Li et al. Progress in Earth and Planetary Science (2018) 5:11 Progress in Earth and https://doi.org/10.1186/s40645-018-0172-y Planetary Science RESEARCH ARTICLE Open Access Daytime F-region irregularity triggered by rocket-induced ionospheric hole over low latitude Guozhu Li1,2* , Baiqi Ning1,2,M.A.Abdu3,4, Chi Wang2,YuichiOtsuka5, Weixing Wan1,JiuhouLei6, Michi Nishioka7, Takuya Tsugawa7, Lianhuan Hu1,8,GuotaoYang2 and Chunxiao Yan2,9 Abstract Unexpected daytime F-region irregularities following the appearance of an ionospheric hole have been observed over low latitude. The irregularities developed initially above the F-region peak height (~ 360 km) with a thickness of about 30 km and an east-west extension of more than 200 km around 1057 LT and then expanded upward to 500 km altitude behaving like the equatorial spread-F (ESF) irregularities of the nighttime ionosphere. These daytime F-region irregularities cannot be explained on basis of an earlier suggestion that the F-region irregularities observed during daytime are the continuation of the irregularities initially generated on the previous night. Based on the coincidence, both in space and time, with the appearance of an ionospheric hole, which was generated after the passage of a rocket, we conclude that the daytime F-region irregularities must have been artificially generated locally through a manifestation of plasma instability triggered by the rocket exhaust-induced ionospheric hole over low latitude. Keywords: Daytime F-region irregularity, Ionospheric hole, Rocket exhaust Introduction in situ plasma density observations made during the Equatorial spread-F (ESF), which contains plasma geomagnetic storms of July 2004, Li et al. (2010)foundthat irregularities of wide-ranging scale sizes, commonly ESF irregularities were generated sequentially after sunset occurs in the nighttime equatorial ionospheric F-region over a large longitude region of about 180° (from American with a maximum occurrence during pre-midnight hours to Southeast Asian longitudes) and continued until late (Kelley 2009 and the references therein). For many years morning (~ 0900 LT). More recently, Huang et al. (2013)re- there have been only a few cases of F-region irregularities ported C/NOFS satellite in situ observations of long-lasting observed over equatorial and low latitudes during daytime ESF irregularities, which initially occurred at post-midnight (e.g., Fukao et al. 2003; Shume et al. 2013 and references and survived until noontime with a lifetime of 12 h. Most of therein). The daytime F-region irregularities can be cate- the daytime F-region irregularities reported in the literatures gorized into two types. One type is due to the long lifetime belong to this type and have been suggested to be triggered of ESF irregularities initially generated on the previous by electric field and/or wind disturbances associated with nightornearsunrisethroughtheRayleigh-Taylor instability geomagnetic activity (e.g., Abdu 2012). during active geomagnetic conditions (Fukao et al. 2003;Li Another type of daytime F-region irregularities was et al. 2010; Huang et al. 2013). By using total electron first observed by the Jicamarca radar (Woodman et al. content (TEC) observations from a globally distributed GPS 1985; Chau and Woodman 2001) and then by the Sao receiver network, together with ionosonde data and satellite Luis and Fuke VHF radars (Shume et al. 2013; Chen et al. 2017) during quiet geomagnetic conditions. These irregularities, which seem unlikely to cause any diffuse * Correspondence: [email protected] echo in ionosonde ionogram, were very rarely observed 1Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China and called as ESF-like irregularities (Chau and Woodman 2State Key Laboratory of Space Weather, National Space Science Center, 2001). Radar observations showed that the backscatter Chinese Academy of Sciences, Beijing, China echoes from these irregularities usually persist for a short Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Li et al. Progress in Earth and Planetary Science (2018) 5:11 Page 2 of 9 time of 1–2 h in radar range-time-intensity (RTI) maps and calculated offline from the complex raw signal returns of the have extremely narrow Doppler spectral width and nearly radar by employing a 256-point FFT algorithm and the zero Doppler velocity, except the case by Shume et al. (2013) moment method (e.g., Ning et al. 2012), with velocity where a large upward Doppler velocity of more than 100 m/ window of ± 160 m/s and velocity resolution of ~ 1.25 m/s. s was detected. Due to the rarity of daytime F-region irregu- larities, and the difficulty in determining when and where Results these irregularities are initially generated, their generation Figure 1a shows the Fuke radar (beam 6) observations mechanism still remains unresolved. on May 30, 2016, when the daytime F-region irregular- In this paper, we report observations of artificially gen- ities were detected. As a comparison, a typical case of erated daytime F-region irregularities on May 30, 2016 nighttime ESF irregularities observed by the radar (beam (the minimum Dst − 16 nT), by the Fuke VHF coherent 6) on October 26, 2015, is shown in Fig. 1b. The F- scatter radar (19.3° N, 109.1° E; dip latitude 14° N). This region peak heights (hmF2) obtained from the Sanya radar, with a multi-beam steering capability, is able to ionosonde (situated at a distance of about 120 km from distinguish the F-region irregularities generated locally Fuke) are superposed in the two RTI plots. As shown in from those drifted from elsewhere. Unlike the daytime Fig. 1a, the daytime F-region irregularities, which got F-region irregularities reported previously that were initiated at 1057 LT, were seen as a backscattering layer often suggested to be the continuation of naturally oc- around 360 km altitude, which then gradually rose to curring nighttime irregularities (e.g., Li et al. 2010; higher altitudes. There are two possibilities that could Huang et al. 2013; Chen et al. 2017), the present F- produce an ascending structure of backscatter echoes in region irregularities, with Doppler spectra similar to that the radar RTI map. One is linked with an irregularity of the ESF-like irregularities (e.g., Chau and Woodman layer elevated to higher altitude in the radar field of 2001), are demonstrated to be initially generated during view, and another is that an irregularity layer situated at daytime. Most interestingly, about 12 min before the constant altitudes but tilted in longitude and drifts generation of F-region irregularities, an ionospheric hole across the radar field of view. For the present case, the identified by TEC reductions with magnitudes of up to Fuke radar multi-beam measurements rule out the latter ~ 23% was detected by a global navigation satellite possibility because no tilted structure was detected in system (GNSS) receiver network over low latitude. The the fan sector maps (which will be shown in Fig. 2). track of the ionospheric hole was found to be in good It is seen from Fig. 1a that in about 36 min, the irregu- coincidence with the track left by passage of a rocket larity layer rapidly evolved into a nearly vertical irregu- both in space and time. We discuss possible mechanisms larity structure extending up to 500 km altitude into the responsible for the present daytime F-region irregularities, topside ionosphere. Such a vertical structure of the day- which evolved from the rocket exhaust-induced iono- time F-region irregularities appears similar to that of the spheric hole. nighttime ESF irregularities shown in panel b. The night- time (post-sunset) ESF structure started from F-region Methods/Experimental bottomside where positive (upward) plasma density The Fuke VHF radar, with a peak power of 54 kW and an gradients favoring for plasma instability development operational frequency of 47 MHz, was installed in 2010 usually exist and grew up to 450 km into the topside. under the support of the Chinese Meridian Project to study However, for the present case, the starting altitude of the the low-latitude ionospheric irregularities (Wang 2010). On daytime irregularity structure is near/above the F-region the day (May 30, 2016) when the F-region irregularities were peak (~ 360 km) where the background plasma density observed, the radar was operated with a 7-beam steering gradient is usually not favorable for plasma instability. mode that formed a field of view of 45° (in east-west direc- The cause why the daytime F-region irregularities tion) to observe E- and F-region plasma irregularities. The started from F-region peak will be discussed in the later major operational parameters were set as follows: a pulse section. Figure 1c–f show the Doppler velocities and repetition frequency of 200 Hz, 13-bit Barker coded pulse, spectral widths as functions of range and time. An coherent integration number 2, and a range resolution of example of Doppler spectra of the daytime irregularities 0.711 km. The seven beams were directed at the azimuth measured at 1133 LT (0415 UT) on May 30, 2016, at angles of 22.5°, 15°, 7.5°, 0°, − 7.5°, − 15°, and − 22.5°, respect- three different height regions, is inset in the left middle ively (at a zenith angle of 28°). Considering the wide 3 dB panels.