The 3D Energetic Electron Spectrometer (3DEES): measuring pitch angle distributions of energetic electrons on a non-spinning spacecraft

Space missions in Earth orbit are frequently exposed to intense and highly variable energetic electron environments. In the outer radiation belt, electrons with energies from ~100 keV to 10 MeV can reach extreme intensities, posing serious risks to spacecraft. Characterizing and forecasting this vast and dynamic environment relies on two complementary strategies: (i) deploying radiation monitors on as many spacecraft as possible, and (ii) applying advanced physics-based space weather models within data assimilation frameworks, where measurements provide boundary conditions. A critical observational input to these models is directionally resolved energy spectra. As electrons are magnetically trapped, directional measurements at a single location can be used to infer their distribution across the radiation belts.

In this context, the 3D Energetic Electron Spectrometer (3DEES) has been designed as a compact, science-class instrument optimized for measuring angle-resolved electron spectra from 0.1 – 10 MeV within Earth’s radiation belts. In addition, it enables quantification of proton fluxes in the 2.5 – 50 MeV energy range.

On 5 December 2024, a demonstrator model of the instrument (capable of simultaneous measurements from six directions) was launched aboard PROBA-3, a non-spinning spacecraft. The mission operates in a highly elliptical orbit with an apogee of 60,530 km, a perigee of 600 km, an inclination of 59°, and an orbital period of 19.7 hours. With these orbital parameters, the satellite is covering parts of the inner belt, outer belt and mostly the border of the magnetosphere. Since 29 July 2025, 3DEES is operated on a regular basis, performing measurements in the radiation belts every orbit when the satellite is at an altitude < 40000 km.

The presentation will provide a brief overview of the 3DEES instrument onboard PROBA-3, present initial results on electron pitch-angle distributions, and illustrate how the instrument captures radiation belt dynamics, such as sudden drop-outs, subsequent flux enhancements and steady decays. It will show that the 3DEES dataset constitutes a valuable new data source for the space weather community, delivering high-quality measurements.