A new fine spectral structure of the solar radio explosions with periodic stripes of accounts observed by CBSM of CMP-II by Li et al. – European astronomer radio community

Fine substantial structures, such as zebras, drifting bursts, peaks and fibers during the last decades of observations of solar radio explosions have been informed. These fine structures are frequently associated with broadband continuum, and type IV bursts, both in metric and decametric wavelengths. They carry important information about the radiation process and coronal conditions.

Previous studies have reported the so -called narrow band stripes (for example, Chernov 2008), characterized by a group of parallel handling stripes with a relatively narrow spectral band (that is, $ \ Delta \ omega/ \ omega <0.1 $). The stripes often appear in the chain, separated by $ \ sim $ <1 s on average and each duration of several dozens of seconds. Usually, they have a negative drift, often accompanied by radiation depression (or absorptions) on the low frequency side. Several mechanisms have been proposed to explain these characteristics, however, the origin of their periodicity and coherence is still difficult to achieve.

The solar bandwood radio spectrometer of Chashan in the meter wavelength (CBSM), an instrument of the Meridian II project phase (CMP-II), has a high temporary frequency resolution (76.29 kHz, 0.84 ms) and sensitivity, which makes it a leading instrument to detect thin structures on the solar radio (spectrum (spectrum (spectrum (spectrum (spectrum (spectrum (spectrum (spectrum (spectrum (spectrum (spectrum (spectrum (spectrum (spectrum (spectrum (spectrum (spectrum (spectrum (spectrum (spectrumChang et al. 2024). With the CBSM data, we identify a new fine spectral structure of the solar burst type IV. The structure is characterized by improvements with accounts (or as a pearl), along a group of periodic stripes of narrow bands, called “stripes with accounts” (see figures 1 and 2). The recurrence of the stripes is $ <1 $ S (occasionally reaching 8S), often drifting high at low frequencies and accompanied by absorptions, with final stripes that appear at the end of the previous ones. Accounts improvements have a periodicity of $ \ Sim $ 0.1s.

For the periodicity and coherence of these stripes, we proposed the following generation mechanism on the basis of double plasma resonance (DPR), and explain the improvements of accounts in terms of modulation by magnetohydrodynamic waves (MHD) of low frequency.

DPR instability can excite superior hybrid waves (UH), in the presence of energy electrons with unstable speed distribution functions, such as beam ring, horseshoe or loss cone. UH waves have the most efficient growth at the frequency close to $ \ omega_ {uh} $ when $ \ omega_ {uh} \ approx s \ omega_ {ce} $, in exaggerated plasmas with $ \ omega_ {pe}/\ omega_ {ce} \ gg 1 $ Li et al. 2019), Where $ s $ is a positive integer. Temporary variations in plasma density ($ n_0 $) and/or the strength of the magnetic field ($ b_0 $) can modulate the values ​​of $ \ omega_ {pe}/\ omega_ {ce} $. Once $ \ omega_ {pe}/\ omega_ {ce} $ varies according to a unit, that is, from $ a $ s+$ or $ s-1 $, we have successive harmonics excitations of ($ s \ pm 1 $) $ \ omega_ {ce} $. These recurrent harmonics (together with the variations of $ N_0 and $ b_0 $) explain the chained stripes, as schemes in Figure 3. The absorption characteristic corresponds to the negative growth of the UH waves.

We attribute the improvements with accounts or pearls to periodic modulations by low frequency MHD waves with a frequency of $ 10 Hz. These modulations vary the magnetic field in the background and/or the plasma density periodically and modulate the growth rate of the UH mode to the same rhythm, which leads to the characteristic of accounts.

We demand a numerical model elaborated to evaluate the growth rates of DPR modes and radiation modes, considering the temporal variation or non -homogeneous distribution of plasma density or the intensity of the magnetic field, as well as the modulations of the low frequency MHD waves, to achieve a better understanding of the spectral structure.

The study informs the possibility of identifying new fine spectral structures with high sensitivity and spectral-temporal resolution instruments, even after persistent observation for several decades. In the future, we will continue looking for novel spectral structures with CBSM.

Figure 1. (a) and (b) CBSM dynamic spectra containing fine rich structures. (c)-(e) zoom views of the selected regions (black frames in (a) and (b)).

Figure 2. (a) An example of stripes with a general undulating morphology in the event of May 8, 2024, with an account structure emerging in the middle portion. Black lines are accessories of striped slopes. (b) Intensity profile at 443 MHz (white discontinuous line (a)). The box (panel (b1)) shows the $ \ Delta $ t interval between adjacent peaks.

Figure 3. Schematic diagram for the generation of striped chains. (a) variations of the frequency of the UH mode as a function of $ \ omega_ {pe}/\ omega_ {ce} $