The instrument in question is the Jovian Plasma Dynamics and Composition analyzer, or JDC, one of six sensors that make up the Particle Environment Package on Juice. JDC is designed to measure positive ions, negative ions and electrons over an energy range from 1 electron volt per charge up to 35 kiloelectron volts per charge. Its most demanding assignment will be to detect the tenuous, extremely cold ionospheres of Jupiter’s icy moons, Ganymede, Europa and Callisto. These ion populations carry energies of only a few electron volts, which makes them notoriously difficult to observe, and the success of that effort depends on the instrument performing flawlessly at the very bottom of its energy range.

Measuring such cold ions is complicated by the spacecraft itself. In space, a vehicle accumulates electric charge through a balance of currents: sunlight knocks photoelectrons off its surfaces and drives it positive, while the ambient plasma electrons, being far more mobile than the ions, tend to drive it negative. In dense plasma regions, the electron current wins and the spacecraft charges negatively. A charged spacecraft repels ions of the same sign, preventing them from ever reaching the detectors. The outflow of cold hydrogen ions from near-Earth space, for instance, went undiscovered for decades precisely because visiting spacecraft were positively charged, and it was finally revealed only through indirect methods. Conversely, a negatively charged spacecraft can attract positive ions and make the lowest energies observable, but it distorts their trajectories and energies in ways that must be carefully modelled and corrected.

During the lunar-Earth gravity assist on 19 and 20 August 2024, Juice passed within two Earth radii of the planet’s centre, cutting straight through the plasmasphere, a cold, dense cloud of ionised upper atmosphere that co-rotates with Earth. This region, filled with hydrogen, helium, oxygen and nitrogen ions at temperatures below one electron volt, resembles in many respects the ionospheres that JDC will encounter at Ganymede and Europa. As the spacecraft cruised through, JDC recorded energy-time spectrograms of the positive ions around it, and the team immediately noticed something odd. The signal showed artificial banding at different energies, and the positions of the peaks did not match the ratios expected for known ion species such as hydrogen, helium and oxygen.

The culprit turned out to be the way JDC sweeps through its energy range. To cover the spectrum efficiently, the instrument uses a triangular voltage sweep on its electrostatic analyzer, stepping upward through the energies and then downward, with the two halves interleaved so that gaps in coverage are filled in and mid-range energies are sampled twice per sweep, doubling the effective time resolution. The design is clever, but the flyby data showed that the upward and downward halves were not interleaving properly. The intended voltages were not being realised in practice. The energy sweep pattern, it emerged, pushed the instrument’s high-voltage power supply outside its engineering specification, and the supply could not settle fast enough at each new step, particularly when dropping from high voltages to near zero.

Fortunately, the team had a way to check. The flight model of JDC is still on the ground; it was replaced on the spacecraft by the nearly identical flight spare after late-discovered electrical grounding issues. The researchers partially disassembled the laboratory model, soldered a dedicated resistive low-capacitance voltage divider directly onto the output of the high-voltage power supply, and recorded the actual voltage waveform during a sweep with a high-resolution oscilloscope inside a vacuum tank. Averaging more than a thousand individual recordings pushed the effective dynamic range beyond 16 bits, enough to capture the entire sweep in a single measurement. The traces revealed clear voltage spikes and, more importantly, a systematic lag: during many energy steps, especially the first ten of the upward sweep, the voltage never reached its intended value before the measurement window closed.

Armed with these laboratory measurements, the team reconstructed the true energies of every step in the flyby data. The correction was made purely from the ground-based voltage measurements, without any reference to the independently measured spacecraft potential, and when the corrected spectrogram was compared with the potential estimate from Juice’s Radio and Plasma Wave Investigation, the agreement was striking. The artificial banding vanished, the ion signal now varied smoothly with the spacecraft potential exactly as a cold plasma should, and the upward and downward halves of the sweep became mutually consistent. The reconstructed energies carry an uncertainty of roughly half an electron volt, dominated by the voltage measurement precision and by small differences between the laboratory model and the flight spare.

To understand what JDC was actually seeing, the team also built a simple simulation of a negatively charged spacecraft ramming through a cold, corotating oxygen plasma. The model showed that when the spacecraft potential is sufficiently negative, it overcomes the ram velocity of about seven kilometres per second and attracts ions toward the spacecraft from all directions, including from behind, where JDC’s field of view is pointed. This explains why the observed ion energies fell below the magnitude of the spacecraft potential, and why the spectrum showed gaps at the lowest energies for part of the flyby. The simulated spectra reproduced the key features of the corrected observations, confirming that the instrument was behaving as physics dictates once the energy scale was fixed.

Correcting future data would have been possible, but it would have been a wasteful compromise: some energy steps would have to be discarded, the spacing between steps would be irregular, and coverage below 10 electron volts would remain sparse. Instead, the team designed three entirely new energy sweep tables, validated each one on the laboratory model, and uploaded them during a general software update of the Particle Environment Package in February 2026. The new sweeps abandon the triangular interleaving in favour of a long, slow descent through the energies, with three sacrificial steps used to raise the voltage rapidly to kilovolt levels before the measurement begins. The result is a stable voltage during every step and an even distribution of energy bins across the range. One table covers the full 1 electron volt to 35 kiloelectron volt span, a second trades some high-energy coverage for reduced power consumption, and a third is dedicated to ionospheric work, densely sampling just 1 to 500 electron volts, the range suggested by Juno’s 2021 flyby of Ganymede as ideal for studying the moons’ ionospheres.

The episode is a powerful argument for in-flight calibration. Calibrating particle detectors at energies of a few electron volts on the ground is extremely difficult because stable, intense ion beams at such low energies are hard to produce, so JDC’s response at the bottom of its range had only ever been extrapolated. Planetary flybys offer something no laboratory can: access to multiple cold particle populations, and, when the spacecraft potential varies with time, a natural calibration source spanning the lowest energies. The lessons learned from Earth’s plasmasphere, combined with a twin instrument in a European laboratory, have turned a potential scientific disappointment into a success story. When Juice finally reaches Ganymede, the only moon in the solar system with its own magnetic field, its plasma analyzer will be ready to capture the faint, cold ions escaping from the icy surface, with an energy scale it can truly trust.

Subject of Research: In-flight calibration of the Juice spacecraft's JDC plasma analyzer using the 2024 Earth plasmasphere flyby to optimize low-energy ion measurements at Jupiter's icy moons

Article Title: Flying through the plasmasphere to optimize low-energy ion measurements

Article References: Stenberg Wieser, G., Wieser, M., Barabash, S., Wittmann, P., Kalla, L., Fränz, M., Roussos, E., Vorburger, A., Wurz, P., Wahlund, J.-E., Brandt, P. C., Futaana, Y., Shimoyama, M., Pontoni, A., Galli, A., Riedo, A., Ho, G., Mitchell, D. G., Clark, G., … Baláž, J. (2026). Flying through the plasmasphere to optimize low-energy ion measurements. Annales Geophysicae, 44(2), 937-948. https://doi.org/10.5194/angeo-44-937-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-937-2026

Keywords: Juice mission, JDC analyzer, plasmasphere, low-energy ions, spacecraft charging, Earth flyby, Ganymede, ionosphere, electrostatic analyzer, in-flight calibration, Jupiter icy moons, space plasma instrumentation