Photo voltaic eruptive occasions, comparable to flares and coronal mass ejections (CMEs), can generate shock waves that propagate via the photo voltaic corona. These shock waves speed up electrons, resulting in Langmuir waves which can be then transformed into radio emission of sort II photo voltaic bursts, that are radio signatures of those shocks. On photo voltaic spectrograms, sort II bursts are usually characterised by elementary and harmonic lanes drifting from excessive to low frequencies inside 0.1-0.4 MHz/s and durations of as much as half-hour (Alissandrakis & Gary 2021).
Along with their typical look, sort II bursts exhibit quite a lot of spectral peculiarities, together with herringbones, spectral breaks, and bumps (Koval et al. 2021, 2023). One other frequent phenomenon is band-splitting, the place the basic and harmonic lanes break up into higher and decrease frequency bands (Vršnak et al. 2002). There are two major interpretations for band-splitting, involving emissions from both the upstream and downstream areas of a shock or distinct elements of the shock (Chrysaphi et al. 2018; Bhunia et al. 2023). The present research focuses on a newly recognized characteristic referred to as “spectral cleaving”, which differs from band-splitting. Spectral cleaving manifests as an precise branching of a sort II burst lane as noticed in an occasion on February 14, 2011.
Knowledge Evaluation
We analyzed dynamic spectra from the Nançay Decametric Array (NDA; France) and the URAN-2 (Ukraine) radio telescope to review the 2011-02-14 sort II photo voltaic burst intimately (see Determine 1). The occasion spectrogram contains the structured sort II burst, entitled as Sort II, which exhibits band-splitting with lower-frequency band (LFB) and upper-frequency band (UFB).
Determine 2 presents the dynamic spectrum recorded by the URAN-2 radio telescope, the place we noticed the LFB of the Sort II present process spectral cleaving at round 13:09:40 UT. Notably, the UFB of the burst remained unaffected by this cleaving, sustaining its typical band-splitting conduct. Along with the Sort II with spectral cleaving, the URAN-2 information revealed a secondary sort II burst, labeled “HB Sort II”, characterised by a herringbone construction and band-splitting.
Determine 1. Dynamic spectrum of the sort II burst on 2011 February 14. The mixed spectrum was obtained from data of the URAN-2 (10-33 MHz) and NDA (33-70 MHz) antenna arrays. The kind II burst and a number of other teams of sort III bursts are denoted within the spectrogram as Sort II and Sort III, respectively. The decrease panel exhibits the photo voltaic X-ray emission measured by GOES 15 on that day. The C9.4 class flare begin, finish, and peak occasions are proven as dashed vertical orange and cyan strains, respectively. The frequent time axis is within the vary 12:40-13:15 UT.
Determine 2. URAN-2 dynamic spectrum with the low-frequency band of the sort II burst, Sort II (LFB), exhibiting the spectral cleaving. The dashed purple strains embody the LFB earlier than and after the cleaving level (CP), which was on the time prompt of 13:09:40.5 UT and at a frequency of 28.12 MHz. The LFB divided right into a decrease lane (LL) and an higher lane (UL) on account of the spectral cleaving. The UL ascends and descends in frequency, making a spectral bump characteristic. The kind II burst with herringbones has a band-splitting with decrease and higher frequency bands, that are indicated as HB Sort II (LFB) and HB Sort II (UFB).
Interpretation
We suggest an preliminary interpretation of the newly recognized spectral cleaving characteristic in sort II bursts. We assume that the electrons chargeable for the sort II emission are accelerated by the shock-drift acceleration mechanism (Holman & Pesses 1983). On this mannequin, the shock’s effectivity at accelerating electrons is determined by the angle ψ1 between the magnetic discipline and the shock regular. The utmost electron acceleration is at very excessive angles (≈90°) and is determined by the shock wave parameters. Thus, sources of a sort II burst emission are assumed to be situated near the areas on a shock entrance through which electrons are most effectively accelerated. This means that these areas are usually not all over the place on the shock entrance, however solely the place ψ1 ≈ 90°, excluding ψ1 ≃ 90°.
We assumed that two radio sources correspond to the LFB and UFB of the Sort II. Furthermore, the LFB radio supply was assumed to be a double supply composed of two parts (see Determine 3a). Then, at about 13:09:40 UT, the Sort II (LFB) divides into two lanes (LL and UL), which is the spectral cleaving (Determine 2). We attribute the spectral cleaving to adjustments within the magnetic discipline orientation forward of the propagating shock entrance. When this orientation varies, the area of the best electron acceleration (sources of the Sort II (LFB) emission) shifts alongside the shock entrance and even diverges, as proven within the state of affairs in Determine 3.
In our case, the shock entrance most probably propagates in a route that’s extremely totally different from the radial route within the gravitationally stratified photo voltaic ambiance. Thus, within the Sort II (LFB) in the course of the time interval of 13:07:30-13:09:40 UT, the situation (ψ1 ≈ 90°) for the best acceleration is fulfilled solely within the areas near the highest of the shock entrance (Determine 3a), the place the double supply is located (purple circles in Determine 3). There electrons are accelerated. These suprathermal electrons gave rise to the following radio emission by way of the plasma emission mechanism. Because the double supply is reasonably compact, the Sort II (LFB) is seen to be not cleaved within the dynamic spectrum.
Later, after 13:09:40 UT, the magnetic discipline strains forward of the shock entrance progressively modified in such a approach that across the high of the shock entrance, ψ1 ≃ 90° (see Determine 3b). Consequently, as a consequence of a lower in electron acceleration effectivity, the radio emission from this location disappeared. Concurrently, two well-separated areas with excessive ψ1 appeared alongside the shock entrance. The primary radio supply was situated at the next altitude within the photo voltaic ambiance and the second supply was at a decrease altitude. Earlier than 13:09:40 UT, they have been the parts of the double supply. As a result of distinction in plasma densities between greater and decrease sources, the LL and UL of the Sort II (LFB) have been generated.
The proposed interpretation emphasizes that variations in magnetic discipline configurations alongside the shock entrance result in the migration of radio sources, which is mirrored in dynamic spectral options like spectral cleaving. This phenomenon could assist clarify the intermittent shapes generally noticed in sort II bursts. Future research utilizing high-resolution radio telescopes like URAN-2, UTR-2, GURT, NenuFAR, and LOFAR are important to deepen our understanding of this course of.
Determine 3. Sketch demonstrating the doable state of affairs of the incidence of the spectral cleaving within the Sort II (LFB) step-by-step: a) Part earlier than the cleaving (~ 13:09:30 UT). b) Part after the cleaving (~13:10:00 UT).
Based mostly on a lately revealed article: A. Koval, M. Karlický, A. Brazhenko, A. Stanislavsky, A. Frantsuzenko, M. Vandas, A. Konovalenko, M. Bárta, I. Bubnov, R. Miteva, and S. Yerin, Spectral cleaving in photo voltaic sort II radio bursts: Observations and interpretation, Astronomy & Astrophysics, 689, A345 (2024) DOI: 10.1051/0004-6361/202451010
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