Nighttime lower ionosphere height estimation from the VLF modal interference distance Jorge Samanes a, b, * , Jean-Pierre Raulin c , Jinbin Cao a , Antonio Magalh~ aes c a School of Space and Environment, Beihang University, Beijing, China b Direcci on de Astrofísica, Comisi on Nacional de Investigaci on y Desarrollo Aeroespacial (CONIDA), Lima, Peru c Centro de R adio Astronomia e Astrofísica Mackenzie (CRAAM), Universidade Presbiteriana Mackenzie, S~ ao Paulo, Brazil ABSTRACT 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 ¼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. 1. Introduction Very Low Frequency (VLF) radio signals (3–30 kHz) are commonly generated by both communication transmitters (VLF narrowband sig- nals) and by natural sources, e.g. lightning discharges (Parrot et al., 2008), geomagnetic storms and substorms (Santolõk et al., 2003; Cao et al., 2005; Zhima et al., 2014). These signals are known to propagate over long distances (ten of megameters) with low attenuation (~2 dB/Mm) within the Earth-Ionosphere waveguide (EIWG) bounded by the Earth's surface (ground or sea) and the lower ionosphere, which consists of the D-region during daytime (~60–75 km) and lower E-region (~75–95 km) after sunset. At daytime, the VLF propagation is particu- larly stable, resulting in quite well-defined phase and amplitude as received by ground-stations. This characteristic has been widely used to study, for instance, the response of the daytime ionospheric D-region to solar transients by estimation of the Wait's parameters (Wait and Spies, 1964) the reference height (H 0 , in km) and the sharpness factor (β, in km 1 ) (e.g., Muraoka et al., 1977; Thomson et al., 2005; Raulin et al., 2006, 2010; McRae and Thomson, 2004). However, few studies have been reported about the nighttime propagation using VLF narrow- band data. Under nighttime conditions, the VLF propagation parameters (phase and amplitude) are significantly more variable than those recorded at daytime. This can be partly due to a more variable reflecting height (Thomson et al., 2007) as a result of the rapid decrease of the electron density at higher altitudes (Schunk and Nagy, 2000), and also by the presence of higher-order propagation modes which may reach the receiver as a consequence of the lower attenuation under nighttime conditions (Wait and Spies, 1964). Thus, a more complicated modal interference pattern exists at night. Because the electronic density rapidly decreases with the altitude, typically from several hundred per m 3 to a few per m 3 (Schunk and Nagy, 2000), it becomes difficult to apply other radio techniques for sounding this region, and its altitude is also too low for orbiting satellites. Although, ‘in situ’ studies using rockets appear to be precise (Mechtly and Smith, 1968; Sechrist, 1968), sounding rockets can be launched at limited times and locations. Since VLF signals are reflected by the lower ionosphere, the received phase and amplitude inherently contain information of the reflection regions, and hence, of the electrical conductivity at both nighttime and daytime conditions. Therefore, the VLF remote sensing has become a powerful tool for studying the lower ionosphere. Thomson et al. (2007) have studied the VLF narrowband nighttime propagation for different frequencies and characteristics, such as nearly all-sea paths with lengths ~ 4–10 Mm and over middle latitudes in both * Corresponding author. Beihang University, New Main Building B114, Beijing Haidian District, XueYuan Road 37, Beijing, China. E-mail address: jsamanes@conida.gob.pe (J. Samanes). Contents lists available at ScienceDirect Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp https://doi.org/10.1016/j.jastp.2017.10.009 Received 12 April 2017; Received in revised form 10 October 2017; Accepted 23 October 2017 Available online xxxx 1364-6826/© 2017 Elsevier Ltd. All rights reserved. Journal of Atmospheric and Solar-Terrestrial Physics xxx (2017) 1–9 Please cite this article in press as: Samanes, J., et al., Nighttime lower ionosphere height estimation from the VLF modal interference distance, Journal of Atmospheric and Solar-Terrestrial Physics (2017), https://doi.org/10.1016/j.jastp.2017.10.009