The annual variation in quiet time plasmaspheric electron density, determined from whistler mode group delays

Whistler mode group delays from the VLF Doppler experiment at Faraday, Antarctica (65°S, 64°W) show an annual variation that has a maximum in December and a minimum in June/July. Assuming signal propagation at constant L (L = 2.5), this implies an annual equatorial electron density (Neq) variation, with December values 3 times higher than in June (during solar minimum—1986). This annual variation in Neq can be modelled from the combined ƒ ƒoF2 medians at each end of the field line (Argentine Islands and Wallops Island), by assuming that diffusive equilibrium is maintained from the F2 layer to the equator over long (≥ 1 month) time scales during quiet magnetic conditions. The use of this model enables the longitude dependence of the annual Neq variation to be investigated. ƒ ƒoF2 data from two other pairs of near-conjugate stations at ∼ 50°E and ∼ 180°E suggest that there are probably no other regions where there are such large annual variations in Neq at L = 2.5. Whistler mode group delays from a similar VLF Doppler experiment at Dunedin, New Zealand (45.8°S, 170.5°E) show an annual variation that is much smaller, and in agreement with the model results at that longitude. At Faraday during solar maximum conditions, the phase of the annual variation is similar to that observed at solar minimum, but the amplitude is smaller, the December–June ratio in Neq being about 2:1.

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