(note: We offer a brief tutorial on upper ionospheric basics to better understand the magnetic 'highway' that the HAARP project and other such research facilities use. The following excerpt, and all related images except where noted, are from Potsdam University. click images for detail)
The Sun constantly emits a stream of plasma, the solar wind, carrying a magnetic field of variable direction, the interplanetary magnetic field, or IMF for short.
The interaction between the solar wind and its IMF in particular with Earth's magnetosphere causes a large-scale plasma movement within the magnetosphere that appears as a permanent and highly variable plasma circulation in the high-latitude polar ionosphere.
When the magnetized plasma of the solar wind encounters the Earth's magnetic field, it is forced to flow around it, leaving behind it a cavity, the magnetosphere, which is separated from the solar wind by a boundary, called the magnetopause. (image right from cse.ssl.berkely,edu)
When the IMF has a southerly direction, it can connect with the Earth's northward-directed field at the front of the magnetosphere, via a universal plasma process, referred to as magnetic reconnection.
This magnetic connection allows solar wind plasma to enter the magnetosphere.
Once connected with the IMF, the terrestrial field lines, and the plasma that sits on them, are carried by the solar wind over the Earth's poles downstream, penetrating deeper and deeper into the Earth's magnetotail, where they eventually reconnect once more to form closed field lines.
The field lines are subsequently transported sunward past the Earth, until they again meet the IMF at the magnetopause, allowing the whole process to start all over (Fig. 1a).
This large-scale plasma and magnetic field circulation extends all the way down to the ionosphere, the ionized portion of Earth's upper atmosphere.
The result is a two-cell pattern, with anti-sunward flow over the polar caps, returning at lower latitudes to the dayside via the dawn and dusk flanks (Fig. 1b).
Since plasma flow in the presence of a magnetic field produces an electric field transverse to the flow, the convection is equivalent to an electric potential across the polar caps, which can be more than 100 kV (100 kiloelectronvolt = 4.4504925e-21 kilowatt hour, from onlineconversion.com) during periods of strongly southward IMF.