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The equatorial electrojet (EEJ) is a narrow ribbon of current flowing eastward in the day time equatorial region of the Earth's Ionosphere. The abnormally large amplitude of variations in the horizontal components in equatorial geomagnetic observatories, as a result of EEJ, was noticed as early as 1920 from Huancayo geomagnetic observatory. Observations by radar, rockets, satellites and geomagnetic observatories are used to study EEJ.

## Causes Edit

The worldwide solar-driven wind results in the so-called Sq (solar quiet) current system in the E region of the Earth's ionosphere (100-130 km altitude). Resulting from this current is an electrostatic field directed E-W (dawn-dusk) in the equatorial day side of the ionosphere. At the magnetic dip equator, where the geomagnetic field is horizontal, this electric field results in an enhanced eastward current flow within ± 3 degrees of the magnetic equator, known as the equatorial electrojet.

## Studies of EEJ from satellite & ground magnetic data Edit

The EEJ phenomenon was first identified using geomagnetic data. The amplitude of the daily variation of the horizontal magnetic intensity ($\Delta$H) measured at a geomagnetic observatory near the dip-equator is 3-5 fold higher than that of data from other regions of Earth. A typical diurnal equatorial observatory data show a peak of strength ~80 nT at 12.00 LT, with respect to the night-time level. Egedal (1947) showed that the enhancement of quiet day solar daily variations in $\Delta$H (Sq(H)) lay within the 50 latitude centered on the dip equator . The mechanism that produced the abnormal current was proposed as a band of current about 300 km in width flowing over the dip equator.

EEJ studies from satellite data were initiated with the arrival of data from the POGO (Polar orbiting Geophysical Observatories) series of satellite (1967-1970). The characteristic signature of the EEJ ,$\Delta$H field is a sharp negative V shaped curve attaining its minimum within 0.5° of the magnetic dip equator. The magnetic data from the satellite missions like Ørsted (1999 - present) and CHAMP (2000 - present) vastly improved our knowledge on EEJ.