Recently, the scientific team of the Kwafu-1 satellite (ASO-S) utilized observations from the White Light Solar Telescope (WST) on the Kwafu-1 satellite to reveal that solar white light flares are not uncommon and capture continuous spectral radiation on the flare ring. The relevant research results have been published in Solar Physics and The Astrophysical Journal Letters.

WST operates in the 360 nm wavelength range, marking the first time that continuous spatial observations have been conducted in this distinctive wavelength range. The 360 nm radiation from the sun is mainly Balmer continuous spectrum radiation from hydrogen atoms. This band is quite important for studying solar white light flares. White light flares refer to a type of flare that exhibits enhanced continuous spectral radiation in visible light. The first solar flare observed in human history, the Carrington flare of 1859, is a typical white light flare. White light flares can be mainly divided into two categories - Type I white light flares exhibit an increase in Balmer continuous spectral radiation (Balmer jump) and an enhancement of Balmer spectral lines. There is a good correlation between white light radiation and hard X-ray radiation; Class II white light flares do not have these features. White light flares are usually large-scale flares (such as X-level and M-level) that release strong energy and have a significant impact on space weather. In addition, stellar flares are mostly white light flares and have similarities with solar white light flares. Therefore, studying white light flares has scientific significance for space weather forecasting and stellar flare research.

From 1859 until the launch of the ASO-S satellite, only about 300 solar white light flare events were reported, which is relatively rare compared to tens of thousands of flares occurring during a solar activity cycle (an average of 11 years). However, WST's all-weather observations in space for just over a year indicate that solar white light flares are not uncommon. The ASO-S scientific team analyzed 205 M1.0 and above flares that occurred from October 2022 to May 2023, and identified 49 360 nm white light flares, with a white light flare occurrence rate of 24%; At the same time, some basic parameters of white light flares are provided, such as white light duration, white light brightening area, maximum and average pixel amplification, etc. This achievement was recommended by Solar Physics editors. With the arrival of solar activity peak years, more and more white light flares are being observed. As of the end of December 2023, more than 120 cases of white light flares have been observed by WST, providing samples for exploring the physical nature of white light flares.

The white continuous spectral radiation of a flare is mainly observed at the foot of the flare ring. For a small number of edge flare events, white continuous spectral radiation can be detected on the flare ring, but is usually weak. The origin height and radiation mechanism of white continuous spectrum radiation that appear at different positions are different. In over a hundred white light flare events observed by WST, most of the 360 nm radiation appeared at the focal point of the flare, such as the X2.1 flare on March 3, 2023; On August 7, 2023, WST captured white continuous spectral radiation on an edge X1.5 flare ring - Balmer continuous spectral radiation on the flare ring has not been previously reported. These radiations are visible in the HMI/SDO 617.3nm Paschen continuous spectrum, while the CHASE continuous spectrum exhibits brightening. The white light radiation at the foot of a flare ring is usually directly or indirectly related to non thermal electron beam heating, while the white light continuous spectrum radiation on the flare ring is related to ring cooling. If the electron density in the flare ring is less than 1012 cm-3, the white light radiation may originate from Thomson scattering processes.

The research work was supported by the Chinese Academy of Sciences strategic leading science and technology project (Class B), the national key research and development plan and the National Natural Science Foundation of China.

Paper link:1、2

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