The study of solar wind acceleration and coronal heating has been a major challenge in solar physics. The main difficulty is that the collisionless characteristic of high-temperature, thin, and fully ionized coronal plasmas lead to the heating and acceleration of the coronal plasmas to be dominated by wave particle interactions, which are the “elementary processes” of the plasma collective interaction at the kinetic scales of plasma particles. Radio observation becomes main information sources of the coronal plasmas, instead of the spectral line observation, which is a main method of inferring the physical situation and processes in the photosphere and chromosphere.
According to their dynamic spectrum, solar radio bursts (SRBs) can be divided into many different types, which can reflect the differences in atmospheric environment and physical mechanisms of the radiation source. For example, Type III SRBs are characterized by rapid frequency drift (relative frequency drift rate greater than 10% per second) and are generated mainly by the direct excitation of high-energy electron beams accompanying flare activities; Type II SRBs, driven by coronal mass ejections, exhibit relatively slow frequency drift (relative frequency drift rate less than 1% per second); The dynamic spectra of Type I SRBs related to strong magnetic fields in solar active regions show almost no frequency drift (Wild et al., 1954; Wild, 1985; Tan et al., 2019). Therefore, radio observations, especially the observations of radiation originating from small-scale emitting sources, can provide effective information on energetic electrons and their kinetic processes, as well as the ambient magnetic plasmas in the coronal plasmas.
The present paper reported a new type of small-scale SRBs, called solar solitary wave radiation (SWR), which were observed recently by the Parker Solar Probe (PSP, Bale et al. 2016; Pulupa et al., 2017) in its observation approaching the Sun. Not only does this new discovery further enrich the types of SRBs, which is a significant progress in the field of solar radio research, but also provides a new physical diagnostic approach for the study of solar coronal heating and wind acceleration, especially for the “elementary processes” of plasma collective interaction in the solar coronal plasmas.
Observation and Analysis
In the data observed by the PSP/RFS during April 1-9, 2019 at a heliocentric distance of about 1/6 AU, we found that there are a large number of small-scale weak SRBs when it passed through an open low-density magnetic channel (Fig. 1). The duration of these small-scale SRBs is on the order of minutes, their frequency ranges from ~10 MHz, drifting towards lower frequencies to ~0.1 MHz, and the intensity is commonly weak, so that they are almost undetectable near 1 AU. Especially, their dynamic spectra exhibit very strong evolution, with a relative frequency drifting rate decreasing from over 1% per second at high frequencies to less than 1% per second at low frequencies (Fig. 2), implying that their emitting sources experienced strongly dynamic processes in their propagating duration.
Figure 1. The solar wind plasma parameters observed by PSP between 2019 April 1 and 9. From top to bottom, the panels are the PSD of the radio radiation (a), the magnetic field (b) and its components (c), the solar wind velocity (d) and its components (e), the plasma density (f) and temperature (g), and the heliocentric distance of PSP in units of solar radius, RS (h). A low-density magnetic channel is clearly displayed between 08:08:34 UT on 2019 April 3 and 13:17:09 UT on April 6 (yellow region).
Based on the commonly adopted empirical models of the solar atmosphere (Mariani & Neubauer, 1990; Hu et al., 1997; Leblanc et al., 1998; Wu & Fang, 2003), we analyzed further the radiation mechanism and evolutionary dynamics of these small-scale SRBs. The results show that the radiation sources of these small-scale SRBs located in the high corona region with the heliocentric distance of 1.1 to 6.1 solar radii, which belongs to the transition region between the quasi-static solar corona and the dynamic solar wind and hence is also the typical acceleration region of the solar wind. In particular, we found that solitary kinetic Alfven waves (SKAWs) with the kinetic scale, which are easily generated in the high solar corona (Wu & Chen, 2020), can self-consistently explain the generation mechanism and strong evolution behavior of the dynamic spectrum of these small-scale SRBs, in which the electric field of SKAWs accelerates electrons to energies of a few tens of keV and the accelerated energetic electrons are trapped in the potential well of SKAWs and move together with SKAWs. It is the trapped energetic electrons that excite the radio radiation of the observed small-scale SRBs and while the frequency drift may be attributed to the propagation of SKAWs. The strongly evolutionary behavior in the dynamic spectra can be reasonably explained by the kinetic dissipation and dynamic evolution of SKAWs.
Figure 2. The weak SRBs observed by PSP during its crossing of the low-density magnetic channel. The top three rows show PSD measured by LOFAR (a), PSP (b), and WIND (c) in two intervals, 08:13–09:03 on 2019 April 3 (left) and 05:23–09:23 on 2019 April 5 (right). (d) The starting (fst; red circles) and ending (flo; blue circles) frequencies for the 385 weak SRBs and the local plasma frequency fpe (back line) along the PSP orbit. (e) The fitting curve (black curve) of maximal PSD for event 5 marked by an arrow in panel (b) through the least-squares fit, where the magenta dots are the location of maximal PSD in the dynamic spectrum. (f) The relative frequency-drift rate for the several typical bursts as marked by arrows in panel (b).
Conclusions
Chen et al 2024 propose that the small-scale SRBs observed by PSP approaching the Sun are a new type of SRBs that are generated by small-scale solitary waves with the particle kinetic scale (i.e., SKAWs), which are considerably different from Type II and III SRBs produced by large-scale shocks and fast electron beams, and hence are called the “solar solitary wave radiation (SWR)”. The discovery of SWRs with the kinetic scale emitting sources is not only a significant progress in the field of solar radio physics, but also provides a powerful new detection and diagnostic tool for further in-depth research on the coronal heating and the solar wind acceleration, especially for the “elementary process” of the plasma collective interaction at the particle kinetic scale in the solar coronal plasmas.
Based on the recent paper : https://doi.org/10.3847/1538-4357/ad0e65
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