Mass: 1.43 kg
Average Power: 1.2 W
Development Institution: University of California, Berkeley (UCB)
SST Leads: D. Larson and R. Lin (UCB)
Purpose: Measures superthermal ions and electrons to remotely sense the expansion of the heated plasma sheet that occurs at substorm onset.
The Solid State Telescope (SST) measures superthermal particle distribution functions, namely the number of ions and electrons coming towards the spacecraft from specified directions with specified energies within the energy range from 25 keV to 6 MeV. These particles are the much more energetic (hence superthermal) than the main magnetospheric population, but are quite important as tracers of acceleration and heating in the magnetosphere. SST observations help us achieve the objectives of the THEMIS mission by providing two different measurements of substorm onset within the Earth's plasma sheet, a region of enhanced particle fluxes and depressed magnetic field strengths within the Earth's magnetotail. The first method is based on the fact that the plasma sheet slowly thins prior to substorm onset, but rapidly expands at onset. Using the so-called 'finite gyroradius' effect, the SST can remotely sense the location and motion of the boundaries of the plasma sheet, in particular the expansion at substorm onset. This is the first time that two SSTs will track this changing boundary from three locations while other instruments at different locations in the magnetosphere are determining the time of substorm onset.
The second method is based on the fact that substorm onset heats and accelerates particles to energies that can be observed by the SST. Thus the appearance of these particles is another indicator of substorm onset. During some substorms, particles may accelerate to high enough energies to populate the Van Allen (radiation) belts around Earth. Scientists use the SST data to track the evolution of the radiation belts, possibly determining the effect of substorms in populating or depleting the radiation belts.
The SST unit is a combination of two SSTs, each of which is a rectangular box with three particle detector plates (silicone solid state detectors) spaced in the center. On one end of the SST is a magnet to deflect electrons and only allow positive ions through. On the other end of the SST is a piece of foil that blocks positive ions and only allows the electrons to travel through. Two shutters with pinholes can be moved in-between the openings to decrease the number of particles that hit the detector. This new attribute of the SST design protects the detector from the copious fluxes of highly energetic "ring current" particles and thereby helps increase the lifetime of the SST. To date, this is the only magnetospheric mission flying such shutters on an SST. The two SSTs are positioned side by side and in reverse such that ions and electrons from the same direction are detected. Like the ESAs, the SSTs are oriented such that they will detect particles over a 78° field of view. As the spacecraft rotates, the two sensors will sample a 360°x148° field-of-view.