Simulated enhancement of solar type II radio bursts during the collision of two shocks associated with coronal mass ejections

Aims. We investigate how solar type II radio bursts can be enhanced when two fast magnetosonic shocks associated with coronal mass ejections (CMEs) collide. This work was motivated by recent observations showing that the radio signature is in the form of intense continuum-like radio emission following an interplanetary type II burst, when a fast CME overtakes a slow CME. Methods. We used a 2D3V fully relativistic, electromagnetic particle-in-cell (PIC) simulation. Results. We found that during the collision between a fast shock with Alfven Mach M-A = 3 and a slow wave with Alfven Mach M-A = 0.75, the radio emission with a broad band can be enhanced with an amplitude about ten times larger than for the undisturbed type II burst. We also found that when a fast shock with Alfven Mach M-A = 3 overtakes a slow shock with Alfven Mach M-A = 1.5, the enhancement of the radio emission is about five times greater than the undisturbed type II burst.