On ion acoustic turbulence and the nonlinear evolution of kinetic Alfvén waves in aurora

Wave data from the recently launched Freja satellite, show a close relationship between Alfvén wave activity and ion acoustic wave activity within auroral energization regions. The Alfvén wave activity becomes especially strong (several 100 mV/m) in localized regions with horizontal scales of a few hundred meters to a km (few tens of Hz in the S/C reference system). In some cases these signatures become more electrostatic‐like and filled with broadband, high frequency electrostatic turbulence up to 15 kHz. This broadband electrostatic turbulence has an amplitude comparable to the larger scale electric signatures, and is identified as ion acoustic waves. We argue that these signatures have evolved from solitary kinetic Alfvén waves (SKAW), and may be important for auroral energization processes.