The Earthʹs quasi-parallel bow shock: Review of observations and perspectives for Cluster

Spacecraft crossings of the Earthʹs quasi-parallel bow shock under typical high Mach number conditions are characterized by the presence of large-amplitude, compressive, low-frequency magnetic pulsations. Thus, instead of the fairly abrupt jump from upstream to downstream states one associates with their quasi-perpendicular counterparts, quasi-parallel shocks are more aptly described in terms of an extended transition from upstream to downstream conditions. The complex, turbulent appearance of the quasi-parallel shock diverted much of the research efforts in the field towards the quasi-perpendicular regime until about 15 years ago. This paper reviews what we have learnt about the quasi-parallel shock transition zone from spacecraft observations at the Earthʹs bow shock in the last decade and a half. Field and particle measurements suggest a prominent role is played by short (∼10 s) large-amplitude (|δB|/|B|>2, typically 3 or more) magnetic structures that evolve out of the upstream wave field and are convected towards the shock by the supersonic solar wind. As these structures grow in amplitude, they steepen, slow down in the shock rest frame and begin to merge with similar structures, leading to the downstream state. A number of questions about the nature of the quasi-parallel shock transition remain and the possible contribution Cluster may make towards their resolution is highlighted.