The Kp index is an indicator of electric currents flowing in the ionosphere that may be associated with visible Aurora. It is actually determined by measuring abnormal deflections in the Earth's magnetic field at the surface caused by these electrical currents, using a ring of magnetic observatories at varying latitudes throughout the world.

The Kp index tells us at what magnetic latitude we should expect to see an Aurora. Magnetic latitude is different from geographic latitude in that it uses the orientation of the Earth's magnetic pole as a reference, instead of the Earth's rotational axis, as 90 degrees latitude. Since the magnetic pole is offset from the rotational axis by over 22 degrees, there is a difference between magnetic latitude and geographic latitude. Auroral currents only care about the Earth's magnetic field - and thus geographic coordinates are irrelevant! Hence the Kp index tells us at what magnetic latitude to expect Aurora.

You can use the form on the Kp page to determine your magnetic latitude based on your geographic latitude and longitude. As a guide, the table below summarizes what different values of the Kp index mean in terms of auroral visibility at different magnetic latitudes:

CURRENT CONDITIONS
[CME] [Solar Wind] [Storms] [Auroral Kp] [Particles]