Traveling ionospheric disturbances as huge natural lenses: Solar radio emission focusing effect

Abstract

We present an in‐depth study into spectral perturbations appearing in solar dynamic spectra and being manifestations of the focusing effect of low‐frequency solar emission by the Earth's ionosphere. Such perturbations are considered to be the result of radio waves focusing by medium scale traveling ionospheric disturbances (MSTIDs). Using the Nançay Decametric Array (NDA) data set, we have conducted a statistical analysis of the spectral structures in solar dynamic spectra within 10–80 MHz. We have detected the spectral structures in the NDA spectral data for 129 observation days from 1999 to 2015. On spectrograms they appear as intensity variations different from well‐known solar radio bursts. The sharp edges with enhanced intensity are distinctive characteristics of the structures for most events. Due to this spectral feature, they are termed as Spectral Caustics (SCs). We have classified the SCs observed by the NDA as several types, based on their spectral morphology, namely: inverted V like, V like, X like, fiber like, and fringe like. We have found that the rate of occurrence of SCs in dynamic spectra depends on the phase of the solar cycle. About 81% of all days with detected SCs fall on active phases of solar cycles 23 and 24 (48% and 33%, respectively). We have also established the seasonal dependence in occurrence of the SCs. It was found that about 95% of days with SCs belong to autumn‐winter months, whereas only near 5% of days with SCs belong to spring‐summer months. This is well correlated with the reported dependence in MSTID occurrence rate.

Simulation of Focusing Effect of Traveling Ionospheric Disturbances on Meter‐Decameter Solar Dynamic Spectra

“…The visible broadening from the convergent point occurs with decreasing frequency that makes the structure wedge shaped. This description agrees well with the SC definition of inverted V‐like type, which has been introduced together with other types of SCs in our recent paper (Koval et al, ). This type of SC can be seen in Figure a near 08:10 UT and in Figure b near 14:10 and 15:00 UT.…”

“…Thus, we infer that the SCs may be observed only in certain periods, mainly in late fall, winter, and early spring. This selectivity in generations of SCs explains the seasonal dependence in the SC appearance, which we found in the NDA data (Koval et al, ). In that paper we have attributed such behavior to similar seasonal variations in the TID occurrence rate (Hernández‐Pajares et al, ; Otsuka et al, ; Tsugawa et al, ).…”

“…Some spectrograms contain multiple SCs belonging to separate types (see description in the text). The dynamic spectra are credited from the paper by Koval et al ().…”

“…The SCs were detected confidently in 129 observational sessions. For the first time the authors have introduced a classification of the SCs and divided them into several particular types, namely, inverted V‐like, V‐like, X‐like, fiber‐like, and fringe‐like SCs, according to their spectral morphology (for details see ; Koval et al, ). Most SCs can be distinguished from well‐known solar radio bursts due to their sharp edges (i.e., rapid enhancement in intensity) in the dynamic spectra.…”

First Light for Station V of the Low-Frequency All-Sky Monitor Radio Telescope

“…Lasting about 40 min, the event clearly has a strong RFl component, with a narrowband feature around 55.6 MHz, and a higher frequency cutoff. The broadband features across the 5 MHz span of the activity includes complex looping and crisscross features, at least qualitatively reminiscent ( [3], [4]) of ionospheric events such as traveling ionospheric disturbances (TIDs), in this case possibly interacting with narrowband transmissions. Two other shorter events of 1-5 min were also seen shortly after midnight on other days, but apart from these three events, most observed days in June showed no similar activity.…”