Multifractal characterization of daily maximum and minimum ionospheric TEC at African equatorial stations

Abstract. The temporal scaling structure of the ionospheric vertical total electron content (vTEC) carries fundamental information about the intrinsic dynamics of the equatorial and low-latitude plasma system. This study applies Multifractal Detrended Fluctuation Analysis (MFDFA) to investigate the intrinsic dynamics of the variability of the ionosphere over African equatorial stations using the daily maximum and minimum vTEC time series recorded at three GNSS stations (Dakar (DAKR), Libreville (NKLG), and Djibouti (DJIG) spanning the equatorial ionisation anomaly (EIA) crest and its flanks. Seasonal trend components are isolated using Seasonal and Trend decomposition with Loess (STL) to separate the contribution of the seasonal cycle from intrinsic ionospheric dynamics. The raw maximum vTEC series exhibit clear semi-annual modulation and multifractality at all stations, with generalised Hurst exponents 0.104 ≤ h (q = 2) ≤ 0.193 and statistically significant singularity spectral widths. A crossover in scaling behaviour near n ≈ 150 days reveals scale-dependent persistence that is stronger at sub-annual scales. The raw minimum vTEC series confirm multifractality but with consistently narrower singularity spectra and weaker q-dependence, reflecting the reduced dynamic range of the nighttime ionosphere in the absence of photo-ionisation. These results reveal a clear daytime-nighttime asymmetry in ionospheric scaling dynamics and establish the EIA crest as a region of persistent, non-seasonal nonlinear variability that manifests across both the daytime and nighttime ionosphere. The findings demonstrate that MFDFA applied to the diurnal extremes of vTEC provides a sensitive and physically interpretable diagnostic of equatorial ionospheric complexity that is not accessible through conventional spectral or trend-based analyses.