Abstract
Ionosonde data from sixteen stations are used to study the semiannual and annual variations in the height of the ionospheric F2-peak, hmF2. The semiannual variation, which peaks shortly after equinox, has an amplitude of about 8 km at an average level of solar activity (10.7 cm flux = 140 units), both at noon and midnight. The annual variation has an amplitude of about 11 km at northern midlatitudes, peaking in early summer; and is larger at southern stations, where it peaks in late summer. Both annual and semiannual amplitudes increase with increasing solar activity by day, but not at night. The semiannual variation in hmF2 is unrelated to the semiannual variation of the peak electron density NmF2, and is not reproduced by the CTIP and TIME-GCM computational models of the quiet-day thermosphere and ionosphere. The semiannual variation in hmF2 is approximately "isobaric", in that its amplitude corresponds quite well to the semiannual variation in the height of fixed pressure-levels in the thermosphere, as represented by the MSIS empirical model. The annual variation is not "isobaric". The annual mean of hmF2 increases with solar 10.7 cm flux, both by night and by day, on average by about 0.45 km/flux unit, rather smaller than the corresponding increase of height of constant pressure-levels in the MSIS model. The discrepancy may be due to solar-cycle variations of thermospheric winds. Although geomagnetic activity, which affects thermospheric density and temperature and therefore hmF2 also, is greatest at the equinoxes, this seems to account for less than half the semiannual variation of hmF2. The rest may be due to a semiannual variation of tidal and wave energy transmitted to the thermosphere from lower levels in the atmosphere.Key words: Atmospheric composition and structure (thermosphere - composition and chemistry) - Ionosphere (mid-latitude ionosphere)
Highlights
Several atmospheric, ionospheric and geomagnetic parameters display a regular semiannual (6-monthly) variation, with maxima near the March and September equinoxes
For hmF2, we found that the largest summer/winter variations occur in southern latitudes, most notably at Port Stanley, where the thermospheric wind eects are strong
At middle and low latitudes, we ®nd that hmF2 has a well-de®ned relationship with solar activity, a clear annual variation, and a clear semiannual variation, comparable in amplitude to the annual variation
Summary
Ionospheric and geomagnetic parameters display a regular semiannual (6-monthly) variation, with maxima near the March and September equinoxes. They include the peak electron density of the ionospheric F2-layer (NmF2) in some parts of the world (Burkard, 1951; Yonezawa and Arima, 1959; Chaman Lal, 1992); the height of the F2-layer peak (hmF2) (Becker, 1967); the neutral air density q in the thermosphere (Paetzold and ZschoÈ rner, 1961); and geomagnetic indices such as Kp and Ap (Bartels, 1963; Green, 1984). As magnetic activity is enhanced at the equinoxes, the median values for equinox months correspond to higher levels of Ap than do those for solstice months; in Sect. 4.2 we discuss the possible eect of this on our results
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