Low-Frequency Waves in the Solar Wind Near Neptune

M. Zhang, J. W. Belcher, J. D. Richardson, V. M. Vasyliunas, R. P. Lepping, N. F. Ness and C. W. Smith

Geophysical Research Letters, 18, 1071-1074 (1991)

Abstract:

Plasma and magnetic field observations from the Voyager 2 spacecraft when it was outbound from Neptune reveal low-frequency waves in the solar wind which are clearly associated with the planet. The waves have frequencies below the proton cyclotron frequency, which is about 0.001 Hz during the periods waves are observed. The waves are present when the interplanetary magnetic field is oriented such that the spacecraft is connected to the bow shock by the magnetic field lines. We have identified the waves to be Alfvenic waves propagating at about 140 degrees to the ambient magnetic field and away from the bow shock. As at the other planets, these downstream waves are thought to be generated in the upstream region, where energetic protons created near the nose of the bow shock excite waves as they stream along solar wind magnetic field lines.