Lower ionosphere monitoring by the South America VLF Network (SAVNET): C region occurrence and atmospheric temperature variability

Abstract

[1] Daily profiles of phase measurements as observed on fixed VLF paths generally show a transient phase advance, followed by a phase delay, for about 90 min after sunrise hours. This is indicative of a reflecting ionospheric C region developing along the terminator line at an altitude below the normal D region. The suggested occurrence of a C region is consistent with rocket measurements made in the 1960s, showing a maximum of the electron density between 64 and 68 km, and by radio sounding in the 1980s. In order to correctly describe the properties of the phase effect associated with the presence of a C region, it is important to understand the subionospheric propagation characteristics of the VLF paths. In this paper, we analyze the variations presented by the temporal properties of the VLF narrowband phase effect and determined a parameter associated with the appearance of the C region at sunrise hours observed by receivers from the South America VLF Network. Periodic patterns emerge from the parameter curves. Two distinct temporal behavior regimes can be identified: one exhibiting slow variations between March and October, and another one exhibiting faster variations between October and March. Solar illumination conditions and the geometrical configuration of the VLF paths relative to the sunrise terminator partly explain the slow variation regime. During periods of faster variations, we have observed good association with atmospheric temperature variability found in the measurements of the Thermosphere Ionosphere Mesosphere Energetics and Dynamics and Sounding of the Atmosphere using Broadband Emission Radiometry satellite instrument, which we assume to be related to the winter anomaly atmospheric phenomenon. However, when comparing the parameter time series with temperature curves, no direct one-to-one correspondence was found for transient events.Citation: Bertoni, F. C. P., J.-P. Raulin, H. R. Gavila´n, P. Kaufmann, R. Rodriguez, M. Clilverd, J. S. Cardenas, and G. Fernandez (2013), Lower ionosphere monitoring by the South America VLF Network (SAVNET): C region occurrence and atmospheric temperature variability,

The Effect of Ozone Shadowing on the D Region Ionosphere During Sunrise

“…Also, the day‐to‐day variability in the size of SPP is larger in winter than in summer. These results agree with previous studies (e.g., Bertoni et al, ). This suggests that the size of SPP measured in quasi‐polar regions has, in general, similar characteristics to those previously measured at low‐ and middle‐latitude regions.…”

“…To undertake this study, VLF phase measurements recorded in northern Finland were used as well as stratospheric and atmospheric temperature, and ozone volume mixing ratio (vmr) data from a near-polar orbiting satellite. The use of these data sets brings the advantage of comparing the results of the present study with those of Bertoni et al (2013) who used similar data sets. This section contains a description of the experimental setup and an explanation of the methodology applied to characterize the SPP observed during sunrise, along with the atmospheric temperature and ozone vertical profile.…”

MF and HF radar techniques for investigating the dynamics and structure of the 50 to 110 km height region: a review

“…In this example, sunrise occurred at 21:05 UT (6.35 am LT). The layer between 60 and 65 km that forms around sunrise is consistent with VLF observations of the C layer described by Bertoni et al (2013) and mentioned above. The layer near 75 km is also commonly observed and, as discussed above, is often associated with a "preferred height".…”

“…The idea of the C layer was first introduced to understand very low frequency (VLF, 3-30 kHz) radar propagation observations. In more recent work, Bertoni et al (2013) briefly review some of these and suggest that a C layer of enhanced electron density does exist near 64 to 68 km for a few hours after dawn, but that its formation is associated with sunrise rather than with cosmic ray ionization. However, in the 50 to 110 km height region, the concept of a clear division between "E" and "D" regions and single "D" and "C" layers is slightly misleading, as the 50 to 100 km height region is stratified (e.g., Reid 1990), and most often consists of multiple regions of enhanced backscatter, with a tendency for this to occur at certain "preferred" heights for radar returns (e.g., Gregory 1961;Titheridge 1962;Reid 1990).…”

Estimation of ionospheric reflection height using long wave propagation

“…Refinements and additional measurements suggest that the observed Hollingworth pattern are also influenced by lower atmospheric layers. It is possible that the influence of the lower layers is even much greater than previously assumed (Bracewell et al, 1951;Entzian, 1967Entzian, , 1972Lauter et al, 1984;Taubenheim et al, 1997). The focus in this article is laid to clear some inconsistencies related to the measuring path from Allouis (47 • N, 2 • E, Central France) as broadcasting station to Kühlungsborn (54 • N, 12 • E, Northern Germany), which is located about 1023 km away and serves as receiving location.…”