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Equatorial Plasma Bubble Seeding by Mstids in the Ionosphere

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Equatorial Plasma Bubble Seeding by Mstids in the Ionosphere Takahashi et al. Progress in Earth and Planetary Science (2018) 5:32 Progress in Earth and https://doi.org/10.1186/s40645-018-0189-2 Planetary Science RESEARCHARTICLE Open Access Equatorial plasma bubble seeding by MSTIDs in the ionosphere Hisao Takahashi1* , Cristiano Max Wrasse1, Cosme Alexandre Oliveira Barros Figueiredo1, Diego Barros1, Mangalathayil Ali Abdu1, Yuichi Otsuka2 and Kazuo Shiokawa2 Abstract Occurrences of equatorial plasma bubbles (EPBs) and medium-scale traveling ionospheric disturbances (MSTIDs) were studied using GPS satellite data-based total electron content mapping, ionograms, and 630 nm all-sky airglow images observed over the South American continent during the period of 2014–2015. In many cases, we observed a close relationship between the inter-bubble distance and the horizontal wavelength of the MSTIDs. The MSTIDs followed by EPBs occurred primarily in the afternoon to evening period under strong tropospheric convective activities (cold fronts and/or intertropical convergence zones). The close relationship between EPBs and MSTIDs suggests that MSTIDs could be one of the seeding sources of EPBs. Keywords: Equatorial plasma bubbles, MSTID, Gravity waves Introduction resulting in the generation of EPBs (Abdu et al. 2015). Equatorial plasma bubbles (EPBs) are a consequence of On the other hand, Kudeki et al. (2007) presented the the development of equatorial ionospheric irregularities importance of eastward thermospheric winds during the under the condition of the Rayleigh–Taylor instability sunset period (the sunset vortex) for the generation of (RTI). During the evening, the passage of the solar F-layer instabilities and EPB seeding. terminator produces F-layer uplifting driven by the For the RTI condition scenario, the development and pre-reversal enhancement of the electric field (PRE). The physical characteristics of EPBs have been extensively upward drift velocity and the F-layer altitude make the studied and discussed and there exists some common RTI conditions favorable for the development of EPBs knowledge (e.g., Abdu 2001). However, regarding RTI (Abdu 2005; Kelley 2009). seeding processes, discussions concerning the dynamical In addition to the RTI condition, there must be a seed- processes are still open. Rottger (1973)reporteda ing process that kicks the F-layer bottom side to initiate quasi-periodic spread F structure in the west–east perturbations. Many previous studies have highlighted extension, which could be explained by atmospheric possible seeding processes, such as contributions of gravity waves propagating in the thermosphere. Singh atmospheric gravity waves in the ionosphere (Rottger et al. (1997) reported a possible mechanism for the 1973; Singh et al. 1997), the passage of the solar termin- periodic ion density perturbation caused by gravity ator (Tulasi Ram et al. 2014), and medium-scale travel- waves, which then develop into plasma bubbles. Most ing ionospheric disturbances (MSTIDs) (Taori et al. recently, Abdu et al. (2015) discussed atmospheric 2015). Tsunoda et al. (2011) pointed out the presence of gravity waves in the ionosphere as a potential source large-scale wave structures (LSWSs) along the magnetic to excite plasma bubbles. equator on the afternoon to evening side. Vertical oscil- Medium-scale traveling ionospheric disturbances lations of the longitudinal waves increase toward the (MSTIDs) are plasma density perturbations propagating evening side and may accelerate F-layer uplifting, in the ionosphere. They have been classified into two groups (Otsuka et al. 2013). The first group is generated * Correspondence: [email protected] via electrodynamical forces under the condition of the 1Instituto Nacional de Pesquisas Espaciais, INPE, São José dos Campos, SP, Perkins instability (Perkins 1973) during the evening to Brazil nighttime. This group is primarily observed in middle 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. Takahashi et al. Progress in Earth and Planetary Science (2018) 5:32 Page 2 of 13 latitudes, and their propagation direction is primarily regions were related to tropospheric convection. Paulino southwest in the northern hemisphere and northwest in et al. (2016) observed nighttime wave propagation in the the southern hemisphere (Otsuka et al. 2004; Shiokawa 630 nm emission layer and reported that these MSTIDs et al. 2005; Duly et al. 2013), even though it depends on have different characteristics compared to those originating the background thermospheric winds. These MSTIDs from the Perkins instability in the ionosphere, suggesting are characterized by a period of 15–60 min, a horizontal the presence of gravity wave propagation in the wavelength of 100–1000 km, and a phase speed of 100– thermosphere. Further, their recent work (Paulino et al. 300 m/s (Hunsucker 1982). The second group is gener- 2018) reinforced the propagation of gravity waves in ated via atmospheric gravity waves propagating in the thethermospherefrombelowviameasurementsofthe F-layer bottom height (Otsuka et al. 2013). The propaga- intrinsic wave parameters. tion direction depends on the gravity wave source region MSTIDs as a possible candidate to generate plasma and the background winds in the lower atmosphere. bubbles have been discussed by several groups. Krall Fukushima et al. (2012) reported that most MSTID et al. (2011) simulated plasma bubble seeding using the events observed in the equatorial and low latitude effect of MSTIDs during the post-sunset hours. Taori Fig. 1 TECMap over South America on the evening of July 05, 2014, at 23:10 UT. The black line indicates the geomagnetic equator, and the broken line indicates the solar terminator line at an altitude of 300 km. The color shade scale is from TEC unit 01 (dark blue) to 40 (dark red). The red-lined elliptical circles depict the location and extension of the EPBs Takahashi et al. Progress in Earth and Planetary Science (2018) 5:32 Page 3 of 13 et al. (2015) observed EPBs and MSTIDs simultaneously that the spacing could be related to the horizontal in the Indian sector and argued that the observed wavelength of the MGWs. Therefore, our present interest MSTIDs could be one of the seeding processes. Obser- is to explore the relationship between the inter-bubble vational evidence of EPB seeding by MSTIDs is, distances and MSTIDs in the ionosphere. however, rare. The conditions and types of MSTIDs In the present study, we analyze EPB seeding and effective in seeding EPBs are still not well known. development under the propagation of MSTIDs. The A close relationship between the inter-bubble distances initial phase of EPB development was detected by moni- (measured via 630 nm airglow images) and the horizontal toring total electron content (TEC) maps (depletions of wavelength of the mesospheric gravity waves (MGWs) TEC larger than 20% of the ambient TEC field). MSTIDs (measured via OH images) has been discussed by Takahashi were identified via small-scale perturbations (amplitudes et al. (2009)andPaulinoetal.(2011). They argue that the of less than 0.5 TEC units) with wavelike structures with gravity waves in the upper mesosphere (~ 90 km) may propagation modes. From the same day presence of propagate up to the lower thermosphere modulating the MSTIDs during the afternoon to evening and EPBs in F-layer bottom height and generating EPBs. Makela et al. the evening to night period, we analyzed their character- (2010) observed the periodic spacing of EPBs via a 630 nm istics to determine common factors, such as horizontal imager at Cerro Tololo (30.2° S, 70.8° W) and mentioned Fig. 3 Temporal variations in the fixed frequency (5–8MHz)true Fig. 2 Signature of the equatorial plasma bubbles observed by the heights, hF (upper), and deviation of hF (band-pass filtered (30 min–3h) ionogram (upper panel) at São Luis (2.6° S, 44.2° W) at 22:20 UT and (lower) observed at São Luis in the afternoon to evening period on July the 630 nm airglow image (lower panel) observed at Cariri (7.4° S, 05, 2014. The vertical dotted line indicates the solar terminator at an 36.5° W) at 02:49 UT (23:49 LT) on July 05–06, 2014 altitude of 300 km Takahashi et al. Progress in Earth and Planetary Science (2018) 5:32 Page 4 of 13 wavelengths and inter-bubble distances. Two case stud- and 50 km for the latter region. We classified the spatial ies and statistical analyses are presented. and temporal variations of dTEC as an MSTID if (1) the amplitude of the oscillation was larger than 0.2 TEC Methods/Experimental units (TECu) and (2) the wave crest extended longer In the present study, we used TEC map (TECMap) data than 500 km followed by a wave valley and there were at obtained by EMBRACE (Brazilian Studies and Monitoring least two propagating wave crests. These criteria are of Space Weather (http://www2.inpe.br/climaespacial/). similar to those presented by Otsuka et al. (2013). The TECMap covers an area of 30–70° W and 10° N–35° S threshold level of 0.2 TECu was chosen because most of with a temporal resolution of 10 min. The spatial the wave structures were identified with amplitudes resolution of the map depends on the density of the larger than that. The dTEC noise level was less than 0.1 ground-based GPS receivers. Spatial resolutions of 50– TECu. The calculation process has been reported by 100 km at low to middle latitudes (15–35° S) and larger Figueiredo et al. (2017). To visualize the perturbation in than 100 km in the equatorial region (10° S–10° N) space and time, N–S- and E–W-sliced dTECs with 0.5° were obtained.
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