Cluster multi-spacecraft observations of flux transfer events
This thesis presents observations of flux transfer events (FTEs) from the four- spacecraft Cluster mission. FTEs are a signature of transient, or variable rate, magnetic reconnection at the Earth's magnetopause causing a bipolar variation in the component of the magnetic field normal to the magnetopause (Bn). The pres ence of four spacecraft allows the velocities of FTEs observed by all four spacecraft to be determined to much greater accuracy than previously possible. Three studies are presented. First, a crossing from the high-latitude dayside magnetosphere into the magnetosheath is studied. A series of bursts of field-aligned magnetosheath-energy plasma were observed in the magnetosphere, but no bipolar Bn signature was present. When the Cluster quartet straddled the magnetopause, traditional bipolar FTE signatures were observed in the magnetosheath, demon strating that the magnetospheric signatures were unorthodox FTEs. The remainder of the thesis contains a survey of FTEs observed by Cluster in 2002/3. Many FTEs were found which occurred under strongly northward IMF, the majority of which occurred on three particular magnetopause crossings. The observed velocities are compared with a model of reconnected flux tube motion. The velocities are consistent with a reconnection line which originates at high latitudes, in a region of high magnetic shear. However, the strict antiparallel reconnection hypothesis does not explain all of the observed velocities a component reconnection line must extend to regions of lower shear. Finally, the velocities of 81 FTEs observed under southward or dawn/dusk- dominated IMF conditions are examined. The observed velocities are generally explained well by the Cooling et al (2001) model. In a small number of cases it is possible to distinguish, within the constraints of the model, between some events which were only consistent with component reconnection, and others which were solely consistent with an antiparallel reconnection site.