Winter anomaly in NmF2 and TEC: when and where it can occur

Yury V Yasyukevich,Nikolay V Chirik,Vladimir V Klimenko,Anna S Yasyukevich,Konstantin G Ratovsky,Maxim V Klimenko

doi:10.1051/swsc/2018036

Yury V Yasyukevich, Nikolay V Chirik + Show 4 more

Open Access

https://doi.org/10.1051/swsc/2018036

Copy DOI

Abstract

For the first time, by using a regression procedure, we analyzed the solar activity dependence of the winter anomaly intensity in the ionospheric F2-layer peak electron density (Nm F2) and in the Total Electron Content (TEC) on a global scale. We used the data from global ionospheric maps for 1998–2015, from GPS radio occultation observations with COSMIC, CHAMP, and GRACE satellites for 2001–2015, and ground-based ionosonde data. The fundamental features of the winter anomaly in Nm F2 and in TEC (spatial distribution and solar activity dependence) are similar for these parameters. We determined the regions, where the winter anomaly may be observed in principle, and the solar activity level, at which the winter anomaly may be recorded in different sectors. A growth in geomagnetic disturbance or in the solar activity level is shown to facilitate the winter anomaly intensity increase. Longitudinal variations in the winter anomaly intensity do not conform partly to the generally accepted Rishbeth theory. We consider the obtained results in the context of spatial and solar cycle variations in O/N2 ratio and thermospheric meridional wind. Additionally, we briefly discuss different definitions of the winter anomaly.

Highlights

1.1 Historical definitionThe term ‘‘anomaly’’ originally meant any deviations from ‘‘solar-controlled behaviour’’ in which the critical frequency foF2 varies regularly with the solar zenith angle v as it does in the well-known Chapman layer (Rishbeth, 1998)
It can be used to reveal the solar activity level, at which the winter anomaly phenomenon starts to be observable in different sectors of the globe, as well as the regions, where the winter anomaly phenomenon may be observed in principle
We note that the geomagnetic disturbances affect the winter anomaly intensity, even if these disturbances occur within 24– 48 h prior to observations. (c) We determined, for the first time, the values of the solar activity level, at which the winter anomaly phenomenon starts to be observable in different sectors of the globe

Summary

Introduction

1.1 Historical definitionThe term ‘‘anomaly’’ originally meant any deviations from ‘‘solar-controlled behaviour’’ in which the critical frequency foF2 (proportional to the square root of the peak electron density, NmF2) varies regularly with the solar zenith angle v as it does in the well-known Chapman layer (Rishbeth, 1998). The winter anomaly is a phenomenon consisting in the fact that mid-latitude daytime NmF2 (or foF2) is greater in winter than in summer at approximately the same solar activity level (Torr & Torr, 1973; Zou et al, 2000). Torr & Torr (1973) using the data of 140 ionosondes in 1958, 1964, and 1969 (high, low and moderate solar activity, respectively) constructed global maps showing the regions where monthly median noon foF2 is greatest in different seasons. From these maps it is seen that the North American sector is the region where foF2 is greatest in winter under any solar activity level. And further in the text as the ‘‘Australian sector’’ we call the domain of the Southern Hemisphere middle latitudes in the range of about 45–135°E

Objectives

Discussion

Conclusion