For the present case, the Fuke radar multi-beam steering measurements clearly reveal that the daytime F-region irregularities were not the continuation of nighttime ESF irregularities but generated locally over the longitudes scanned by the radar during 1057–1151 LT (0339–0433 UT). Before we discuss possible mechanisms responsible for the daytime F-region irregularities, it may be relevant to mention the two possibilities for the generation of nighttime F-region irregularities over low latitude: (1) The ESF plasma bubble generation after sunset in equatorial F-region through the generalized Rayleigh-Taylor instability and their magnetic field line mapping to low latitudes in both hemispheres where meter scale irregularities develop locally through the non-linear cascade process (see also Otsuka et al. 2002; Patra and Phanikumar 2009). This is the case for the F-region irregularities observed after sunset during equinoctial months (ESF high occurrence season) over Southeast Asia low latitude (for example Fuke and Sanya) (e.g., Li et al. 2012, 2016); and (2) Meter scale irregularities generated locally over low latitude not being associated with large scale plasma bubbles. The F-region irregularities observed at midnight/post-midnight during summer months over low/middle latitudes in East/Southeast Asia have been suggested to be generated locally by the medium-scale traveling ionospheric disturbance (MSTID) and driven by gradient-drift instability mechanism (e.g., Yokoyama et al. 2011 and the references therein).
For the present F-region irregularities that developed during daytime, the key question lies in the fact that the normally present perturbation factors under the daytime F-region conditions are expected to be too weak to favor any instability development over equatorial and low latitudes (e.g., Kelley and Ilma 2016). As regards the initiation and growth of the Rayleigh-Taylor instability over magnetic equator and of the MSTID-driven gradient drift instability over low latitude, an important requirement is plasma density perturbation with its associated polarization electric field being present at the ionospheric F-region. However, during daytime, the E-region conductivity is obviously larger than that of nighttime that can lead to a reduction of the F-region polarization electric fields (Kelley 2009). Under such condition, the factors, for example the plasma density gradient and/or its associated polarization electric field required to initiate instability growth during daytime, operating against the inherently stable conditions of the daytime ionosphere, should have abnormally larger amplitude than that required for instability growth in the nighttime ionosphere.
In this context we may note from Fig. 1g that an obvious TEC reduction was observed just prior to the detection of the daytime F-region irregularities. To obtain the spatial coverage of TEC reduction, we investigated the TEC data obtained from GNSS receivers and Beidou geostationary satellite receivers situated around Southern China. Figure 3a shows the geographic locations of ionospheric pierce points (IPP, shown as curves and dots) for the GNSS satellite-receiver links (with elevation angle > 30°) measured during the period 0315–0345 UT (1033–1103 LT), May 30, 2016. Note that the dots represent the fixed IPPs for the Beidou geostationary satellite-receiver links. From this plot we can see that TEC reductions (marked by blue) were detected at latitudes higher than ~ 12–13° N, around the longitudes where the daytime F-region irregularities were observed. Figure 3b shows the temporal variation of TEC reductions measured at the fixed IPPs from higher to lower latitudes (marked by blue dots in Fig. 3a). Two notable features can be seen from this plot: (1) the TEC reductions initially appeared around 1045 LT (0327 UT, marked by a blue dotted vertical line) in a wide latitude region, about 12 min before the generation of daytime F-region irregularity; and (2) the time when the TEC maximum reductions were observed (marked by a red dotted vertical line) corresponds to the initial generation of daytime F-region irregularity (with the duration indicated by a blue horizontal bar). During the presence of daytime F-region irregularities (3-meter scale), no amplitude scintillations (small S4) and obvious fast fluctuations of TEC (small ROTI) were detected around the region where the 3-meter scale irregularities were observed. Due to the significant recovery of background plasma density (increased by ~ 10 TECu in 1 h), the absolute perturbation density induced by the irregularities could be very low and hard to be detected in TEC.
As regards the generation of TEC reductions, we note from the cctv news (tv.cctv.com) that China launched a sun-synchronous satellite using a Long March-4B rocket from Taiyuan on May 30, 2016. The arrow superposed in Fig. 3a represents the approximate trajectory of the rocket projected onto the Earth’s surface (from news press). The rocket passed over southern China (near the location where the daytime F-region irregularities were observed) around 1045 LT (0327 UT), being well coincident with the detection of TEC reductions both in space and time. Further details on the rocket trajectory and exhaust information are beyond the scope of this study and will not be addressed here. It is well known from earlier studies that the rocket exhaust, which consists of CO2, H2O, and N2, could cause quick reduction in the background ionospheric F-region primary ion (O+) density through chemical reactions and produce an ionospheric hole (e.g., Bernhardt et al. 2001; Mendillo et al. 2008; Nakashima and Heki 2014). The coincident observations of the TEC reductions and the rocket passage, both in time and space, reveal that the TEC reductions must be associated with a background ionospheric hole produced by the rocket exhaust. The hole should be centered around the rocket release altitude (e.g., Bernhardt et al. 1998). For the present case, the rocket altitude (near the location where the daytime F-region irregularities were observed) is approximately 350 km, right near the F-region peak height.
Based on the sequential observations of ionospheric hole, followed by the appearance of thin irregularity layer evolving into plume-like irregularities expanding to high altitude, which occurred promptly after the passage of the rocket over Southern China, we may conclude that the daytime F-region irregularities in the present study must have been artificially triggered by the rocket exhaust-induced ionospheric hole around the F-region peak (~ 350 km). Although earlier rocket release experiments have shown the occurrences of artificially generated F-region irregularities (e.g., Bernhardt et al. 2012), the present F-region irregularities were observed at daytime with a form similar to that of naturally generated ESF in radar RTI map. Based on the observations of TEC reductions shown in Fig. 3b, the fractional decrease in the local electron density, constituting the present ionospheric hole, could provide plasma density gradients significantly larger than that required for nighttime F-region irregularity development. In Fig. 3b, we note that the maximum TEC reduction is about 7 TECu that represents a perturbation by 23%, of the total unperturbed TEC value, which is ~ 30 TECu. This value is significantly larger than the perturbation (of about 2–5%) nominally required for F-region irregularity development under typical post-sunset/nighttime conditions (e.g., Zalesak et al. 1982; Yokoyama et al. 2011). The deep ionospheric hole representing significant depletion in the background plasma density that is obtained here in fact describes a plasma density depletion structure necessarily with large upward density gradient around the F-region peak. By using the simultaneous observations of 3-meter scale irregularities and of plasma density depletions with the low latitude Kototabang VHF radar and all-sky airglow imager, respectively, Otsuka et al. (2004) reported that the most intense backscatter (induced by 3-meter scale irregularities) came from the deepest plasma density depletion region. They suggested that the 3-meter scale irregularities were generated through the lower-hybrid-drift instability. For the present study, the lower-hybrid-drift instability could also operate in the deepest depletion region with large density gradients and produce the 3-meter scale irregularities.
According to the empirical F-region plasma drift model (Fejer et al. 2008), the plasma drifts upward during daytime under the background eastward electric field. Through investigating the daytime 150 km echoes from Gadanki and Kototabang VHF radars, Patra et al. (2012) reported that the upward drifts over Kototabang (10° away from Fuke in longitude) range between 10 and 20 m/s at 1100 LT. For the present case, the uplift of the irregularity layer with a velocity of about 30 m/s, shown in Fig. 2d–f, is larger than the upward drifts of background plasma shown in Patra et al. (2012). Under the force of gravity and the Pedersen current due the background eastward electric field, a relatively more intense polarization zonal (eastward) electric field could be set up in the deep ionospheric hole characterized by a significant TEC perturbation, 23% of the total unperturbed TEC. Earlier Millstone Hill ISR measurements have provided direct observational evidences on the sustenance of daytime F-region polarization electric field (Buonsanto 1994). For the present case, if the polarization electric field generated in the deep ionospheric hole was not well shorted out by the E-region conductivity, it would enhance the upward drift. It has been shown that the sustainability of F-region polarization electric field would depend upon the ratio of the field line-integrated conductivity of the F-region to that of the sum of F- and E-regions (that is, ∑PF /∑PE + F) (Kelley 2009). Whereas the E-region conductivity at daytime is obviously larger than that of nighttime, detailed calculation by Abdu et al. (2015) based on a low-to-mid latitude ionospheric model simulated by the SUPIM (Sheffield University Plasmasphere Ionosphere Model, Bailey et al. 1997) has shown that the value of the ratio ∑PF/∑PE + F can be of the order of 0.8–0.9 under near-midday conditions, thereby assuring the prevalence of a major portion of the F-region polarization electric field. We surmise that the extremely large density gradient as characterized by the ionospheric hole, together with the potentially survived eastward polarization electric field, could be intense enough to upset the stability of the normal daytime ionosphere, thereby leading to the accelerated growth of F-region irregularities that promptly followed, as observed by the Fuke radar. Further efforts will be made to better understand this type of phenomenon in the future.