Subjects: Astronomy submitted time 2023-07-14 Cooperative journals: 《天文学报》
Abstract： 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 to 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.
Subjects: Astronomy submitted time 2023-07-14 Cooperative journals: 《天文学报》
Abstract： Radio bursts are ubiquitous in the cosmic plasma. Solar radio emission mainly comes from the outer atmosphere of the sun. It is an induced radiation phenomenon generated by the interaction between energetic electrons and solar atmospheric plasma. Different dynamic spectra of solar radio bursts (SRBs) contain physical information of the plasma structure and state in the radiation source region. Therefore, the radiative mechanism of radio bursts has always been the object of research. There are two kinds of coherent radiation mechanisms related to solar radio bursts: one is the plasma radiation mechanism based on electron Langmuir frequency; the other is the electron cyclotron maser (ECM) radiation mechanism based on the electron cyclotron frequency. Although these two radiation mechanisms were proposed almost at the same time, based on the understanding of the coronal environment and the ECM mechanism at that time, the ECM radiation mechanism did encounter some difficulties in explaining SRBs. Until 1979, Wu $\&$ Lee introduced the relativistic effect and used the ECM radiation to explain the earth's Auroral Kilometric Radiation (AKR). Since then, the ECM emission has attracted wide attention. Considering some difficulties in applying the ECM emission mechanism to SRBs, we proposed a series of modified models in recent years. Firstly, the cutoff in the energy spectrum of the power-law electrons can effectively drive the ECM instability without relying on the anisotropic distribution of electron velocity. Secondly, considering the influence of Alfv\'{e}n wave perturbations which are prevalent in space and celestial plasmas, a self-consistent ECM emission mechanism excited by energetic electron beams is developed. On this basis, this paper summarizes the application of the ECM emission mechanism in traditional SRB phenomena from type I to V and microwave SRBs in recent years.
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