08 Nov 2023
 | 08 Nov 2023
Status: this preprint is open for discussion.

Calibrating estimates of ionospheric long-term change

Christopher John Scott, Matthew N. Wild, Luke Anthony Barnard, Bingkun Yu, Tatsuhiro Yokoyama, Michael Lockwood, Cathryn Mitchel, John Coxon, and Andrew Kavanagh

Abstract. Long-term change in the height of the ionospheric F2 layer, hmF2, is predicted to result from increased levels of tropospheric greenhouse gases. Sufficiently long sequences of ionospheric data exist to investigate this long-term change, recorded by a global network of ionosondes. However, direct measurements of ionospheric layer height with these instruments is not possible. As a result, most estimates of hmF2 rely on empirical formulae based on parameters routinely scaled from ionograms. Estimates of trends in hmF2 using these formulae show no global consensus. We present an analysis in which data from the Japanese ionosonde station at Kokubunji were used to estimate monthly median values of hmF2 using an empirical formula. These were then compared with direct measurements of the F2 layer height determined from Incoherent Scatter measurements made at the Shigaraki MU observatory, Japan. Our results reveal that the formula introduces diurnal, seasonal and long-term biases in the estimates of hmF2 of ≈ ±10 % (±25 km an altitude of 250 km). These can be explained by the presence of underlying F1 layer ionisation not accounted for in the formula. We demonstrate, that for Kokobunji, the ratio of F2/F1 peak electron concentrations is strongly controlled by changes in geomagnetic activity represented by the am index. Changes in thermospheric composition in response to geomagnetic activity have been shown to be highly localised. We conclude that localised changes in thermospheric composition modulate the F2/F1 peak ratio, leading to differences in hmF2 trends. We further conclude that the influence of thermospheric composition on the underlying ionosphere needs to be accounted for in these empirical formulae if they are to be applied to studies of long-term ionospheric change.

Christopher John Scott et al.

Status: open (until 21 Dec 2023)

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Christopher John Scott et al.

Christopher John Scott et al.


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Short summary
Long-term change in the ionosphere are expected due to increase in greenhouse gases in the lower atmosphere. Empirical formulae are used to estimate height. Through comparison with independent data we show that there are seasonal and long-term biases introduced by the empirical model. We conclude that estimates of long-term changes in ionospheric height need to account for these biases.