The physics of solar spectral imaging observations in dm-cm wavelengths and space weather by Tan et al.

Recent years, several new generation solar radio telescopes operating in the centimeter decimeter (dm-cm) wavelengths have emerged in the world, including the Mingantu Spectral Radioheliograph (MUSER, 0.4-15GHz) (Yan et al. 2021), the Expanded Owens Valley Solar Array (EOVSA, 1-18GHz) (Gary et al. 2018), and the Siberian Radio Heliograph (SRH, 3-24GHz) (Altyntsev et al. 2020). Due to the fact that the solar radio emission in dm-cm wavelengths mainly originates from the solar burst source region and the primary propagation region of released energy and accelerated high-energy particles, the observations of these telescopes will provide unprecedented opportunities for us to explore and study major issues in solar physics and space weather, such as the origin of solar bursts, initial energy release, and perturbations to the solar-terrestrial environments.

  1. Main physical problems from spectral-images in dm-cm wavelengths
  • Direct magnetic field diagnostics in the chromosphere and corona. By utilizing dm-cm wave spectral imaging observations, combined with our recently compiled magnetic field diagnostic functions (Tan, RAA, 2022), a complete magnetic map for the chromosphere and corona over the whole solar disk, including the quiet region, active regions, and flaring source regions, will be obtained.
  • The origin of microwave quasi-periodic pulsations (QPP). Microwave QPP is widely occurred in all phases of solar flares, but their origins and formation mechanisms are still controversial. Where are their source regions? What is the relationship between their sources and magnetic fields?
  • The origin of microwave zebra patterns (ZPs). Microwave ZPs are the most complex, intriguing and very interesting spectral phenomena in solar radio astronomy. Where did the microwave ZPs originate? What physical processes are hidden behind them?
  • Particle acceleration and propagation characteristics related to small-scale microwave bursts (SMBs), including the microwave spikes, dots, and narrowband type III bursts. They are always occurring in great number and in groups.
  • The origin of microwave fiber bursts. The peculiar feature of microwave fiber bursts is that their medium frequency drifting rate is much lower than the radio type III bursts excited by high-energy particle beams, and much higher than the radio type II bursts excited by CME and shock waves. How are these fiber bursts excited?
  • The origin of microwave type III burst groups. As is well known, radio type III bursts are typically formed by high-energy electron beams moving along an open magnetic field. They typically occur in the low frequency range below 2.0GHz, which is formed in the higher corona and interstellar space. However, observations showed that radio type III bursts often occur in microwave bands above 2.0GHz, and they always appear in groups. It is difficult to imagine the existence of this open field in the microwave source region that generates such a high frequency. So, how do these microwave type III burst groups occur? What are the differences and connections between these microwave type III bursts and low-frequency radio type III bursts?
  • The origin of some unique and complex radio spectrum structures and the physical processes behind them. Such as the hand-shaped structure, α-shaped pattern structure, and imbricate ZP structures shown in the following figure. What physical processes do these complex spectral structures reflect?

Figure 1, Some unique and complex microwave spectrum structures: hand-shaped structure (left), α-shaped pattern structure (middle), and imbricate ZP structures (right).

The answers to the above questions not only help us understand the physical nature of solar eruptions, but also help us reveal the basic laws of plasma physics.

  1. Possible application on space weather

The solar dm-cm wave spectral imaging observations may help us to answer the aforementioned physical problems, and this will provide important theoretical and observational inputs for space weather prediction. These application mainly focus on the following three aspects:

  • Detecting and confirming radio precursors of solar flares.
  • Detecting and confirming radio precursors of coronal mass ejections.
  • The radio signals of solar energetic particle flows.
  1. Conclusion

With the advent of MUSER, EOVSA, SRH and even the future FASR, we will obtain spectral imaging observations with very broadband dm-cm wavelengths (from 0.4 GHz to 24 GHz). By using these unprecedented high-quality observational data, we will get a new understanding of a series of frontier problems in solar physics, and apply these new insights to the prediction of catastrophic space weather events, while also promoting the development of plasma physics.

*Based on the recent article:

Tan B.L., Yan Y.H., Huang J., Zhang Y., Tan C.M., Zhu X.S. The physics of solar spectral imaging observations in dm-cm wavelengths and the application on space weather, 2023, Adv. Space Res., 72:5563-5576. DOI: 10.1016/j.asr.2022.11.049