Abstract
Controversy exists over the potential effects of long-term increases in greenhouse gas concentrations on the ionospheric D-region at 60–90 km altitudes. Techniques involving in-situ rocket measurements, remote optical observations, and radio wave reflection experiments have produced conflicting results. This study reports a novel technique that analyses long-distance subionospheric very low frequency radiowave observations of the NAA 24.0 kHz transmitter, Cutler, Maine, made from Halley Station, Antarctica, over the period 1971–2016. The analysis is insensitive to any changes in the output power of the transmitter, compensates for the use of different data logging equipment, and can confirm the accuracy of the timing systems operated over the 45 year long record. A ~10% reduction in the scale size of the transmitter nighttime interference fringe pattern has been determined, taking into account the quasi-11 year solar cycle. Subionospheric radiowave propagation modeling suggests that the contraction of the interference fringe pattern about the mid-latitude NAA transmitter is due to a 3 km reduction in the effective height of the nighttime ionospheric D-region over the last 45 years. This is consistent with the effect of enhanced infra-red cooling by increasing greenhouse gases.
Highlights
The long term increase in greenhouse gas concentrations in the atmosphere during the 20th and 21st centuries are expected to drive an increase in temperature in the troposphere, i.e., 0–~15 km altitude[1]
There has been an observed decrease in low frequency (LF) radio wave reflection heights between 80–90 km[e.g.17]
One technique suggested for monitoring the lowest altitudes of the D-region, but not undertaken until now, is the analysis of very low frequency (VLF) radio wave propagation over very long distances[9]
Summary
Data are analysed from a VLF magnetic loop antenna system located at Halley Station, Antarctica (75°36′S, 26°12′W) that has been in operation since 1967. A least squares best fit to the times was made, and a time of the amplitude fade calculated for 28 November in each year In this way data gaps where either a recording was not made, or the transmitter was off-air on the day, were compensated for. The standard deviation of the mean time of the fades in each year was 1–2 minutes, and the population sample size ranged from 5–20 points, which converts to a distance along the great circle path of ~ ±15–50 km (the terminator sweeps along the Halley to NAA GCP at ~35 km/min in November). Data analysed in this study (both the paper records and electronic files) are available at the British Antarctic Survey Polar Data Centre (http://psddb.nerc-bas.ac.uk/data/access/)
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