Space plasma spectrometers measure the number of particles present in space and their individual velocities (velocity magnitude and direction). This is done to construct the particle velocity distribution functions (VDFs), which are essential for any scientific study of the space environment.
Plasma spectrometers traditionally rely on spacecraft spin to sample particles coming from all directions. The VDF acquisition rate is therefore limited by the spin rate. This limitation can be overcome by the use of electrostatic deflectors at the entrance of the instrument. The operation of such an instrument can then be optimized through the use of beam tracking.
Beam tracking is a strategy in which one relies on previous measurements to predict and measure only the velocity range and the directions of the sky from which particles are thought to arrive.
Application to the solar wind
Beam tracking is perfect for measuring the solar wind: the solar wind comes from only a part of the sky and has a limited energy range, although its mean energy and direction can change significantly. From a Sun-pointing platform a spectrometer can acquire solar wind VDFs at a rate limited only by technology.
As part of the Phase A study for the THOR mission (a candidate for the Cosmic Vision M4 slot of the ESA Science program), BIRA-IASB has designed – in collaboration with IRAP (Toulouse) – the CSW cold solar wind spectrometer, an instrument with unprecedented capabilities.
BIRA-IASB has devised the “beam tracking” strategy that makes this instrument so innovative. A software simulator has been used to validate the design and to show that beam tracking allows rapid acquisition (< 100 ms) of high angular (1.5°) and energy resolution (<7%) VDFs, while being able to gracefully recover from potential beam loss situations.
The advantages of beam tracking are a faster VDF acquisition for a given angular/energy resolution or a higher angular/energy resolution for a given acquisition rate. As it is especially well suited for measuring the solar wind, CSW or a similar instrument would be perfect for space weather monitoring.
Unfortunately, the THOR mission candidate was not selected for implementation, so this instrument will not be built – at least, not yet.
Want to know more?
- Cara, A., Lavraud, B., Fedorov, A., De Keyser, J., DeMarco, R., Marcucci, M.F., Valentini, F., Servidio, S., Bruno, R. (2017). Electrostatic analyzer design for solar wind proton measurements with high temporal, energy, and angular resolutions: Solar Wind Ion Instrument. Journal of Geophysical Research: Space Physics. https://doi.org/10.1002/2016JA023269
- De Keyser, J., Lavraud, B., Přech, L., Neefs, E., Berkenbosch, S., Beeckman, B., Fedorov, A., Marcucci, M.F., De Marco, R., Brienza, D. (2018). Beam tracking strategies for fast acquisition of solar wind velocity distribution functions with high energy and angular resolutions. Annales Geophysicae, 36(5), 1285–1302. https://doi.org/10.5194/angeo-36-1285-2018