Tracing shock waves: type II radio emission on 27th of September 2001

Abstract

This study focus on atypical Type II radio bursts observed in conjunction with three simultaneous coronal mass ejections (CMEs) on September 27, 2001. These CMEs originated from a single active region (AR) and were linked to relatively weak solar flares. Analysis of the CME sequences revealed distinct periods of interplanetary (IP) Type II radio emissions, characterized by pronounced increases in intensity. The first radio enhancement, lasting 20 minutes, exhibited very low density and frequency (1.65–1.5 MHz) at a height range of (7.8–8.2) solar radii (). Subsequently, the second radio signature persisted for 40 minutes with a frequency range of (900–700) kHz and a height range of (10.9–12.6) . The third radio signature spanned 1 hour and 20 minutes, featuring a frequency range of (660–390) kHz and a height range of (13.2–18.6) . The fourth enhancement extended over 3 hours, ranging from (550–250) kHz in frequency and (14.6–25.0) in height. We concluded that the initial low-density radio signature resulted from a shock wave generated by reconnection of magnetic field lines, without an intense flare or extreme ultraviolet imaging telescope (EIT) wave. This shock wave then accelerated subsequent CMEs. Alternatively, the radio burst could have formed in the wake of the initial slow CME, creating a low-density environment. The second radio enhancement coincided with the accelerated propagation of CME1’s core and was attributed to enhanced radio emission resulting from the Type II shock encountering filament material. The third radio enhancement aligned with the concept of a CME bow shock, indicating that the shock was positioned at the leading front of the CME. This enhancement occurred when the shock met remnant material from earlier CMEs, yet the shock continued propagating at a constant speed. The fourth enhancement progressed to higher frequencies due to the merging of CME1’s core with CME2, propagating along CME3’s path. This comprehensive analysis provides valuable insights into the complex dynamics and interactions associated with these unique Type II radio bursts and their correlation with coronal mass ejections.