Low-frequency waves upstream and downstream of the terrestrial bow shock

摘要

Disturbances in the solar wind caused by the Earth's bow shock lead to excitation of low-frequency waves both in the upstream and the downstream regions of the terrestrial bow shock, but their physical processes are different between these two regions and the wave properties are therefore different. Although, these waves have been investigated using a number of spacecraft, unambiguous identification of wave modes and their propagation directions remain as a task in interpreting spacecraft data. The difficulty stems from the fact that the measurements have been made at one or at best two points in space. In wave analyses it is essential to distinguish between temporal and spatial effects in the observed fluctuations, since wave frequencies and propagation speeds are subject to the Doppler shift. The Cluster mission overcomes this difficulty and achieves a four point measurement for the first time in space.