Case for a new process, not mechanism, for cusp irregularity production

Two plasma instability mechanisms are currently thought to dominate formation of plasma irregularities in the F region high‐latitude and polar ionosphere: the gradient‐drift driven instability and velocity‐shear driven instability. The former mechanism is accepted as accounting for structuring plasma in polar cap patches and the latter for structuring plasma in polar cap Sun‐aligned arcs. Recent work has established a dominant patch formation process, involving magnetic reconnection driving strong plasma shears repeatedly observed in the cusp. Proceeding from this, we present the case for a needed new plasma structuring process (not new mechanism), whereby shear‐driven instabilities first rapidly structure the entering plasma, after which gradient drift instabilities build on these large “seed” irregularities. Correct modeling of cusp and early polar cap patch structuring will not be accomplished without allowing for this compound process. This compound process also explains previously unexplained characteristics of cusp and early polar cap patch irregularities.