The instrument in question, JUICE-UVS, is an imaging spectrograph sensitive to extreme- and far-ultraviolet light between 50 and 204 nanometers, built by the Southwest Research Institute as the fifth in a family of six UV spectrographs flown on planetary missions. Light enters through one of three apertures: a main airglow port, a smaller high-resolution port tucked behind a deployable door, and a pinhole solar port offset by 60 degrees that attenuates the Sun’s glare so the instrument can eventually observe at 5 astronomical units from the Sun. An off-axis paraboloidal mirror focuses incoming light onto a slit measuring 0.1 degrees by 7.3 degrees, after which a toroidal diffraction grating spreads the UV spectrum across a microchannel plate detector coated with UV-sensitive cesium iodide. Because the optics and housing are all machined from aluminum 6061, the system is athermal, holding its focus without active temperature control.

Before the LEGA encounter, the team had verified basic performance by observing UV-bright stars and the faint glow of interplanetary hydrogen and helium. But stars are point sources, and a full characterization of the instrument’s response across all wavelengths and all positions in its field of view demands extended targets with discrete spectral features spanning the entire bandpass. That is precisely what the Moon and Earth delivered. During the lunar flyby, JUICE-UVS captured sunlight reflected from the Moon’s surface beginning roughly two minutes after the field of view crossed the terminator onto the sunlit hemisphere. Light levels rose so quickly that only the first 30-second exposure could use the main aperture; the rest of the sequence shifted to the high-resolution port to avoid overwhelming the detector.

The lunar data served two calibration purposes at once. First, the team summed counts over the 155 to 197 nanometer range, deliberately avoiding the intensely bright Lyman-alpha line at 121.6 nanometers and a reflected solar oxygen line at 130.4 nanometers where the flux temporarily saturated the detector’s ability to process every photon. The resulting map traced a narrow strip across craters including Kapteyn E, Langrenus W and Langrenus N, revealing sharp contrasts between sunlit and shadowed terrain. Because the spacecraft’s motion carried the slit mostly along its length rather than across it, the mapped region was narrow, but matching the observed crater positions to their known locations confirmed that the instrument pointing model used by the mission is accurate. The team could even resolve the shadow inside the 12.5-kilometer-wide Langrenus N crater.

Second, the lunar spectra provided a check on the instrument’s effective area, the quantity that converts raw counts into physical brightness. After correcting for exposure time, slit solid angle, viewing geometry using the Lommel-Seeliger photometric function, and dividing by a solar spectrum measured the same day by NASA’s TIMED/SEE instrument, the researchers obtained far-ultraviolet reflectance spectra for two regions: terrain near Langrenus crater and a patch of Mare Fecunditatis. Comparing these with published spectra from LAMP, the Lyman Alpha Mapping Project on NASA’s Lunar Reconnaissance Orbiter, showed excellent agreement at wavelengths below 165 nanometers. A possible absorption feature near 170 nanometers, seen in most LAMP spectra but absent from the JUICE data, remains an open question, potentially reflecting real differences in surface properties, calibration uncertainties in either instrument, or the roughly 16 percent uncertainty in the solar reference spectrum over that interval.

The Earth flyby offered an even richer calibration target. For nearly 35 minutes, JUICE-UVS observed from the nightside at an altitude of about 13,550 kilometers, across the dawn terminator, and onto the brilliant dayside, finishing with a view of the sunlit limb. Earth’s dayglow spectrum is crowded with emission lines from hydrogen, helium, oxygen and nitrogen at precisely known wavelengths, making it an ideal ruler for wavelength calibration. Using a high-resolution pixel list exposure of the dayside, the team fitted Gaussian profiles to a set of strong, unblended emission lines at multiple positions along the slit, then interpolated the results into a two-dimensional wavelength solution best described by a fifth-order polynomial. The measured pixel size varies from about 0.07 nanometers at the edges of the bandpass to about 0.11 nanometers near the center, consistent with pre-launch calibration.

The airglow measurements themselves carried scientific weight. Tracking the brightness of hydrogen Lyman-alpha and the two neutral oxygen emissions at 130.4 and 135.6 nanometers, the team found nightside values closely matching a classic 1991 compilation by R. R. Meier, while dayside peaks reached 21 kilorayleighs at 130.4 nanometers and 4.4 kilorayleighs at 135.6 nanometers. The 135.6-nanometer value exceeded contemporaneous measurements from TIMED’s Global Ultraviolet Imager, and the discrepancy is instructive rather than alarming: as the instrument’s boresight swung away from nadir, it sampled longer atmospheric columns. The optically thin 135.6-nanometer emission kept brightening with increasing off-nadir angle, while the optically thick 130.4-nanometer emission plateaued, driving the ratio of the two lines below the nadir-pointed reference value. This viewing-geometry effect, first modeled in the 1990s from Dynamics Explorer 1 data, is exactly the diagnostic JUICE-UVS will exploit at Jupiter’s moons.

Three days after the flyby, a 360-degree spacecraft roll gave UVS one final Earth observation: a 7.5-degree swath covering altitudes between roughly 2 and 15 Earth radii. The image captured the Earth’s extended hydrogen geocorona glowing in Lyman-alpha, along with the Pleiades and UV-bright stars in Taurus. The measured stellar positions matched catalog predictions almost perfectly, and the geocoronal brightness profile agreed with decades of prior observations. Searches for high-altitude helium and oxygen emissions came up empty at detectable levels. Together, these checks validated the instrument’s wavelength scale, effective area, and pointing with a completeness that stars alone could never provide.

Most importantly, the LEGA campaign was a dress rehearsal for Jupiter. During future flybys of Ganymede, Europa and Callisto, UVS will perform disk scans and limb stares to map surface ice, tenuous atmospheres, and extended hydrogen and oxygen coronas. The oxygen line ratio technique demonstrated at Earth will distinguish O2-dominated atmospheres from those containing water vapor or atomic oxygen, the same method that revealed a localized water atmosphere on Ganymede’s subsolar point. The lunar map, acquired from an altitude within the range planned for real flybys, previews the instrument’s mapping resolution of roughly 4 to 6 kilometers from 1,100 kilometers up. One technique remains undemonstrated: absorption spectroscopy via stellar and solar occultations, which the team has flagged as the highest-priority UVS activity for the final Earth gravity assist in January 2029. With the instrument confirmed healthy and its calibration sharpened, JUICE-UVS now waits, pointed toward a 2031 arrival at the largest planet in the solar system.

Subject of Research: Ultraviolet spectrograph observations and in-flight calibration of ESA's JUICE spacecraft during the 2024 lunar-Earth gravity assist

Article Title: Ultraviolet observations of the Earth and Moon during the JUICE Lunar-Earth flyby

Article References: Molyneux, P. M., Davis, M. W., Giles, R. S., Greathouse, T. K., Retherford, K. D., Versteeg, M. H., Ferrell, S. A., Becker, T. M., Gladstone, G. R., Trantham, B. J., Kammer, J. A., Freeman, M. A., & Persyn, S. C. (2026). Ultraviolet observations of the Earth and Moon during the JUICE Lunar-Earth flyby. Annales Geophysicae, 44(2), 1093-1108. https://doi.org/10.5194/angeo-44-1093-2026

Image Credits: AI Generated

DOI: 10.5194/angeo-44-1093-2026

Keywords: JUICE, JUICE-UVS, ultraviolet spectroscopy, lunar-Earth gravity assist, Moon, Earth airglow, geocorona, instrument calibration, Jupiter icy moons, ESA, Southwest Research Institute, Ganymede

STIX is not a conventional telescope. It is an indirect imaging spectrometer, a device that never forms a direct picture of the Sun. Instead, it observes the bremsstrahlung X-rays emitted by hot flare loops and accelerated electrons in the energy range from 4 to 150 kiloelectronvolts, using pixelated cadmium telluride detectors with an energy resolution of about 1 kiloelectronvolt. The imaging trick relies on 30 pairs of grids, each a stack of tungsten foils etched with fine slits, placed in front of the detectors. As X-rays stream through a grid pair, they cast a shifting Moiré pattern of light and shadow onto the detector behind it. By measuring the contrast and phase of that pattern, each sub-collimator, as a detector-grid unit is called, records one Fourier component of the X-ray source, much as a radio interferometer records a single visibility. Combine enough of these components and an image of the flare emerges from the mathematics.

The trouble is that every Fourier component depends on knowing precisely how transparent each grid pair really is. Before Solar Orbiter launched, engineers measured the grids optically, characterizing the pitch, slit width, and orientation of each one, and those measurements have underpinned the data analysis software ever since. But optical measurements have a fundamental blind spot: they can only see the top and bottom layers of a stack. The STIX grids for the 24 sub-collimators with the coarsest angular resolution are built from tungsten foils 33 or 50 micrometers thick, stacked to reach a total thickness of 400 micrometers, with the pattern etched into each layer. If the layers are not perfectly aligned, or if the etching leaves slits that vary slightly from layer to layer, the effective opening of the grid narrows in ways no optical inspection of the outer surfaces can reveal. Unlike its predecessors on Yohkoh, RHESSI, and the Chinese ASO-S mission, STIX also lacked a full pre-launch X-ray calibration at the small incident angles relevant to solar flare observations, a constraint of time and budget.

The new work turns that limitation into an opportunity. The key insight is that STIX carries a built-in flux monitor it was not entirely designed to be one: the Coarse Flare Locator, or CFL. This sub-collimator uses an H-shaped front grid and an empty rear grid, and for certain flare locations on the solar disk, the shadow cast by the front grid leaves up to three of the CFL’s large detector pixels fully illuminated. A fully illuminated pixel, by definition, sees the total incident X-ray flux, unmodulated by any grid. By comparing the flux recorded by the fully illuminated CFL pixels with the flux transmitted through each imaging sub-collimator, the team could compute the true effective transmission of every grid pair directly from flight data. The transmission is simply the ratio of the transmitted flux to the total flux, a beautifully direct measurement that sidesteps the uncertainties of the ground-based optical characterization.

Building the calibration dataset required patient mining of the STIX archive. From roughly 25,000 flares recorded between January 2021 and February 2025, the team selected events with at least one fully illuminated large CFL pixel, accurate flare locations, and enough counts to keep statistical errors at or below about 3 percent. The winnowing left 91 flares, a small fraction of the total, because only flares at particular solar latitudes illuminate the CFL pixels completely. The team focused on the 10 to 15 kiloelectronvolt band, where the transmission of the Solar Black coating on the spacecraft heatshield is high enough that any inhomogeneities introduce only second-order errors, and where photon statistics remain strong. Careful corrections for background counts, detector live time, and onboard energy channel binning were applied, and a self-consistency check of the CFL pixels themselves showed the total flux measurements are accurate to about 2.3 percent.

What the flares revealed was unambiguous. The measured transmission of the sub-collimators is systematically lower than the values predicted from the optical grid characterization, and the internal shadowing effect, the progressive narrowing of the effective slit width as photons arrive at increasing angles, is far weaker than a model of perfect grids would predict. Simple geometric simulations explain why. When the team modeled grids with random etching and stacking imperfections of a few micrometers, drawn from Gaussian distributions with a standard deviation of about 2 micrometers, the simulated on-axis transmission dropped and the shadowing effect flattened out, exactly matching the pattern in the flight data. For sub-collimator 5, with slits nominally 83 micrometers wide, imperfections of this scale reduce the effective slit width by roughly 6 micrometers. In some sub-collimators, such as 6b, a slight tilt of the stacked layers, plausibly caused by tilted alignment pins during assembly, produces a sloped transmission profile that the simulations reproduce as well.

The practical payoff is substantial. When the team applied the new transmission calibration to 25 independent flares observed between March and December 2025, the total flux estimates from the different sub-collimators agreed within about 2 percent, whereas the old calibration produced systematic biases ranging from 3 to 39 percent for the coarse-resolution sub-collimators and up to 175 percent for the finest ones. A second test on the powerful X2.2-class flare of December 8, 2024 drove the point home: photon spectra derived independently from the individual sub-collimators now agree within roughly 2 percent across the 8 to 20 kiloelectronvolt range, a dramatic improvement over the 6.9 percent scatter of the old calibration. The new calibration also means that derived photon fluxes are about 13 percent higher than previously computed, and fitted emission measures, a measure of the amount of hot plasma in the flare, increase by a similar 13 percent. Fitted flare temperatures, reassuringly, do not change, and the team emphasizes that conclusions from previously published STIX spectral analyses remain valid.

There are honest caveats. The calibration is valid only at low energies, below about 20 kiloelectronvolts, and only for offset angles between roughly minus 0.5 and plus 0.5 degrees, because at higher energies the tungsten grids become partially transparent and the CFL statistics grow too thin. Below 8 kiloelectronvolts, inhomogeneities in the Solar Black coating introduce discrepancies that the team hopes to self-calibrate in a future study using the dependence of transmission on flare location. Three sub-collimators, 3a, 5a, and 5b, showed energy-dependent discrepancies likely tied to detector rather than grid calibration and are recommended for exclusion from spectral analyses for now. The high-energy grid calibration, crucial for studying whether flare X-ray emission is anisotropic, will follow in a companion paper, and the team notes that cross-calibration better than 10 percent between Solar Orbiter and Earth-orbiting observatories is needed to settle that question.

Perhaps the most enduring lesson is a design philosophy. The authors strongly recommend that future indirect X-ray imagers include a dedicated total flux monitor, so that every recorded flare can contribute to self-calibration, as the Hard X-ray Imager aboard ASO-S already does. A flux monitor with sufficient effective area would even allow the same technique to be extended to high energies. For now, the new transmission model will be released in the upcoming version of the STIX analysis software, quietly improving every flare image and spectrum the mission produces. It is a striking demonstration that a spacecraft instrument, far from any laboratory, can diagnose its own imperfections using nothing more than the raw fury of the Sun, and that a few micrometers of tungsten misalignment, invisible to any optical microscope on Earth, can be measured from 100 million kilometers away by letting solar flares do the talking.

Subject of Research: In-flight self-calibration of the tungsten grid transmission of the STIX hard X-ray imaging spectrometer on board Solar Orbiter using solar flare observations

Article Title: In-Flight Self-Calibration of the STIX Grid Transmission

Article References: Massa, P., Volpara, A., Stiefel, M. Z., Bilgili, E., Degen, M., Limousin, O., Hurford, G. J., & Krucker, S. (2026). In-Flight Self-Calibration of the STIX Grid Transmission. Solar Physics, 301(9), Article 138. https://doi.org/10.1007/s11207-026-02739-6

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02739-6

Keywords: Solar Orbiter, STIX, solar flares, X-ray imaging, instrument calibration, tungsten grids, Solar Physics, space instrumentation, hard X-rays, spectroscopy, ESA, helio physics