# Global GNSS-RO Electron Density in the Lower Ionosphere
###### Abstract
Because the \(D\)/\(E\)-layer conductivity can vary substantially between day and night, as well as between solar minimums and maximums over the 11-year cycle, it can short-circuit the \(F\)-region dynamo during the day [6, 7, 8] and modulate the GEC with the atmosphere throughout the day. However, it remains unclear if GEC has a significant feedback effect on thunderstorms, electrified clouds, or even on global cloud cover. If this feedback exists,the coupling between the ionosphere and the troposphere would act an important process to change Earth's radiation and climate system [9; 10; 11]. The \(D\)-region plasma is weakly ionized during daytime, mainly from photoionization of nitric oxide (NO) by the solar Lyman-\(\alpha\) (Ly-\(\alpha\)) radiation [12]. When the photoionization is absent at night, the \(D\)-region electron density (\(N_{e}\)) decreases substantially but is maintained at a low level from complex sources [13]. During the quiet time, the daytime \(D\)-region \(N_{e}\) is balanced largely by positive molecular and cluster ions, where the upper \(D\)-layer mostly consists of molecular ions and the lower is dominated by cluster ions [14]. A strong solar activity can bring hard \(X\)-rays (wavelength \(<\) 1 nm) to ionize mesospheric N\({}_{2}\) and O\({}_{2}\), but even in this situation, the recombination rates are so high in the \(D\)-region that the environment tends to relax back to the neutral condition quickly once the ionization sources are gone. Hence, the number concentration in this altitude region is dominant by neutral molecules. The \(D\)/\(E\)-region ionosphere can be often perturbed by irregular small-scale dynamic and electromagnetic variability and instability. In the \(E\)-region, for instance, sporadic \(E\) (\(E_{s}\)) can occur as a transient disturbance to the background \(N_{e}\) profile at 90-120 km altitudes [15]. The wind shear is thought to play a critical role in \(E_{s}\) formation by converging the plasma density via the \(\mathbf{v}\times\mathbf{B}\) drift in this region [16; 17]. Global distribution and seasonal variation of \(E_{s}\) have been obtained and studied extensively with global navigation satellite system radio occultation (GNSS-RO) [18; 19; 20] and ground-based observations [21]. These sporadic layers can even occur in the \(D\)-layer [22]. At high latitudes, influences on the \(D\)/\(E\)-region come from magnetospheric disturbances through polar electron precipitation with enhanced ionization. This process is thought to play a role in increasing HO\({}_{\rm x}\) and NO\({}_{\rm x}\) productions through atmospheric chemical chain reactions involving electrons and positive and negative ions [23; 24]. The short-lived HO\({}_{\rm x}\) tends to produce direct effects on the O\({}_{3}\) loss that are localized in space (i.e., the mesosphere) and time (i.e., precipitation events) [25; 26], whereas the long-lived NO\({}_{\rm x}\) can be transported down to the stratosphere to produce indirect effects on the O\({}_{3}\) loss [27; 28]. The \(D\)/\(E\)-region \(N_{e}\) data are rare and limited, largely because of poor instrument sensitivity to low \(N_{e}\) concentration and incapability of remote sensing from space to profile a sharp \(N_{e}\) vertical gradient (i.e., four orders of magnitude over 30 km). Inconvenient and costly access to this altitude region makes in situ measurements infrequent and sparse. Thus, the majority of lower-ionospheric \(N_{e}\) measurements were obtained from sounding rockets (\(D\)/\(E\)-region) and ionosondes (\(E\)/\(F\)-region), which constitute the basis of empirical models such as FIRI (Faraday-International Reference Ionosphere) [29] and IRI-2016 (International Reference Ionosphere 2016) [30]. The objective of this study is to develop a robust algorithm to retrieve global \(D\)/\(E\)-region \(N_{e}\) from the high-rate GNSS-RO data that have been increasing in spatiotemporal sampling from a large number of SmallSat/CubeSat constellations.