Research Progress of the Heliospheric Radio Emissions

The heliospheric radio emissions are the strongest radio emissions phenomenon in the solar system, with a radiation power of at least ${10}^{13}$ W, which can provide important physical information of high energy electron beam and magnetic plasma structure near the heliospheric boundary. Since the first detection by the Voyager spacecraft in 1983, those radio emissions have widely and continuously attracted much attention from researchers. There are generally two types of the heliospheric radio emissions: instantaneous or drifting emission with relatively high frequency, and continuous emission or non-drifting emission with relatively low frequency. Usually, both types of emissions start from about 2 kHz. For the drifting emission, it has the characteristic of drifting toward high frequency, the drifting rate is about 1–3 kHz/yr, the frequency range is 1.8–3.6 kHz, and the duration is about 100–300 days. For the non-drifting emission, it has no obvious frequency drift, the frequency range is 1.8–2.6 kHz, and the duration is about 3 yr. It is generally believed that the heliospheric radio emissions are related to shock. In this paper, the possible source region of the radio emissions, the emission mechanisms, and the source of shock related to the emissions are introduced. Furthermore, the existing scientific problems and the future perspectives on the research of heliospheric radio emissions are discussed.