Mathematical Modeling of the Formation of Type IIId Solar Decameter Radio Bursts with Echo Components

Abstract

Using numerical simulation, we investigate the possibility that echo components of type IIId solar decameter bursts can be produced through refraction of radio emission in a coronal plasma with large-scale regular electron density inhomogeneities. It is shown that the observed time profiles of radio burst intensity with three or four maxima can be related to the presence in the middle corona of streamers and a localized regular nonuniformity whose electron density exceeds background electron density by a factor of more than 3 -4. The nonuniformity scale in this case is comparable with the optical diameter of the solar disk. The observed radio burst echo components with delays longer than 3 s are explained by the production of additional radio emission propagation modes within a "transverse" refraction waveguide arising between the localized electron density nonuniformity and deeper layers in the corona. As these additional modes are reflected from the streamers, they can reach the Earth. Calculations of the time profile of radio burst intensity take into consideration the influence of scattering by turbulent coronal inhomogeneities and of collisional absorption. Comparison of the modeling results with the observational data shows that the calculated values of some profile parameters differ from the observed values. One of the possible reasons is that the method used does not take into account the diffraction leakage of radio emission through the large-scale nonuniformity.

Low‐frequency heliographic observations of the quiet Sun corona

“…Further research efforts with using the 2-D heliograph were basically concentrated on studying the Type IIId radiobursts with echo components (Abranin et al 1982(Abranin et al , 1984Abranin, Baselyan & Tsybko 1996, 1997. The echo components are formed due to the reflection from a deeper layer of a spherically symmetric www.an-journal.org corona, as well as a result of the refraction of radio waves on some large-scale coronal structures (Afanasiev 2006). As for early 2-D heliograph results about the quiet-Sun radiation at 10-30 MHz, obtained with help of the UTR-2, there is the only work in the form of preprint (Abranin & Bazelyan 1986).…”

Heliograph of the UTR-2 Radio Telescope

“…The assumption [28] was therefore made that the echo-component of stria-bursts is formed not merely due to the reflection from a deeper layer of a spherically symmet-ric corona, but also as a result of the refraction of radio waves on some large-scale coronal structures. This hypothesis has been supported in [30] where the author has mathematically simulated the shaping of Type IIId bursts. During the systematic researches of the Sun with the UTR-2 array system and positional observations with the two-dimensional heliograph for almost two decades, more than 10 solar Type IIId burst storms were recorded.…”