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Description
The frequency spectrum of the short transient radio pulses from air showers contains valuable information. Its shape depends on several factors, most importantly on the lateral distance of the observer from the shower axis, the arrival direction, i.e. the zenith angle, and on the distance between core position and shower maximum $X_{\mathrm{max}}$. To exploit this wealth of information, we investigated the spectral content of CoREAS simulations for showers covering energies from $10^{18.4}$ eV up to $10^{20.2}$ eV and zenith angles from $65.0\,^\circ$ up to $85\,^\circ$. We derived a full parameterization of the spectral shape in the 30-80 MHz frequency band, exploited by several experiments, including the radio antennas of the Pierre Auger Observatory. For both geomagnetic and charge-excess components of the signals, the parameterization is expressed as a function of the distance to the shower axis and the geometrical distance between core and $X_{\mathrm{max}}$. These pieces of information can be employed in event reconstruction to constrain the geometry, i.e. the core position.