Multi-day Persistence of Equatorial F-region Field-Aligned Irregularities: Evidence from Radar Observations over Indonesia
Dyah Rahayu Martiningrum1,2*, Suhaila M Buhari1*, Prayitno Abadi2,3, Ihsan N Muafiry2, Anwar Santoso4,5, and Rezy Pradipta6

1. Dept. of Physics Universiti Teknologi Malaysia, Skudai, Johor Bahru, 81310, Malaysia
2. Research Center for Climate and Atmosphere, National Research and Innovation Agency, Bandung, Indonesia
3. School of Electrical Engineering, Telkom University, Bandung, Indonesia
4. Research Center for Space, National Research and Innovation Agency, Bandung, Indonesia
5. Physics Department, Faculty of Mathematics and Natural Sciences, Gadjah Mada University,
Yogyakarta, Indonesia
6. Institute for Scientific Research, Boston College, Chestnut Hill, Massachusetts, USA

Abstract

This study examines nighttime field-aligned irregularities (FAI) in the F-region ionosphere over the Equatorial Atmosphere Radar (EAR) site in West Sumatra, Indonesia, using observations from 2011 to 2016. FAI events are identified from radar backscatter intensity and classified daily as present or absent. A combinatorial framework is applied to quantify multi-day occurrence patterns over 1 to 6-day sequences, enabling direct assessment of temporal structure, including persistence and transition behaviour. The analysis is performed for equinox (March and September) and solstice (June and December) periods. FAI occurrence peaks during equinoxes and is strongly suppressed during solstices. Beyond this seasonal contrast, the results reveal clear and systematic multi-day persistence and transition patterns. Solstice periods are dominated by extended non-occurrence sequences, while equinox periods show more frequent transitions and a more balanced distribution of occurrence patterns. These findings demonstrate that FAI variability follows organized temporal structures rather than random behaviour. The observed persistence is consistent with ionospheric preconditioning, where background plasma conditions evolve gradually across days, as well as with seasonal modulation of the Rayleigh-Taylor instability through variations in pre-reversal enhancement of the zonal electric field. The proposed approach provides a concise and robust framework for quantifying multi-day temporal structure in ionospheric irregularities, strengthening both their statistical characterization and physical interpretation, with direct implications for future forecasting efforts.

Keywords: ionospheric irregularities, day-to-day variability, equatorial ionosphere, combinatorics, persistence

Topic: Earth Physics and Space Science