Sort III radio bursts are probably the most highly effective and customary kind of photo voltaic radio bursts, shifting quickly from excessive to low frequencies. Along with the quick frequency drift from excessive to low frequencies, the elemental–harmonic (F-H) frequency pairs’ construction of their dynamic spectra is the opposite most essential noticed characteristic of photo voltaic kind III radio bursts. On this Letter, utilizing the radio information noticed by PSP throughout encounter phases from the first orbit to the ninth orbit (E01-E09) with 7 s (E01-E05) and the improved 3.5 s (E06-E09) temporal resolutions, we report the consultant spectral traits of F-H pairs of IP kind III radio bursts. These spectral traits can present some constraints of their supply areas and will probably be useful for us to grasp the physics of their formation.
Remark and Information Evaluation
On the idea of the frequency-drift charge, the F-H pairs of IP kind III radio bursts may be divided into two classes, that’s, the quasi-parallel F-H pairs with quasi-parallel F and H frequency-drift charge curves (see, e.g., Determine 2 (c)) and the nonparallel F-H pairs with nonparallel F and H frequency-drift charge curves, and the previous is the dominating one.
Evaluating the spectral traits of those F and H pairs, such because the frequency-drift charge, emission depth, relative bandwidth, length, and fine construction, they could be divided into six typical varieties for the case of F-H pairs with quasi-parallel frequency-drift charge (Determine 1 and Determine 2). Nonetheless, for the reason that emission mechanism of radio bursts relies upon sensitively on the native plasma parameters within the supply areas, the understanding of the physics that causes the distinction within the spectral traits requires additional detailed evaluation. Within the current research, we’re simply making an attempt to supply some preliminary interpretations for the spectral traits based mostly on the brand new electron cyclotron maser (ECM) emission mechanism that takes account of the results of the ambient Alfvén waves (Wu et al. 2012, Wu 2014, Chen et al. 2017, Chen et al. 2021) as a possible bodily mannequin of the formation of them as follows:
- F being Shorter Period than H: One of many potential causes is that based mostly on the ECM emission mechanism the F radiation is emitted at electron cyclotron frequency $f_{ce}$ of the native plasma, which probably could be very near the electron plasma frequency $f_{pe}$, and F solely emits when the $f_{ce}$ is larger than the $f_{pe}$, whereas H just isn’t restricted by this limitation. That’s, F is extra prone to be minimize off, which prevents it from persevering with to radiate.
- F being Structured and H being Clean: The X-mode emission within the F frequency band generally can’t escape as a result of emission frequency decrease than the X-mode cutoff frequency. Subsequently, the F elements of the F-H pairs are dominated by the O-mode emission, and the H elements encompass the O-mode and X-mode emissions. In consequence, based mostly on the brand new ECM emission mannequin, the F elements seem to have appreciable striae due to a significant modulation of the O-mode development charge deduced by Alfvén wave, whereas the H elements have little striae on account of little impact on the X-mode development charge by the Alfvén
- Emission Depth of H being Greater or Decrease than F: Primarily based on the brand new ECM emission mannequin, the expansion charges of the O1 (F wave within the O-mode), O2, and X2 (second H waves within the O- and X-modes) mode emissions, in several approach, not solely sensitively rely upon the native parameters of the background plasma and the speed distribution of the quick electron beams but additionally are significantly modulated by the depth of the ambient Alfvén waves. This might be most likely one of many predominant causes that the relative depth of the F and H elements varies dependently on particular person occasions as a result of the depth of the ambient Alfvén waves usually varies vastly within the photo voltaic corona and photo voltaic wind.
- H being of Wider Frequency Bandwidth than F: Typically talking, the bandwidth variation of H is twice bigger than that of F with the identical frequency change brought on by magnetic or density perturbation. The truth that F has a decrease radiation bandwidth than H could also be on account of its susceptibility to scattering losses at decrease frequencies. Alternatively, the F part with decrease frequencies under the noise stage has been subtracted as a result of the noise stage will increase because the frequency decreases (Jebaraj et al. 2023).
Determine 1. Six typical F-H pairs of IP kind III radio bursts noticed by PSP. (a) F-H pairs with comparable spectral traits between F and H (occasion 1). (b) F-H pairs with F being structured and H being easy (occasion 2). (c) F-H pairs with F being shorter length than H (occasion 3). (d) F-H pairs with the emission depth of H being larger than F (occasion 4). (e) F-H pairs with H being wider bandwidth than F (occasion 5). (f) F-H pairs with the emission depth of F being larger than H (occasion 6). Tailored from Chen et al. (2024)
Determine 2. The spectral traits of all of the six F-H pairs radio bursts. Panels (a)–(b) and (c) present the F-H frequency ratio $f_H/f_F$ and the frequency-drift charge $D_{abs}$. The maximal flux densities corresponding to every given frequency $mathrm{Flux_{tmax}}$ for occasions 1–6 are proven in panels (d)–(i), respectively. Panels (j)–(o) current the relative bandwidth as a perform of the central frequency for occasions 1–6, respectively. The black, blue, orange, purple, inexperienced, and crimson colours denote occasions 1–6 proven in Figures 1(a)–(f), respectively. The outcomes representing H are shifted to the frequency of $f_H/2$.
Conclusions
The consequence exhibits that the prevalence charge of F-H pairs rises significantly with the rise within the variety of IP kind III radio bursts detected by the PSP or the enhancement within the time decision of the RFS instrument. Specifically, the relative prevalence charge of F-H pairs will get larger (i.e., ∼50%) as the quantity of IP kind III radio bursts noticed by PSP will increase or because the time decision of the RFS is enhanced, akin to PSP throughout E02 and E07-E09.
The outcomes additionally present that within the majority of the F-H pairs, the F elements have extra evident striae construction, shorter length, and narrower bandwidth in contrast with the H elements. Furthermore, the relative intensities of the F and H elements of those F-H pairs exhibit advanced variations, together with the stronger F part, the stronger H part, in addition to the F and H elements with almost equivalent depth. The detailed and complete formation mechanisms of the F-H pairs of IP kind III radio bursts are intently associated to the native plasma circumstances, together with the ambient AW fluctuations, within the supply area and are price discussing in future work.
Primarily based on the current paper by Ling Chen, Bing Ma, Dejin Wu, Zongjun Ning, Xiaowei Zhou, and Stuart D. Bale, Spectral Traits of Elementary–Harmonic Pairs of Interplanetary Sort III Radio Bursts Noticed by PSP, ApJL, 975: L37, 2024. DOI:10.3847/2041-8213/ad89c2
References
Chen, L., Ma, B., Wu, D., et al. 2021, ApJL, 915, L22
Chen, L., Wu, D. J., Zhao, G. Q., et al. 2017, JGRA, 122, 35
Wu, C. S., Wang, C. B., Wu, D. J., et al. 2012, PhPl, 19, 082902
Wu, D. J. 2014, PhPl, 21, 064506
Jebaraj, I. C., Krasnoselskikh, V., Pulupa, M., et al. 2023, ApJL, 955, L20
