Abstract: Most of the parameters of microwave devices operating on the basis of electron cyclotron resonance (ECR) are determined by the characteristics of the generated plasma. The use of well-studied single-mode resonators for maintaining the discharge in many cases turns out to be insufficiently effective, because the single-mode condition imposes severe restrictions on the maximum linear dimensions of the used resonator at a given frequency of microwave waves. Therefore, in recent years, the features of the maintenance of discharges with the simultaneous excitation of several vibration modes have been actively studied. In this work, we study a microwave discharge (2.45 GHz) in a multimode resonator placed in a magnetic mirror (magnetic trap), in which a strongly nonequilibrium anisotropic low-temperature plasma is generated. It is shown that the structure of the electromagnetic field in the resonator changes with an increase in the pressure of the plasma-forming gas, which is accompanied by a change in the electron density and temperature. © 2022, Pleiades Publishing, Ltd.