Auroral Electrojet Research Studies

What is this 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 flow 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.

Following picture shows the location of the auroral electrojet in general, and also shows subionospheric propagation paths used for VLF-based electrojet detection over North America and North Atlantic.

Location of the Auroral Electrojet

Energetic Electron Precipitation

EEP represents a significant form of coupling between the ionosphere and magnetosphere, which is itself an important component of Space Weather. The magnetosphere can be thought of as a current generator which drives the ionosphere as an electrical 'load', in which significant energy dissipation takes place.

There is abundant evidence that during geomagnetic storms and substorms large fluxes of energetic electrons are directly injected into the auroral and subauroral regions from source regions in the magnetosphere. This precipitation is a major source of energy input into the lower ionosphere and cause substantial changes in the chemistry of this region. In addition these precipitating electrons produce secondary ionization in the D and E regions of both the auroral and subauroral ionospheres, significantly increasing the electrical conductivity of these regions. As a result of this increased conductivity, larger electric currents are produced there.

The importance of energetic particle precipitation in the electrojet expansion during disturbed periods has been demonstrated by Kikuchi and Evans [1983], who showed that the precipitation is completely correlated in time with the development of the electrojet current system.

VLF Measurements of the Auroral Electrojet

A key feature of E > 300 keV precipitation is the fact that it penetrates deeply into the D region of the ionosphere, well below the normal reflection height for VLF waves which propagate in the earth ionosphere waveguide. The enhanced ionization which is produced by the precipitated flux strongly perturbs the phase and amplitude of the propagation VLF waves. As shown by a number of studies [Kikuchi, 1981; Kikuchi and Evans, 1983; Kikuchi et al., 1983; Cummer et al., 1994 (AGU Outstanding Student Paper Award); Cummer et al., 1996] these perturbations are readily measurable with contemporary instruments.

Cummer et al. [1996] found that clear perturbations in both the VLF amplitude and phase data were associated with electrojet movements over the magnetometer stations.

The path configurations were such that any southward expansion of the electrojet during substorms could be remotely monitored by measuring the phase perturbations produced in the earth-ionosphere waveguide signals along the various paths.

Application of VLF Technique

VLF data over Eastern Canada has been studied for correlations with auroral electron precipitation regions over the path [Cummer et al., 1994]. It is well known that the E region conductivity enhancements caused by this precipitation are a major component in the increase of auroral electrojet currents, and as such the edge of the precipitation region is well-correlated with the edge of the elctrojet. X-ray images of the region were taken by the AXIS instrument on the UARS satellite, and from these it can easily be determined whether the precipitation region is over a VLF propagation path.

Following figure shows an example of simultaneous AXIS images, VLF data, and magnetometer data over the course of a single night. The VLF path studied was from the NLK transmitter to Gander, Newfoundland. Three x-ray images were taken over the region of interest, and it can clearly be seen that only during the third image was there electron precipitation occuring over the VLF path. Simultaneous with this last image is a significant amplitude drop in the NLK signal.

Early Warning

In many cases the VLF technique can provide hours of early warning of possible electrojet intrusion to mid-latitudes over the North American continent. The electrojet is a current system that is approximately fixed in inertial space with the earth rotating under it, and a strong electrojet activity is common in the local midnight sector of the auroral region. During major storms the electrojet can remain at mid-latitudes for as many as six hours. Since the VLF technique can detect the electrojet boundary over the Atlantic, it can provide 2 to 6 hours warning of possible strong electrojet activity when the North American continent enters the midnight sector.