Abstract The evolution of night‐time F region irregularities over the geomagnetic equatorial station, Trivandrum (geographic coordinates: 8.5°N, 76.93°E; geomagnetic coordinates: 0.48°N, 149.94°E) is investigated using HF radar, GNSS and digisonde data for moderate to high solar activity levels during magnetically quiet and disturbed periods. The important results are as follows. (a) Significant inhibition of F region irregularities of all scale sizes during disturbed periods. (b) Demarcation of higher threshold values (for cascading) of ESF spread and/or duration during disturbed periods. (c) Existence of significant linear relationship between h’F just before ESF and maximum altitude of spread, for equinoxes and solstices of disturbed periods. (d) Observation of BT‐TS, TS and BT/TS structures in HF radar during disturbed periods with inverse relation between vertical drift velocity and spectral width. (e) Cascading occurred for ∼97% structures in magnetically quiet period and ∼43% structures in disturbed period. (f) Cascading occurred for 75% of days in Main Phase (MP) and 30% of days in Recovery Phase (RP), due to enhanced neutral densities during RP. (g) Threshold values of neutral densities above which cascading is inhibited are 3.4 × 10 8 /cc and 2.4 × 10 8 /cc respectively for disturbed periods of equinox and solstice as a result of larger E region Pedersen conductivity in the latter, which necessitates a much lower collision frequency for cascading. (h) During magnetically quiet period the neutral density thresholds are 4.2 × 10 8 /cc and 4.7 × 10 8 /cc respectively for equinox and solstice. Reduced neutral density threshold during disturbed period is ascribed plausibly to the disturbance induced background ionization reduction.
Vishnupriya et al. (Tue,) studied this question.