Tracking Ionospheric Changes during Solar Eclipses: Concepción Historical Data

Abstract. Solar eclipses offer a unique natural experiment to probe ionospheric responses to sudden reductions in solar radiation. This study examines the Concepción (36.79° S, 73.03° W/Chillán (36.64° S, 71.99° W) ionosphere during 16 selected eclipses, out of 21 identified events between 1958 and 2024, using a historical ionogram dataset spanning several decades. Critical frequencies (foE, foF1, foF2) and virtual heights (h’E, h’F1, h’F2/F) were extracted from digitized and scaled ionograms to quantify eclipse-induced perturbations. Diurnal variations show typical dips in the E- and F1-layer critical frequencies, while F2-layer responses are more complex and variable. Regression analysis was performed exclusively on critical frequencies, revealing a nearly linear decrease of foE with increasing solar obscuration, whereas virtual heights exhibited inconsistent behavior due to neutral winds, plasma transport, and other dynamical processes. High-cadence observations, available for select events, provided further insight into short-term ionospheric variability. Only the 2 July 2019 and 14 December 2020 eclipse responses were previously published. The study highlights the rescue and standardization of historical ionograms, many originally on fragile or hazardous 35 mm film, emphasizing the scientific value of long-term datasets. Predictions for the 06 February 2027 eclipse indicate an expected foE decrease of ~0.7 MHz at Chillán, offering a timely opportunity to validate the regression models and assess predictive skill across solar cycle conditions.