Long‐term ionospheric trends based on ground‐based ionosonde observations at Kokubunji, Japan

Abstract

[1] Using monthly medians of ionosonde observations taken over a period including more than four solar cycles at Kokubunji (35.71°N, 139.49°E), Japan and a linear regression model to eliminate solar and geomagnetic effects, we derive the long-term trends of critical frequencies ( foE, foF1, and foF2), the E region height (h 0 E), and the F2 peak height (hmF2). The results show increasing trends for foE (+0.002 MHz/yr), h 0 E (+0.189 km/yr), and foF1 (+0.0107 MHz/yr) and decreasing trends for foF2 (À0.0058 MHz/yr at noon and À0.0016 MHz/year at midnight) and hmF2 (À0.398 km/yr at noon and À0.505 km/year at midnight). We have also analyzed seasonal and diurnal trend variations. These trends differed for various times of day and months, even with altering signs (except for hmF2). An interesting phenomenon was observed for the first time; that is, the morphologies of the overall seasonal trend variations of foE and foF1 were opposite each other to some extent, although the trends remained positive. A similar phenomenon was found for the seasonal trend variation of hmF2 between local noon and midnight. In addition, comparison of the results from four regression models indicated that the differences induced by linear and quadratic models are slightly significant for foF2 but not for other parameters. On the basis of the average of foF2 data obtained over 5 hours around noon, we have assessed the performance of each of these regression models. The performance differed depending on the month. The results showed that the quadratic model and the linear model that took saturation into consideration performed better than the linear ones which did not consider saturation. In addition, it was indicated that the effect of geomagnetic activity was not significant in regression models at this station.

Ionospheric F1 layer long-term trends and the geomagnetic control concept

“…1 shows that one may get positive or negative trends with different magnitudes when different phases of geomagnetic activity are put together. The statement of Xu et al (2004) that "the effect of geomagnetic activity was not significant" is just a result of a rude method application, as the geomagnetic activity effects are strong and cannot be missed. Further, rough methods applied for trend analyses may result in different sign of foF1 and foF2 trends (Bremer, 1998;Xu et al, 2004).…”

“…In these papers it is stressed: a) a positive trend in the foF1 long-term variation, b) a relationship with the global cooling of the thermosphere due to the greenhouse effect (Bremer, 1998(Bremer, , 2001Sharma et al, 1999), c) the insignificance of the geomagnetic activity (Xu et al, 2004).…”

“…The majority of analyses have been devoted to long-term trends in the ionospheric F2 and E-layer parameters since these observations are the most abundant and consistent, while trends in the F1-layer have been considered, as far as it is known, only in some papers (Bremer, 1998(Bremer, , 2001Sharma et al, 1999;Xu et al, 2004). In these papers it is stressed: a) a positive trend in the foF1 long-term variation, b) a relationship with the global cooling of the thermosphere due to the greenhouse effect (Bremer, 1998(Bremer, , 2001Sharma et al, 1999), c) the insignificance of the geomagnetic activity (Xu et al, 2004).…”

Different long-term trends of the oxygen red 630.0 nm line nightglow intensity as the result of lowering the ionosphere F2 layer

“…The presence of the negative long-term trend of the ionosphere F2 layer electron density peak height hmF2 was detected at many ionosphere stations in different regions of the world (Ulich and Turunen, 1997;Bremer, 1998;Jarvis et al, 1998;Marin et al, 2001;Xu et al, 2004). This lowering of the ionosphere F2 layer may be the result of global cooling of the upper atmosphere (Roble and Dickinson, 1989;Rishbeth, 1990) and related changes in its dynamics.…”

“…These phenomena are thought to be a possible consequence of lowering the ionosphere F2 layer, which was observed at several ionosphere stations (Ulich and Turunen, 1997;Bremer, 1998;Jarvis et al, 1998;Marin et al, 2001;Xu et al, 2004). The lowering in the ionosphere F2 layer, was predicted by Roble and Dickinson (1989), Rishbeth (1990), Rishbeth and Roble (1992) as the result of cooling in the upper atmosphere which should accompany global warming in the lower atmosphere.…”