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

Israel Silber,Colin Price,Craig J Rodger

doi:10.5194/acp-16-3279-2016

Israel Silber, Colin Price + Show 1 more

Open Access

https://doi.org/10.5194/acp-16-3279-2016

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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 NOx 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.

Highlights

Earth’s middle and upper atmosphere exhibit several dominant large-scale oscillations in many measured parameters
Examination of the apparent time periods of the large oscillations shows that they appear to correspond to the annual oscillation (AO) and semi-annual oscillation (SAO)
We examined the OH∗ data (1◦ width) at the latitudes that match the middle of the two transmitter– receiver great circle path (GCP), during the same time period as the very low frequency (VLF) data (2005–2010 for DN-NWC, and 2009–2012 for Mt. Hermon (MH)-NSY), by using Eq (1), and finding the SAO maximum phase

Summary

Introduction

Earth’s middle and upper atmosphere exhibit several dominant large-scale oscillations in many measured parameters. These oscillations can be found at all latitudes, from the Equator to the mid- and high latitudes. The charged part of the atmosphere (i.e., the ionosphere), experiences the SAO, which was observed and derived in and from measurements of several parameters, e.g., the ionospheric lower transition height at ∼ 180–260 km (a level where atomic and molecular ion concentrations become equal) (e.g., Lei et al, 2004); the electron and plasma density within the daytime D, E, and F regions of the ionosphere (e.g., Lauter and Nitzsche, 1967; Bremer and Singer, 1977; Forbes et al, 2000; Peters and Entzian, 2015); and ionospheric total electron content (TEC) (e.g., Zhao et al, 2007; Opio et al, 2015).

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