Spread F and statistics

The Equatorial Spread-F (ESF) phenomenon appears as the spreading of the F-Layer trace in the ionogram observed at the stations near the magnetic equator at night time. The ESF is a manifestation of the plasma bubble irregularities that occur under unique electro-dynamical conditions of the post-sunset ionosphere. We can say that the irregularities indicate the ionospheric disturbance. The major mechanism that plays the important role is the well known generalized Rayleigh-Taylor (R-T) instability, which is mainly caused by pre-reversal electric field enhancement (PRE) indicating the uplift of F-layer bottom side [3].

The ESF are categorized into 3 types :

  • range spread-F (RSF), spreading on the range side (indicating the rise of plasma bubble from the bottomside of the ionosphere),
  • frequency spread-F (FSF), spreading on the frequency side (indicating the disturbance at higher altitudes above the bottomside) and
  • mixed spread-F (MSF)

based on U.R.S.I. Handbook of Ionogram Interpretation and Reduction

Ionogram showing examples of

(a) FSF trace (b) RSF trace and (c) MSF trace


RSF Occurrence rate, 2004, Chiangmai, Thailand



RSF Occurrence rate, 2004, Chumphon, Thailand


Monthly mean percentage of the ESF occurrence in September, 2009,

at Chumphon station


The %RSF occurrence is higher than the %FSF occurrence. Besides, the FSF onsets occur later than the RSF onsets. The %FSF occurrence has the peak after midnight. While the %RSF occurrence has the peak before midnight.