Density cavity formation by large-scale shear Alfvén waves

Abstract

Using two-dimensional particle-in-cell (PIC) simulations, we study the formation of density cavities in plasmas driven by transversely large-scale Alfvén waves (LSSAWs) when they reflect from a boundary, like in the topside auroral ionosphere. The ponderomotive force, associated with the resulting standing waves, causes strong modification in the density distribution, including density cavities. Based on our findings we suggest that the ubiquity of density cavities in the auroral plasma is possibly a direct consequence of the ubiquity of the LSSAWs. In the cavities the amplitudes of the transverse wave electric (|E⊥|) and magnetic (|B⊥|) fields are, respectively, diminished and enhanced so that E⊥≪VAB⊥, where VA is the Alfvén velocity, as recently reported from a statistical analysis of FAST data at an altitude of ∼4000 km (Chaston C. C. et al., Phys. Rev. Lett., 100 (2008) 175003).