomagnetic streamer belt flows, where the spacecraft frequently encountered enhanced solar mind turbulence, interplanetary shocks, and current sheet crossings. The electromagnetic wave bursts (types 1 and 2) are likely to be whistler mode. The occurrences of apparently electrostatic waves during periods of expanding solar wind are coincident with significant reductions in the electron heat flux intensity. The generation mechanism of these electrostatic waves is still under investigation, but the observations may imply that these waves reduce the intensity of the heat flux through enhanced wave particle scattering associated with a heat flux instability.