Direct Observations of Traveling Ionospheric Disturbances as Focusers of Solar Radiation: Spectral Caustics

Abstract

The solar radiation focusing effect is related to the specific phenomenon of propagation of the Sunemitted HF and VHF waves through terrestrial ionosphere. This natural effect is observed with ground-based radio instruments running within 10-200 MHz range, as distinctive patterns -the Spectral Caustics (SCs) -on the solar dynamic spectra. It has been suggested that SCs are associated with medium-scale traveling ionospheric disturbances (MSTIDs). In this paper, we present the first direct observations of SCs induced by MSTIDs, using solar dynamic spectra with SCs obtained by different European radio telescopes on January 8, 2014 and simultaneous two-dimensional detrended total electron content (dTEC) maps over Europe. Spatial examination of dTEC maps as well as precise timing analysis of the maps and the dynamic spectra have been performed. First, we found several pairs of one-to-one (TID-SC) correspondences. The study provides strong observational evidence supporting the suggestion that MSTIDs are the cause of SCs.

lensing from small-scale travelling ionospheric disturbances observed using lofar

“…The features are pseudo-periodic, with one appearing roughly every 10 minutes over approximately an hour. These repeated intensity enhancements suggest focusing from a succession of electron density minima, such as would be associated with a TID and has been reported in solar radio observations by Koval et al (2019). The features consistently display a broadening towards lower frequencies with clear diffraction fringes, as well as a time asymmetry with lower frequencies leading the higher frequencies.…”

“…In their classification, feature 3 would be described as 'inverted V like', features 1 and 5 as 'X like' and features 2 and 4 appear similar to their 'fiber like' caustics. Many of their observations have been shown to correlate with the minima of passing MSTIDs (Koval et al, 2019) and they were able to explain at least the 'inverted V like' caustics as arising from focusing from the minima of such TIDs (Koval et al, 2018). The absence of fringes in their observations is likely due to the larger angular size of the Sun compared to Cygnus A.…”

“…For example, Koval et al (2017Koval et al ( , 2018Koval et al ( , 2019 showed strong focusing of solar radio emission due to the passage of MSTIDs. However, much less attention has been paid to small-scale TIDs (SSTIDs) (i.e.…”

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.…”