Waves upstream and downstream of interplanetary shocks driven by coronal mass ejections

Abstract

In this work we study the waves in regions adjacent to ten interplanetary (IP) shocks formed by the interactions between interplanetary coronal mass ejections and the solar wind. We analyze the STEREO data for the years 2007–2010. Shocks in our sample have low magnetosonic Mach numbers (Mms ≤ 2.3), their criticality ratios range between 0.8 and 2.3 and θBn are between 38° and 85°. We find ultra‐low frequency (ULF, 0.01 Hz–0.05 Hz) waves and higher‐frequency (HF, ≥ 1 Hz) whistler precursors upstream of these shocks. Downstream of them we observe irregular ULF fluctuations and regular HF waves with similar frequencies as in the upstream case. We find that IP shocks with relatively small Mms can excite waves in large regions in front of them (2.2 × 10−3 AU–4.6 × 10−3 AU), thereby forming large ULF wave foreshocks. We do not find any evidence for the steepening of these waves. We do observe suprathermal (E ≤ 30 keV) proton foreshocks upstream of some of the shocks in the sample. The extensions of suprathermal proton foreshocks range between 0.02 AU and 0.1 AU. However, not all foreshocks with suprathermal ions show ULF waves or vice versa. The extensions of ULF and proton foreshocks can be very different. Enhanced ULF waves and suprathermal protons can be observed upstream of local quasi‐perpendicular shocks. We propose that the observed discordance between the shock geometries and the presence of the foreshock phenomena may be explained in terms of temporal and spatial variations of the local geometry of the IP shocks.