In-flight calibration of RADEM, the JUICE mission radiation monitor
Pith reviewed 2026-05-15 18:05 UTC · model grok-4.3
The pith
In-flight calibration using galactic cosmic rays allows RADEM to reconstruct accurate proton fluxes from solar particle events.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
By increasing detector thresholds in flight and matching observed count rates to expectations from the Badhwar-O'Neill 2020 galactic cosmic ray model together with the instrument response functions, the team obtains new calibration slopes that differ from ground values by up to an order of magnitude in several cases. These updated coefficients enable a bow-tie flux reconstruction algorithm that converts RADEM count rates into proton fluxes. The resulting fluxes during solar energetic particle events agree within a factor of two with measurements from the Solar and Heliospheric Observatory, confirming that RADEM supplies accurate proton data in interplanetary space.
What carries the argument
Threshold-dependent count-rate comparison to galactic cosmic ray model predictions, used to derive new calibration coefficients and enable bow-tie flux reconstruction.
If this is right
- RADEM now supplies accurate proton flux measurements throughout interplanetary space.
- The instrument supports both single-spacecraft studies and coordinated multi-mission analyses of solar energetic particles.
- New calibration coefficients differ from ground values by as much as an order of magnitude.
- Electron flux reconstruction remains unreliable with the current method and requires further development.
Where Pith is reading between the lines
- The same threshold-adjustment technique could correct ground-to-flight discrepancies on other radiation monitors without dedicated calibration hardware.
- Reliable RADEM proton data during the cruise phase could improve real-time space-weather monitoring for missions traveling to the outer solar system.
- The order-of-magnitude shifts between ground and flight coefficients suggest that similar instruments on future spacecraft should plan for in-flight recalibration as a standard step.
Load-bearing premise
The galactic cosmic ray model and detector response calculations match the actual particles and instrument behavior seen during the calibration observations.
What would settle it
Proton fluxes reconstructed from RADEM during a solar event that differ by more than a factor of two from simultaneous measurements by another spacecraft would falsify the accuracy of the in-flight calibration.
read the original abstract
The RADiation-hard Electron Monitor (RADEM) aboard the Jupiter Icy Moons Explorer (JUICE), launched on 14 April 2023, measures high-energy protons and electrons during the cruise phase and will continue throughout the nominal mission. Initial in-flight observations could not be explained by pre-flight ground calibration, motivating an in-flight calibration campaign. We calibrated the RADEM sensors using galactic cosmic rays by progressively increasing detector thresholds, thereby modifying their response to high-energy particles. Threshold-dependent in-flight count rates were compared with theoretical expectations derived from the Badhwar-O'Neill 2020 galactic cosmic ray model and corresponding response functions. These results were used to derive new in-flight calibration coefficients and to develop a flux reconstruction algorithm based on the bow-tie method. In several cases, the in-flight calibration slopes differ by up to an order of magnitude from ground calibration values. Proton fluxes from solar energetic particle events reconstructed with this method agree within a factor of two with independent measurements from the Solar and Heliospheric Observatory. These results demonstrate that RADEM provides accurate proton flux measurements in interplanetary space and is well suited for both single-spacecraft analyses and coordinated multi-mission studies of solar energetic particles. While electrons were clearly detected during the JUICE Lunar-Earth gravity assist, reliable reconstruction of their fluxes requires further analysis.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript describes an in-flight calibration of the RADEM instrument aboard JUICE using galactic cosmic ray observations obtained by progressively raising detector thresholds. Observed count rates are compared to predictions from the Badhwar-O'Neill 2020 GCR model and pre-computed response functions to derive new calibration coefficients. A bow-tie flux reconstruction algorithm is developed, and proton fluxes from solar energetic particle events are shown to agree within a factor of two with independent SOHO measurements. Electron detection during the Lunar-Earth gravity assist is noted, but reliable electron flux reconstruction is stated to require further work.
Significance. If the calibration holds, the work provides validated proton flux measurements for the JUICE cruise and Jupiter phases, enabling both single-spacecraft and multi-mission studies of solar energetic particles. The in-flight approach corrects discrepancies with ground calibration and demonstrates a practical bow-tie reconstruction method. The factor-of-two agreement with SOHO data offers supporting evidence, though the dependence on an external GCR model and response functions requires explicit validation to confirm independence from modeling assumptions.
major comments (2)
- [Abstract] Abstract: The central claim that reconstructed proton fluxes agree within a factor of two with SOHO data is presented without reported statistical or systematic uncertainties, error budgets, or identification of the specific SEP events and time intervals used. This omission prevents assessment of whether the agreement is statistically meaningful or sensitive to threshold choices.
- [Abstract] Abstract: Calibration coefficients are obtained by fitting count rates to predictions from the Badhwar-O'Neill 2020 model; the manuscript does not describe any sensitivity tests to variations in the GCR model parameters or response functions, leaving open the possibility that the derived slopes (which differ by up to an order of magnitude from ground values) are partly model-dependent rather than purely instrument-driven.
minor comments (1)
- [Abstract] Abstract: The final sentence on electron detection during the gravity assist is brief and could usefully indicate the observed count-rate levels or energy range to provide context for why further analysis is required.
Simulated Author's Rebuttal
We thank the referee for the constructive comments on our manuscript. We address each major point below and will revise the abstract (and supporting sections) to incorporate the requested details on uncertainties, events, and sensitivity tests.
read point-by-point responses
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Referee: [Abstract] Abstract: The central claim that reconstructed proton fluxes agree within a factor of two with SOHO data is presented without reported statistical or systematic uncertainties, error budgets, or identification of the specific SEP events and time intervals used. This omission prevents assessment of whether the agreement is statistically meaningful or sensitive to threshold choices.
Authors: We agree that the abstract requires additional detail to make the factor-of-two agreement assessable. In the revised manuscript we will specify the SEP events and time intervals used, report the associated statistical and systematic uncertainties, and include a concise error budget. This will show that the agreement holds within the stated uncertainties and is not critically sensitive to threshold selection. revision: yes
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Referee: [Abstract] Abstract: Calibration coefficients are obtained by fitting count rates to predictions from the Badhwar-O'Neill 2020 model; the manuscript does not describe any sensitivity tests to variations in the GCR model parameters or response functions, leaving open the possibility that the derived slopes (which differ by up to an order of magnitude from ground values) are partly model-dependent rather than purely instrument-driven.
Authors: We acknowledge the absence of explicit sensitivity tests in the current abstract. We will add a statement to the revised abstract and expand the methods section to describe sensitivity tests performed on the Badhwar-O'Neill 2020 parameters and response functions. These tests will quantify the impact on the derived slopes and demonstrate that the order-of-magnitude differences from ground calibration are driven primarily by the in-flight data rather than model assumptions. revision: yes
Circularity Check
No significant circularity
full rationale
Only the abstract is available and contains no equations, response functions, or derivation steps. Calibration compares in-flight count rates to predictions from the external Badhwar-O'Neill 2020 GCR model (not authored by the present team) and validates reconstructed fluxes against independent SOHO measurements. No self-definitional steps, fitted inputs renamed as predictions, or load-bearing self-citations appear. The argument relies on external benchmarks and is therefore self-contained.
Axiom & Free-Parameter Ledger
free parameters (1)
- in-flight calibration slopes
axioms (1)
- domain assumption Badhwar-O'Neill 2020 galactic cosmic ray model accurately describes the high-energy particle spectrum during the observations
discussion (0)
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