TY - JOUR
T1 - Semi-annual oscillation (SAO) of the nighttime ionospheric D region as detected through ground-based VLF receivers
AU - Silber, Israel
AU - Price, Colin
AU - Rodger, Craig J.
N1 - Funding Information:
This research was supported by the Ministry of Science and Technology, Israel. The authors wish to thank the Stanford University VLF group for their support in the construction of the VLF receiver station at Mt. Hermon. The authors thank the TIMED/SABER team for providing the data used in this paper.
Publisher Copyright:
© Author(s) 2016.
PY - 2016/3/14
Y1 - 2016/3/14
N2 - 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.
AB - 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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U2 - 10.5194/acp-16-3279-2016
DO - 10.5194/acp-16-3279-2016
M3 - Article
AN - SCOPUS:84960970923
VL - 16
SP - 3279
EP - 3288
JO - Atmospheric Chemistry and Physics
JF - Atmospheric Chemistry and Physics
SN - 1680-7316
IS - 5
ER -