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Using an analogy method the frequencies of new modes of the electromagnetic planetary-scale waves (with a wavelength of 10<sup>3</sup> km or more), having a weather forming nature, are found at different ionospheric altitudes. This method gives the possibility to determine spectra of ionospheric electromagnetic perturbations directly from the dynamic equations without solving the general dispersion equation. It is shown that the permanently acting factor-latitude variation of the geomagnetic field generates fast and slow weakly damping planetary electromagnetic waves in both the E- and F-layers of the ionosphere. The waves propagate eastward and westward along the parallels. The fast waves have phase velocities (1–5)km s<sup>–1</sup> and frequencies (10<sup>–1</sup>–10<sup>–4</sup>), and the slow waves propagate with velocities of the local winds with frequencies (10<sup>–4</sup>–10<sup>–6</sup>)s<sup>–1</sup> and are generated in the E-region of the ionosphere. Fast waves having phase velocities (10-1500)km s<sup>–1</sup> and frequencies (1–10<sup>–3</sup>)s<sup>–1</sup> are generated in the F-region of the ionosphere. The waves generate the geomagnetic pulsations of the order of one hundred nanoTesla by magnitude. The properties and parameters of the theoretically studied electromagnetic waves agree with those of large-scale ultra-low frequency perturbations observed experimentally in the ionosphere.<br><Br> <b>Key words.</b> Ionosphere (ionospheric disturbances; waves propagation; ionosphere atmosphere interactions)