Estimating the daytime Equatorial Ionization Anomaly strength from electric field proxies

Abstract

[1] 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.

Relationships between pre-sunset electrojet strength, pre-reversal enhancement and equatorial spread-F onset

“…A statistical analysis performed by Rama Rao et al (1997) showed that the peak density of the F2-layer near and inside the EIA crests was strengthened during evening hours on ESF days. Stolle et al (2008) investigated a typical response time of observed EIA to EEJ variations and they showed that EIA decays significantly within approximately 2 h after CEJ onsets. Considering the results reported by Stolle et al (2008), the evening CEJ modifies the plasma density distribution around and inside the EIA crests around sunset, since the evening CEJ that makes the pre-sunset IEEJ negative already starts in the afternoon (see Fig.…”

“…Since the ESF irregularity causes severe disturbances for various communication and navigation systems, a precise forecast of ESF onsets is important from a point-of-view of space weather. Although an initial phase of ESF development has been explained by the linear Rayleigh-Taylor instability theory (e.g., Sultan, 1996), the precise forecast of ESF onsets has not been established. One of the reasons is that the seeding mechanism of an initial perturbation which is necessary for ESF onsets has not been fully understood, although the gravity wave has been suggested as the possible seeding source (Abdu, 2001, and references therein).…”

Sq and EEJ—A Review on the Daily Variation of the Geomagnetic Field Caused by Ionospheric Dynamo Currents

“…Studies have shown that the intensity and latitude of the equatorial anomaly crests vary with the intensity of the equatorial electrojet (Dunford 1967;MacDougall 1969;Rastogi and Rajaram 1971;Rush and Richmond 1973;Raghavarao et al 1978;Balan and Iyer 1983;Huang et al 1989;Rastogi and Klobuchar 1990;Rama Rao et al 2006;Stolle et al 2008). The most comprehensive study was made by Stolle et al (2008), involving 5 years of CHAMP electron density observations along with radar and geomagnetic field measurements. The following are empirical formulae, given by Stolle et al (2008), relating geomagnetic field perturbations to equatorial anomaly parameters:…”

“…The most comprehensive study was made by Stolle et al (2008), involving 5 years of CHAMP electron density observations along with radar and geomagnetic field measurements. The following are empirical formulae, given by Stolle et al (2008), relating geomagnetic field perturbations to equatorial anomaly parameters:…”

An empirical model of ionospheric total electron content (TEC) near the crest of the equatorial ionization anomaly (EIA)

“…The amplitude of the equatorial electrojet (EEJ) is considered to be the proxy index of the equatorial electric field (see Stolle et al 2008). The EEJ refers to an enhanced ionospheric current flowing at an altitude of~100 km in the narrow latitudinal belt around the magnetic equator, ranging from 5°N to 5°S (Chapman 1951).…”