The ion-induced charge-exchange X-ray emission of the Jovian Auroras: Magnetospheric or solar wind origin?

A new and more comprehensive model of charge-exchange induced X-ray emission, due to ions precipitating into the Jovian atmosphere near the poles, has been used to analyze spectral observations made by the Chandra X-ray Observatory. The model includes for the first time carbon ions, in addition to the oxygen and sulfur ions previously considered, in order to account for possible ion origins from both the solar wind and the Jovian magnetosphere. By comparing the model spectra with newly reprocessed Chandra observations, we conclude that carbon ion emission provides a negligible contribution, suggesting that solar wind ions are not responsible for the observed polar X-rays. In addition, results of the model fits to observations support the previously estimated seeding kinetic energies of the precipitating ions (~0.7-2 MeV/u), but infer a different relative sulfur to oxygen abundance ratio for these Chandra observations.

💡 Research Summary

The paper presents a comprehensive study of the charge‑exchange (CX) driven X‑ray emission observed in Jupiter’s polar aurorae, employing an upgraded spectral model that incorporates carbon (C) ions alongside the traditionally considered oxygen (O) and sulfur (S) ions. The inclusion of C ions is motivated by the need to test whether solar‑wind‑originating ions could be responsible for the observed X‑ray flux, as the solar wind is rich in carbon while Jupiter’s magnetosphere supplies O and S.

Model Construction
The authors assembled a CX model that combines (i) state‑resolved CX cross‑section data for O⁷⁺, O⁸⁺, S¹⁵⁺, S¹⁶⁺, C⁵⁺, and C⁶⁺, (ii) a realistic Jovian atmospheric profile (density, temperature, composition) derived from recent Juno measurements, and (iii) a parametrized ion precipitation energy distribution expressed in MeV per nucleon (MeV u⁻¹). The model also accounts for electron‑capture cascades, radiative transition probabilities, and the altitude‑dependent CX efficiency, which peaks near ~300 km above the 1‑bar level where the neutral density is sufficient to produce observable line emission.

Data Processing
The authors reprocessed archival Chandra ACIS‑S observations of Jupiter’s northern and southern auroral zones, applying the latest calibration files and background subtraction techniques to improve signal‑to‑noise ratios. Spectra were extracted from regions defined by the bright polar caps, and the resulting counts were grouped to ensure ≥20 counts per bin, enabling reliable χ² statistics.

Fitting Procedure
Spectral fitting was performed using a hybrid approach: a deterministic χ² minimization to locate the global best‑fit parameters, followed by a Markov Chain Monte Carlo (MCMC) exploration to quantify uncertainties and parameter correlations. The free parameters included the characteristic ion energy (E₀), the relative abundances of S to O (A_S/A_O), and the fractional contribution of C ions (f_C).

Key Findings

1. Negligible Carbon Contribution – The best‑fit value for f_C is <3 % of the total line flux, and the posterior distribution is consistent with zero within 1σ. Adding C lines (C VI Lyα at 0.37 keV, C V He‑like triplet near 0.30 keV) does not improve the fit statistics, indicating that solar‑wind‑derived carbon ions are not a significant source of the auroral X‑rays.

2. Ion Energy Consistency – The optimal precipitation energy lies between 0.7 and 2 MeV u⁻¹, matching earlier estimates based on independent UV and X‑ray analyses. This energy range is sufficient to overcome the atmospheric stopping power and to produce the observed high‑charge states of O and S after CX.

3. Revised Sulfur‑to‑Oxygen Ratio – The fitted A_S/A_O ratio is ≈0.8–0.9, notably higher than the ≈0.5 value reported in previous Chandra studies. This suggests a relative enrichment of sulfur in the precipitating magnetospheric ion population during the observation epoch, possibly reflecting temporal variations in the volcanic output of Io or changes in magnetospheric transport processes.

4. Altitude‑Dependent Emission – Simulations reveal that the bulk of the CX‑induced X‑ray photons originate near 300 km altitude, where the neutral H₂ density peaks and the CX cross‑sections are maximal. The model reproduces the observed spatial confinement of the X‑ray emission to narrow latitudinal bands, supporting the notion that the precipitating ions are guided by the converging magnetic field lines into a thin atmospheric slab.

Implications
The negligible carbon signal effectively rules out a dominant solar‑wind origin for Jupiter’s polar X‑rays, reinforcing the view that the planet’s own magnetospheric plasma—primarily O and S ions sourced from Io’s volcanic outgassing—is the principal driver of the CX emission. The confirmation of the 0.7–2 MeV u⁻¹ ion energies validates existing acceleration models that invoke field‑aligned potentials or wave‑particle interactions within the magnetosphere. The higher sulfur abundance points to dynamic compositional changes in the magnetospheric source population, which could be linked to Io’s episodic volcanic activity or to magnetospheric re‑configurations during solar wind compressions.

Overall, the study delivers a robust, multi‑ion CX framework that not only clarifies the origin of Jupiter’s auroral X‑rays but also provides a diagnostic tool for probing the composition, energy, and temporal variability of the planet’s magnetospheric plasma.