Type II Interplanetary Bursts

The purpose of this work was to find out why not all type II bursts, usually observed in the meter wavelength range, switch to the longer wavelength range (decameter, hectometer, and kilometer). Most of the statistical work only partially addressed this issue. Many authors assumed that all shock waves are of the piston type and only high-frequency type II bursts can be associated with explosive shock waves that fade before they reach interplanetary space. At the same time, the authors did not take into account the mechanism of radio emission generation. In this paper, a number of type II bursts are considered and it is established that the conditions for generation are preserved if the shock front has no obstacles to propagation at the boundary of the transition from frequencies around 20 MHz to a range less than 14–16 MHz, and the radiation can already be detected by spectrometers of the WIND/WAVES and STEREO spacecraft. Bursts can be generated in separate sections of the shock front, where certain conditions are met: exceeding the critical Mach number and the perpendicular of the shock front. Such conditions may also explain the patchy structure of type II radiation bands. These conclusions are in good agreement with the conditions for the development of the Buneman instability of particles in the front of a collisionless shock wave. It is also important to take into account the interaction of the shock front with the coronal mass ejection.