High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 428

When the IMF has a strong northward component, the horizontal currents
associated with closure of the field-aligned NBZ current system can be observed
from satellites but, from the ground, it is only observable in the summer polar
cap. What is detectable on the ground is a low-intensity version of the two-cell
current system that exists when B z is negative or very small. In addition, there
is evidence for an antisunward current in the center of the polar cap.
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