SUTRI: Solar Upper Transition Region Imager

• SUTRI (SU-01) is a solar EUV imager dedicated to capturing the upper transition region at the Ne VII 46.5 nm line, filling a critical observational gap.
• It utilizes a Ritchey–Chrétien design with Sc/Si multilayer coatings and a back-illuminated CMOS sensor to achieve an effective spatial resolution of approximately 8″ at a 30-second cadence.
• The mission enables detailed studies of TR–coronal coupling, flare and CME precursors, and mass and energy transport in the solar atmosphere, with data openly available for research.

The Solar Upper Transition Region Imager (SUTRI, mission SU-01) is a normal-incidence extreme ultraviolet (EUV) solar imager onboard the SATech-01 satellite, launched to sun-synchronous orbit in July 2022. It is the first instrument dedicated to imaging the upper solar transition region (TR) at the Ne VII 46.5 nm resonance line, a regime around 0.5 MK largely unsampled by previous solar missions. SUTRI’s combination of large field-of-view, moderate spatial resolution, and single-line EUV imaging provides a new window for exploring TR/coronal coupling, flare and CME precursors, and the energy and mass transport in the solar atmosphere (Bai et al., 2023).

1. Optical System and Imaging Concept

SUTRI employs a Ritchey–Chrétien optical design with a hyperbolic 18 cm primary and secondary mirror, optimized for normal incidence at the Ne VII line. The optical train is housed in an 85 cm × 38 cm × 38 cm sealed tube. The effective focal length is 1.09 m, yielding a full-disk field of view (FOV) of 41.6′ × 41.6′ on the 2048 × 2048 CMOS array (pixel size 6.5 μm; ≈1.22″ / px). Diffraction-limited angular resolution at 46.5 nm is approximately 0.07″, but dominant image-degrading factors are pointing stability (~0.0005°/s), thermal/mechanical drift, and spatial sampling, limiting the on-orbit effective resolution to about 8″.

The spectral passband is enforced by a pair of narrow-band (3 nm FWHM) interference filters centered on 46.5 nm, and by a state-of-the-art scandium/silicon (Sc/Si) multilayer coating on both M1 and M2. The theoretical maximum reflectivity at 46.1 nm is ~30.7%, with the multilayer period Λ = 24.55 nm and thickness ratio Γ = d_Sc/Λ = 0.68. These mirror coatings are key to enabling high normal-incidence throughput in the otherwise demanding spectral regime.

2. Detector, Filters, and Throughput

SUTRI’s camera is a back-illuminated GSENSE2020BSI CMOS sensor with 2048 × 2048 pixels of 6.5 μm pitch. The device offers four gain settings: high gain 1 (0.47 e⁻/DN, read noise ~4 e⁻, full well ~1.8 ke⁻), high gain 2 (5.29 e⁻/DN, ~23 e⁻, ~17 ke⁻), and two lower gain modes. At 46.5 nm, the quantum efficiency is ≥20% (measured in laboratory calibration at the National Synchrotron Radiation Lab). In operation, detector temperature is maintained at –5 °C, with a cleaning mode at +20 °C to mitigate contamination by annealing.

The optical stack includes a 200 nm Al front filter, a 3 nm (FWHM) multilayer Al/Mg/Al rear filter (77/102/77 nm), and the Sc/Si mirrors. The total on-axis system efficiency at the Ne VII line is approximately 0.25% (T_front × R_M1 × R_M2 × T_rear ≈ 0.18 × 0.30 × 0.30 × 0.16). This ensures suppression of out-of-band UV, visible, and IR background at the required sensitivity. The PSF is dominated by sampling and pointing, yielding a measured FWHM of ~8″ in bright point targets.

3. Observing Performance and Data Pipeline

SUTRI typically operates at a cadence of one image every 30 s (exposure 1–25 s, adjustable up to 300 s), producing ~15 GB of uncompressed data per day (2×8 MB images per orbit, sunlit time ~16 h/day). The field of view allows imaging of the whole solar disk with ample margin for tracking off-limb events. Daily operation is limited by the 60 min day and 36 min eclipse for each ~96 min orbit.

Raw (Level 0.0, 16-bit FITS) data are downlinked and processed at HSOS/NAOC. The pipeline applies sequential calibrations:

• Level 0.9: Dark-field correction (using eclipse frames), row-stripe and cosmic-ray filtering, hot/bad pixel and dust corrections, co-alignment, limb fitting, derotation, and timing corrections.
• Level 1.0: Flat-field correction (from periodic off-point maneuvers and Chae–Kuhn algorithm).
• Level 1.5: Radiometric calibration to physical units (photons cm⁻² s⁻¹ sr⁻¹).

All data (levels 0.9/1.0/1.5) are made publicly available, along with quick-look movies, via https://sun10.bao.ac.cn/SUTRI/ and through the Chinese Virtual Observatory.

4. In-Orbit Performance Metrics

Parameter	Value/Remarks
Optical design	Ritchey–Chrétien, D = 18 cm, f = 1.09 m
Angular resolution	Theoretical 0.07″; measured on-orbit ~8″
Field of view	41.6′ × 41.6′ @1.22″/pixel
Wavelength	Ne VII 46.5 nm, Δλ ≈ 3 nm (FWHM)
Mirror coating	Sc/Si, Λ = 24.55 nm, R_peak ≈ 30%
Filters	Front: Al 200 nm; Rear: Al/Mg/Al 77/102/77 nm
Detector	CMOS GSENSE2020BSI, 2048² px, 6.5 μm/px
Gain/noise	4 gains, 4–64 e⁻ noise, full well 1.8–57 ke⁻
Cadence	Up to 3 fps; routine 30 s/image
Daily observation	~16 h/day (day+eclipse/orbit = 96 min)
Data volume	~15 GB/day (uncompressed)
Data levels	L0 → L0.9 (prelim) → L1.0 (flat) → L1.5 (phys)

SUTRI’s spatial resolution (~8″) and cadence are matched to dynamics of sub-MK and transition-region structures but also enable flare and CME tracking. Routine calibration and strict contamination controls (sealed aperture door, “detector cleaning,” cleaning protocols) have maintained optical performance—no significant degradation observed over the first four months of operations.

5. Science Motivations and Early Results

The Ne VII 46.5 nm line forms at T ≈ 0.5 MK, sampling the “upper transition region” at a regime between the chromospheric–transition 30.4 nm channel (T ≈ 0.1 MK) and the lower corona imaged by conventional EUV channels (e.g., AIA 17.1 nm, T ≈ 0.8 MK). Existing space-borne imagers do not provide resolved synoptic imaging at this intermediate TR temperature; SUTRI thus fills a critical diagnostic gap.

Early SUTRI results demonstrate:

• Expanded quiet-Sun network lanes compared to AIA 160 nm, consistent with magnetic funnel expansion through the upper TR.
• Active region loop systems connecting features in IRIS/AIA 30.4 nm (~0.1 MK) and AIA 17.1 nm (~0.8 MK).
• Ubiquitous downflows at AR loop legs in the Ne VII channel, providing evidence for TR condensation and coronal mass cycle processes.
• Filament/prominence absorption features at 0.5 MK for mass diagnostics.
• Flare footpoint and ribbon evolution mapped at intermediate temperatures.
• Global wave and CME-associated dimming, enabling improved emission-measure inversions when combined with higher-T channels.

The unique dataset enables high-fidelity studies of TR–coronal dynamics, energy, and mass transport, as well as space weather precursor signatures such as filament destabilization and CME onsets.

6. Broader Significance and Data Accessibility

SUTRI advances observational heliophysics by enabling, for the first time, direct imaging of the upper transition region with high temporal and spatial coverage. All data products are released without restriction, supporting research into:

• Coupling across solar atmospheric layers: chromosphere–TR–corona.
• Pre-flare and pre-CME dynamics: identification and early-warning of eruptive events.
• Differential emission measure diagnostics using combined SUTRI, AIA, and IRIS datasets.
• Statistical studies of global TR structure, network, and active regions.

The synergy of SUTRI’s single-wavelength normal-incidence design, Sc/Si multilayers, and high-throughput, stable CMOS imaging establishes a new technical and scientific baseline for EUV imaging in the upper solar atmosphere (Bai et al., 2023).