Measuring the solar wind: better, faster

Space Physics
In the context of the THOR space mission proposal, IRAP (Toulouse) and BIRA-IASB designed the fastest and most accurate solar wind measurement apparatus ever. BIRA-IASB devised the “beam tracking” strategy that makes this instrument so innovative. Simulations demonstrate that beam tracking allows rapid data acquisition (< 100 ms) of high angular (1.5°) and energy resolution (<7%) solar wind velocity distributions, while being robust against beam loss. Unfortunately, the THOR mission candidate was not selected for implementation, so the instrument remains a dream—at least for the time being.

Beam tracking

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.

Perspectives

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 ​Open Access​

 

Simulation of plasma measurements of the solar wind on a spinning spacecraft using beam tracking
Simulation of plasma measurements of the solar wind on a spinning spacecraft using beam tracking: (a) and (b) energy spectrum of the simulated and measured solar wind; (c) energy as a function of time, where the blue line represents the true solar wind value, the magenta circles and triangles indicate the center and the bounds of the sampled energy range, and the red diamonds give the mean energy as determined by the spectrometer; (d) azimuth; (e) elevation; (f) spin phase; (g) density; (h, i) velocity in the spacecraft frame of reference (x axis pointing to the Sun, spacecraft spinning in the y-z plane), and (j) temperature; (x) energy–elevation, (y) energy–azimuth, and (z) azimuth–elevation projections of the VDF at the end of the simulation.  

Click to open movie 1 (.mp4) and movie 2 (.mp4) that show the measurement process.