Solar Wind Electric Fields in the Ion Cyclotron Frequency Range

In earlier papers (Kellogg and Lin 1997, Kellogg 2000, Kellogg et al 2001) which attempted to understand what effects were responsible for the violation of the conservation of magnetic moment in the solar wind it was pointed out that electric fields in the range which would resonate with the ion cyclotron frequency have not been adequately investigated. In most spinning spacecraft, fields in this frequency range are overwhelmed by the photoelectric variation of the antenna potentials. More generally, it is expected that there are fluctuating forces which replace collisions, not only to maintain isotropy, but also to make valid the assumptions leading to MHD, since that theory seems to work in the solar wind. The violation of conservation of magnetic moment is a convenient measure of these effects which can be treated quantitatively, but is not the only one. A first attempt at measurement of electric fields in the relevant frequency range was made with the URAP instrument on Ulysses. The spin axis of Ulysses was pointed near the sun, reducing photoelectric variation, but the instrument used a digital transform for on-board frequency analyses whose leakage from the spin frequency was so high that only occasionally were signals in the relevant frequency range visible above this unwanted signal (Lin et al 2003) . The RPWS experiment on the Cassini spacecraft seemed to offer an excellent opportunity as Cassini is three-axis stabilized (Kellogg et al, 2001). However the measurements seemed to be contaminated by an instability on the spacecraft wake (Kellogg et al, 2003), and it was uncertain how much of the observed signal represented the undisturbed solar wind. The present work analyzes data from the Cluster satellites. These are spinning. However the antennas consist of small spheres. These are on the ends of wires which are not part of the sensor, and which are biased by a current which nearly cancels the photoelectron emission. These antennas have much reduced photoelectric variation which has allowed the observations of low frequency electric fields presented here.