Investigation of the Indirect Neupert Effect in Five Solar Flares

The Neupert effect is characterized by the strong correlation between the time derivative of the soft X-ray (SXR) flux and the hard X-ray (HXR) or microwave flux in solar flares. Based on the standard flare model, the nonthermal electrons responsible for SXR, HXR, as well as microwave emissions and those producing solar meter-wave type III radio bursts originate simultaneously from magnetic reconnection with the opposite directions, i.e., downward and upward. Therefore, it is reasonable that the upward nonthermal electrons exhibit the same characteristics as the downward electrons. Observationally, the meter-wave type III radio bursts should be correlated with the SXR derivative, which is the indirect Neupert effect. In this paper, we analyzed the correlations between the solar meter-wave type III radio bursts observed by Chashan broadband solar radio spectrometer at meter wavelengths (CBSm) and the SXR derivative at 1-8 Å measured by Geostationary Operational Environmental Satellite (GOES), the HXR at 15-25 or 25-50 keV measured by Spectrometer/Telescope for Imaging X-rays (STIX), and the microwave measured by Nobeyama Radio Polarimeters (NoRP) in five solar flares. Firstly, we find a high correlation between the meter-wave type III radio bursts and the SXR derivative, indicating that these five flares follow the indirect Neupert effect. Secondly, we find a high correlation between the meter-wave type III radio bursts and both HXR and microwave emissions, which is consistent with the magnetic reconnection to accelerate the bi-directional electrons.