CUSP: CubeSat Solar Polarimeter

• CUSP is an Earth-orbiting mission designed to measure hard X-ray polarization from solar flares using a dual-phase Compton polarimeter with plastic and GAGG(Ce) scintillators.
• It employs sophisticated event coincidence and calibration techniques to extract polarization degree and angle, enabling insights into magnetic reconnection and particle acceleration.
• The mission supports operational space weather forecasting through rapid, time-resolved polarimetric measurements and rigorous multi-physics CubeSat qualification.

The CUbesat Solar Polarimeter (CUSP) is an Earth-orbiting space mission developed within the Italian Space Agency’s Alcor Program to measure the linear polarization of solar flare X-rays in the 25–100 keV energy band. Envisioned as a constellation of two CubeSats—and currently proceeding with a one-satellite baseline—the project utilizes a dual-phase Compton scattering polarimeter combining plastic and GAGG(Ce) scintillators, sophisticated readout electronics, and a mechanical design engineered for the severe dynamic and thermal environment of launch and orbit. CUSP’s objective is to probe mechanisms of magnetic reconnection and particle acceleration in the Sun’s flaring magnetic structures and to support space weather forecasting via rapid, high-sensitivity, time-resolved polarimetric measurements.

1. Scientific Objectives and Rationale

CUSP’s central scientific goal is to perform high-sensitivity measurements of the linear polarization degree (PD) and angle (PA) in hard X-rays from solar flares (Fabiani et al., 2022, Angelis et al., 1 Aug 2025, Fabiani et al., 1 Aug 2025). These measurements provide direct diagnostic access to:

• The geometry and strength of magnetic fields in the flaring regions,
• The directional distribution (anisotropy) of accelerated electron populations, and
• The partitioning between thermal and non-thermal bremsstrahlung emission, which leaves distinct polarization fingerprints.

Discriminating between competing solar flare models (e.g., differing in electron beaming and magnetic topology) is possible through polarization, which breaks degeneracies that remain when only using spectroscopic or timing data (Fabiani et al., 4 Jul 2024, Cesare et al., 4 Jul 2024). The real-time return of this information is critical for operational space weather frameworks such as ASI’s SPace weather InfraStructure (ASPIS) (Fabiani et al., 2022).

2. Instrument Architecture and Physical Principles

CUSP’s payload is a dual-phase Compton scattering polarimeter (Cesare et al., 4 Jul 2024, Angelis et al., 1 Aug 2025, Fabiani et al., 1 Aug 2025). Its detection concept is based on the polarization dependence of the Compton effect, described by the Klein–Nishina differential cross section:

$$\frac{d\sigma}{d\Omega} = \frac{r_0^2}{2} \left( \frac{E'}{E} \right)^2 \left( \frac{E}{E'} + \frac{E'}{E} - 2\sin^2\theta\cos^2\phi \right)$$

where:

• $E, E'$ are, respectively, the energies of the incident and scattered photons,
• $\theta$ is the polar scattering angle,
• $\phi$ is the azimuthal angle relative to the polarization vector.

Polarized X-rays preferentially scatter perpendicular to the polarization vector, creating a modulation of the azimuthal event distribution. The instrument reconstructs the scattering azimuth by time-coincident detection in two sub-detectors:

• Plastic scintillators (scatterers): Arrayed (8×8 or 64 rods) for maximizing Compton recoil probability.
• GAGG(Ce) scintillators (absorbers): 32 rods surround the scatterer array, optimized for efficient photoelectric capture of the scattered photon.

Associated electronics include:

• Multi-Anode Photomultiplier Tubes (MAPMTs) for the plastics (read out by the MAROC-3A ASIC).
• Avalanche Photodiodes (APDs) coupled to the GAGGs (read by SKIROC-2A ASIC).
• Coincidence electronics interface with an onboard processing unit to associate events with submillisecond timing (Angelis et al., 1 Aug 2025).

A simplified payload schematic is:

$$\begin{array}{c} \text{Incident Hard X-ray Photon}\ \downarrow\ \textbf{Plastic Scintillator} \rightarrow \text{MAPMT~(64~channels)}\ \downarrow\ \textbf{GAGG Scintillator} \rightarrow \text{APDs~(32~channels)}\ \downarrow\ \text{Coincidence Electronics} \rightarrow \text{Data Processing} \end{array}$$

The satellites rotate at a nominal 1 revolution per minute about the Sun axis during observations, averaging out systematic gain variations and enabling full modulation curve sampling (Fabiani et al., 2022, Fabiani et al., 4 Jul 2024).

3. Technical Development and Calibration

Design and Structural Qualification

The instrument and CubeSat (6U/6U-XL) platform leverage an integrated multi-physics workflow:

• Mechanical Design: 3D CAD (SolidWorks) models are simplified to retain structurally critical elements, enabling efficient yet accurate FEA for static (up to 20g), modal (first mode >100–270 Hz), and random vibration (PSD curves per ECSS and NASA GEVS) analyses (Lombardi et al., 4 Jul 2024, Lombardi et al., 4 Aug 2025).
• Thermal Analysis: ANSYS/Thermica simulations capture orbital "hot" and "cold" scenarios; topological optimization of the payload-platform interface enhances resilience.
• Environmental Qualification: Structural models (“mechanical demonstrators”) are slated for a comprehensive regime of quasi-static loading, sine/random vibration, and potential shock tests, validating the design versus launch loads and on-orbit cycling (Lombardi et al., 4 Aug 2025).

Sensor System Characterization

Key to instrument performance is the calibration and stabilization of the optical readout system:

• APD gain and energy resolution: Calibrated using $^{55}\mathrm{Fe}$, $^{241}\mathrm{Am}$, and $^{109}\mathrm{Cd}$ sources; APD gain ($G_{APD}$) increases as temperature decreases. A closed-loop feedback system is being developed to compensate for thermal effects by adjusting APD bias voltage ($V_{bias}$) in real-time (Cologgi et al., 4 Jul 2024, Angelis et al., 1 Aug 2025).
• Plastic scintillator and MAPMTs: Gain and energy calibration performed with multi-line X-ray sources; signal shaping optimized in MAROC-3A ASIC (fast shaper modes).
• High Voltage Power Supplies: Custom DC/DC units (SMHV series) provide up to −1 kV (MAPMT) and +500 V (APD) with compact format and controlled output ripple. Temperature-dependent performance is characterized to ensure bias stability in orbit (Lacerenza et al., 1 Aug 2025).

Event Selection and Coincidence Tagging

Detector performance metrics rely on the efficiency of correctly identifying true Compton events. Simulations and calibrations separately determine:

• Coincidence efficiency ($\epsilon_{coinc}$): Probability of detecting both primary scatter and secondary absorption per event.
• Tagging efficiency ($\eta_{tag}$): Practical probability of registering a valid light signal in both detectors given electronics thresholds. The effective area, as a function of energy, is then:

$$A_{eff}(E) = A_{geom} \cdot \epsilon_{coinc}(E) \cdot \eta_{tag}(E)$$

(Cesare et al., 4 Jul 2024, Angelis et al., 1 Aug 2025, Kumar et al., 1 Aug 2025)

4. Simulation, Performance Analysis, and Sensitivity

Exhaustive Geant4 Monte Carlo simulations underpin instrument optimization and performance prediction (Cesare et al., 4 Jul 2024, Kumar et al., 1 Aug 2025). Principal steps include:

• Detailed Mass Model Construction: The full payload geometry (active/passive detector elements, support, shielding) is modelled and converted from CAD to GDML for Geant4 input.
• Event Generation and Physics Tracking: Simulations deploy both polarized and unpolarized monochromatic or solar-flare-like spectra to propagate X-rays through the mass model, recording coincidence-tagged energy deposits and scattering coordinates.
• Stokes Parameter Extraction: For each event, $q_i = 2\cos{2\phi_i}$, $u_i = 2\sin{2\phi_i}$; with N events, $q = \sum q_i/N$, $u = \sum u_i/N$. The modulation, polarization degree, and angle are retrieved as $m = \sqrt{q^2 + u^2}$, $p = m/\mu_{100}$, $\phi = \frac12 \tan^{-1}(u/q)$.
• Spurious Modulation and Geometric Corrections: The rigid square geometry induces systematic artifacts (e.g., variable modulation factor along diagonal/corner), separated using methods based on Stokes parameter subtraction (between 0° and 90° polarization) and simulation based deconvolution.
• Backscatter Shielding Optimization: Molybdenum shielding layers (200/400 μm) mitigate unpolarized background but reduce effective area, introducing a design trade-off: modulation factor improvement (~1.7%) versus area loss (~5%).

A key performance metric is the Minimum Detectable Polarization (MDP), at 99% confidence:

$$\mathrm{MDP}_{99\%} = \frac{4.29}{\mu R} \sqrt{ \frac{R + B}{T} }$$

where $R$ is source count rate, $B$ is background, $T$ is integration time (Fabiani et al., 1 Aug 2025).

Simulations show a few percent MDP is achievable for X-class flares, exceeding previous hard X-ray polarimetry efforts (Fabiani et al., 4 Jul 2024, Kumar et al., 1 Aug 2025, Angelis et al., 1 Aug 2025).

5. Mission Implementation and Programmatic Framework

CUSP is built as a 6U or 6U-XL CubeSat, with a mass and volume envelope driven by the scientific payload (Fabiani et al., 1 Aug 2025). Platform synergies include:

• Attitude Determination and Control (ADCS): High precision pointing (better than ±2°), critical for solar-tracking and minimizing systematic biases.
• Power Systems: Deployable solar panels yielding peak power ~30 W, battery capacity up to 84 Wh.
• Communication: UHF (command/telemetry) and S-band (science downlink up to 5 Mbps), with ground infrastructure at the University of Tuscia (tracking accuracy 0.1°, multiple daily passes).
• Constellation Architecture: While baseline is single-satellite, a two-CubeSat implementation is under consideration to increase Sun coverage and polarimetric sensitivity by √2; phased orbits may optimize continuous solar observation (Fabiani et al., 2022, Fabiani et al., 4 Jul 2024).

CUSP is currently in the 12-month Phase B (Dec 2024–Dec 2025), focused on:

• Subsystem prototyping and qualification,
• Payload engineering qualification model (EQM) development,
• Calibration and test campaigns at INAF-IAPS facilities (formerly for IXPE),
• Finalization of launch and mission scenarios, anticipating flight (subject to ASI approval) in late 2027/early 2028 (Angelis et al., 1 Aug 2025, Lombardi et al., 4 Aug 2025).

6. Impact, Innovations, and Future Prospects

CUSP establishes several methodological standards for hard X-ray CubeSat astrophysics:

• Compton polarimetry in a CubeSat: Demonstrates that high-precision, time-resolved X-ray polarization is achievable within nanosatellite mass, power, and telemetry constraints.
• Integrated multi-physics qualification: Iterative coupling of mechanical, thermal, and electronic simulation with 3D-printed, optimized structures paves a path for future advanced CubeSat missions (Lombardi et al., 4 Jul 2024, Lombardi et al., 4 Aug 2025).
• Novel “optical frame”: Ensures alignment of critical detector modules, vital given the sensitivity of polarization measurements to mechanical shifts induced by launch and in-orbit environment (Lombardi et al., 4 Aug 2025).
• Feedback-controlled detector stabilization: Closed-loop APD gain compensation across large orbital thermal cycles is essential for robust instrument response (Cologgi et al., 4 Jul 2024).
• Data return for operational networks: CUSP’s rapid, real-time data provision integrates with existing and emerging space weather frameworks.

The methodologies and technical solutions developed—particularly mass model-driven Geant4 simulations, mechanical/thermal optimization, and coincident polarimetric event tagging—represent transferable advancements for subsequent missions targeting heliophysics, space weather, and X-/gamma-ray astrophysics.

CUSP’s potential to deliver statistically significant measurements of the polarization of hard X-ray solar flares positions it to play a fundamental role in advancing both theoretical and operational solar physics in the context of nanosatellite technology (Fabiani et al., 2022, Fabiani et al., 4 Jul 2024, Fabiani et al., 1 Aug 2025).