Time‐varying ionosonde trend: Case study of Sodankylä h_mF₂ data 1957–2014

Abstract

We discuss trend analysis of nonstationary ionosonde hmF2 time series measured at Sodankylä Geophysical Observatory (67.4∘N, 26.7∘E), Finland, 1957–2014. We model the hmF2 with a dynamic regression time series model with the following components: a slowly varying background level, seasonal variations, and solar effects. We analyze the time series with a dynamic linear state space model. Such an approach allows model components to vary in time, allowing us to study the dynamic stochastic nature of the underlying long‐term trend. This feature is lacking in most time series models used in atmospheric and environmental long‐term trend analyses. Our objective is to understand the long‐term hmF2 trend with respect to increased levels of carbon dioxide and methane in the atmosphere. Based on model estimates, this phenomenon is predicted to cool the thermosphere and leads to decrease of the altitude of the so‐called F2 layer peak. After accounting for the effects of solar activity variations on the data, we see that the estimated trend shows an almost 30 km decrease of the F2 layer peak during the observation period. The decrease of the peak during 1990–2010 is significantly greater than during earlier observation period.

Stability of solar correction for calculating ionospheric trends

“…A similar problem was found for ionospheric parameters hmF2 (e.g. Roininen et al, 2015) and foF2 (e.g. Laštovička, 2013).…”

Long-term variations of the upper atmosphere parameters on Rome ionosonde observations and their interpretation

“…Such trend hardly can be reliably detected keeping in mind the inaccuracy of (14-17) km of h m F 2 determination with modern digisondes, obtained in a comparison with ISR observations (Chen et al, 1994). Anyway a trend of 1 km/decade strongly contradicts a 30 km h m F 2 decrease obtained at Sodankylä over the period and attributed to the CO 2 cooling (Roininen et al, 2015).…”

“…mit.edu/madrigal/ give h m F 2 = 220-230 km. However, this formula directly or with some corrections is widely used for h m F 2 trend analyses (Bremer, 1998;Jarvis et al, 1998;Roininen et al, 2015). In Rome the usage of this formula gives a strong negative trend which does not correspond to f o F 2 longterm variations.…”

“…An analysis by Ulich (2000) has shown that the overall inaccuracy of such conversion is about 20 ± 10 km depending on geophysical conditions, however such expressions are widely used in trend analyses (e.g. Bremer, 1998Bremer, , 2001Jarvis et al, 1998;Cnossen & Franzke, 2014;Roininen et al, 2015). It should be stressed that various improvements of the initial Shimazaki (1955) expression include the f o F 2 /f o E ratio which by itself manifests long-term variations therefore the usage of such expressions for h m F 2 trend analyses is questionable.…”

“…They have shown that even under the double CO 2 increase scenario (which we are very far from) the predicted ionospheric effects should be small. But their results have stimulated researchers to relate the observed ionospheric long-term trends to the thermosphere greenhouse cooling (Ulich & Turunen, 1997;Sharma et al, 1999;Alfonsi et al, 2001Alfonsi et al, , 2002La stovička et al, 2008;Qian et al, 2008Qian et al, , 2009La stovička et al, 2012;Danilov & Konstantinova, 2013;Mielich & Bremer, 2013;Konstantinova & Danilov, 2015;Roininen et al, 2015). The mechanisms of the thermospheric and ionospheric trends may be different and serious contradictions with the CO 2 hypothesis confirm this (Perrone & Mikhailov, 2016).…”