Decimetric broadband radio bursts (with frequencies of tons of of MHz to a couple GHz) have been noticed from the Solar and different flaring stars. Within the photo voltaic case, such bursts have been labeled because the decimetric type-IV (t-IVdm) bursts. Their spectra typically include substantial advantageous buildings, akin to pulsations, spikes, and numerous absorption buildings. This makes them distinct from their metric and centimetric counterparts.
As a result of lack of imaging information at decimetric wavelengths, earlier research decided the supply area orientation primarily based on the main sunspot speculation (Aschwanden 1986; Zlobec et al. 1987). This results in contradictory conclusions on the mechanism of t-IVdm bursts. Over the previous eight a long time of photo voltaic radio astronomy, solely photo voltaic spikes which might be extremely transient, extremely polarized, and intensely vivid are broadly attributed to electron cyclotron maser emission (ECME), whereas a number of different forms of coherent photo voltaic bursts, akin to type-I, II, and III, have been attributed to plasma emission.
We analyzed a flare-induced t-IVdm burst on 20110924 with medium-strong ranges of polarization and from sources close to a sunspot. The occasion was noticed by NRH at a number of imaging frequencies and by SDO/AIA at EUV passbands. The aim is to establish the underlying radiation mechanism.
Determine 1: Overview of the t-IVdm radio burst on 2011 September 24. (a) GOES gentle X-ray fluxes of the M7.1-class flare at 1.0–8.0 Å. (b) The dynamic spectra combining the info from the San Vito observatory (100–175 MHz), the Bleien observatory (175–870 MHz), and the Ondrejov observatory (870–2000 MHz). Bursts I and II are two parts of the t-IVdm occasion. (c) The temporal profiles of the utmost TB and the diploma of polarization (d) with a ten s cadence at eight NRH frequencies.
The occasion consists of two main sub-bursts (bursts I and II), every lasting ∼20 minutes. The observations reveal three options: (1) Each burst parts are extremely intermittent, with well-defined higher and decrease frequency cutoffs at ∼1–2 GHz and 200–300 MHz, respectively. (2) The utmost brightness temperature (TB) is ∼2 × 1011 Okay for Burst I and ∼4×1010 Okay for Burst II. (3) The polarization is left-handed with ranges
reaching ∼70%–100%, with an general rising development with frequency. Primarily based on these observations, we conclude that the emission is coherent for the reason that incoherent gyrosynchrotron radiation of photo voltaic flares can not yield such intermittency with excessive TB > 1011 Okay and powerful polarization at such frequencies (e.g., Dulk 1985).
Determine 2: The AIA and HMI information of the occasion. (a)–(c) and (e)–(g) The AIA photos at 94, 131, and 171 Å are overlaid with NRH 95% contours of the utmost TB
at eight frequencies (228–445 MHz) for consultant moments of bursts I and II. (d) and (h) The HMI magnetogram superposed by extrapolated NLFFF
area strains. The coloured sections of area strains correspond to the sector strengths rising from ∼40.7 to ∼79.5 G and the corresponding 2 Ωce rising from 228 to 445 MHz. Coloured circles show NRH 95% contours at 13:04:17 (d) and 13:45:07 UT (h). The yellow sphere crossing the cyan area line marks the situation the place
energetic electrons are injected for the particle-transport simulation.
The NRH sources moved systematically in the course of the burst, exhibiting at the least two leaps which point out the beginning of bursts I and II. Round 13:00 UT, the sources moved to the area above the flaring loops, and at ∼13:20 UT, they leaped towards the western leg of the loops, proper above the rightmost sunspot with a unfavorable polarity. Throughout the entire course of, the NRH sources aligned properly with one another, and higher-frequency sources are nearer to the disk.
We draw two main conclusions: (1) NRH sources of each bursts (I and II) lie alongside the sector strains pointing towards the sunspot with sturdy left-handed polarization (Determine 1(d)), so each bursts are of X mode, and (2) the sources align properly with
the coloured 2 Ωce part. Thus, the most certainly radiation mechanism is the harmonic X mode (X2) through ECME for the reason that various coherent plasma emission course of would produce O mode for the elemental department or weak polarization for the
harmonic department (e.g., Chen et al. 2022).
We additional modeled the transport of downward-streaming energetic electrons alongside a coronal loop. We discovered that almost all electrons get mirrored in converging sunspot fields throughout the altitude vary of 20–100 Mm. This agrees with the well-defined spectral ranges of such bursts. The ECME-radiating energetic electrons exhibit a shell-like velocity distribution perform (VDF) as an alternative of the commonly presumed loss-cone distribution.
This examine supplies important proof that the flare-induced t-IVdm burst is induced by ECME within the harmonic X mode, and drastically expands the appliance of ECME in photo voltaic radio astronomy and supplies photo voltaic samples for comparable bursts from different flaring stars.
Primarily based on the current paper by Lv M., Zhong Z., Kong X., Ning, H., Yu, F., Wang B., Tan B., Victor, M., Alexey, Okay., Track, H., Zheng R., Chen Y., A Flare-related Decimetric Sort-IV Radio Burst Induced by the X2 Radiation of Electron Cyclotron Maser Emission, ApJL, 989, L24, DOI: https://doi.org/10.3847/2041-8213/adf5c6
References
Aschwanden, M.: 1986, SoPh, 104, 57
Zlobec, P., Messerotti, M., Li, H. et al.: 1987, SoPh, 114, 375
Dulk, G.: 1985, ARA&A, 23, 169
Chen, Y., Zhang, Z., Ni, S., et al.: 2022, ApJL, 924, L34
