Ionospheric nighttime enhancements are manifested in an increase of the electron density at nighttime. This paper studies the latitudinal variation of the specific local time of postmidnight enhancement peaks using ionosondes distributed at low latitudes. To obtain the parameters of the ionosphere, we manually extracted ionograms recorded by ionosondes. Cases show that there are significant latitudinal variations in the observed local time of the postmidnight enhancement peaks. Results show that the lower the geomagnetic latitude, the earlier the enhancement peak occurred in the geomagnetic northern hemisphere. Additionally, the enhancement peaks occurred earlier in the geomagnetic southern hemisphere than that in the geomagnetic northern hemisphere for these present cases. We suggest that the combined effect of the geomagnetic inclination and transequatorial meridional wind might be the main driving force for latitudinal variation of the local time of the occurrence.

[1] During the total solar eclipse of 22 July 2009, two ground-based high-frequency radio systems were applied to observe the ionospheric variations over Wuhan, China. The ionosonde recorded ionograms every 5 min; the Wuhan Ionospheric Oblique Backscattering Sounding System recorded echo range and Doppler every 1 min. Comparing the observations of the two radio systems, we found that the periodic fluctuation on the top penetration frequency of regular Es (fEs) and on the Doppler velocity of the spread Es appeared and disappeared simultaneously on the eclipse day. Moreover, wavelet analysis results show that the fEs curve and the Doppler velocity of the spread Es contain the same period of ∼35 min. The spread Es occurred owing to the off-vertical reflections from the tilted layer at the Es latitude. Atmospheric gravity waves are considered to be generated during the solar eclipse and propagate upward to deform the Es layer and produce the moving wave-like structures in the layer. The tilted plasma concentration in the moving wave-like structures induced the off-vertical HF radiowave reflections and added the Doppler modulation of also ∼35 min period on the reflected radiowave. Furthermore, the Doppler values of regular Es indicate that the layer at lower altitude was weakly disturbed and the layer at higher altitude was deeply modulated, which is in agreement with the theory that the amplitude of gravity waves increases with height.

Spread F on ionograms has been considered to be a phenomenon mainly occurred at nighttime. This study presented a case study of daytime spread F observed by the ionosonde installed at Puer (PUR; 22.7°N, 101.05°E; dip latitude 12.9°N), where daytime spread F that lasted for more than 2 h (about 08:30 LT~10:45 LT) was observed on 14 November 2015. To investigate the possible mechanism, ionograms recorded at PUR and Chiang Mai (18.76°N, 98.93°E; dip latitude 9.04°N) were used in this study. We found that traveling ionospheric disturbances were observed before the occurrence of daytime spread F. Meanwhile, the movement of the peak height of the ionosphere was downward. We suggested that downward vertical neutral winds excited by traveling atmospheric disturbances/atmospheric gravity waves might play a significant role in forming daytime spread F over PUR during geomagnetic storms.

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[1] Recent advances in computing technologies have renewed interest in the intelligent systems for automatic interpretation of ionograms, images obtained by remote sensing of the ionospheric plasma. The ionogram "autoscaling" techniques based on the template matching method, previously rendered unrealistic for their computing complexity, have now become feasible. This work presents an automatic scaling technique for extracting the main features of the F 1 and F 2 layers of the ionosphere, such as critical frequency and virtual height, from vertical incidence ionograms that do not distinguish O/X polarization of the echoes. The proposed technique uses the quasi-parabolic segments (QPS) to model the electron density profile shapes that are used to synthesize a pool of candidate traces. Moreover, the empirical orthogonal functions and image technique are applied to reduce the size of the candidate traces so that the auto-scaling algorithm can run in realistic time. With the template matching algorithm, the technique will provide the initial parameters of the QPS model for the F 1 and F 2 layers, which are then fine-tuned to obtain the better fitting parameters. In order to evaluate the performance of this technique, a large data set of ionograms recorded in Wuhan at daytime and nighttime in winter, summer, and equinoctial months, are analyzed and investigated. The automatic scaling results are compared with manually scaling results. Our results indicate that the proposed technique described in this paper is reliable and efficient and will facilitate the statistical study of temporal and spatial ionospheric characteristics over Wuhan.