## 4 Discussion
In other words, if the dimensionless parameters like \(f_{p}\tau_{p}\) or \(\
u\tau_{p}\) are not small compared to unity in ionospheric plasma, they should be considered as key simulation parameters, and their values must be reproduced in laboratory plasmas. The presence of the magnetic field makes the pattern of transformation of the EMP form and spectrum much richer. In the experiments on propagation of short pulses in relatively rare plasma and weak magnetic fields (\(\tau_{p}<<\)\(f_{p}^{-1}\),\(f_{c}^{-1}\)), one should expect sucheffects as the Faraday rotation of the polarization plane or birefringence (Cotton-Mouton effect), which are manifested with sufficiently long paths. If the plasma is dense and magnetic field is not very weak, or the pulse is not too short, and the resonance frequencies \(f_{p},f_{c}\) fall within the main frequency band of the EMP, the laboratory experiment allows simulating the effects that take place in the case of propagation of EMPs with durations from 100 ns to 10 \(\upmu\)s through magnetized ionospheric plasma. The laboratory experiment makes it possible to obtain an illustrative physical pattern of linear transformation of the form of the EMP propagating in a medium, which is complicated from the viewpoint of dispersion, e.g., magnetized plasma. In particular, an interesting result was obtained with the EMP propagating in low-density plasma in the presence of the cyclotron resonance (see Figure 4b). It has already been noted that the presence of the cyclotron resonance absorption leads to a train of damped oscillations with a frequency close to \(f_{c}\) appearing on the \"tail\" of a short EMP (shorter than the cyclotron rotation period). When analyzing the signal in the time domain, the appearance of this train can be interpreted as follows. The spectrum \(\hat{s}_{T}\) of the transmitted signal (here, \(\hat{s}_{T}\) is the complex amplitude \(\hat{S}_{T}(f)=(2\pi)^{-1/2}\int_{-\infty}^{\infty}u_{T}(t^{\prime})\text{ exp}\,(2\pi ift^{\prime})dt^{\prime}\)) can be represented as the difference of the spectrum of the signal transmitted through the GCL without plasma (\(\hat{s}_{0}\)) and the absorption spectrum (\(\hat{s}_{A}\)):  
\[\hat{s}_{T}=\hat{s}_{0}-\hat{s}_{A} \tag{3}\]  
The parameters of the absorption spectrum \(\hat{s}_{A}\) (the central frequency \(f_{0}\) and the width \(\delta f<<\)\(f_{0}\)) are determined by the properties of the absorption line. Applying the linear procedure of the inverse Fourier transform (\(u_{T}(t)=(2\pi)^{1/2}\int_{-\infty}^{\infty}\hat{S}_{T}(f^{\prime})\text{exp} \,(-2\pi if^{\prime}t)df^{\prime}\)) to equality (3), we obtain the time-domain representation of the signal transmitted through the GCL:  
\[u_{T}(t)=u_{0}(t)-u_{A}(t) \tag{4}\]  
Here, \(u_{0}(t)\) is a transmitted signal without plasma which can be measured in experiments directly (see Figure 4). The \"virtual\" signal \(u_{A}(t)\) corresponds to the resonance absorption and has a narrow spectrum with the width \(\delta f\). Therefore, it should be a quasimonochromatic packet with a duration of at least \(1/\delta f\) and carrier frequency \(f_{0}\)-\(f_{ce}\). The initial signal \(u_{0}(t)\) has a short duration, \(\tau_{p}<<1/\delta f\). Therefore, the resulting signal is a superposition of a short (initial) pulse and the consequent narrowband train with the frequency \(f_{0}\)-\(f_{ce}\). The fact that the narrowband signal is excited after the initial pulse follows from the principle of causality and is clearly seen in the experiment (see Figure 4). To illustrate this consideration, Figure 7 shows fragments of the waveforms obtained in the experiment in vacuum (without plasma) and with rare magnetized plasma, as well as the spectra of fragments of the waveforms corresponding to the initial EMP and the narrowband train of the damped oscillations. An alternative physical explanation for the formation of a packet of damped oscillations with a frequency about \(f_{c}\) is as follows.