Investigating a Characteristic Time Lag in the Ionospheric F-region's Response to Solar Flares

TL;DR

This study investigates the short time delays between solar EUV flare emissions and the Earth's ionospheric response, revealing key correlations and seasonal effects that enhance understanding of solar-terrestrial interactions.

Contribution

It provides the first detailed analysis of the characteristic time lag in the ionospheric F-region's response to EUV solar flares, highlighting the influence of flux parameters and seasonal factors.

Findings

Delays under 1 minute for 10 powerful solar flares.

Strong negative correlation (-0.85) between delay and He II flux change.

Longer delays observed near summer solstice.

Abstract

X-ray and EUV solar flare emission cause increases in the Earth's dayside ionospheric electron density. While the response of the lower ionosphere to X-rays is well studied, the delay between EUV flare emission and the response of the ionospheric F-region has not been investigated. Here, we calculate the delays between incident He II 304 Angstrom emission, and the TEC response for 10 powerful solar flares, all of which exhibit delays under 1 minute. We assess these delays in relation to multiple solar and geophysical factors, and find a strong negative correlation (-0.85) between delay and He II flux change and a moderate negative correlation (-0.55) with rate of increase in He II flux. Additionally, flare magnitude and the X-ray-to-He II flux ratio at peak He II emission show strong negative correlations (-0.80 and -0.75, respectively). We also identify longer delays for flares occurring closer to the summer solstice. These results may have applications in upper-ionospheric recombination rate calculations, atmospheric modelling, and other solar-terrestrial studies. We highlight the importance of incident EUV and X-ray flux parameters on the response time of the ionospheric electron content, and these findings may also have implications for mitigating disruptions in communication and navigation systems.