1. Climate change in the D-region
Mark A. Clilverd1, Roger Duthie1, Craig J. Rodger2, Rachael L. Hardman1,4, and Keith H. Yearby3
1British Antarctic Survey (NERC), Cambridge, UK.
2Department of Physics, University of Otago, Dunedin, New Zealand.
3Department of Control Engineering, University of Sheffield, Sheffield, UK.
4Now at School of Mathematics and Statistics, University of Sheffield, Sheffield, UK.
STP14: Session Long term variability of the whole atmosphere
Toronto, Wednesday, 11th July 2018

2. The expected effect of greenhouse gas increases.
Laštovička et al., Ann. Geophys
Trends in the [lower] ionosphere below 100 km, derived from LF radio propagation data, have been studied primarily in Europe.
Trends in the lowermost part of the ionosphere could be investigated with the use of the VLF radio wave propagation measurements over long distances. As far as we know, such an analysis has not yet been made.
A very important point to be discussed in relation to long-term trend studies is the various uncertainties in data and also the long-term non-stability of measurements.

3. What experiment stays the same for 40+ years?
The NAA VLF transmitter
1MW output, since 1961 (~20 MW input).
24.0 kHz since 1983.
17.8 kHz before that.

4. Making measurements of VLF radio waves:
Radio transmissions at Very Low Frequencies (VLF) largely trapped between the conducting ground (or sea) and the lower part of the ionosphere (70-90 km) , forming the Earth-ionosphere waveguide.
Changes in the ionosphere cause changes in the received signal. The nighttime D-region is primarily generated by the ionisation of NO by solar Lyman-α (121.6 nm) scattered by the neutral hydrogen in the Earth’s geo-corona.

5. DEMETER view of NWC at 19.8 kHz
From: J.-A. Sauvaud et al., Geophys. Res. Lett., 35, L09101, doi: /2008GL033194, 2008.
‘provides a nice example of diffraction pattern’

6. Halley, VLF receiver operating since 1971

7. NAA at Halley: detecting the minima
Night
Sunrise modal minima
Day
: Paper charts on a multi-channel receiver
NAA trace
Time (UT)

8. LWPC amplitude versus distance
Previous work on sunrise modal minima times (Clilverd et al., Radio Sci., 34(4), , 1999)
LWPC amplitude versus distance
Variation of NAA modal minima over a year
Measured just under the ionosphere
Sunrise at a receiver at 75km altitude.
Measured near ground level

9. Changing Frequency
[NAA was 17.8 kHz in 1971, but changed to 24.0 kHz in 1983 staying the same since.]
Changing frequency changes the locations of the nighttime modal minima. But we can model the change.
NAA, 24.0 kHz
NAA, 17.8 kHz

10. The timing of sunrise at the Tx in Nov/Dec, and fringe distance from Tx
10:43:40 UT on 28 November. Equivalent to the terminator crossing overhead of the transmitter at an altitude of 71.5km +/- 2km (i.e., +/- 30 s)
LWPC was used to investigate the sensitivity of the NAA nighttime fringe distance to Wait’s electron density profile parameters: h′ and β.
It was found that the NAA fringe distance was relatively insensitive to β, but a contraction of 10% could be achieved through a reduction in nighttime h′ by 3 km.
Both sunspot activity samples show gradients of ~5 km/year, i.e., 10% change in fringe distance in 45 years.

11. Summary
Using a VLF transmitter in the same location, same power, we have investigated where modal minima occur on the great circle path to Halley over 40+ years.
The transmitter fringe pattern at km altitude (lower D-region) appears to be contracting at a rate of 4-6 km/year.
The 10% fringe pattern contraction is consistent with a height change in the nighttime D-region of ~3 km (downwards). At a fixed altitude electron density would increase. This is consistent with the predicted effects of greenhouse cooling /contraction of the mesosphere.
Reference:
Clilverd, M. A., R. Duthie, C. J. Rodger, R. L. Hardman, and K. Yearby, Long-term climate change in the D-region, Nature: Scientific Reports, 7:16683, doi: /s , 2017.

12. Uncertainties 2012: We use 10 s averages, timing <1 second
1971: 0.5 mm or ~30 seconds, time accurate to within 30 secs.
Path length 13,000 km – 4 hours for terminator to sweep along path – ~50 km/min
Movement of modal feature is ~70 km/1 km in H prime.
Timing uncertainties are therefore about 25km, which is ~0.3 km in H prime.
Halley Nov 2015 modal min times are unstructured, and need further investigation.