TITLE:
Quiet-Time Equatorial Plasma Fountain Observed by ICON
AUTHORS:
Ildiko Horvath, Brian C. Lovell
KEYWORDS:
EIA, Field-Perpendicular Electrodynamic E × B Drift, Field-Aligned Mechanical Drift, PRE, Forward Plasma Fountain, Reverse Plasma Fountain
JOURNAL NAME:
Atmospheric and Climate Sciences,
Vol.16 No.4,
October
28,
2026
ABSTRACT: This study’s main aim is to increase our understanding of how ICON could observe the signatures of forward fountain and reverse fountain at lower sunspot numbers within ICON’s observational capability and operational limitation. Multi-instrument and multi-point observations demonstrate the Equatorial Ionization Anomaly (EIA) and the EIA’s breaking down, and the signatures of underlying forward fountain and reverse fountain seen in the radial and meridional ICON drift measurements. Driver mechanisms investigated include the 1) equatorial electrodynamic field-perpendicular (⊥) upward/downward (E × B)⊥ drift driving fountain-related vertical plasma flows over the dip equator where the angle of inclination (I) is zero and 2) mechanical total field-aligned (‖) upward/downward (VmechasinI)‖ drift driving field-aligned plasma diffusions near the dip equator. Jicamarca radar measurements taken over the dip equator and ∆H values computed for the Asian sector validated the (E × B)⊥ drift’s daily variation and the evening PRE. Total electron content maps verified the EIA’s spatial variation at ~350 km altitude. Results show that shortly before/after the PRE, the EIA appeared under stronger/weaker solar conditions as a two-peak/single-peak structure. Their underlying forward fountain signatures are demonstrated with the enhanced upward (E × B)⊥ drift measured within ±1.25˚ (dip) latitudes and with the decreased total downward (VmechasinI)‖ drift’s vertical and meridional drift components. Reverse fountain related signatures were demonstrated with the enhanced downward (E × B)⊥ drift measured within ±1.25˚ and with the total upward (VmechasinI)‖ drift’s vertical and meridional drift components. From these observational results we conclude that a ±1.25˚ (dip) latitude restriction should be applied to the ICON drift data to appropriately monitor the vertical (E × B)⊥ drift over the dip equator and to properly define the PRE.