Variability of the ionosphere

Hourly foF2 data from over 100 ionosonde stations during 1967-89 are examined to quantify F-region ionospheric variability, and to assess to what degree the observed variability may be attributed to various sources, i.e., solar ionizing Aux, meteorological influences, and changing solar wind conditions. Our findings are as follows. Under quiet geomagnetic conditions (K-p < 1), the 1-sigma (sigma is the standard deviation) variability of N-max about the mean is approx. +/-25-35% at 'high frequencies' (periods of a few hours to 1-2 days) and approx. +/-15-20% at 'low frequencies' (periods approx. 2-30 days), at all latitudes. These values provide a reasonable average estimate of ionospheric variability mainly due to "meteorological influences" at these frequencies. Changes in N-max due to variations in solar photon flux, are, on the average, small in comparison at these frequencies. Under quiet conditions for high-frequency oscillations, N-max is most variable at anomaly peak latitudes. This may reflect the sensitivity of anomaly peak densities to day-to-day variations in F-region winds and electric fields driven by the E-region wind dynamo. Ionospheric variability increases with magnetic activity at all latitudes and for both low and high frequency ranges, and the slopes of all curves increase with latitude. Thus, the responsiveness of the ionosphere to increased magnetic activity increases as one progresses from lower to higher latitudes. For the 25% most disturbed conditions (K-p > 4), the average 1-sigma variability of N-max about the mean ranges from approx. +/-35% (equator) to approx. +/-45% (anomaly peak) to approx. +/-55% (high-latitudes) for high frequencies, and from approx. +/-25% (equator) to approx. +/-45% (high-latitudes) at low frequencies. Some estimates are also provided on N-max variability connected with annual, semiannual and Ii-year solar cycle variations. (C) 2000 Elsevier Science Ltd. All rights reserved.