Abstract
Annual, seasonal and semiannual variations of F2-layer electron density (NmF2) and height (hmF2) have been compared with the coupled thermosphere-ionosphere-plasmasphere computational model (CTIP), for geomagnetically quiet conditions. Compared with results from ionosonde data from midlatitudes, CTIP reproduces quite well many observed features of NmF2, such as the dominant winter maxima at high midlatitudes in longitude sectors near the magnetic poles, the equinox maxima in sectors remote from the magnetic poles and at lower latitudes generally, and the form of the month-to-month variations at latitudes between about 60°N and 50°S. CTIP also reproduces the seasonal behaviour of NmF2 at midnight and the summer-winter changes of hmF2. Some features of the F2-layer, not reproduced by the present version of CTIP, are attributed to processes not included in the modelling. Examples are the increased prevalence of the winter maxima of noon NmF2 at higher solar activity, which may be a consequence of the increase of F2-layer loss rate in summer by vibrationally excited molecular nitrogen, and the semiannual variation in hmF2, which may be due to tidal effects. An unexpected feature of the computed distributions of NmF2 is an east-west hemisphere difference, which seems to be linked to the geomagnetic field configuration. Physical discussion is reserved to the companion paper by Rishbeth et al.Key words: Atmospheric composition and structure (thermosphere-composition and chemistry) - Ionosphere (mid-latitude ionosphere; modelling and forecasting)
Highlights
1.1 F2-layer anomaliesThis work discusses the F2-layer`anomalies'', the term being generally taken to mean any departure from simple solar-controlled behaviour
The principal anomalies observed at midlatitudes may be characterized as follows: Winter or seasonal anomaly: NmF2 is greater in winter than in summer by day, but the anomaly disappears at night, NmF2 being greater in summer than in winter; Semiannual anomaly: NmF2 is greater at equinox than at solstice; Annual or non-seasonal anomaly: in the world as a whole December NmF2 is on average greater than June NmF2, both by day and by night
At middle and low latitudes, the coupled thermosphere±ionosphere±plasmasphere model (CTIP) electric ®eld distribution is taken from the model of Richmond et al (1980), which includes some seasonal variation but no solar cycle variation
Summary
This work discusses the F2-layer`anomalies'', the term being generally taken to mean any departure from simple solar-controlled behaviour. These phenomena were detected in the 1930s and were well described by the end of the 1950s; for historical details see Rishbeth (1998). Using data from many ionosonde stations, these authors give a detailed picture of the anomalies in noon and midnight NmF2 and of how they vary with latitude and solar activity. To give a very broad summary for midlatitudes at moderate solar activity: the annual anomaly in NmF2 is about () 20%, the semiannual anomaly about () 15±20%. Lower midlatitude: Wallops Island (38°N, 75°W), Hobart (43°S, 147°E)
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