Virtual Instrument Team

 The Virtual Instrument Team aids the definition of THOR instruments making use of kinetic simulations of ion and electron distributions and generating synthetic observations based on instrument characteristics. An example of the output of the analysis is given below. The figure depicts the counting rate of a virtual proton detector per square centimeter and steradian in the magnetosheath region just downstream of the bow shock. The mesh depicts the foreseen angular and speed resolution of this virtual instrument. The proton distribution function used as input is obtained from FMI's global hybrid Vlasov simulation code Vlasiator.
In particular, three-dimensional velocity distributions of protons, electrons and heavy ions (alpha particles) as obtained from direct numerical simulations of plasma turbulence are used to simulate the response of a virtual top-hat analyzer which mimics the real measurements of particle instruments on board THOR, using realistic angular and energy resolution and geometric factor. In the picture below, an example of this kind of analysis is shown, by using the output of HVM simulations of turbulence in the solar wind: in the box on the left we report the original proton velocity distribution from the kinetic simulation in the Cartesian velocity plane and in the spacecraft frame. This velocity distribution is then moved into energy-angular coordinates and sampled by a top-hat simulator. Results obtained by the virtual CSW instrument on board THOR (middle panel) are compared with the response of the particle instrument on board the WIND satellite (right panel).

Cold Solar Wind Instrument

In order to simulate the response of the CSW instrument, we set up the virtual top-hat to operate with the following resolution and plasma parameters (Valentini et al., New Journal of Physics 2016; De Marco et al., Journal of Instrumentation 2016) and fed it with the output of velocity distributions for protons and alpha particles from HVM turbulence simulations:
  • 96 energy-per-charge intervals with ∆E/E = 7%.
  • Field of view 48° and angular resolutions up to 1.5°
  • Geometric factor G = 2.2 × 10−5cm2sr
  • Accumulation time Tacc = 0.25ms.
  • Proton density                    8.2 cm-3
  • Proton thermal speed          35 km/s
  • Solar-wind speed                400 km/s
  • Alpha particle abundance    5%

Non-Maxwellianity indicator: The departure of a particle velocity distribution from the Maxwellian (or bi-Maxwellian) shape is described by the parameter:
where f is the measured velocity distribution and fbiMaxw is the corresponding bi-Maxwellian distribution, calculated from the moments of f (density n, temperatures parallel and perpendicular to magnetic field and bulk speed). In the following plots we show the non-Maxwellian indicator for protons (left) and alpha particles (right), from the HVM simulation (red curve), from the CSW virtual top hat (green curve) and from the CSW virtual top hat with Poisson noise (blue curve).