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The initiation and sustaining of field emission processes from a liquid surface are based mainly on the effect of field amplification at the surface microtips. Such microtips form under different types of hydrodynamic instabilities. However, the threshold field of such instabilities has a very high value. At the same time, there are some situations when one needs to excite the emission using an arbitrary low amplitude of the external voltage. In this paper, in order to solve this problem, the process of preliminary excitation of the surface by an ultrasonic field on the liquid emitter surface has been studied. The theory developed has shown that, as a result of parametric instability, the formation of surface microstructures is possible. Such structures can be produced at very low levels of ultrasonic power. This fact is of particular importance for large surface area cathodes. The dependence of the geometric parameters of the surface structures both on frequency and power of the ultrasonic field is determined theoretically. Experiments with field pulses in the nanosecond range confirm the decrease of the explosive emission mechanism when preliminary ultrasonic excitation is used.