The data of the geophysical observation complex at Barentsburg observatory on Spitsbergen archi�pelago, together with the data from other stations and satellite observations, were used to interpret a sharp increase in the frequency interval in the electromagnetic noise spectral resonance structure (SRS) in the Hertz range that took place during a Substorm that occurred on December 24, 2005. It has been shown that such a change in SRS is related to a decrease in the electron density in the ionospheric F region, which agrees with the ionospheric Alfvén resonator theory. In turn, a decrease in electron density is probably related to the fact that the station was in the auroral cavity region related to the field�aligned current flowing into the ion�osphere.