Geomagnetic activity threshold for F2-layer negative storms onset: Seasonal dependence

[1] Geomagnetic activity threshold expressed in ap index units for F 2-layer negative storms onset at middle latitudes exhibits seasonal (winter/summer) variation with summer thresholds being larger than winter ones. That is, for a given magnitude of the ionospheric disturbance, the associated magnetic activity must be larger in summer than in winter. There are also relative minima (or plateau) in the threshold annual variation during equinoctial periods. An analysis has shown that seasonal effect is due to seasonal difference in the thermosphere (O/N 2 ratio) reaction to geomagnetic disturbances, while the equinoctial plateau in the threshold annual variations may be related to the thermospheric circulation and atomic oxygen abundance changes during the equinoctial transition periods

Morphology of NmF2 nighttime increases in the Eurasian sector

A statistical analysis of two peaks (pre-midnight and post-midnight) occurrence in NmF2 daily variations was made on a latitudinal chain of four ionosonde stations in the Eurasian longitudinal sector. Overall 6182 cases of the first and 5600 cases of the second peak occurrence were analyzed using all available foF2 observations for the years of solar maximum and minimum. Well-pronounced and systematic variations with season and solar activity were revealed in the occurrence probability of the peaks, their amplitude and timing. The pattern of both peaks occurrence is similar during winter and equinoxes for midlatitude stations implying one and the same mechanism of their formation. The pre-midnight summer peak shows specific variations in particular during solar maximum pointing to a different mechanism controlling its appearance. Possible mechanisms of both peaks formation are discussed.Key words: Ionosphere (electric fields and currents; ionosphere-magnetosphere interactions; mid-latitude ionosphere

A revised approach to the ƒ<i>o</i>F2 long-term trends analysis

A new approach to extract foF2 long-term trends, which are free to a great extent from solar and geomagnetic activity effects, has been proposed. These trends are insensitive to the phase (increasing/decreasing) of geomagnetic activity, with long-term variations being small and insignificant for such relatively short time periods. A small but significant residual foF2 trend, with the slope Kr = - 2.2 × 10-4 per year, was obtained over a 55-year period (the longest avail-able) of observations at Slough. Such small trends have no practical importance. On the other hand, negative (although insignificant) residual trends obtained at 10 ionosonde stations for shorter periods (31 years) may be considered as a manifestation of a very long-term geomagnetic activity increase which did take place during the 20th century. All of the revealed foF2 long-term variations (trends) are shown to have a natural origin related to long-term variations in solar and geomagnetic activity. There is no indication of any manmade foF2 trends.&nbsp; Key words. Ionosphere (ionosphere-atmosphere interactions, ionospheric disturbances