🔭 astrophysics

The Sunrise Chromospheric Infrared Spectro-Polarimeter SCIP: an instrument for SUNRISE III

The paper introduces the Sunrise Chromospheric Infrared Spectro-Polarimeter (SCIP), a high-resolution instrument designed for the 2024 Sunrise III balloon mission to perform seeing-free, high-precision spectro-polarimetric observations of solar magnetic fields and velocities in the photosphere and chromosphere using near-infrared wavelengths.

Original authors: Y. Katsukawa, J. C. del Toro Iniesta, S. K. Solanki, M. Kubo, H. Hara, T. Shimizu, T. Oba, Y. Kawabata, T. Tsuzuki, F. Uraguchi, K. Shinoda, T. Tamura, Y. Suematsu, T. Matsumoto, R. T. Ishikawa, Y. Na

Published 2026-03-19

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Y. Katsukawa, J. C. del Toro Iniesta, S. K. Solanki, M. Kubo, H. Hara, T. Shimizu, T. Oba, Y. Kawabata, T. Tsuzuki, F. Uraguchi, K. Shinoda, T. Tamura, Y. Suematsu, T. Matsumoto, R. T. Ishikawa, Y. Naito, K. Ichimoto, S. Nagata, T. Anan, D. Orozco Suárez, M. Balaguer Jiménez, A. C. López Jiménez, C. Quintero Noda, D. Álvarez García, J. L. Ramos Mas, B. Aparicio del Moral, A. Sánchez Gómez, D. Hernández Expósito, E. Bailón Martínez, J. M. Morales Fernández, A. J. Moreno Mantas, A. Tobaruela, I. Bustamante, A. Álvarez Herrero, J. Piqueras Carreño, I. Pérez Grande, A. Korpi-Lagg, A. Gandorfer, T. Berkefeld, P Bernasconi, A. Feller, T. L. Riethmüller, H. N. Smitha, V. Martínez Pillet, B. Grauf, A. Bell, M. Carpenter

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Sun as a bustling, chaotic city. To understand how its "weather" works—how energy moves, how magnetic storms form, and why the atmosphere heats up—we need to take a very close look at its streets and buildings. But the Sun is far away, and Earth's atmosphere is like a thick, wobbly blanket of fog that blurs our view.

To solve this, scientists launched a giant, high-altitude balloon called Sunrise III. It floats above 99% of Earth's atmosphere (higher than commercial jets), giving it a crystal-clear, "fog-free" view of the Sun.

The main star of this story is a new instrument attached to the balloon's telescope called SCIP (Sunrise Chromospheric Infrared spectroPolarimeter). Think of SCIP as a super-powered, multi-spectral detective camera designed to solve the mystery of the Sun's magnetic fields.

Here is how SCIP works, broken down into simple concepts:

1. The Detective's Toolkit: Seeing the Invisible

Magnetic fields on the Sun are invisible to the naked eye. However, when light passes through a magnetic field, it gets "twisted" slightly. This is called polarization.

The Analogy: Imagine looking at a river through a pair of special sunglasses. If the water is calm, the light looks normal. But if there are hidden currents (magnetic fields), the light ripples in a specific pattern that only your special glasses can see.
SCIP's Job: SCIP doesn't just take a picture; it measures these "ripples" in the light. By analyzing how the light is twisted, SCIP can map the strength and direction of the Sun's magnetic fields, from the surface (photosphere) up into the lower atmosphere (chromosphere).

2. The "Magic Glasses": Seeing Through the Fog

SCIP looks at two specific "colors" (wavelengths) of light that are invisible or blurry from the ground: 770 nm and 850 nm (near-infrared).

The Problem: On the ground, Earth's atmosphere acts like a pair of dirty glasses that block these specific colors, especially around 770 nm.
The Solution: Because the balloon floats in the stratosphere, it is above the "dirty glasses." This allows SCIP to see these specific colors clearly for the first time from a high-altitude platform. These colors act like a "key" that unlocks information about different layers of the Sun's atmosphere.

3. The Spinning Wheel: Capturing the Twist

To measure the polarization (the "twist" in the light), SCIP uses a clever trick involving a spinning waveplate.

The Analogy: Imagine a lighthouse beam spinning rapidly. As it spins, it flashes different patterns of light. SCIP spins a special crystal plate (the waveplate) very fast. As it spins, it changes how the light enters the camera.
The Result: The camera takes 16 rapid snapshots for every single rotation of the wheel. By comparing these snapshots, the computer can mathematically reconstruct the magnetic field's shape and strength. It's like taking a video of a spinning fan to figure out exactly how fast it's going and what shape the blades are.

4. The "Scanner": Painting a Picture

SCIP is a spectrograph, which means it doesn't take a wide photo of the whole Sun at once. Instead, it looks at a thin slice (a slit) of the Sun, like looking through a keyhole.

The Problem: If you only look through a keyhole, you miss the rest of the room.
The Solution: SCIP has a Scan Mirror (SMM). Think of this as a tiny, ultra-fast mirror that tilts back and forth. It sweeps the "keyhole" view across the Sun's surface, building up a 2D map pixel by pixel.
Speed: It moves so fast and so precisely that it can map a large area of the Sun without the image getting blurry, even though the balloon is floating in the air.

5. The Brain and the Body

The instrument is split into two main parts:

The O-Unit (The Body): This sits right behind the telescope. It holds the mirrors, the spinning wheel, the cameras, and the slit. It is built to be incredibly stable and light, using special materials that don't expand or shrink much with temperature changes.
The E-Unit (The Brain): This sits inside the balloon's gondola (the basket). It contains the computers and electronics. It acts as the conductor, telling the spinning wheel when to spin, the mirror when to tilt, and the cameras when to snap photos. It also processes the data on the fly, doing complex math to turn raw images into magnetic field maps before sending them down to Earth.

6. Why Does This Matter?

The Sun's atmosphere is a place of constant drama. Tiny jets of gas shoot up, and waves crash through the magnetic fields. Scientists want to know: How does energy get from the surface to the upper atmosphere?

SCIP is designed to watch these events in real-time. By measuring the magnetic fields and gas movements simultaneously in different layers of the Sun, it helps scientists understand the "engine" that drives solar weather.

Summary

In short, SCIP is a high-tech, balloon-borne camera that floats above the clouds. It uses spinning crystals and fast mirrors to take incredibly sharp, 3D "magnetic maps" of the Sun. It looks at specific colors of light that ground telescopes can't see, allowing scientists to finally get a clear, detailed look at how the Sun's magnetic fields drive its dynamic and energetic atmosphere.

Technical Summary: The Sunrise Chromospheric Infrared Spectro-Polarimeter (SCIP) for Sunrise III

Problem and Scientific Motivation
Understanding the dynamic phenomena in the solar atmosphere, such as jets and magnetohydrodynamic (MHD) waves, requires precise measurements of magnetic fields and velocities from the photosphere up through the chromosphere. Previous observations, particularly those from the first two flights of the Sunrise balloon-borne observatory (2009 and 2013), were limited by a lack of spectro-polarimetric capability in the chromosphere. While the Imaging Magnetograph eXperiment (IMaX) provided high-resolution photospheric magnetic maps, the chromosphere remained largely imaged rather than spectropolarimetrically analyzed. This gap hinders the quantitative understanding of energy transfer and acceleration mechanisms in the solar wind. Furthermore, ground-based observations of key near-infrared spectral lines (specifically the K I doublet around 770 nm) are severely compromised by atmospheric oxygen absorption, rendering them inaccessible for precise polarimetry. To address these limitations, the third flight of the Sunrise observatory (Sunrise III), conducted in 2024, required a new instrument capable of seeing-free, high-precision spectro-polarimetry in the near-infrared.

Methodology and Instrument Design
The Sunrise Chromospheric Infrared Spectro-Polarimeter (SCIP) was developed as a focal-plane instrument to observe near-infrared spectral lines around 770 nm and 850 nm. The instrument is designed to operate on a 1-meter aperture Gregorian telescope at a stratospheric altitude (>33 km), eliminating atmospheric seeing.

Optical Architecture: SCIP utilizes a long-slit spectrograph based on a modified Czerny-Turner design in a quasi-Littrow configuration. It employs an echelle grating to simultaneously observe two wavelength bands: Channel 1 (CH1) centered at 850 nm (19th diffraction order) and Channel 2 (CH2) centered at 770 nm (21st diffraction order). A slit-jaw (SJ) optical system provides a 2D image of the slit plane for guiding and context.
Polarimetry: Full Stokes vector measurements are achieved using a rotating waveplate (Polarization Modulation Unit, PMU) at the optical entrance and polarization beam splitters (PBS) placed in front of the detectors. The PMU rotates at a constant speed (0.512 sec/rotation), modulating the polarization state. The system uses three back-illuminated CMOS cameras (two for the spectrograph channels, one for the slit-jaw) with 2048×2048 pixels.
Mechanisms: A Scan Mirror Mechanism (SMM), located in the Image Stabilization and Light Distribution (ISLiD) unit, tilts a convex mirror to scan the field of view (FoV) perpendicular to the slit, enabling 2D mapping.
Electronics and Control: The Electronics Unit (E-unit), housed in a pressurized vessel on the gondola, contains the Digital Processing Unit (DPU), Analog/Mechanics/Heaters Driver (AMHD), and Power Converter Module (PCM). The DPU, featuring an FPGA and a system controller, synchronizes the PMU rotation, SMM scanning, and camera exposures. It performs onboard polarization demodulation, accumulation, and data compression.
Thermal Control: Given the stratospheric environment, the O-unit (Optics) is thermally isolated from the gondola using Single-Layer Insulation (SLI) and active heaters to maintain near-room temperature (~20°C). The cameras are actively cooled via dedicated radiators and copper straps to maintain sensor temperatures between 15°C and 30°C with high stability.

Key Contributions and Performance Specifications
The paper details the design, integration, and ground testing of SCIP, highlighting the following technical achievements:

Spectral Coverage: The instrument covers the Ca II infrared triplet (849.8 nm and 854.2 nm) and the K I doublet (766.4 nm and 769.9 nm), along with multiple Fe I and Si I lines. This allows for seamless coverage of the photosphere and chromosphere.
Resolution and Sensitivity:
- Spatial Resolution: 0.21 arcseconds (diffraction-limited at 850 nm).
- Spectral Resolution: $1 \times 10^5$ (with sampling up to $2 \times 10^5$ ).
- Polarimetric Sensitivity: 0.03% (1 $\sigma$ ) of the continuum intensity, achieved with a 10-second integration time per resolution element in the 850 nm band (using 2-pixel binning) and without binning in the 770 nm band.
Field of View: The slit length covers 58 arcseconds, with a scanning capability to map a 58" $\times$ 58" area.
Calibration: Extensive ground-based polarization calibration was performed using tunable lasers and polarizers to derive the instrument response matrix. The system accounts for the rolling shutter effect of the CMOS sensors and the polarization properties of the telescope and ISLiD.
Testing: The instrument underwent rigorous optical alignment, synchronization tests (verifying PMU, SMM, and camera timing), and Thermal-Vacuum Tests (TVT) to validate performance under flight-like conditions. The TVT confirmed the thermal model and verified that focus shifts between air and vacuum could be corrected.

Results and Operational Status
The paper reports that SCIP was successfully integrated into the Sunrise III payload. During the flight in July 2024, the instrument operated successfully, acquiring spectropolarimetric data for various solar targets, including the quiet Sun, sunspots, and plages. The ground testing demonstrated that the instrument met its design requirements for wavefront error, spatial resolution, and polarization sensitivity.

Significance
The authors state that SCIP provides a novel capability to observe near-infrared spectral lines that are sensitive to magnetic fields in both the photosphere and chromosphere. By combining these observations with data from the other Sunrise III instruments (SUSI and TuMag), SCIP is expected to elucidate the mechanisms driving energy transfer and time-dependent phenomena across the solar atmosphere. The successful operation of SCIP represents a significant step forward in high-resolution, seeing-free solar polarimetry, particularly in the near-infrared regime where ground-based observations are limited by atmospheric absorption.