Solar eruptive events, such as flares and coronal mass ejections (CME), can generate shock waves that spread through the solar crown. These shock waves accelerate electrons, which leads to Langmuir waves that then becomes the emission of type II solar bursts, which are radio signatures of these clashes. In solar spectrograms, type II explosions are generally characterized by fundamental and harmonic lanes that move from high at low frequencies within 0.1-0.4 MHz/sy durations of up to 30 minutes (up to 30 minutes (up to 30 minutes (up to 30 minutes (Alissandrakis and Gary 2021).
In addition to their typical appearance, type II bursts exhibit a variety of spectral peculiarities, which include hairs of harsh, spectral breakage and blows (Koval et al. 2021, 2023). Another common phenomenon is the band’s division, where the fundamental and harmonic lanes are divided into higher and lower frequency bands (Vršnak et al. 2002). There are two main interpretations for band division, which involve emissions from the upstream and downstream regions of a crash or different parts of the crash (Chrysaphi et al. 2018; Bhunia et al. 2023). The current study focuses on a recently identified feature called “spectral split”, which differs from the band division. The spectral split is manifested as a real branch of a type II gusts as observed in an event on February 14, 2011.
Data analysis
We analyze Dynamic spectra of the Nançay (NDA; France) and the URAN-2 radio (Ukraine) to study the solar outbreak type II 2011-02-14 in detail (see figure 1). The event spectrogram includes the structured gust of type II, titled as type II, which shows the band division with low frequency band (LFB) and superior frequency band (UFB).
Figure 2 presents the dynamic spectrum recorded by the URAN-2 radio, where we observe the LFB of type II that is subjected to spectral split around 13:09:40 ut. In particular, the UFB of the explosion was not affected by this split, maintaining its typical band division behavior. In addition to type II with spectral split, the URAN-2 data revealed a secondary explosion of type II, labeled as “HB type II”, characterized by a spike structure and band division.
Figure 1. The dynamic spectrum of type II burst on February 14, 2011. The combined spectrum was obtained from the records of the URAN-2 antenna matrices (10-33 MHz) and NDA (33-70 MHz). Type II explosion and several groups of type III bursts are denoted in the spectrogram as type II and type III, respectively. The lower panel shows the solar X -ray emission measured by GOS 15 that day. The start of class C9.4, the end, the end and the maximum hours are shown as orange lines and discontinuous vertical cyan, respectively. The common time axis is in range 12: 40-13: 15 ut.
Figure 2. The Uran-2 dynamic spectrum with the low frequency band of the type II gust, type II (LFB), which shows the spectral split. The discontinuous purple lines cover the LFB before and after the split point (CP), which was at the time of 13: 09: 40.5 ut and a frequency of 28.12 MHz. The LFB was divided into a lower lane (LL) and a Upper lane (UL) as a result of spectral split. The UL ascends and descends in frequency, which makes a characteristic of spectral protuberance. Type II explosion with hairs of harsh has a band division with lower and upper frequency bands, which are indicated as HB type II (LFB) and HB type II (UFB).
Interpretation
We propose an initial interpretation of the recently identified spectral split characteristic in bursts of type II. We assume that electrons responsible for type II emission are accelerated by the shock drift acceleration mechanism (Holman & Pesses 1983). In this model, the efficiency of shock in acceleration electrons depends on angle ψ1 between the magnetic field and the normal clash. The maximum electron acceleration is at very high angles (≈90 °) and depends on the parameters of the shock wave. Therefore, it is assumed that the sources of an emission of type II gusts are close to the regions in a shock front in which the electrons accelerate more efficiently. This implies that these regions are not everywhere on the shock front, but only where ψ1 ≈ 90 °, excluding ψ1 ≃ 90 °.
We assume that two radio sources correspond to LFB and UFB of type II. In addition, it was assumed that the Radio LFB source was a double source composed of two components (see Figure 3a). Then, at approximately 13:09:40 UT, type II (LFB) is divided into two lanes (LL and UL), which is the spectral split (Figure 2). We attribute spectral split to changes in the orientation of the magnetic field before the propagation shock front. When this orientation varies, the most effective electrons acceleration region (type II emission sources (LFB)) changes along the shock front or even diverges, as shown in the scenario of Figure 3.
In our case, the shock front is probably propagated in a direction that is very different from the radio direction in the gravitational stratified solar atmosphere. Therefore, in type II (LFB) during the period of time of 13: 07: 30-13: 09: 40 ut, the condition (ψ1 ≈ 90 °) For the most effective acceleration it is fulfilled only in the regions near the top of the shock front (Figure 3a), where the double source (red circles in Figure 3) is located. The electrons are accelerated. These supratermic electrons gave rise to the subsequent radio emission through the plasma emission mechanism. Since the double source is quite compact, it is seen that type II (LFB) is not scheduled in the dynamic spectrum.
Later, after 13:09:40 UT, the magnetic field lines in front of the shock front gradually changed in such a way that around the top of the shock front, ψ1 ≃ 90 ° (see Figure 3b). Consequently, due to a decrease in electron acceleration efficiency, the radio emission of this location disappeared. Simultaneously, two well -separated regions with high ψ1 It appeared along the shock front. The first radio source was located at a higher altitude in the solar atmosphere and the second source was at a lower altitude. Before 13:09:40 UT, they were the components of the double source. Due to the difference in plasma densities between higher and lower sources, the LL and the UL of type II (LFB) were generated.
The proposed interpretation emphasizes that variations in magnetic field configurations along the shock front lead to the migration of radio sources, which is reflected in dynamic spectral characteristics such as spectral split. This phenomenon can help explain the intermittent forms commonly observed in type II gusts. Future studies that use high-resolution radio telescopes such as URAN-2, UTR-2, GURT, NENUFAR and LOFAR are essential to deepen our understanding of this process.
Figure 3. Sketch demonstrating the possible scenario of the appearance of the spectral split in type II (LFB) step by step: a) phase before the split (~ 13:09:30 ut). b) Phase after splitting (~ 13: 10: 00 ut).
Based on a recently published article: A. KOVAL, M. Karlický, A. Brazhenko, A. Stanislavsky, A. Frantsuzenko, M. Vandas, A. Konovalenko, M. Bárt The solar type II radio donkeys: observations and interpretation, Astronomy and astrophysics, 689A345 (2024) DOI: 10.1051/0004-6361/202451010
References
Alissandrakis, CE, and Gary, from, 2021, front. Astron Space Sci., 7, 77
Bhunia, S., Carley, Ep, Oberoi, D. and Gallagher, PT, 2023, A&A, 670, A169
Chrysaphi, N., Kontar, EP, Holman, GD and Temmer, M., 2018, APJ, 868, 79
Holman, gd and pesses, me, 1983, APJ, 267, 837
KOVAL, A., Karlický, M., Stanislavsky, A. et al., 2021, APJ, 923, 2
KOVAL, A., Stanislavsky, A., Karlický, M. et al., 2023, APJ, 952, 1
Vršnak, B., Magdalenić, J., Aurass, H., and Mann, G., 2002, A&A, 396, 673
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