The electric currents of the central plasma sheet play a pivotal role in the dynamics of the Earth’s magnetosphere. I describe new instrumentation developed for measuring its properties, and analyze data from existing instruments. The analysis shows the structure and physical current-carrying mechanisms of the quiescent central plasma sheet in new detail.

Electric field observations are critical for this work. I discuss two aspects of space-based double-probe electric field experiments: the probe design and the signal processing. I develop a numerical model that self-consistently solves for the interaction between the probes and the nearby plasma environment, including the effects of the spacecraft and its attendant photoelectrons. I also describe the signal processing hardware developed for the 5-satellite THEMIS mission, known as the Digital Fields Boards (DFB). THEMIS was launched in February 2007, and all 5 DFBs are working as intended.

Since THEMIS is only recently launched, I analyze data from the 4-satellite Cluster mission, which has similar instrumentation. With Cluster data, the position of the current sheet relative to the satellite can be determined, allowing direct comparisons between observations and models. To encompass the wide variety of possible current-carrying mechanisms, I develop a kinetic model based on the quasi-isotropic formalism of Schindler and Birn [2002]. The model fits many of the observed sheets well.

The observations reveal a wide variety of current-carrying mechanisms. Some of the thinnest currents consist entirely of a pair of electron Hall currents which together form a bifurcated current sheet driven by strong inward-pointing electric fields.