Semi-annual oscillation (SAO) of the nighttime ionospheric D region as detected through ground-based VLF receivers

Abstract

Abstract. Earth's middle and upper atmosphere exhibits several dominant large-scale oscillations in many measured parameters. One of these oscillations is the semi-annual oscillation (SAO). The SAO can be detected in the ionospheric total electron content (TEC), the ionospheric transition height, the wind regime in the mesosphere-lower thermosphere (MLT), and in the MLT temperatures. In addition, as we report for the first time in this study, the SAO is among the most dominant oscillations in nighttime very low frequency (VLF) narrowband (NB) subionospheric measurements. As VLF signals are reflected off the ionospheric D region (at altitudes of ∼ 65 and ∼ 85 km, during the day and night, respectively), this implies that the upper part of the D region is experiencing this oscillation as well, through changes in the dominating electron or ion densities, or by changes in the electron collision frequency, recombination rates, and attachment rates, all of which could be driven by oscillatory MLT temperature changes. We conclude that the main source of the SAO in the nighttime D region is NO x molecule transport from the lower levels of the thermosphere, resulting in enhanced ionization and the creation of free electrons in the nighttime D region, thus modulating the SAO signature in VLF NB measurements. While the cause for the observed SAO is still a subject of debate, this oscillation should be taken into account when modeling the D region in general and VLF wave propagation in particular.

Variability of the Brunt–Väisälä frequency at the OH<sup>∗</sup>-airglow layer height at low and midlatitudes

“…The semi-diurnal tide reaches its highest amplitudes from November to February. These results are supported by Silber et al (2017), who show in their Fig. 7 that tidal amplitudes in general are relatively low during summer for 4 years of GRIPS (Ground-based Infrared P-branch Spectrometer) data in Tel Aviv (32.1 • N, 34.8 • E).…”

“…Also the phases of the diurnal and semi-diurnal tides vary to some extent (see again Pancheva et al, 2009, andMukhtarov et al, 2009, for measurements between 50 • S and 50 • N). Silber et al (2017) depict in their Fig. 9 that the phases of the diurnal and the semi-diurnal tide are at least over some time relatively stable (the tides are approximated by a cosine starting at 12:00 LT; the phases approach values near zero on average).…”

On the Use of VLF Narrowband Measurements to Study the Lower Ionosphere and the Mesosphere–Lower Thermosphere

“…They concluded that the origin for these observations is in changes in the nighttime electron density due to the effect of neutral minor constituents in the mesosphere, probably atomic oxygen. However, recently Silber et al [2016] observed a dominant (>3.3 dB from peak-to-peak) semi-annual oscillation in VLF NB midnight amplitudes of two long (>1900 km) TRGCPs (located in both hemispheres). Each of these datasets spanned over more than 4 years.…”

“…Unlike Pintado et al [1987], they suggested that NOx transport from the lower thermosphere is the main driver of this oscillation in the D-region. The disagreement between Silber et al [2016] and the studies that utilized the waveguide ray theory for short TRGCPs [e.g., Straker, 1955] with regards to the semi-annual oscillation during nighttime can be explained by the different methodologies, and the discrepancy in the probed region's altitude, due to the different transmitter-receiver distance, and hence the wave's incident angle [e.g., Hargreaves and Roberts, 1962;Inan and Inan, 2000;Hunsucker and Hargreaves, 2002].…”

Periodic Oscillation of VLF Transmitter Signals Measured in Low and Middle Latitude Regions

“…To overcome these limitations, we have divided the VLF measurements into different frequency ranges, and employed the EEMD and LS methods in this study. It is by using these approaches that periodicities that are shorter than those reported in previous studies (e.g., Macotela et al, 2019;Sharma et al, 2017;Silber et al, 2016) have been found from our VLF measurements, allowing for the analysis of atmospheric waves.…”