Type II Interplanetary Radio Bursts
Conventional ground observations of solar radio bursts are usually limited to a range above 10 MHz in frequency or below some tens of meters in wavelength because of the shielding effect of the ionosphere, and their source origin is thought to be located less than 0.05 AU or 10 solar radio in the solar corona. Recent spacecraft measurements in situ allowed us to cover low-frequency radio bursts down to frequencies as low as 10 kHz, including deca-, hecto-, and kilo-meters in wavelength. Their source positions are extended to interplanetary space not only in the solar corona, thus being referred to as interplanetary radio bursts (IRB). Based upon in situ observations from the HELIOS 1 and 2 Solar Probes, there has been found a new type of interplanetary radio bursts with electron plasma oscillations (EPO) that are indicated as type II in close association with interplanetary shocks, superthermal electrons, and low/medium energy protons, and correspond to ground observations of the type II solar radio burst generated in the coronal region. While these EPO-IRB events are produced at the front of a large scale of interplanetary hydromagnetic shocks, there has been found another kind of EPO-IRB events associated with a small sclae of sporadic electrostatic shocks that are characterized by superthermal electrons in a way similar to type II IRB in a highly turbulent downstream region behind a hydromagnetic shock near a HELIOS perihelion.