Effects of the Great American Solar Eclipse on the lower ionosphere observed with VLF waves

Abstract The altitude of the ionospheric lower layer (D-region) is highly influenced by the solar UV flux affecting in turn, the propagation of Very Low Frequency (VLF) signals inside the waveguide formed between this layer and the Earth surface. The day/night modulation observed in these signals is generally used to model the influence of the solar irradiance onto the D-region. Although, these changes are relatively slow and the transitions are “contaminated” by mode coalescences. In this way, a rapid change of the solar irradiance, as during a solar eclipse, can help to understand the details of the energy transfer of the solar radiation onto the ionospheric D-layer. Using the ”Latin American VLF Network” (LAVNet-Mex) receiver station in Mexico City, Mexico, we detected the phase and amplitude changes of the VLF signals transmitted by the NDK station at 25.2 kHz in North Dakota, USA during the August 21, 2017 solar eclipse. As the Sun light was eclipsed, the rate of ionization in the ionosphere (D-region) was reduced and the effective reflection height increased, causing a considerable drop of the phase and amplitude of the observed VLF waves. The corresponding waveguide path is 3007.15 km long and crossed almost perpendicularly the total eclipse path. Circumstantially, at the time of the total eclipse a C3 flare took place allowing us to isolate the flare flux from the background flux of a large portion of the disk. In this work we report the observations and present a new model of the ionospheric effects of the eclipse and flare. The model is based on a detailed setup of the degree of Moon shadow that affects the entire Great Circle Path (GCP). This relatively simple model, represents a new approach to obtain a good measure of the reflection height variation during the entire eclipse time interval. During the eclipse, the maximum phase variation was -63.36 ° at 18:05 UT which, according to our model, accounts for a maximum increase of the reflection height of 9.3 km.