Spatiotemporal Characteristics of Very-long-periodic Pulsations in Solar Meterwave Bursts: Implications for Their Origins

We traced the origin of very-long-periodic pulsations (VLPs) in type-I burst chains on 2024 February 14. Seven successive and repetitive pulsation structures appeared in radio dynamic spectra in the metric waveband, which were simultaneously measured by CBSm, DART, and MUSER-L. A quasi-period at about 160 −6+11 s, determined by the fast Fourier transform, was detected in the frequency range of about 210–280 MHz. Imaging observations from DART and Solar Dynamics Observatory (SDO) reveal that the type-I burst chains occur above two groups of sunspot umbrae connected by coronal loops. A quasi-period of approximately 170 s was also identified in the sunspot umbrae and coronal loops. The burst chains exhibit strong circular polarization and high brightness temperature, and they show spatiotemporal correlation with emerging magnetic flux. The number densities at the loop top and double footpoints can produce radio emission and generate type-I burst chains in the frequency range of 210–280 MHz. Our observations support the scenario that plasma emission serves as the primary generation mechanism of type-I bursts, with VLPs most likely being modulated by the slow magnetoacoustic waves originating from sunspot umbrae. The observed frequency drift of burst chains may reflect the density attenuation along coronal loops.