Dynamic Pressure Pulses in Earth's Dayside Magnetosheath

Solar wind mass, energy and momentum can be transferred to Earth's magnetosphere at the magnetopause with the shocked magnetosheath acting as an interface between the two regions. In particular the magnetosheath pressure is important in terms of the position and motion of the magnetopause, which in turn can have effects throughout the dayside magnetosphere. A variety of transient phenomena often occur in the magnetosheath and in this thesis one example is studied, namely pulses in the magnetosheath dynamic pressure, using multipoint spacecraft observations to investigate their origins and magnetospheric impacts and illuminate dayside magnetospheric dynamics. Simultaneous observations in the solar wind, foreshock and magnetosheath reveal an interval of dynamic pressure pulses that did not exist upstream of the bow shock in the pristine solar wind or foreshock and appear consistent with previous simulations of solar wind discontinuities interacting with the bow shock, which predict large amplitude pulses when the local geometry of the shock changes. A statistical study of these structures, however, reveals their predominant origin near the quasi-parallel shock, typically under steady interplanetary magnetic fields, suggestive that the foreshock is important in their generation. The enhanced pressure on the magnetopause due to these pulses can perturb the boundary, exciting ultra-low frequency waves in the magnetosphere and travelling convection vortices in the ionosphere, similar to the response to pressure variations of solar wind origin. However, in this case the response is smoother and on longer timescales than the sharp, impulsive pressure variations and often a collective effect of numerous pulses. Conditions at the magnetopause are often inferred from suitably time lagged measurements of the pristine solar wind taken far upstream of Earth at the L1 Lagrangian point. However, such methods cannot predict the precise locations and times of dynamic pressure pulses in the magnetosheath, which directly drive magnetospheric dynamics.Open Acces