## 3 Experimental Results
The signal spectrum becomes closer to the spectrum of the initial EMP, but deep lines of absorption at the electron cyclotron resonance \(f=\)\(f_{c}\) and its second harmonic 2\(f_{c}\) appear in it. The color diagrams presented in Figure 6 serve as an illustration for the variation in the EMP spectrum in the case of EMP propagation in magnetized plasma. Figure 6a uses a color code to show the spectra of an EMP passing through the GCL as a function of time after the end of the ionizing pulse \(T\) in the plasma decay process. The diagram is a continuous sequence of EMP spectra (as in Figure 5) obtained with a gradual EMP delay change relative to the end of the ionizing pulse. The frequency scale is plotted along the vertical axis. For the sake of convenience, time dependences of the plasma frequency \(f_{p}(t)\), cyclotron resonance frequency \(f_{c}(t)\), and its second harmonic \(f_{c}(t)\), as well as the upper hybrid resonance frequency \(f_{uh}(t)=(\)\(f_{p}^{2}+f_{c}^{2})^{1/2}\)[7], are superimposed on the diagrams. The diagram demonstrates the division of the EMP frequency spectrum in plasma into two bands: one, \(f<\)\(f_{c}(t)\), in the whistler range and the other, \(f>\)\(f_{p}\), \(f_{uh}\), in the high-frequency plasma transparency region. At later plasma decay times (\(T>20\) ms) or at low \(N_{\rm e}\), the EMP spectrum demonstrates the effects of cyclotron absorption in narrow bands near the first and second harmonics of \(f_{c}\). Cyclotron absorption is clearly seen even at a very low electron density, \(N_{e}\sim 10^{5}\) cm\({}^{-3}\), at \(T=39\) ms (Figure 7). Figure 5: Spectra of the signals transmitted through the GCL filled with “dense” (top panel) or “rarefied” (bottom panel) magnetized plasma. Figure 6b shows the sum of the spectral amplitudes of the transmitted (\(S_{\rm T}\), energy units) and reflected (\(S_{\rm R}\), energy units) signals (see Figure 3) versus plasma decay time \(T\). The value \(S_{T}+S_{R}\) is normalized to the spectral amplitude of the signal transmitted through the GCL without plasma (\(S_{\rm E}\)). This diagram allows one to evaluate the balance of the EMP energy transmitted through, reflected from, and absorbed in the GCL filled with magnetized plasma. Note that the amplitude-frequency characteristics of the signal reflected from the GCL are uneven due to details of its design [19; 20]. This diagram shows that, first, the EMP energy is dissipated in plasma at the frequencies close to \(f_{c}\) and \(2f_{c}\), i.e., resonance absorption takes place in rarified plasma. Second, the EMP energy is absorbed in plasma of medium density in a wide frequency range between the lower (\(f<f_{c}\)) and upper transparency subbands. Figure 6: (**a**) Spectrum of the UWB EMP transmitted through the plasma-filled GCL as a function of the pulse delay relative to the end of the ionizing pulse. The figure also shows the time dependences of the plasma frequency, upper hybrid frequency, and cyclotron resonance frequencies (first and second harmonics). (**b**) Sum of the spectra of the transmitted and reflected UWB EMPs as a function of pulse delay relative to the end of the ionizing pulse. The spectrum amplitude is normalized on the spectrum of the signal transmitted through the GCL without plasma. Figure 7: (**a**) Form of the pulse transmitted through the GCL filled with rarefied plasma (\(N_{e}<10^{6}\) cm\({}^{-3}\),\(f_{p}<10\) MHz \(<<\)\(f_{c}\)) as compared with the form of the pulse transmitted through the line without plasma filling. The interval corresponding to the period of the cyclotron rotation of electrons is also shown. (**b**) Spectra of the pulses shown in Figure (**a**), as well as the spectrum of the “tail” of the pulse transmitted through the plasma.