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

Israel Silber, Colin Price, Craig J. Rodger

Research output: Contribution to journalArticle

7 Citations (Scopus)

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.

Original languageEnglish (US)
Pages (from-to)3279-3288
Number of pages10
JournalAtmospheric Chemistry and Physics
Volume16
Issue number5
DOIs
StatePublished - Mar 14 2016

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semiannual oscillation
thermosphere
oscillation
mesosphere
electron
middle atmosphere
upper atmosphere
wave propagation
recombination
ionization
collision
temperature
ion
modeling

All Science Journal Classification (ASJC) codes

  • Atmospheric Science

Cite this

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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.",
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Semi-annual oscillation (SAO) of the nighttime ionospheric D region as detected through ground-based VLF receivers. / Silber, Israel; Price, Colin; Rodger, Craig J.

In: Atmospheric Chemistry and Physics, Vol. 16, No. 5, 14.03.2016, p. 3279-3288.

Research output: Contribution to journalArticle

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