Theory and simulation of low‐frequency plasma waves and comparison to Freja satellite observations

One‐dimensional models of obliquely propagating nonlinear plasma waves were formulated and solved both analytically and numerically to interpret recent Freja satellite observations of low‐frequency plasma waves detected in the low‐altitude auroral magnetosphere. Analytic calculations revealed four types of steady state waves solutions. Time dependent initial value numerical calculations were compared to the steady state solutions and to Freja observations. One type of steady state wave solution emerged in the long time limit from the initial sinusoidal waves; however, the initial value simulations agreed best with the observations during the nonlinear steepening phase of the initial waveform at a time well before a steady state was reached. From this result we concluded that many of the low altitude auroral waves Freja has detected were oblique inertial Alfvén waves that had nonlinearly steepened due to propagation into a region of lower Alfvén speed. The nonlinear steepening was found to produce very large parallel currents. The current is sufficiently high to excite parallel electron drift instabilities, which may lead to electron and ion energization and enhanced dissipation of auroral arc energy.