| dc.description |
Nonlinear excitation of ultra-low frequency (ULF) atmosphere-ionosphere waves by extremely low frequency (ELF) seismic acoustic bursts is discussed. An analysis of the nonlinear transformation of ELF acoustic waves (f ~ 100 Hz) into ULF (f ≤ 1Hz) waves is presented. The ELF wave is excited as the burst-like envelope of a finite transverse scale by the seismic motion of the Earth’s surface. Then, it propagates upwards and is subject to both nonlinearity and dissipation. Nonlinearity leads to the generation of higher harmonics and, thus, to a saw-tooth-like wave structure, and also to an increase in the ULF part of the wave spectrum. This process takes place at the altitudes of z = 0–100 km. For altitudes z > 100 km, the ELF wave dissipates fully, and only ULF acoustic wave can reach the ionosphere dynamo layer (120–200 km), where this wave excites fluctuating currents. Those currents are the sources of Alfvén and fast magnetosonic plasma waves that penetrate into the higher ionosphere and magnetosphere. The simulations of the nonlinear transformation of the acoustic waves are based on the slowly varying profile equation derived from the hydrodynamic equations of air motion. A high efficiency of the nonlinear transformation of ELF acoustic waves into ULF ones is shown. Like a possible linear transformation, this nonlinear transformation causes ULF magnetic field perturbations in the ionosphere of 10<sup>-4</sup>–10<sup>1</sup> nT at altitudes z ~ 200 km, possibly accompanied by temperature variations. |
