On the Growth and Evolution of Nocturnal Ionospheric Irregularities Using HF Radar Observations Along With Digisonde and GNSS Data Over Trivandrum, India

Abstract The growth and evolution of nocturnal ionospheric plasma irregularities are analyzed over the geomagnetic equatorial station Trivandrum (8.5°N, 77°E, dip latitude 1.9°N) using multi‐instrumental techniques during various seasons of moderate and high solar activity periods. Irregularities are classified into three categories based on their structure and the vertical extent of radar backscattered echoes as Bottom Type, Top Side, and Bottom Type‐Top Side structures. The presence or absence of energy cascading across different scale sizes is identified from the presence of irregularities as Equatorial Spread F (ESF) in digisonde, scintillation in GNSS and backscattered echoes in HF radar with the signals manifesting sequentially or otherwise in these instruments. Threshold values of the duration (2.25 hr) and altitudinal extent (430 km) of ESF irregularities (observed with the digisonde) required for effective energy cascading across scale sizes are identified. The bottom type irregularity layers are observed to form in the equatorial region, Trivandrum, when the post‐sunset F region height varied within the range of 280–370 km which is broader in comparison to those observed at low latitudes. An empirical model is developed using post‐sunset ionospheric height to predict the maximum altitudinal extent of ESF irregularities. This serves as an indicator of the energy cascading process across scale sizes. The altitudinal extent of irregularities predicted by the empirical model also serves as a proxy for the occurrence of plasma bubbles, which has significant adverse implications for communication and navigation systems.