[Paper Review] Toward Next Generation Solar Coronagraph: Diagnostic Coronagraph Experiment

Kyung‐Suk Cho, Heesu Yang+9 more|arXiv (Cornell University)|Jun 11, 2020

Solar and Space Plasma Dynamics1 references4 citations

TL;DR

This paper presents the development and validation of the Diagnostic Coronal Experiment (DICE), a ground-based solar coronagraph system designed to measure electron temperature and velocity in the low solar corona using narrowband imaging at 3934 Å and 4305 Å. By analyzing the thermal broadening and Doppler shifts of Fraunhofer lines in the corona, DICE successfully produced a temperature map of the eastern limb, revealing higher temperatures in streamers and lower temperatures at coronal hole boundaries, though scattering from prominences contaminated western limb data.

ABSTRACT

Korea Astronomy and Space Science Institute (KASI) has been developing a next-generation coronagraph (NGC) in cooperation with NASA to measure the coronal electron density, temperature, and speed simultaneously using four different filters around 400 nm. KASI organized an expedition team to demonstrate the coronagraph measurement scheme and the instrumental technology through the 2017 total solar eclipse (TSE) across the USA. The observation site was in Jackson Hole, Wyoming, USA. We built an eclipse observation system, so-called Diagnostic Coronal Experiment (DICE), which is composed of two identical telescopes to improve a signal to noise ratio. The observation was conducted with 4 wavelengths and 3 linear polarization directions according to the planned schedule in a limited total eclipse time of about 140 seconds. Polarization information of corona from the data was successfully obtained but we were not able to obtain global information of coronal electron temperature and speed in the corona due to a low signal-to-noise ratio of the optical system and a strong emission from the prominence located in the western limb. In this study, we report the development of DICE and observation results from the eclipse expedition. TSE observation and analysis by using our own developed instrument gave an important lesson that a coronagraph should be carefully designed to achieve the scientific purpose of this study. And it was a very useful experience in the way for the success of follow-up NASA-KASI joint missions called the Balloon-borne Investigation of the Temperature and Speed of Electrons in the Corona (BITSE) and COronal Diagnostic EXperiment (CODEX).

Motivation & Objective

• To develop a ground-based diagnostic coronagraph system for measuring electron temperature and velocity in the low solar corona.
• To validate the optical, filter, and control systems of a next-generation coronagraph using a total solar eclipse as a testbed.
• To assess the feasibility of using narrowband imaging of Fraunhofer lines to derive plasma temperature and radial velocity in the corona.
• To identify systematic errors and technical challenges for future space-based missions like CODEX.

Proposed method

• Design and integration of a compact optical system with a 50 mm aperture lens, bandpass filters, polarizers, and CCD detectors.
• Implementation of a computer-controlled motorized tracking system to maintain solar alignment during the eclipse.
• Use of the core Flight System (cFS) for embedded control software, enabling reliable operation in time-critical conditions.
• Application of temperature and velocity diagnostics via the ratio of intensities at two wavelength bands: 3934 Å (sensitive to thermal broadening) and 4305 Å (sensitive to Doppler shift).
• Conducting lab tests to verify system resolution (RMS spot diameter <14.8 μm) and intensity accuracy (<±1%) under controlled conditions.
• Performing image stacking and Monte Carlo simulations to improve signal-to-noise ratio and estimate temperature uncertainty.

Experimental results

Research questions

• RQ1 Can narrowband imaging of Fraunhofer lines at 3934 Å and 4305 Å accurately retrieve electron temperature and radial velocity in the low corona?
• RQ2 How do stray light and lens flare from prominences affect the accuracy of temperature measurements in coronal imaging?
• RQ3 To what extent do asymmetries in streamer geometry (front vs. back side) influence temperature ratio measurements compared to symmetric models?
• RQ4 What are the dominant systematic errors in temperature retrieval that are not accounted for in standard tolerance analyses?
• RQ5 How effective is the cFS software framework for real-time control of a portable solar coronagraph system?

Key findings

• The DICE instrument successfully produced a temperature distribution map of the eastern limb of the corona, showing higher temperatures within streamers and lower temperatures at the boundaries with polar coronal holes.
• Temperature estimates from the DICE data showed consistency with known coronal trends, particularly in the east limb region.
• Significant contamination from scattering and lens flare caused by prominences on the west limb led to unreliable temperature measurements in that region.
• The observed ratio variations exceeded expected uncertainties from Monte Carlo simulations, suggesting unaccounted systematic errors such as stray light, diffraction patterns, or flat-fielding issues.
• One camera failure during the eclipse limited data collection, highlighting the importance of robust power distribution and pre-flight system-level testing.
• The use of cFS for embedded control proved effective and reliable, supporting future deployment of similar systems on high-altitude balloons and space missions.