Field lines from inside the auroral ovals thread outward through regions known as the "polar caps" and thence away from the sun through regions known as the "lobes" of the magnetospheric tail. Conventional wisdom gained from numerous satellite flights holds that the polar caps and lobes are essentially devoid of plasma, and in fact the plasma density there is very low compared with either the ionosphere or the solar wind.

A spacecraft in sunlight emits electrons by the photoelectric effect and tends to charge positively so as to retain those emitted photoelectrons and maintain zero net electrical current. In the polar caps and lobes, the plasma density is so low that the photoelectric effect charges spacecraft by as much as 40-50 V positive. This prevents low energy positively charged ions from even reaching the spacecraft. Thus it is no wonder that low energy ion plasmas have not been observed in the polar caps!

The POLAR spacecraft includes a plasma source device that, when operated, effectively regulates the potential of the spacecraft at +1.8 V, thereby allowing all but the very lowest energy ions to reach the spacecraft. The result is quite spectacular.