The First Flare Observation with a New Solar Microwave Spectrometer Working in 35–40 GHz

The microwave spectrum contains valuable information about solar flares. Yet, the present spectral coverage is far from complete and broad data gaps exist above 20 GHz. Here we report the first flare (the X2.2 flare on 2022 April 20) observation of the newly built Chashan Broadband Solar millimeter spectrometer (CBS) working from 35 to 40 GHz. We use the CBS data of the new Moon to calibrate, and the simultaneous NoRP data at 35 GHz to cross-calibrate. The impulsive stage has three local peaks with the middle one being the strongest and the maximum flux density reaches ∼9300 solar flux unit at 35–40 GHz. The spectral index of the CBS data (αC) for the major peak is mostly positive, indicating the gyrosynchrotron turnover frequency (νt) goes beyond 35–40 GHz. The frequency νt is smaller yet still larger than 20 GHz for most of the other two peaks according to the spectral fittings with NoRP-CBS data. The CBS index manifests the general rapid-hardening-then-softening trend for each peak and gradual hardening during the decay stage, agreeing with the fitted optically thin spectral index (αtn) for νt < 35 GHz. In addition, the obtained turnover frequency (νt) during the whole impulsive stage correlates well with the corresponding intensity (It) according to a power-law dependence (${I}_{t}\propto {\nu }_{t}^{4.8}$) with a correlation coefficient of 0.82. This agrees with earlier studies on flares with low turnover frequency (≤17 GHz), yet it is being reported for the first time for events with a high turnover frequency (≥20 GHz).