Patterns of F2-layer variability

The ionosphere displays variations on a wide range of time-scales, ranging from operational time-scales of hours and days up to solar cycles and longe r. We use ionosonde data from thirteen stations to study the day-to-day var iability of the peak F2-layer electron density, NmF2, which we use to defin e quantitative descriptions of variability versus local time, season and so lar cycle. On average, for years of medium solar activity (solar decimetric flux approximately 140 units), the daily fluctuations of NmF2 have a stand ard deviation of 20% by day, and 33% by night. We examine and discuss the p atterns of behaviour of ionospheric and geomagnetic variability, in particu lar the equinoctial peaks. For further analysis we concentrate on one typic al midlatitude station, Slough. We find that the standard deviations of day -to-day and night-to-night values of Slough NmF2 at first increase with inc reasing length of the dataset, become fairly constant at lengths of 10-20 d ays and then increase further (especially at equinox) because of seasonal c hanges. We found some evidence of two-day waves, but they do not appear to be a major feature of Slough's F2 layer. Putting together the geomagnetic a nd ionospheric data, and taking account of the day-to-day variability of so lar and geomagnetic parameters, we find that a large part of F2-layer varia bility is linked to that of geomagnetic activity, and attribute the rest to 'meteorological' sources at lower levels in the atmosphere. We suggest tha t the greater variability at night is due to enhanced auroral energy input, and to the lack of the strong photochemical control of the F2-layer that e xists by day. (C) 2001 Elsevier Science Ltd. All rights reserved.