STEP is an instrument to measure supra thermal electrons and protons/ions. It consists of two identical detector units mounted on a common electronics unit on the y-deck of the spacecraft. The FOVs of both detector units are unobstructed and co-aligned and base on a pinhole mapping. Both detector units point sunwards into the direction of the nominal Parker spiral.

The instrument uses passively cooled silicon solid-state detectors (SSDs) to measure supra-thermal particles in the energy range from about 3keV up to 65keV. The use of SSDs with ultra-thin ohmic contacts provides high sensitivity compared to traditional electrostatic analyzers used for solar wind electron instruments, as has been shown with STE on STEREO. This also allows continuous measurements at all applicable energies and provides a high duty cycle as no stepped measurements are required to determine the energies. The sensitive or active area of the detectors can be changed in flight to adapt the geometry factor to the in-situ particle fluxes. In addition, the SSDs are pixelated to gain angular resolution in two directions. The ion-electron separation is performed as follows:

One of the otherwise identical detector units is equipped with a magnetic deflection system to suppress the electrons at all relevant energies. This detector unit yields the proton/ion fluxes only. The second detector unit is not affected by the magnetic fields and measures all incoming particles in the applicable energy ranges. The difference of both readings gives the electrons fluxes. It should be pointed out that the magnetic fields inside one of the detector unit do not have an effective influence on the ion trajectories: The magnets are designed in a way that protons/ions with identical properties reach the identical pixel of the SSD in both detector units whether magnetic fields are present or not. This is schematically shown for both units in the Figure below (blue: electrons, red: ions). The combination of a baffle at the entrance of each detector unit together with the pinhole minimizes scattered light to the detectors.

Because electron fluxes at supra-thermal energies are many orders of magnitude lower than solar wind plasma core or halo/strahl electrons, STEP is the only kind of instrument that can provide these measurements at high cadence. Solar Orbiter goes close to the Sun, where the Parker spiral field will be much closer to the radial direction and the magnetic field gradient will be much larger, so charged particles will tend to be focused along the field line. Thus, to have a high probability of detecting the field-aligned supra-thermal electrons, it is highly desirable to look as close to the radial direction as possible. This means that ions will also be in the field of view.



Figure: Schematic view of the working principle of STEP showing representative particle trajectories (blue: electrons, red: ions) in both detector units. The left unit contains the magnets, the yoke, and the support structure to suppress the electrons.


Test and calibrations