A definite class of interplanetary (IP) sort II bursts that present single-lane, wide-band diffuse emission has been recognized, however how these bursts are shaped remains to be unclear. The radio burst on June 17-18, 2003 was analysed by Bastian (2007) , who instructed that the wide-band and diffuse a part of the burst (sort II-S in Fig. 1) could possibly be on account of synchrotron emission. The later a part of the burst (sort II-P) could possibly be recognized as plasma emission due to the fundamental-harmonic (F-H) lane pair. An identical-looking two-part burst has been found to have occurred on Might 17, 2012. On this later occasion, there are radio information out there from three totally different spacecraft, STEREO-A, STEREO-B, and Wind. The wide-band diffuse emission was seen solely within the STEREO-A dynamic spectrum, i.e., solely from one viewing angle. This research investigates the circumstances the place two-part sort II bursts are shaped, to seek out out potential explanations for the untypical radio emission buildings.
Determine 1. Left: The June 17-18, 2003 sort II burst occasion that included a potential synchrotron-emitting supply (sort II-S) and a plasma emission supply (sort II-P), the determine was revealed by Bastian (2007). Proper: The Might 17, 2012 sort II burst occasion that confirmed an identical two-part construction within the radio emission.
Observations
The 2003 and 2012 occasions have been each related to high-speed, halo-type CMEs. Within the 2003 occasion a M6.8 GOES class flare was situated at S07E55, however no EUV wave was noticed. Within the 2012 occasion a M5.1 GOES class flare was situated close to N11E42 within the STEREO-A view (at N11W76 in Earth view), and an EUV wave was noticed. For the 2012 occasion, Shen et al. (2013) instructed that there could possibly be two eruptions (CMEs) current, and later Grechnev et al. (2024) concluded that there could possibly be two wave-like disturbances and a later halo-shock concerned, suggesting that there could possibly be three separate sort II bursts as effectively.
Within the current research, the height-time evolution of the diffuse and F-lane sort II emission, in contrast with the LASCO CME top evolution, look comparable in each occasions (Fig. 2, left panel). Nevertheless, within the 2012 occasion the STEREO-A CME heights are a lot decrease than the LASCO CME heights (Fig. 2, proper panel). The sort II-P emission heights (F-lane, marked with stars) are near the STEREO-A CME heights more often than not.
Determine 2. Left: Peak-time plots for the 2 occasions, displaying SOHO/LASCO CME heights with the radio supply heights for the two-part sort II bursts. Proper: CME heights from SOHO/LASCO and STEREO-A for the 2012 occasion, with radio supply heights of the sort II-P (F-lane) emission. Pink dashed line signifies the heights of a white-light shock area close to the CME main entrance.
Questions
Comparability of the diffuse sort II emission heights and the LASCO CME heights recommend that the radio emission originates from a CME bow shock, though within the 2003 occasion there’s a rising top distinction. So, might the diffuse radio emission in some circumstances be on account of synchrotron radiation?
In these two occasions, the diffuse sort II burst weakens and disappears from the spectrum close to the time when the F-H sort II burst turns into seen. Pohjolainen, Allawi, and Valtonen (2013) discovered that in most diffuse sort II bursts the emission lasts longer and it may be adopted to a lot decrease frequencies. So, is the ending of the diffuse burst a precondition for the formation and look of the F-H burst? Are two-part sort II shocks created by one single CME, or are they created by a couple of CME and several other shocks, because the 2012 occasion suggests?
The radio emission within the 2012 occasion was noticed from three totally different viewing angles, however solely considered one of them confirmed the diffuse emission. Are there directivity results within the diffuse emission, like in sort IV bursts? Sort IV bursts are advanced and might include a number of emission mechanisms, as synchrotron-emitting electrons get trapped inside a shifting magnetic cloud (Mohan et al., 2024) .
Conclusions
A number of explanations exist for the creation of those radio bursts. However, with out imaging at DH wavelengths the true radio supply places aren’t recognized. Homologous occasions current a problem to concept and simulations, as find out how to produce comparable emission options.
Based mostly on the current paper by Silja Pohjolainen, Two-Half Interplanetary Sort II Photo voltaic Radio Bursts, Sol. Phys. 300, id.25, 2025, DOI:10.1007/s11207-025-02449-5
References
Bastian, T. S.: 2007, ApJ 665, 805
Grechnev, V.V., Kiselev, V.I., Uralov, A.M., et al.: 2024, Sol. Phys., 299, id.129
Mohan, A., Gopalswamy, N., Kumari, A., et al.: 2024, ApJ, 971, id.86
Pohjolainen, S., Allawi, H., Valtonen, E.: 2013, A&A, 558, id.A7
Shen, C., Li, G., Kong, X., et al.: 2013, ApJ, 763, id.114
