Ionospheric responses to the 14 October 2023 annular solar eclipse over Brazil: A case study of fixed-frequency isoline variations

Abstract. This study investigates the ionospheric response to the 14 October 2023 annular solar eclipse using data from stations deployed in the Brazilian equatorial and low latitudes. While most research focuses on total electron content (TEC) and ionization reduction, this work uniquely examines the temporal evolution of fixed-frequency isolines (3–8 MHz) to evaluate variations in the vertical motion and structural dynamics of the ionosphere. Observations were conducted at three stations with varying degrees of obscuration i.e., Araguatins (89 %), Jataí (55 %), and São José dos Campos (38 %). Results demonstrate a significant reduction in the altitude of iso-frequency lines at all stations following the onset of the partial eclipse, with the most pronounced effects occurring at Araguatins due to its proximity to the eclipse path. A time lag of approximately 1.5 hours was observed between maximum obscuration and the minimum altitude of the isolines. Interestingly, a slight initial increase in altitude was detected at all sites immediately after the start of the partial eclipse, likely due to plasma convergence toward the eclipse path driven by thermospheric cooling. The recovery phase was found to be latitudinally dependent as well, Araguatins (equatorial) recovered faster than the low-latitude stations, a phenomenon attributed to the interplay between the equatorial fountain effect and eclipse-induced atmospheric pressure gradients. These findings highlight that even an annular eclipse with partial obscuration significantly disturbs the ionospheric dynamics over 1,500 km away from the central shadow path, emphasizing the high sensitivity of the Earth's ionosphere to transient solar occultation.