Observations of short large‐amplitude magnetic structures at a quasi‐parallel shock

Abstract

We have conducted a detailed analysis of a set of events termed short large-arnphtude magnetic structures (SLAMS) observed at an encounter of the quasi-parallel bow shock by the AMPTE UKS and IBM 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 iYmne 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 amphtude 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, cychcally 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. conditions and that these instabilities cause the shock to cyclically re-form. pulsations was dictated primarily by the Larmor radii of the backstreaming particles found in abundance upstream of the bow shock but downstream of the foreshock boundary. This boundary is defined by the upstream field line which is tangent to the curved bow shock, or by the trajectory of field-aligned ion beams of a given energy originating at the point of tangency. The turbulence associated with the shock apparently saturates some distance, typically 10 R•, downstream of this ion foreshock boundary [Bonifazi et al., 1983], downstream of which nearly isotropic distributions of "diffuse" energetic ions are found. The curved nature of the bow shock led early workers to suggest that the turbulent appearance of the quasi-parallel shock was due to debris which originated under more quasi-perpendicular configura-4209

Ion Acceleration at the Quasi-Parallel Bow Shock: Decoding the Signature of Injection

“…The longperiod pulsations are generated by backstreaming ions in the foreshock, andtheir wave phase velocities are directed upstream in the plasma frame, but they are swept back toward the shock with the solar wind flow (e.g., Eastwood et al 2005). These pulsations can grow into large-amplitude structures in the area immediately upstream of the shock (sometimes referred to as short large-amplitude magnetic structures, or SLAMS), which can trigger a reformation of the shock layer, contributing to the turbulent appearance of the shock transition region (Schwartz 1991;Schwartz & Burgess 1991;Schwartz et al 1992). This upstream ULF wave field also leads to variations in the instantaneous θ Bn of the shock, which locally changes the dynamics of the shock, leading to short-scale variations between parallel and perpendicular conditions.…”

Cluster magnetic field observations at a quasi-parallel bow shock

“…They are found to propagate sunward in the plasma frame, but are convected anti-sunward by the solar wind. They display mixed polarisation, biased towards right-hand polarised signatures (in the spacecraft frame), often with the remnant of a left-handed (in the spacecraft frame), high frequency whistler wave on the leading edge, similar to that found at the lower amplitude, steepened ULF waves observed in the foreshock (Schwartz et al, 1992) commonly called "shocklets".…”

“…A combination of satellite observations (e.g. Gosling et al, 1989;Thomsen et al, 1990;Schwartz et al, 1992) and simulation work (e.g. Burgess, 1989;Thomas et al, 1990;Scholer, 1993) have led to a picture of a shock undergoing cyclic reformation, composed of a patchwork of magnetic field enhancements called SLAMS (Short, Large-Amplitude, Magnetic Structures) and regions between the SLAMS where the magnetic field is disturbed and the plasma is partially thermalised (e.g.…”

Time domain analysis of plasma turbulence observed upstream of a quasi‐parallel shock

“…The solar wind speed and magnetic field remained stable [Schwartz et al, 1992]. The spacecraft separation vector was nearly parallel with the GSE x axis.…”

“…The spacecraft separation vector was nearly parallel with the GSE x axis. During the whole period under investigation the orientation of averaged IMF remained in the quasiparallel regime and the angle between the magnetic field and the model shock normal was 10ø-150 [Schwartz et al, 1992]. The stability of these conditions enables the application of the NARMAX nonlinear system identification methodology.…”