OBSERVATIONS OF SHORT LARGE-AMPLITUDE MAGNETIC-STRUCTURES AT A QUASI-PARALLEL SHOCK

We have conducted a detailed analysis of a set of events termed short large-amplitude magnetic structures (SLAMS) observed at an encounter of the quasi-parallel bow shock by the AMPTE UKS and IRM satellites. Both the satellite configuration and the solar wind conditions are favorable for the case study presented here. We have identified isolated SLAMS, surrounded by solar wind conditions, and embedded SLAMS, which lie within or form the boundary with regions of significant heating and deceleration. The duration, polarization, and other characteristics of SLAMS are all consistent with their growth directly out of the ULF wave field, including the common occurrence of an attached whistler as found in ULF shocklets. The plasma rest frame propagation speeds, where they can be determined, and two-spacecraft time delays for all cases show that the SLAMS attempt to propagate upstream against the oncoming flow, but are convected back downstream. The speeds and delays vary systematically with SLAMS amplitude in the way anticipated from nonlinear wave theory, as do their polarization features. Inter-SLAMS regions, and boundary regions with the solar wind, contain hot deflected ions of lesser density than within the SLAMS. The amplitude of the SLAMS requires an active growth mechanism. Following earlier inferences about the limited transverse extent of SLAMS, we highlight the importance of determining the thickness of the transition zone over which SLAMS grow and the bulk heating and deceleration is effected. From this case study it appears that, at least under some circumstances, the quasi-parallel shock cannot be regarded as an undulating, cyclically re-forming simply connected surface. Instead, the transition zone is better represented as a set of ULF waves, some of which grow to become SLAMS which gradually decelerate and merge to form the downstream state.