Currents flowing in the ionosphere induce an external magnetic field component. Sources of these currents include wind-driven motion of ionospheric plasma that produces a daily variation field known as Sq (‘solar quiet’). This takes the form of two vortices, fixed north and south of the Sun-Earth line, on the dayside of the Earth, as the Earth rotates beneath. Along the geomagnetic equator an equatorial electrojet is formed, due to a high conductivity channel related to the near-horizontal field morphology. Below (or above) this electrojet the field amplitude can be enhanced by several hundred nT, within about five degrees i.e. a few hundred km of the magnetic equator.
At auroral latitudes (approximately 55-65 magnetic degrees), the auroral electrojet is formed at the open/closed field line boundary. This electrojet is fed by field-aligned currents that connect the ionosphere to the magnetospheric flanks and to the night-side tail and partial ring current sheet. The magnitude of the resulting induced fields is very dynamic and can be many hundreds of nT as observed at ground level or in low Earth orbit during periods of disturbed geomagnetic activity.
In the lower magnetosphere there are inter-hemispheric (field-aligned) currents of several nT at around 400 km altitude. Plasma ‘bubbles’ can also cause localised magnetic variations of a few nT to be measured by low-Earth orbit satellite altitudes. At high and polar latitudes the Earth’s field is also open to the solar wind and cusp currents, also known as Region 0 currents flow.