Interaction Of Low-frequency Waves Research Articles

One type of three-wave interaction of low-frequency waves in magnetoactive plasma of the solar atmosphere

We study the process of occurrence of “quasi-mode” decay instability of kinetic Alfven waves (KAW) in the chromosphere of a solar active region before a flare, namely, in plasma of magnetic loops near their footpoints. The decay of a primary KAW into a kinetic ion-acoustic wave and a secondary KAW was considered as a specific type of three-wave interaction. Necessary conditions for the KAW decay instability occurrence were found for two semiempirical models of the solar atmosphere with the use of a modified expression for the growth rate of instability in the case of nonlinear interaction of low-frequency waves with an abnormally low excitation threshold. It was shown that the main criteria for the development of this instability significantly depend on the amplitude of external magnetic field in the region under study as well as on a model of the solar atmosphere.

Nonlinear transformation of seismic waves into ultra-low frequency atmospheric acoustic waves

A nonlinear mechanism of the transformation of low-frequency seismic waves into ultra-low frequency atmospheric acoustic waves is discussed. The spectrum of the seismic waves caused by the earthquake is wide but only ultra-low frequency acoustic waves reach the ionosphere. This process can be amplified due to the nonlinear interaction of low frequency waves and their transformation into ultra-low frequency acoustic wave at the difference frequency. It is shown that thus the efficiency of this transformation is high.

Nonlinear Transformation of Seismic Waves into Ultra-Low Frequency Atmospheric Acoustic Waves

A nonlinear mechanism of the transformation of low-frequency seismic waves into ultra-low frequency atmospheric acoustic waves is discussed. The spectrum of the seismic waves caused by the earthquake is wide but only ultra-low frequency acoustic waves reach the ionosphere. This process can be amplified due to the nonlinear interaction of low frequency waves and their transformation into ultra-low frequency acoustic wave at the difference frequency. It is shown that thus the efficiency of this transformation is high.

Observations of near-bottom currents in the Gulf of Riga, Baltic Sea

Near-bottom currents and temperature were measured at 2 and 4 m above bottom (m.a.b.) in the Gulf of Riga, a semi-enclosed sub basin of the Baltic Sea. Measurements were taken during three periods of different vertical stratification and weather conditions. Water depth was 54 meters. The studied area comprised the outer region of the bottom boundary layer, as judged by the observed Ekman veering of the current vector.¶Local wind was the primary driving force of near-bottom currents in the deepest area of the Gulf of Riga. Imposed wind stress induced surface-layer flow to the right, while compensating return flow developed in the bottom boundary layer. The response of current field was dominated by low-frequency variability with a clear indication of topographic steering and minor variability due to near-inertial frequency processes. The near-inertial frequency processes were generated also by interaction of low-frequency waves with the topography. The low-frequency and near-inertial frequency processes were of equal value in producing vertical shear of horizontal velocity in the near-bottom layer. The quasi-steady flow in autumn was forced by strong net inflow to the Gulf of Riga through the Irbe Strait. Lateral advection of water is important for the deep-water renewal that is due to dominating low-frequency currents.