Estimating the daytime Equatorial Ionization Anomaly strength from electric field proxies

Abstract

The Equatorial Ionization Anomaly (EIA) is a significant feature of the low-latitude ionosphere. During daytime, the eastward electric field drives a vertical plasma fountain at the magnetic equator creating the EIA. Since the eastward electric field is also the driving force for the Equatorial Electrojet (EEJ), the latter is positively correlated with the EIA strength. We investigate the correlation between the zonal electric field and the EIA in the Peruvian sector and compare the results with correlations of the EEJ versus EIA strength. Analyzing 5 years of Challenging Minisatellite Payload (CHAMP) electron density measurements, plasma drift readings from the Jicamarca Unattended Long-term Investigations of the Ionosphere and Atmosphere (JULIA) radar, and magnetic field observations at Huancayo and Piura, we find the EEJ strength and the zonal electric field to be suitable proxies for the EIA intensity. Both analyses reveal high correlation coefficients of cc > 0.8. A typical response time of the EIA to variations in the zonal electric field is ∼ 1-2 h, and it is ∼2-4 h after EEJ strength variations. Quantitative expressions are provided, which directly relate the EIA parameters to both proxies. From these relations, we infer that an EIA develops also during weak Counter Electrojets (CEJs), but no EIA forms when the vertical plasma drift is zero. For positive EEJ magnetic signatures to form, a minimum eastward electric field of 0.2 mV/m is required on average. The above-mentioned delay between EIA and EEJ variations of ∼3 h is further confirmed by the investigation of the EIA response to transitions from CEJ to EEJ, e.g., during late morning hours. Copyright 2008 by the American Geophysical Union.