**1. Introduction**

Total electron content (TEC) obtained from Global Navigation Satellite Systems (GNSS) can be used to ascertain the impact of space weather events on communication and navigation systems. The spatio-temporally wide range of capabilities of GNSS make it useful in studying these impacts. One technique in using GNSS to reveal weather impacts is by detrending the original TEC [1]. That is, a best fit model or method is first fitted or applied to the original TEC, then the difference between the original TEC and the best fit model is computed. The difference obtained can determine amplitude, frequency, and other signal changes in case of a weather event.

Over time, different best fit models have been used. Each best fit method produces different results, which may or may not necessarily detect the impact a weather event makes on the signal or adequately show the occurrence of the event. In using GNSS for space weather event studies, extensive studies have been done on geomagnetic storms [2–4], earthquakes [5,6], and typhoons [7–9]. The fitting methods have been mostly used in these studies to detect travelling ionosphere disturbance (TID) and other ionosphere irregularities.

Thunderstorms/lightning, a troposphere weather event, have recently garnered interest from the scientific community in the context of harnessing the spatio-temporal capabilities of GNSS to help understand some characteristics of this weather event. Thunderstorm studies in the mid-latitude US plains by Lay, et al. [10] pointed out that ionosphere

**Citation:** Osei-Poku, L.; Tang, L.; Chen, W.; Mingli, C. Evaluating Total Electron Content (TEC) Detrending Techniques in Determining Ionospheric Disturbances during Lightning Events in A Low Latitude Region. *Remote Sens.* **2021**, *13*, 4753. https://doi.org/10.3390/rs13234753

Academic Editors: Serdjo Kos, José Fernández and Juan F. Prieto

Received: 18 October 2021 Accepted: 22 November 2021 Published: 24 November 2021

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gravity waves (IGW) damp out before a thunderstorm dies out, contrary to a recent study by Rahmani, et al. [11] in the same study area but at a different time period. Ogunsua, et al. [12] also reported on the ionosphere's response to thunderstorms in the West African and Congo sector of the equatorial region, stating that thunderstorm impact on the ionosphere at nighttime is negligible compared to the daytime. Mahmud M [13], who also studied thunderstorm events over Southern Africa, showed that a strong correlation exists between hourly lightning and ionosphere irregularity event occurrence. Recent studies by Tang, et al. [14] and Liu, et al. [15] showed the characteristics of thunderstorm generated ionosphere gravity waves (IGW) in the Southern China region.

In contributing to the GNSS and thunderstorm related studies, this study assesses two commonly used fitting methods, the Savitzky–Golay and polynomial methods, to detect lightning events as they have been used to detect the presence of other space events. These two methods have been demonstrated in the existing literature to best indicate the incidence of weather events. The study area is Hong Kong, a low latitude region in the southern China coastal region. In the following sections, a description of the detrending methods is briefly discussed. A two-step approach, used to detect and distinguish lightning from non-lightning events using the detrending methods, is introduced, followed by the results. Discussions on the results and derived conclusions are then presented. Aside from the obvious visual changes in amplitude, statistical means were also used to choose the best fitting method.
