Long-Term Study on Medium-Scale Traveling Ionospheric Disturbances Observed over the South American Equatorial Region

Abstract

Using data collected by the GNSS dual-frequency receivers network, de-trended TEC maps were generated to identify and characterize the medium-scale traveling ionospheric disturbances (MSTIDs) over the South American equatorial region (latitude: 0∘ to 15∘ S and longitude: 30∘ to 55∘ W) during solar cycle 24 (from January 2014 to December 2019). A total of 712 MSTIDs were observed during quiet geomagnetic conditions. The Frequency of occurrence of MSTID is high during the solar maximum and low in the minimum phase. This might be due to the solar cycle dependence of gravity wave activity in the lower atmosphere and gravity wave propagation conditions in the thermosphere. The predominant daytime MSTIDs, representing 80% of the total observations, occurred in winter (June-August season in the southern hemisphere) with the secondary peak in the equinox; while the evening time MSTIDs, representing 18% of the entire events, occurred in summer (December to February season) and equinox (March to May and September to November), and the remaining 2% of the MSTIDs were observed during nighttime. The seasonal variation of the MSTID events was attributed to the source mechanisms generating them, the wind filtering and dissipation effects, and the local time dependency. The horizontal wavelengths of the MSTIDs were mostly concentrated between 500 and 800 km, with the mean value of 667 ± 131 km. The observed periods ranged from 30 to 45 min with the mean value of 36 ± 7 min. The observed horizontal phase speeds were distributed around 200 to 400 m/s, with the corresponding mean of 301 ± 75 m/s. The MSTIDs in the winter solstice and equinoctial months preferentially propagated northeastward and northwestward. Meanwhile, during the summer solstice, they propagated in all directions. The anisotropy of the propagation direction might be due to several reasons: the wind and dissipative filtering effects, ion drag effects, the primary source region, and the presence of the secondary or tertiary gravity waves in the thermosphere. Atmospheric gravity waves from strong convective sources might be the primary precursor for the observed equatorial MSTIDs. In all seasons, we noted that the MSTIDs propagating southeastward were probably excited by the likely gravity waves generated by the intertropical convergence zone (ITCZ).

Climatology of Medium-Scale Traveling Ionospheric Disturbances (MSTIDs) Observed with GPS Networks in the North African Region

“…They reported an increase in amplitude with an increase in solar activity, and that the amplitude tends to increase with increasing auroral electrojet (AE) index, and also found that MSTIDs amplitude is dominantly high at daytime. The increasing MSTIDs with increase in solar activity in the current result also agrees with Essien et al (2021) where they reported a long-term study of MSTIDs occurrence rate during 2014-2019 over South American equatorial region (Lat: 0°-15°S and Long: 30°-55°W). They concluded that MSTIDs increases with increasing solar activity.…”

“…However, the nighttime MSTIDs exhibited highest occurrence rate observed in June solstice during * 2100-0200 LT and * 1900-0200 LT at northwest and northeast, respectively. In the current study, the magnitude of MSTIDs generally increases with increase in solar activity (Essien et al, 2021;Oinats et al, 2016). Our MSTIDs seasonal occurrence results show a larger part of agreement with the MSTIDs investigation conducted by Tsugawa et al (2006) who reported MSTIDs occurrence over South-East Asian sector (Japan).…”

“…In addition, Figs. 6 and 9 have clearly revealed the progression in the growth rate of MSTIDs with increase of solar activity which agrees with Oinats et al (2016) and Essien et al (2021) as stated earlier. The MSTIDs maximizes at 2014, the peak of solar activity of solar cycle #24 and minimizes at 2008, the beginning of the minimum year of the cycle.…”

Daytime Medium Scale Traveling Ionospheric Disturbances (MSTIDS) Over the Andes Mountains at Equatorial and Low Magnetic Latitudes

“…Theoretically, this phenomenon was first investigated in the 1960s by the pioneering work of Hines (1960), although the first observation of the wavelike disturbances was in the 1940s (Munro, 1948). In the last 10 years, dozens of theoretical and observational studies in different geographic regions have been carried out using diverse instruments, such as ionosondes (e.g., Amorim et al., 2011; Sherstyukov et al., 2018), satellites (e.g., Bolmgren et al., 2020), incoherent scatter radars (e.g., Negale et al., 2018; Nicolls et al., 2010, 2014; Vadas & Nicolls, 2009), and all‐sky airglow imagers (e.g., Figueiredo et al., 2018b; Martinis et al., 2019; Paulino et al., 2016; Smith et al., 2013; Terra et al., 2020), total electron content (e.g., Azeem et al., 2015, 2017; Chen et al., 2019; Essien et al., 2021, 2022; Figueiredo et al., 2018a; Sivakandan et al., 2021), and Doppler radar sounder (Crowley & Rodrigues, 2012; Vadas & Crowley, 2010).…”

Intertropical Convergence Zone as the Possible Source Mechanism for Southward Propagating Medium-Scale Traveling Ionospheric Disturbances over South American Low-Latitude and Equatorial Region

“…In the middle latitudes, day-and nighttime MSTIDs have been reported to be generated via atmospheric gravity waves propagating in the F-layer bottom height [10][11][12][13] and Perkins instability [14][15][16], while at high latitudes, MSTIDs have been attributed to the geomagnetic storms and polar electroject [17]. Several authors have suggested that the possible source regions for equatorial and low-latitude MSTIDs, especially those propagating southeastward, are linked to the gravity waves in the thermosphere that originate from the ITCZ [10,[18][19][20]. Knowing and understanding the sources of MSTIDs is of utmost importance for unraveling the complexities of the neutral atmosphere and ionosphere coupling and, eventually, the prediction of the climatology of the occurrence of MSTIDs in the future.…”