Abstract
The effect of including solar cycle 23 in foF2 trend estimation is assessed using experimental values for Slough (51.5°N, 359.4°E) and Kokobunji (35.7°N, 139.5°E), and values obtained from two models: (1) the Sheffield University Plasmasphere-Ionosphere model, SUPIM, and (2) the International Reference Ionosphere, IRI. The dominant influence on the F2 layer is solar extreme ultraviolet (EUV) radiation, evinced by the almost 90% variance of its parameters explained by solar EUV proxies such as the solar activity indices Rz and F10.7. This makes necessary to filter out solar activity effects prior to long-term trend estimation. Solar cycle 23 seems to have had an EUV emission different from that deduced from traditional solar EUV proxies. During maximum and descending phase of the cycle, Rz and F10.7 seem to underestimate EUV solar radiation, while during minimum, they overestimate EUV levels. Including this solar cycle in trend estimations then, and using traditional filtering techniques, may induce some spurious results. In the present work, filtering is done in the usual way considering the residuals of the linear regression between foF2 and F10.7, for both experimental and modeled values. foF2 trends become less negative as we include years after 2000, since foF2 systematically exceeds the values predicted by a linear fit between foF2 and F10.7. Trends become more negative again when solar cycle 23 minimum is included, since for this period, foF2 is systematically lower than values predicted by the linear fit. foF2 trends assessed with modeled foF2 values are less strong than those obtained with experimental foF2 values and more stable as solar cycle 23 is included in the trend estimation. Modeled trends may be thought of as a ‘zero level’ trend due to the assumptions made in the process of trend estimation considering also that we are not dealing with ideal conditions or infinite time series.
Highlights
Solar extreme ultraviolet (EUV) irradiance is the main ionization source of the F2 region of the Earth’s ionosphere (Tobiska 1996; Chen et al 2012), explaining around 90% of the variance of parameters such as the critical frequency of the F2 region and the peak height of electron concentration
This is done by estimating the frequency of the F2 region (foF2) residuals from the regression between the experimental values and the solar activity index F10.7, that is foF2res 1⁄4 foF2 exp–ða F10:7 þ bÞ; where a and b are constants determined with least squares
In the case of experimental trend values, they are negative first and become less negative as solar cycle 23 is included in the trend estimation
Summary
Solar extreme ultraviolet (EUV) irradiance is the main ionization source of the F2 region of the Earth’s ionosphere (Tobiska 1996; Chen et al 2012), explaining around 90% of the variance of parameters such as the critical frequency of the F2 region (foF2) and the peak height of electron concentration (hmF2). The global pattern of experimental hmF2 and foF2 of several worldwide stations is highly complex and cannot be entirely reconciled with the greenhouse hypothesis. Some reasons for this are as follows: On one side, other sources of upper atmosphere trends exist (such as geomagnetic activity long-term variation and Earth’s magnetic field secular variations), which act jointly with the greenhouse effect. The methods used to extract trend values differ from author to author and usually rely on some filtering process that may bias the trend results
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