Kind-II radio bursts are normally noticed beneath 400 MHz with slowly-drifting slender basic and/or harmonic bands. Occasions with beginning frequencies increased than 400 MHz are not often reported (e.g., Pohjolainen et al. 2008). Such high-frequency Kind-IIs might stem from the CME interplay with surrounding dense constructions within the corona, similar to streamers, ray-like or loop construction, or from sources within the decrease corona.
Determine. 1. Dynamic spectrum of the high-frequency wide-band type-II burst recorded by the ORFEES radio spectrograph (670 – 144 MHz). The utmost brightness temperatures (TBmax) at completely different NRH frequencies are plotted with white strains.
Right here we report an uncommon wide-band excessive frequency type-II burst on 19 Might 2022 with beginning frequency as excessive as 670 MHz and an instantaneous bandwidth as broad as 300 MHz, as recorded by the ORFEES spectrograph (see Determine 1). Such bandwidth is way wider than ordinary occasions. As well as, at any particular frequency the burst lasts for two minutes. That is additionally for much longer than ordinary bursts. Simultaneous observations of extreme-ultraviolet (EUV) and radio imaging from the Nançay Radio Heliograph (NRH) can be found for this occasion. This enables us to measure the placement of radio sources over the EUV shock construction and to discover the reason for the high-frequency wide-band function.
By over-plotting the 90 and 95% contours of TBmax onto the closest-in-time AIA 211 Å working distinction pictures (see Determine 2), we made 4 observations: (1) the type-II sources are co-spatial with the nostril entrance of the EUV shock wave construction (being barely separated, by 0.01 Rʘ; (2) the sources mainly overlap with one another earlier than 12:04 UT (with out spatial separation 0.001 Rʘ; later they change into spatially dispersed, with lower-frequency sources being farther away from the disk; (3) as already talked about, the sources at a number of frequencies seem concurrently, and so they distribute inside a broad area of 150 ̶ 200 arcsecs; and (4) throughout most time of the burst, the sources are spatially dispersed but nonetheless centered across the dip of the shock entrance that corresponds to the CME-shock transit throughout the intense dense loop tops.
Determine 2. Temporal evolution of NRH sources superposed onto the closest-in-time AIA 211 Å pictures. The radio sources are represented by the 90% and 95% of TBmax contours.
The dip is probably going as a result of robust compression brought on by the shock propagation into shiny and dense loops the place the Alfvénic velocity (in addition to the shock velocity) is comparatively low. In accordance with the EUV and white gentle knowledge, shiny and dense loops that join the 2 spots are spatially correlated with the shock dip, indicating the shock has transited throughout them with a powerful interplay. On this case, electron acceleration is sort of environment friendly based on numerical simulations by Kong et al. (2015, 2016). We recommend that the type-II burst originates from the shock transit throughout these magnetically closed-loop constructions that types the shock dip (see Determine 3).
Determine 3. Sketch of the shock-loop system with the dip area and the corresponding type-II radio sources.
The wide-band function of the type-II burst signifies that the sources stem from a area with a wide variety of density; both the density fluctuations of the supply have massive amplitudes or the supply extends over a broad and extremely inhomogeneous area. Our observations favor the latter state of affairs: that the wide-band type-II burst stems from the broad sources centered across the shock dip.
We recommend that the burst represents the harmonic department, for the reason that basic one experiences much-stronger scattering/absorption impact and for a lot of different coronal bursts the harmonic department usually represents the stronger one. At 12:03:41 UT, the higher and decrease frequencies of the burst are 440 and 180 MHz, respectively. Then, the densities inside the type-II supply at 12:03:41 UT ought to fluctuate from ~109 to ~108 cm3, with a ratio of ~10. That is a lot bigger than any attainable compression ratio of a magnetohydrodynamic shock, indicating that the wide-band function will not be attributable to density variations throughout the shock layer. Additional research on comparable occasions are crucial for a deeper understanding of their origin.
Based mostly on a not too long ago printed article: V. Vasanth, Yao Chen and G. Micahlek, Awide-band high-frequency type-II photo voltaic radio burst, Astronomy & Astrophysics, 2025, 720, A15. DOI: 10.1051/0004-6361/202554430
References
Kong, X., Chen, Y., Guo, F., et al. 2015, ApJ, 798, 81
Kong, X., Chen, Y., Feng, S., et al. 2016, ApJ, 830, 37
Pohjolainen, S., Pomoell, J., & Vainio, R. 2008, A&A, 490, 357
*Full checklist of Authors: V. Vasanth, Yao Chen and G. Micahlek
