DH Type II Radio Bursts During Solar Cycles 23 and 24: Frequency-Dependent Classification and Their Flare-CME Associations

We present the characteristics of DH type II bursts for the Solar Cycles 23 and 24. The bursts are classified according to their end frequencies into three categories: Low-Frequency Group (LFG; 20 kHz ≤ f ≤ 200 kHz), Medium-Frequency Group (MFG; 200 kHz  $< f \leq 1$  MHz), and High-Frequency Group (HFG; 1 MHz  $< f \le 16$  MHz). We find that the sources for LFG, MFG, and HFG events are homogeneously distributed over the active region belt. Our analysis shows a drastic reduction of the DH type II events during Solar Cycle 24, which includes only 35% of the total events (i.e., 179 out of 514). Despite having smaller number of DH type II events in the Solar Cycle 24, it contains a significantly higher fraction of LFG events compared to the previous cycle (32% versus 24%). However, within the LFG group, the cycle 23 exhibits significant dominance of type II bursts that extend below 50 kHz, suggesting rich population of powerful CMEs traveling beyond half of the Sun–Earth distance. The events of LFG group display strongest association with faster and wider (more than 82% events are halo) CMEs, whereas at the source location, they predominantly trigger large M/X class flares (in more than 83% cases). Our analysis also indicates that CME initial speed or flare energetics is partly related to the duration of type II burst and that survival of CME-associated shock is determined by multiple factors/parameters related to CMEs, flares, and state of coronal and interplanetary medium. The profiles relating CME heights with respect to the end frequencies of DH type II bursts suggest that for HFG and MFG categories, the location for majority of CMEs (≈ 65%–70%) is in well compliance with ten-fold Leblanc coronal density model, whereas for LFG events, a lower value of density multiplier (≈ 3) seems to be compatible.