Abstract
It seems that the wind shear theory is accepted for the explanation of sporadic E at mid and low latitudes. Some examples from Arecibo are displayed to show this. The effect of lunar tides should then modify the wind-shear theory in a manner that yields the observed features of the lunar tide in the critical frequency foEs and the height h'Es of the sporadic E. This is shown to imply that the phase of the lunar tide in h'Es should be the same as the phase of the lunar tide in the eastward wind and that the phase of the lunar tide in foEs is three hours later. Hourly values of foEs, f bEs (the blanketing critical frequency) and h'Es from several observatories are analysed for the lunar semidiurnal tide. It is found that the phase of the tide in foEs is often about 3 hours later than for h'Es in agreement with the theory. Seasonal variations in the tide are also examined with the statistically most significant results (largest amplitudes) usually occurring in summer. After reviewing the many difficulties associated with determining the lunar tide in Es, both experimentally and theoretically, the analysed phase results are compared with what might be expected from Hagan's global scale wave model. Agreement is only fair (a success rate of 69% among the cases examined) but probably as good as might be expected.Key words. Ionosphere (ionosphere â atmosphere interactions â ionospheric irregularities), Meteorology and atmosphere dynamics (waves and tides)
Highlights
The lunar tides in the height of sporadic E, h¢Es, and of its critical frequency, foEs, were investigated many years ago
(1994) produced a simulation of the lunar atmospheric tide which yielded fair agreement with observed lunar tides in winds at heights of 80 to 100 km (Stening and Vincent et al, 1989, Stening et al, 1994) but this model did not extend much above 100 km
There does not appear to be any satisfactory direct measurement of lunar tides in neutral winds at heights of 100 to 140 km where sporadic E is found, but recent simulations do extend into that height region
Summary
The lunar tides in the height of sporadic E, h¢Es, and of its critical frequency, foEs, were investigated many years ago. As the ®rst term dominates in this equation, the lunar tide in z0 comes to have the same phase as the lunar tide in ix, namely a maximum at about 7 lunar h This phase was measured by Tarpley and Balsley (1972) at the magnetic equator during daytime and has been con®rmed in simulations of the lunar tide using the TIEGCM model (Stening et al, 1999). The latter model gives a good simulation of the lunar tide in the F region of the ionosphere near the equator and so the value of 7 h is well supported. I shall ®rst look at a few new observations of sporadic E which will con®rm the wind-shear theory, postulate and discuss some theories which might explain the observed lunar phases of h¢Es and foEs and examine a range of new determinations of these phases for comparison with the theory
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