Wavenumber-4 structures observed in the low-latitude ionosphere during low and high solar activity periods using FORMOSAT/COSMIC observations

Abstract

Abstract. The main purpose of this study is to investigate the four-peak structure observed in the low-latitude equatorial ionosphere by the FORMOSAT/COSMIC satellites. Longitudinal distributions of NmF2 (the density of the F layer peak) and hmF2 (ionospheric F2-layer peak height) averages, obtained around September equinox periods from 2007 to 2015, were submitted to a bi-spectral Fourier analysis in order to obtain the amplitudes and phases of the main waves. The four-peak structure in the equatorial and low-latitude ionosphere was present in both low and high solar activity periods. This kind of structure possibly has tropospheric origins related to the tidal waves propagating from below that modulate the E-region dynamo, mainly the eastward nonmigrating diurnal tide with wavenumber 3 (DE3, "E" for eastward). This wave when combined with the migrating diurnal tide (DW1, "W" for westward) presents a wavenumber-4 (wave-4) structure under a synoptic view. Electron densities observed during 2008 and 2013 September equinoxes revealed that the wave-4 structures became more prominent around or above the F-region altitude peak ( ∼ 300-350 km). The four-peak structure remains up to higher ionosphere altitudes (∼ 800 km). Spectral analysis showed DE3 and SPW4 (stationary planetary wave with wavenumber 4) signatures at these altitudes. We found that a combination of DE3 and SPW4 with migrating tides is able to reproduce the wave-4 pattern in most of the ionospheric parameters. For the first time a study using wave variations in ionospheric observations for different altitude intervals and solar cycle was done. The conclusion is that the wave-4 structure observed at high altitudes in ionosphere is related to effects of the E-region dynamo combined with transport effects in the F region.

Seasonal Variations of Low‐Latitude Migrating and Nonmigrating Diurnal and Semidiurnal Tides in TIMED‐SABER Temperature and Their Relationship With Source Variations

“…In the present study, both have downward phase propagation from lower heights indicating that the tides have source in the lower atmosphere. Recently, Onohara et al () suggested that the wave number 4 signature in the ionosphere can be due to the combination of DE3, stationary planetary wave (SPW) of wave number 4 with migrating tides. Earlier, Oberheide et al () revealed that SE2 and SPW4 could also form wavenumber 4 signatures.…”

The Seasonal and Longitudinal Variations of Nighttime OI 135.6‐nm Emission at Equatorial Ionization Anomaly Crests Observed by the DMSP/SSUSI

“…From the Table 1, the amplitudes of Wavenumbers 1 and 2 components are generally greater than Wavenumbers 3 and 4. Even so, only Wavenumbers 3 and/or 4 structures have been widely investigated (e.g., Chang et al, 2013; Fang et al, 2009; Immel et al, 2006; Jin et al, 2008; Onohara et al, 2018; Pedatella et al, 2012; Ren et al, 2009; Sagawa et al, 2005; Wang et al, 2015; Xiong & Lühr, 2013). Why the previous researchers did not put too much attention into the Wavenumbers 1 and 2?…”

“…However, some exclusive studies (e.g., England et al, 2006; Fang et al, 2009; Hagan et al, 2007; Jin et al, 2008; Ren et al, 2009; Wan et al, 2008) suggested that the Wavenumber 4 structure is due to the dynamo action of the eastward propagating diurnal tide of wave number 3 (DE3), which is mainly excited by the latent heat released in the tropical troposphere (Hagan & Forbes, 2002). Besides, some other factors, such as the semidiurnal nonmigrating tides of zonal wave number 2 (SE2), stationary planetary wave number 4 (SPW4) (e.g., T. Chen et al, 2019; Oberheide et al, 2011; Onohara et al, 2018; Pedatella et al, 2012), may also produce the Wavenumber 4 structure in the ionosphere.…”

Dynamical coupling between the low-latitude lower thermosphere and ionosphere via the non-migrating diurnal tide as revealed by concurrent satellite observations and numerical modeling

“…When viewed at quasi-fixed local time (LT) from slowly precessing satellites, this feature manifests as a 4-peak longitude structure. Subsequent studies examined the LT and seasonal variations of related WN4 and WN3 structures (e.g., Ren et al, 2009), and investigated the underlying mechanisms in further depth (e.g., Maute et al, 2012;Mukhtarov & Pancheva, 2011;Oberheide et al, 2011;Onohara et al, 2018).…”