Saturnʹs ULF wave foreshock boundary: Cassini observations

Even though the solar wind is highly supersonic, intense ultra-low frequency (ULF) wave activity has been detected in regions just upstream of the bow shocks of magnetized planets. This feature was first observed ahead of the Earthʹs bow shock, and the corresponding region was called the ULF wave foreshock, which is embedded within the planetʹs foreshock. The properties as well as the spatial distribution of ULF waves within the Earthʹs foreshock have been extensively studied over the last three decades and have been explained as a result of plasma instabilities triggered by solar wind ions backstreaming from the bow shock. Since July 2004, the Cassini spacecraft has characterized the Saturnian plasma environment including its upstream region. Since Cassiniʹs Saturn orbit insertion (SOI) in June 2004 through August 2005, we conducted a detailed survey and analysis of observations made by the Vector Helium Magnetometer (VHM). The purpose of the present study is to characterize the properties of waves observed in Saturnʹs ULF wave foreshock and identify its boundary using single spacecraft techniques. The amplitude of these waves is usually comparable to the mean magnetic field intensity, while their frequencies in the spacecraft frame yields two clearly different types of waves: one with frequencies below the local proton cyclotron frequency ( Ω H + ) and another with frequencies above Ω H + . All the wave crossings described here, clearly show that these waves are associated to Saturnʹs foreshock. In particular, the presence of waves is associated with the change in θ Bn to quasi-parallel geometries. Our results show the existence of a clear boundary for Saturnʹs ULF wave foreshock, compatible with θ Bn ∼ 45 ° surfaces.