Nighttime lower ionosphere height estimation from the VLF modal interference distance

We have studied the dynamics of the nighttime lower ionosphere height through continuous monitoring of the VLF modal interference distance (so-called distance D). Since the distance D is related to the nighttime propagation modes within the Earth-Ionosphere waveguide, it provides information of the nighttime reflection height (h_N). We have used a long-term VLF narrowband database of almost 8 years (2006–2014) from a long transequatorial VLF propagation path between the transmitter NPM (Hawaii, 21.4 kHz) and the receiver ATI (Atibaia, Brazil). Our results show that h_N assumes lower values during northern hemisphere wintertime as compared with summertime. By using the Lomb-Scargle periodogram, periodicities around 180 (SAO), 365 (AO) and 800 (QBO) days have been found, being the periodicity around 180 days stronger than all other oscillations. Since these large-scale oscillations are commonly observed in several measurable parameters of the mesosphere-lower thermosphere (MLT) region, our results suggest that the nighttime lower ionosphere can be strongly influenced by the dynamics of the MLT region. The effect of the long-term solar activity on h_N is also studied, resulting in high negative correlation (R = −0.91). This effect makes h_N decrease around 1.2 km from low to high solar activity. This result suggests a control of the solar radiation on the nighttime lower ionosphere, and hence, on the electron density at night.