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).

On January 15, 2022, we observed various unusual atmospheric wave events over South America: Atmospheric pressure waves (Lamb mode) around 12:30 to 17:30 UT, tsunamis along the Chilean coast at around 17:00 to 19:00 UT, and ionospheric disturbances between 11:30 and 20:00 UT. We understand that these events were generated by the Tonga volcanic eruption that occurred at (20.55°S, 175.39°W) in South Pacific Ocean at 04:15 UT. Several traveling ionospheric disturbances (TIDs), the horizontal wavelengths of 330 to 1174 km and the phase speed of 275–544 m/s were observed before and after the Lamb wave passed over the continent and the arrival of the tsunami on the Chile coast. The observed TID characteristics suggest us that these waves might be generated by the two atmospheric events, Lamb wave and gravity waves induced by the tsunamis. This is the first time to report the signature of ionospheric disturbances over the South American continent generated by the huge volcanic eruption. Graphical Abstract

To cite this version:T.T. Ayorinde, A. B Rabiu, Christine Amory-Mazaudier. Inter-hourly variability of total electron content during the quiet condition over Nigeria, within the equatorial ionization anomaly region. AbstractThe inter-hourly variability (IHV) of the Total Electron Content (TEC) over Nigeria during the quiet days (Ap<4) of the year 2013 was examined using ground-based GPS receivers installed at seven (7) different locations across Nigeria by the Nigerian Global Navigation Satellite Systems (GNSS) Reference Network (NIGNET) operated by the office of the surveyor general of Nigeria. Nigeria is a country that lies within equatorial ionospheric anomaly (EIA) region. The IHV was calculated by converting the observed hourly slant TEC (STEC) value into the hourly vertical TEC (VTEC) and the differencing (∆TEC) with its corresponding hourly value from the previous day. There is a clear variation which depicts the expected temporal variability. The IHV in TEC in all the stations ranges between 0-20 TECU (TEC Units). The seasonal variation of the IHV of TEC over Nigeria maximizes (5-20 TECU) during Equinoctial months and minimizes (1-10 TECU) during the Solstice months.The IHV of TEC in September equinox period is higher than that of March equinox. Minimum value of IHV (~7 TECU at equinoxes and ~5 TECU at Solstice) was recorded at the Office of Surveyor General of the Federation (OSGF) station and the maximum value (~12 TECU at equinoxes and ~16 TECU at Solstice) was recorded at the Birni Kebbi Federal Polytechnic (BKFP) station which may be due to the fact that BKFP at 0.72° dip latitude is closer to the dip equator.

On January 15, 2022, we observed various unusual atmospheric wave events over South America: Atmospheric pressure waves (Lamb mode) around 12:30 to 17:30 UT, tsunamis along the Chilean coast at around 17:00 to 19:00 UT, and ionospheric disturbances between 11:30 and 20:00 UT. We understand that these events were generated by the Tonga volcanic eruption that occurred at (20.55°S, 175.39°W) in South Pacific Ocean at 04:15 UT. Several traveling ionospheric disturbances (TIDs), the horizontal wavelengths of 330 to 1174 km and the phase speed of 275 to 544 m/s were observed before and after the Lamb wave passed over the continent and the arrival of the tsunami on the Chile coast. The observed TID characteristics suggest us that these waves might be generated above the mesosphere far from the observation site. This is the first time to report the signature of ionospheric disturbances over the South American continent generated by the huge volcanic eruption.