Solar flares, one of the most intense explosive phenomena in the solar atmosphere, release large amounts of energy in a short period, resulting in increased electromagnetic radiation in multiple bands. These events pose significant threats to Earth’s space environment and human activities in space. High-energy electrons accelerated during flares emit millimeter-wave radiation via gyrosynchrotron radiation, providing unique information about the underlying magnetic fields and energy release processes (Fleishman et al. 2018). However, observational data in the millimeter-wave range has been lacking, particularly above 20 GHz. To address this gap, a research team from the Institute of Space Sciences, Shandong University has successfully developed a 50 to 55 GHz millimeter-wave radiometer spectrometer for solar flare detection, complementing blank data from the relevant frequency band.
Figure 1: The 50-55 GHz millimeter wave radiometer spectrometer for solar flare detection is installed at an altitude of 4200 meters.
The millimeter wave radiation emitted during solar flares is very efficient and sensitive to high-energy electrons. This radiation not only reveals information about the accelerated electrons, but also provides unique information about the structures of the magnetic field and the energy release processes during flares. Traditional solar radio instruments operate primarily below 18 GHz, with limited data available above this frequency range. This gap has hindered a comprehensive understanding of the processes of high-energy electron acceleration and magnetic reconnection in solar flares. The new 50-55 GHz radiometer spectrometer aims to fill this observation gap, offering unprecedented data to study solar flares in the millimeter wave range.
Developed by the Institute of Space Sciences of Shandong University, the 50–55 GHz millimeter-wave radiometer spectrometer began regular observations in October 2024. The system features several innovative designs and technical advantages: High sensitivity and dynamic range: Equipped with a 50 cm diameter Cassegrain antenna, the system has a noise figure of less than 2.5 dB and a dynamic range greater than 30 dB. It can detect both silent solar signals and intense bursts of flares with high precision. High temporal resolution: The system achieves a temporal resolution of 0.001 to 1 second, allowing it to capture rapid changes during solar flares and provide detailed temporal information for flare studies. Direct detection scheme: Unlike traditional superheterodyne receivers, this system employs a direct detection method, which eliminates non-linear effects and noise induced by analog devices (Chang et al. 2024). This design improves the dynamic range of the system and prevents signal saturation during intense flare events. Multi-channel parallel acquisition: The system features a custom-designed 4-channel parallel acquisition digital receiver with 32-bit resolution. This design ensures high sensitivity and allows detection of weaker flare bursts, providing more complete data for solar flare research.
Figure 2: Detailed architecture and signal routing of the system. It includes four parts: antenna feeder subsystem, analog front subsystem, digital receiver subsystem and upper computer subsystem. The design schemes for analog and digital receivers were presented, as well as the data processing work carried out within the set of field-programmable gates.
In December 2024, the system successfully captured the world’s first solar flare data in the 50 GHz band, recording a class X1.5 flare event. The data revealed radiation intensity and polarization characteristics in this frequency range, demonstrating the system’s ability to observe high-energy electron emissions during flares. The observed flare exhibited a maximum flux density of approximately 450 SFU at 50.5–51.5 GHz, consistent with typical optically thin negative spectra. This achievement not only fills the observation gap in the millimeter wave range, but also provides crucial data for establishing solar radiation models in this frequency band.
Figure 3:The observation results of the solar flare on December 30, 2024 are shown. And the spectral index map obtained from observations at 35-54 GHz shows a significant evolution of soft-hard-harder during the flares.
Spectral indices derived from the flare data showed a “soft-hard-harder” pattern during the gradual, peak, and decay phases of the explosion. This evolution suggests an accumulation of higher energy electrons in the flare loops, a common feature in solar flares. The consistency between the data from the new system and those from other instruments (such as the Chashan Broadband Solar millimeter spectrometer, CBSmm)Shang et al. 2022) confirms the reliability of the calibration scheme and lays the foundation for future detailed calibrations and data analysis.
Based on a recently published article: Xu, Z. Zhao, Q. Liu, Q. Li, Z. Wu, G. Lu, Y. Su, Y. Chen and F. Yan, A 50–55 GHz millimeter-wave radiometer spectrometer for solar flare detection, ApJS 279(1), 29 (2025), DOI: 10.3847/1538-4365/ade0b5
References:
Fleishman, G.D., Nita, G.M., Kuroda, N., et al. 2018, 301, ApJ, 859, 17
Chang, B. Wang, G. Lu, et al., 2024, ApJS, 272, 21
Shang, K. Xu, Y. Liu et al., 2022,ApJS, 258.25
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