First flight model of the ElectroStatic Analyzer (ESA) instrument (round structure) and the Instrument Data Processing Unit (IDPU), the heart of THEMIS's integrated instrument electronics and data processing (vertical box with lots of connector slots, designed for connecting to the various instrument sensors).
Mass: 2.96 kg
Average Power: 1.7W
Development Institution: University of California, Berkeley (UCB)
ESA Lead: C. W. Carlson (UCB)
Purpose: Measure thermal electrons and ions to identify and track high-speed flows through the magnetotail and identify pressure pulses
The Electrostatic Analyzers (ESAs) measure how many electrons and ions they detect with a specified energy from a certain direction at a given time (the particle distribution function) over the energy range from ~3 eV to 30 keV. These thermal electrons and ions are the particles responsible for creating the aurora. The ESA measurements allow scientists to derive the density, velocity, and temperature of the ambient electrons and ions (plasma). Knowing these quantities at the five THEMIS probes will allow scientists for the first time to determine the time of substorm onset.
The ability to measure the flux of particles in a 360° field of view during substorm onset creates the opportunity for scientists to better understand the trigger mechanism of substorm onset. Together with the electromagnetic wave and superthermal particle measurements, the thermal particle measurements will help elucidate the way in which electromagnetic waves and particles interact during and after substorm onset. In particular the multipoint ESA measurements can be used to distinguish between two competing substorms models. In the first model, substorms begin with an implosion in the near-Earth magnetotail that launches an anti-sunward moving pressure front (across which the pressure drops abruptly, like those associated with a storm front). In the second model, an explosion in the distant tail launches high-speed sunward-moving plasma flows. Timing the ESA observations permits us to determine when and where substorms begin, and to distinguish between the sequences of events in the two models, key objectives of the mission.
The ESA sensors accept charged particles entering over a "fan-shaped" 180°x5° field of view. As the spacecraft rotates, the entire 360° field-of-view is sampled. One can imagine this as a lighthouse in reverse with light entering the ESA aperture instead of leaving the lighthouse. Two ESA sensors comprise the ESA instrument on each probe, one to measure ions and the other measure electrons. The ESA is made of two hemispherically shaped plates, one nested within the other making a shell for particles to move within. An opening with a "hat" at the top of the hemisphere guides particles into this shell. The plates (and "hat") are electrified such that they are at a different voltage. This causes the electrons and ions, the charged particles, to move in a circle. Only the charged particles with just the right energy will follow the curve of the instrument's hemispheric shell and make it to the particle detector at the exit. At this point, the detector registers the number of particles that hit it. By varying the voltages, we can find out how many particles there are within each specified energy.