The influence of the atmospheric refractive index on radio Xmax measurements of air showers

The refractive index of the atmosphere, which is n ≈ 1:0003 at sea level, varies with altitude and with local temperature, pressure and humidity. When performing radio measurements of air showers, natural variations in n will change the radio lateral intensity distribution, by changing the Cherenkov angle. Using CoREAS simulations, we have evaluated the systematic error on measurements of the shower maximum Xmax due to variations in n. It was found that a 10% increase in refractivity (n – 1) leads to an underestimation of Xmax between 8 and 22 g/cm2 for proton-induced showers at zenith angles from 15 to 45 degrees, respectively.