M3DI: A miniaturized three-dimensional ion velocity and mass imager for the topside ionosphere

Precise characterization of low-energy ions in the topside ionosphere is important for understanding the dynamic coupling between the ionosphere and the magnetosphere but is often compromised by the restrictive volume constraints of modern small satellite missions. To address this challenge, the Miniaturized Three-Dimensional Ion Velocity and Mass Imager (M3DI) was developed to measure three-dimensional velocity distribution functions of major ionospheric ion species (H+, He+, O+, and NO+) in the 1-30 eV energy range while achieving a 70% volume reduction compared to its predecessor. The M3DI integrates an electrostatic analyzer, a deflector, post-acceleration optics, and a magnet-based mass spectrometer within a compact form factor. The optimization of the detector geometry utilizing a trapezoidal micro-pixel anode addresses the inherent azimuthal spread of ion trajectories within the compressed flight path. This geometric adaptation mitigates radial signal broadening and yields a mass resolution of R > 8 for primary ionospheric species (O+ and NO+). Laboratory validation utilizing a Ne+ beam confirms that the prototype functions accurately within the predicted performance regime, with ion landing positions aligning precisely with simulation models. These results demonstrate that the M3DI delivers the robust in situ measurements required for future constellation missions investigating magnetosphere-ionosphere coupling.