Excitation of ion-acoustic turbulence and its role in auroral processes at Freja altitude

Abstract

The Freja spacecraft wave experiment provides data on solitary kinetic Alfvén wave structures co-located with ion acoustic like turbulence observed in the auroral zone. These structures are electromagnetic in nature with a transverse scale size of nearly 1 km. The scale length is of the order of the electron inertial length in the auroral zone. The typical E x B y ratio associated with the wave events is of the order of the inertial Alfvén speed. The SKAW events are often associated with broadband emissions identified as O + and H + ion-acoustic waves. Following the Vlasov-Maxwell formalism, the mechanism of the excitation of the ionacoustic turbulence by the auroral electrons is examined. These electrons are accelerated by the parallel electric field associated with the SKAW structures. Conditions for the unstable growth of the ion acoustic mode are calculated. I show that the conditions favourable to the unstable growth of these waves do exist in the auroral region. I also calculate the heating rate of auroral plasmas due to the anomalous resistivity effects resulting from ion-acoustic turbulence.