A direct evidence for kilometric continuum generation through mode conversion

The origin of the Kilometric Continuum (KC) is usually attributed to the linear mode conversion window theory, yet direct evidence has been lacking. Here we present an event where electrostatic waves, Z-mode, and KC were observed simultaneously near the magnetic equator by the Van Allen Probes. We identify the radio window (the region for mode conversion taking place) at L = 4.059 by solving the fully-thermal dispersion relation. Ray tracing simulations show that the backward-propagating electrostatic mode can smoothly transition to Z-mode. Then, Z-mode can convert to KC when its direction shifts to parallel or anti-parallel propagation at the radio window, which aligns with observations. This study provides direct evidence that supports the linear mode conversion theory as an effective mechanism for KC generation.