Abstract
Abstract. Earlier revealed morphological features of the foF2 and hmF2 long-term trends are interpreted in the scope of the geomagnetic control concept based on the contemporary F2-layer storm mechanisms. The F2-layer parameter trends strongly depend on the long-term varying geomagnetic activity whose effects cannot be removed from the trends using conventional indices of geomagnetic activity. Therefore, any interpretation of the foF2 and hmF2 trends should consider the geomagnetic effects as an inalienable part of the trend analysis. Periods with negative and positive foF2 and hmF2 trends correspond to the periods of increasing or decreasing geomagnetic activity with the turning points around 1955, and the end of 1960s and 1980s, where foF2 and hmF2 trends change their signs. Such variations can be explained by neutral composition, as well as temperature and thermospheric wind changes related to geomagnetic activity variations. In particular, for the period of increasing geomagnetic activity (1965–1991) positive at lower latitudes, but negative at middle and high latitudes, foF2 trends may be explained by neutral composition and temperature changes, while soft electron precipitation determines nighttime trends at sub-auroral and auroral latitudes. A pronounced dependence of the foF2 trends on geomagnetic (invariant) latitude and the absence of any latitudinal dependence for the hmF2 trends are due to different dependencies of NmF2 and hmF2 on main aeronomic parameters. All of the revealed latitudinal and diurnal foF2 and hmF2 trend variations may be explained in the frame-work of contemporary F2-region storm mechanisms. The newly proposed geomagnetic storm concept used to explain F2-layer parameter long-term trends proceeds from a natural origin of the trends rather than an artificial one, related to the thermosphere cooling due to the greenhouse effect. Within this concept, instead of cooling, one should expect the thermosphere heating for the period of increasing geomagnetic activity (1965–1991).Key words. Ionosphere (ionosphere-atmosphere interactions; ionospheric disturbances)
Highlights
IntroductionThe problem of long-term variations (trends) in the ionospheric parameters is widely discussed in recent publications (see reviews by Danilov, 1997, 1998; Givishvili and Leshchenko, 1994, 1995; Givishvili et al, 1995; Ulich and Turunen, 1997; Rishbeth, 1997; Danilov and Mikhailov, 1998, 1999; Bremer, 1992, 1998; Upadhyay and Mahajan, 1998; Sharma et al, 1999; Foppiano et al, 1999; Mikhailov and Marin, 2000; Deminov et al, 2000; Marin et al, 2001)
The problem of long-term variations in the ionospheric parameters is widely discussed in recent publications
The foF2 and hmF2 trend morphology earlier revealed by Danilov and Mikhailov (1999), Mikhailov and Marin (2000), and Marin et al (2001), was interpreted in the framework of the geomagnetic control concept proposed by Mikhailov and Marin (2000)
Summary
The problem of long-term variations (trends) in the ionospheric parameters is widely discussed in recent publications (see reviews by Danilov, 1997, 1998; Givishvili and Leshchenko, 1994, 1995; Givishvili et al, 1995; Ulich and Turunen, 1997; Rishbeth, 1997; Danilov and Mikhailov, 1998, 1999; Bremer, 1992, 1998; Upadhyay and Mahajan, 1998; Sharma et al, 1999; Foppiano et al, 1999; Mikhailov and Marin, 2000; Deminov et al, 2000; Marin et al, 2001). The world-wide network of ground-based ionosonde observations provides excellent experimental material for such an analysis, since many of the ionosondes have been operating for 3–5 solar cycles using one method of ionospheric sounding to obtain the set of main ionospheric characteristics. Most of these observations are collected and available from WDC-C at RAL (Chilton, UK) and from NGDC, Boulder, USA. It should be stressed that different authors use different approaches to ex-
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