Short‐period fluctuations in F region ionization observed at Arecibo

Small fluctuations in the critical frequency of the F region plasma have been measured at Arecibo with frequency resolution better than 1 kHz and time resolution of 1 s. For periods in the 10‐ to 20‐min range, f0F2 fluctuations fall off with fluctuation frequency (f) as f−k, with k ≥ 4.5. While no oscillation is observed in the period range 4–8 min, significant oscillations with amplitude of Δf0F2 ≈ 500 Hz are observed with a period of τ ≈ 3 min, corresponding to a fractional change in number density of 1–2 parts in 104. Several possible sources of these fluctuations are explored: acoustic waves in the F region, hydromagnetic or Alfven‐type oscillations, and small and large scale electric field fluctuations. Coordinated experiments employing incoherent scatter, magnetograms, and preferably an HF‐CW doppler sounder would be able to resolve the origin and nature of these intriguing waves observed for the first time in f0F2 fluctuations.