Ionospheric signatures of dayside reconnection processes

This thesis presents a study of the ionospheric response to dayside magnetopause reconnection. The principal data set employed is the ionospheric convection velocities from the CUTLASS HF radars. In addition to this, images of the ultraviolet aurora from the VIS Earth Camera and the Far Ultraviolet Imager aboard the Polar spacecraft were examined. In situ measurements have shown that there is a time-dependent, periodic nature to magnetic reconnection, with time scales of the order of minutes; the transient nature of magnetopause reconnection is reflected in the ionospheres. In this thesis the plasma velocity fluctuations in the dayside ionosphere were found to be in full agreement with the repetition rates of bursts of reconnection at the magnetopause, as well as with the repetition rates of polarised moving visible auroral forms seen from Earth. The first-ever full vector measurements of ionospheric convection velocity within the footprint of the reconnecting flux tubes revealed that, despite being transient, magnetic reconnection was a large scale process during which layers of magnetic flux were successively peeled from the magnetopause. Directly measured particle precipitation revealed that particles originating the reconnection region was found to be present on the same magnetic field lines as the HF radar signature of the reconnected field lines. The multi-instrument study of the ionospheric responses to reconnection demonstrated that the measured ionospheric convection velocities are inextricably linked to measured ultraviolet aurora. Energetic particle precipitation from the magnetosphere into the atmosphere via field-aligned currents excites auroral emissions, and field-aligned current estimates from measured ionospheric convection velocities were found to be in excellent agreement with the aurora.