Long-term climate change in the D-region

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.

Figure 1

The amplitude of NAA received at Halley in 2015.Amplitude fading due to the passage of the sunrise/sunset terminator along the transmitter-receiver great circle path can be seen as features with blue/black colouring. During November three periods of decreased amplitude can be seen during early morning at 08–11 UT, changing in time as sunrise times change seasonally. Black horizontal stripes indicate transmitter off-times (typically 12–20 UT).

Figure 2

A map of the great circle path from the NAA transmitter (red circle) to Halley Station in Antarctica (blue diamond). A representative nighttime amplitude variation along the path of NAA is shown as a red line. Low amplitude levels occur when the line approaches the great circle path. Sunrise terminator times for 28 November are shown by magenta lines, indicating when sunrise occurs at the transmitter, and at a modal interference minima located ~2000 km from the transmitter. Map generated using Matlab (ver R2016b, https://www.mathworks.com/products/matlab.html).

Figure 3

(a) The times of sunrise fades overhead of the NAA transmitter during the 45 year study period. The 10:43:30 UT average, and 45 s either side are indicated by the blue dashed and dotted lines respectively. (b) The calculated distance of the penultimate sunrise amplitude fade from the NAA transmitter. Normally distributed standard deviation errors bars are shown as vertical lines, and a linear best fit line indicates an interference fringe pattern contraction of 3.6 km/yr over the whole dataset. A fit is also shown for just the 1990–2016 data points (blue, dashed-dot line) indicating a contraction of 2.6 km/yr. (c) The calculated distance of the penultimate sunrise amplitude fade from the NAA transmitter separated into periods of low (red) and medium (blue) sunspot activity levels. Interference fringe pattern contractions of 4–5 km/yr can be seen.