Case study 1: July 05, 2014
Figure 1 shows a TECMap over South America on the evening of July 05, 2014. The universal time (UT) of 23:10 corresponds to a local time (LT) of 20:10 at 45° W. The broken line indicates the solar terminator crossing the F layer at 300 km altitude, after which the ionization production ceased. One can easily recognize that, along the geomagnetic equator line, TEC depletion extended from 35° W to 55° W. This is the trough of the post-sunset equatorial ionization anomaly (PS-EIA), which is caused by F-layer plasma uplift due to an increase in the zonal electric field, known as PRE, during the sunset period. If the vertical drift velocity exceeds a certain level, the bottom side gradient region of the F layer becomes unstable under the RTI condition and starts to generate plasma irregularities. Note that the TECMap (Fig. 1) shows periodic TEC decreasing along the magnetic equator, some of which extended toward the southeast as highlighted by the elliptical circles numbered 1, 2, and 3. The depletions, however, did not extend up to the PS-EIA crest region. These periodic depletions appear to be an initial phase of the plasma bubbles that are developed along the geomagnetic field lines. The distance between depletions 1 and 2 is approximately 640 ± 50 km.
On this evening, both ionosondes at São Luís and Fortaleza registered satellite traces (at 21:50 UT) followed by spread F after 22:00 UT. In Fig. 2 (upper panel), the ionogram at São Luis at 22:20 UT (19:20 LT) is presented. The spread F continued up to 04:00 UT (01:00 LT). The 630 nm emission, observed by an all-sky imager at Cariri (7.4° S, 36.5° W), also registered plasma bubbles, which are shown in Fig. 2 (lower panel) as a snapshot at 02:49 UT (23:49 LT). Due to the cloudy conditions on this evening, airglow images could not be taken in the beginning phase of the plasma bubble development (at around 22:00 UT). Using a sequence of images collected from 02:40 UT to 03:30 UT, a zonal drift velocity of 76 m/s and an inter-depletion distance of 485 km were obtained. These are characteristic of plasma bubbles. The inter-bubble distance is, however, shorter than that measured by TECMap at 23:00 UT. Normally, the inter-bubble distances tend to decrease from the evening to the midnight time zones, as observed by Barros et al. (2018).
In order to investigate any dynamical forcing of the F layer prior to and during the terminator passage, temporal variations in the true height of the ionogram-fixed frequencies (5, 6, 7, and 8 MHz) observed at São Luis were plotted in Fig. 3 (upper panel). The vertical drift speed at around 21:00 UT was 16 ± 2 m/s. Note that three frequencies (6, 7, and 8 MHz) showed a common oscillation during the period of 15:00–20:00 UT. The d(hF) was obtained using a band-pass filter (0.5–3 h) in order to eliminate long and large hF gradients during the evening period, which is shown in Fig. 3 (lower panel). One can see a periodic oscillation with a period of ~ 70 min at around 18:00 UT with the phase propagating downward. This indicates that there was gravity wave propagation in the F layer during the afternoon sector (see also Abdu et al. 2009). Periodic oscillations shorter than 0.5 h cannot be seen in Fig. 3 (lower panel) due to the limitation of the data sampling interval (10 min) and the applied band-pass filtering, as mentioned above.
The oscillation of the F layer could also be seen in the dTEC maps. Figure 4 shows a keogram of dTEC, i.e., a latitudinally sliced (0–30° S) dTEC map at 50° W as a function of time from 17:00 UT to 24:00 UT. Due to the different spatial and temporal resolutions (as mentioned in the previous section), the keogram is divided in two latitudinal zones, from 0 to 15°S with a temporal resolution of 5 min and from 15° S to 30° S with a temporal resolution of 1 min. The broken line indicates the solar terminator at an altitude of 300 km. During the period between 19:30 UT and 21:45 UT (highlighted by a box), one can see periodic perturbations of dTEC with a period of ~ 30 min in both the 0–15° S and 15–30° S keograms. It is clear that the perturbations are propagating from the south (~ 22° S) to the north up to ~ 5° S. The keogram of the longitudinal (E–W) slice at 7° S (not presented here) did not show any clear propagation mode. In the 5–15° S region, therefore, it appears that there was a wave propagating northward. Due to the low spatial and temporal resolution of the 0–15° S keogram, we could not obtain wave parameters with sufficient accuracy in this region. Conversely, from the 15–30° S keograms (lower panel), which have better spatial and temporal resolutions, it was possible to obtain the wave characteristics from the time interval of 20:30 UT to 21:30 UT: the horizontal wavelength was 732 ± 50 km, the period was 26 ± 5 min, and the propagation direction was 3.5 ± 5° from the north. According to the classification of ionospheric perturbations, this event can be classified as an MSTID (Hunsucker 1982). The dTEC perturbations after 22:00 UT in the 15–20° S region are most likely the signature of the plasma bubbles seen in the ionogram.
It appears that the wave structure observed in the 0–15° S region (upper panel) is similar to those observed in the 15–30° S region (lower panel). The MSTID observed by the ionogram (Fig. 3) and the dTEC keogram (Fig. 4) suggests that these waves perturbed the F-layer bottom height from 19:30 UT to 21:40 UT and generated plasma bubbles at around 22:00 UT. The observed horizontal wavelength of the MSTID was ~ 730 km, and the inter-bubble distance was approximately 640 km. Note that the occurrence of EPBs during the months of June–July in this longitudinal zone is rare (Nishioka et al. 2008).
Case study 2: March 08, 2015
Figure 5 presents TECMaps on the evening of March 08, 2015. The upper panel (at 23:40 UT) shows three consecutive bubbles (elliptic circles) behind the solar terminator (dotted line). These bubbles are newly formed and in the developing phase. The inter-bubble distance between bubbles 1 and 2 is approximately 770 km along the geomagnetic equator. Two hours later at 01:40 UT (lower panel), five well-developed bubbles can be seen. Note that they are separated by approximately 640 km, which is a smaller distance than those observed at 22:40 UT. Such periodic structures of EPBs have been frequently observed over South America (Takahashi et al. 2016). There have been discussions concerning the periodic structure of EPBs (e.g., Rottger 1973; Huang et al. 2013). Tsunoda et al. (2011) proposed the occurrence of a large-scale wave structure (LSWS) along the geomagnetic equator.
In order to investigate possible periodic disturbances in the F-layer bottom heights, dTEC keograms are shown in Fig. 6. From the upper panel (0–15° S), one can see three groups of dTEC perturbations with amplitudes of ± 0.2 TECu. The first perturbation occurred at 18:00 UT propagating equatorward. The second perturbations occurred at around 19:30–20:10 UT and do not show a clear propagation mode but rather a local oscillation at ~ 5–10° S. The third perturbation occurred at 21:10 UT propagating equatorward. Due to the low temporal and spatial resolution of dTEC map, it was not possible to obtain the wave characteristics from the keogram.
On the other hand, the keogram of the 15–30°S regions (lower panel) did show several wave structures from 17:30 UT to 23:00 UT. Note that there are southward propagating waves at around 17:50 UT, 19:50 UT, and 21:10 UT. It appears that these waves started from low latitudes (5–7° S). However, there is another wave structure at 22:00–23:00 UT (highlighted by a box) starting from ~ 25° S with a perturbation amplitude of ~ 0.3 TECu. It propagated north and intersected with the solar terminator at 300 km (broken line). We calculated its wave characteristics: a period of 22 min, a horizontal wavelength of 760 ± 50 km, a phase speed of 570 m/s, and a direction of propagation of 32° from the north. These are typical characteristics of an MSTID. Note that the horizontal wavelength is very close to the inter-bubble distance (770 km) observed at 23:40 UT soon after the terminator passed.
The ionogram at Fortaleza registered satellite traces starting at ~ 22:10 UT, and the spread F started at 23:00 UT. Figure 7 shows temporal variations in the true heights of the 5, 6, 7, and 8 MHz radio waves and d(hF). One can see that there are two distinct oscillation forms at around 18:00 UT and 19:30 UT, these are coincident with the dTEC keogram in Fig. 6 (upper panel). Conversely, the wave structure observed at 22:00–23:00 UT in the dTEC keogram (Fig. 6, lower panel) cannot be seen in the d(hF) oscillation. This might be due to the overlap of the strong evening F-layer uplifting. This observational evidence suggests that the plasma bubbles 1, 2, and 3 in Fig. 5, with an inter-bubble distance of ~ 770 km, are closely related to the F-layer perturbation generated by the MSTID at around 22:00–23:00 UT, which might have crossed the solar terminator at a longitude of 45–50° W.