Rapid pulsation growth at quasi-parallel collisionless shocks.

Abstract

The bow shock is a shock wave that forms ahead of the Earth's magnetosphere in the collisionless supersonic flow of the solar wind. The quasi-parallel shock forms when the interplanetary magnetic field makes an angle of less than 45 degrees with the shock normal. Observations show the presence of magnetic pulsations, energetic diffuse ions and cooler, denser specularly reflected ions at the shock. Shock reformation involving the growth of magnetic pulsations is a key component of the structure of Earths quasi-parallel bow shock. This thesis aims to explore the rapid growth mechanism of these pulsations and hence increase our understanding of the structure of the quasi- parallel shock, which is generally less well understood than quasi-perpendicular shock. Using a hybrid simulation model of the interaction between shock reflected ions and upstream waves we study the effect of varying the parameters which control the growth of waves into large amplitude pulsations. A number of features are observed providing an insight into the growth mechanisms. Reflected ion beams can cause strong coupling with the pulse due to nonlinear cyclotron effects, and cause a narrow feature to develop. On the other hand, the diffuse ions show a relatively weaker interaction with the pulse and no narrow feature is generated. A strong interaction with the diffuse ions requires an unrealistically high density. However, in both cases, it is clear that the ULF wave does not simply just grow, as the pulse does not remain with the ions that cause the interaction. Instead the pulse splits and launches a new beamward (i.e., in the beam direction) wave in the simulation frame. This result may explain the spacecraft observation that pulsations appear to slow down in the spacecraft frame. The newly launched propagating wave seems to have some of the same properties as solitons. Comparisons are made with observations from the Cluster spacecraft. Hence, the results give us a greater insight into the wave-particle dynamics in the reformation process and implications for particle acceleration at the quasi-parallel shock.