The A index is a linear measure of the Earth’s field. As a result of this, its values extend over a much wider range. It is derived from the K index by scaling it to give a linear value which is termed the “a” index. This is then averaged over the period of a day to give the A index. Like the K index, values are averaged around the globe to give the planetary Ap index.
Values for the A index range up to 100 during a storm and may rise as far as 400 in a severe geomagnetic storm.