Big L Days in GNSS TEC Data

Big L days are days when the lunar semidiurnal variation M2 in the ionosphere is strongly enhanced by a factor of 2 or more. The worldwide network of ground-based receivers for the Global Navigation Satellite System (GNSS) has monitored the ionospheric total electron content (TEC) since 1998. The derived world maps of TEC are provided by the International GNSS Service (IGS) and allow the study of the characteristics of big L days in TEC. In the data analysis, the signal of the lunar semidiurnal variation M2 in TEC is separated from the solar semidiurnal variation S2 by means of windowing in the spectral domain. The time series of the M2 amplitude often shows enhancements of M2 (big L days) a few days after sudden stratospheric warmings (SSWs). The M2 amplitude can reach values of 8 TECU. The M2 composite of all SSWs from 1998 to 2024 shows that the M2 amplitude is enhanced after the central date of the SSW. Regions in Southern China and South America show stronger effects of big L days. Generally, the effects of big L days on TEC show latitudinal and longitudinal dependencies.