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 than 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.