M.Phil.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.STFC studentship