Microwave Bright Points with Multiple Periodic Oscillations in the Solar Polar Coronal Hole during Solar Cycle Minimum

Abstract During solar cycle minimum, polar coronal holes show a prominent radio brightening cap. The analysis of polar microwave enhanced radiation in polar coronal holes is helpful for understanding the magnetic field characteristics and the origin of the solar wind. Using the Koshix synthesis method on Nobeyama Radioheliograph data, we identified microwave bright points (BPs) superposed on the microwave brightening cap of the northern polar coronal hole. These microwave BPs manifested intermittently at fixed locations, with their radiation intensities displaying multiperiodic oscillations (20 s, 50 s, and 2.5 minutes). Multiwavelength analysis revealed that nearly all BPs were adjacent to or coincide with 171 Å open structures, indicating a strong correlation between microwave BPs and solar wind, propagating along the open field lines. At lower layers, BPs were associated with bright ribbons in the chromosphere and local enhanced magnetic structures in photosphere. The 20 s lifetime of microwave BP enhancement corresponds to their shortest oscillation period, which, in the temporal characteristics of Alfvén waves, implies a localized heating or a small-scale magnetic reconnection process modulated by Alfvén waves at the root of open magnetic field lines. The microwave 2.5 minute and 171 Å 5 minute oscillations likely stem from chromospheric and photospheric oscillation leakage/propagation, respectively. And longer-period (12 minute) oscillations in 171 Å open structures may link to large-scale coronal hole evolution. The multiperiod oscillatory processes in the polar coronal hole imply complex plasma dynamics inside, which contributes to our understanding of the origin and propagation of solar wind along the open structures in the coronal hole.