Proton cyclotron waves at Mars and Venus

Proton cyclotron frequency waves are present upstream of the Mars bow shock and have been interpreted as due to the ionization of exospheric hydrogen and the subsequent acceleration of the proton by the solar wind electric field. These waves are left handed polarized on the spacecraft frame and propagate at a small angle to the magnetic field, presumably growing from the free energy of newly accelerated protons. A second consequence of this acceleration is the loss of the planet's hydrogen exosphere. Proton cyclotron waves have not previously been reported for Venus. Herein, we report the occurrence of such waves in the Venus magnetosheath. However, we do not find clear examples of such waves in the solar wind upstream of the Venus shock. At Mars, the waves are found up to at least 12 planetary radii to the side of the planet. A process that could produce such an extended pattern of newly created ionization and resultant waves is a multi-step one; ionization near the planets, acceleration of ions by the solar wind electric field, reneutralization of ions allowing them to travel across field lines, and a second ionization far from the planet producing the waves. (c) 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.