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

## TL;DR

This study provides the first direct observational evidence linking medium-scale traveling ionospheric disturbances to spectral caustics, which are patterns caused by solar radiation focusing effects in the ionosphere, using radio spectra and TEC maps.

## Contribution

It presents the first direct observations connecting MSTIDs with spectral caustics, confirming their causal relationship through simultaneous spectral and TEC data analysis.

## Key findings

- Confirmed one-to-one correspondence between MSTIDs and spectral caustics.
- Provided strong observational evidence supporting MSTIDs as the cause of spectral caustics.
- Used multi-instrument data to analyze ionospheric disturbances and solar radiation focusing effects.

## Abstract

The solar radiation focusing effect is related to the specific phenomenon of propagation of the Sun-emitted 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.