Progress has been made in the study of the radiation mechanism of solar radio bursts at the Purple Mountain Observatory

The solar radio burst phenomenon is the most direct physical manifestation of the solar high-energy electron activity, and it is also the most important observation method for diagnosing the physical state of the solar activity area and the acceleration and propagation process of high-energy electrons. Determining and establishing the correct radiation mechanism for bursts is the physical premise and theoretical basis for accurate diagnosis. Since the end of the 1950s, the long-term controversy over the two mechanisms of “plasma radiation” and “electron swirling vein radiation” has been the main problem plaguing the research on the radiation and diagnostic research of solar radio bursts.

Recently, the “Sun and Solar System Plasma” research team at the Purple Mountain Observatory of the Chinese Academy of Sciences has made important progress in overcoming this problem. Using the full-dynamic theory PIC numerical simulation code technology (ACRONYM), the research team deeply studied the excitation efficiency and corresponding nonlinear saturation level of high-energy electron beams with low-energy cut-off power law spectra under different background magnetization plasma parameters. The results of the study show that high-energy electron beams with low-energy cut-off power law energy spectra can effectively excite Langmuir waves and electron cyclotropic vascular instabilities at the same time, which are the basis for generating plasma radiation and electron cyclocyclic vascular radiation, respectively. However, in the strongly magnetized plasma in which the electron gyrotronic radiation mechanism plays a leading role, the saturation level of the radiated electromagnetic wave is much higher than the saturation level in the weakly magnetized plasma in which the plasma radiation mechanism plays a leading role (see Figure 1). This result means that the electron gyrotron pulsation radiation mechanism directly amplified based on the linear instability of the plasma is indeed an effective coherent radiation mechanism that can lead to the phenomenon of solar radio bursts, while the plasma radiation mechanism based on the nonlinear coupling conversion of Langmuir waves is difficult to produce effective radio burst radiation. The preliminary analysis of the reasons for this may be that the nonlinear coupling conversion efficiency of Langmuir’s wave-to-radiated electromagnetic waves is too low, so that most of its main energy is dissipated in the plasma heating process. The results of this study will help us to further clarify the long-standing controversy in the study of the radiation mechanism of solar radio bursts.

Figure 1: Wave vector-frequency energy spectrogram of high-energy electron beams excited by transverse electromagnetic waves in different magnetized plasmas (different columns) of low-energy cut-off power law energy spectra. Different rows show the excitation of lateral electromagnetic waves at different times. In each panel, the ω>0 (ω<0) region represents the energy spectrum of the right-handed (left-handed) electromagnetic wave. In the right-handed region of ω>0, from bottom to top, the dispersion relationship curves of whistle sound waves (gray dot line), Z mode wave (gray dotted line), and X mode wave (gray dotted line), which are superimposed at the magnetized cold plasma limit, are plotted. The dispersion relationship curve of the O-modulus (gray dotted-dot line) is superimposed and plotted in the left-handed region of ω<0. Note: All panels use the same colorbar.

The research was supported by the National Natural Science Foundation of China, and the results have recently been published in The Astrophysical Journal, an authoritative journal in the field of international astronomy. (Source: Purple Mountain Observatory, Chinese Academy of Sciences)

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