Electrojet
Encyclopedia
An electrojet is an electric current
which travels around the E region of the Earth's ionosphere
. There are two electrojets: above the magnetic equator (the equatorial electrojet
), and near the Northern and Southern Polar Circles (the Auroral Electrojets). Electrojets are Hall currents carried primarily by electrons at altitudes from 100 to 150 km. In this region the electron gyro frequency (Larmor frequency) is much greater than the electron-neutral collision frequency. In contrast, the principal E region ions (O2+ and NO+) have gyrofrequencies much lower than the ion-neutral collision frequency.
Kristian Birkeland
was the first to suggest that polar electric currents (or auroral electrojets) are connected to a system of filaments (now called "Birkeland current
s") that flow along geomagnetic field lines into and away from the polar region.
.
During magnetically quiet periods, the electrojet is generally confined to the auroral oval. However during disturbed periods, the electrojet increases in strength and expands to both higher and lower latitudes. This expansion results from two factors, enhanced particle precipitation and enhanced ionospheric electric fields.
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
which travels around the E region of the Earth's ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
. There are two electrojets: above the magnetic equator (the equatorial electrojet
Equatorial electrojet
The equatorial electrojet 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 measured at equatorial geomagnetic observatories, as a result of EEJ, was noticed as early...
), and near the Northern and Southern Polar Circles (the Auroral Electrojets). Electrojets are Hall currents carried primarily by electrons at altitudes from 100 to 150 km. In this region the electron gyro frequency (Larmor frequency) is much greater than the electron-neutral collision frequency. In contrast, the principal E region ions (O2+ and NO+) have gyrofrequencies much lower than the ion-neutral collision frequency.
Kristian Birkeland
Kristian Birkeland
Kristian Olaf Birkeland was a Norwegian scientist. He is best remembered as the person who first elucidated the nature of the Aurora borealis. In order to fund his research on the aurorae, he invented the electromagnetic cannon and the Birkeland-Eyde process of fixing nitrogen from the air...
was the first to suggest that polar electric currents (or auroral electrojets) are connected to a system of filaments (now called "Birkeland current
Birkeland current
A Birkeland current is a set of currents which flow along geomagnetic field line connecting the Earth’s magnetosphere to the Earth's high latitude ionosphere. They are a specific class of magnetic field-aligned currents. Lately, the term Birkeland currents has been expanded by some authors to...
s") that flow along geomagnetic field lines into and away from the polar region.
Equatorial Electrojet
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 within ± 3 degrees of the magnetic equator, known as the equatorial electrojetEquatorial electrojet
The equatorial electrojet 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 measured at equatorial geomagnetic observatories, as a result of EEJ, was noticed as early...
.
Auroral Electrojet
The term 'auroral electrojet' is the name given to the large horizontal currents that flow in the D and E regions of the auroral ionosphere. Although horizontal ionospheric currents can be expected to flow at any latitude where horizontal ionospheric electric fields are present, the auroral electrojet currents are remarkable for their strength and persistence. There are two main factors in the production of the electrojet. First of all, the conductivity of the auroral ionosphere is generally larger than that at lower latitudes. Secondly, the horizontal electric field in the auroral ionosphere is also larger than that at lower latitudes. Since the strength of the current is directly proportional to the vector product of the conductivity and the horizontal electric field, the auroral electrojet currents are generally larger compared to those at lower latitudes.During magnetically quiet periods, the electrojet is generally confined to the auroral oval. However during disturbed periods, the electrojet increases in strength and expands to both higher and lower latitudes. This expansion results from two factors, enhanced particle precipitation and enhanced ionospheric electric fields.