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Calibrating the absolute energy scale of air showers initiated by ultra-highenergy cosmic rays is an important experimental issue. Currently, thecorresponding systematic uncertainty amounts to 14-21% using the fluorescencetechnique. Here we describe a new, independent method which can be applied ifultra-high energy photons are observed. While such photon-initiated showershave not yet been identified, the capabilities of present and future cosmic-raydetectors may allow their discovery. The method makes use of the geomagneticconversion of UHE photons (preshower effect), which significantly affects thesubsequent longitudinal shower development. The conversion probability dependson photon energy and can be calculated accurately by QED. The comparison of theobserved fraction of converted photon events to the expected one allows thedetermination of the absolute energy scale of the observed photon air showersand, thus, an energy calibration of the air shower experiment. We providedetails of the method and estimate the accuracy that can be reached as afunction of the number of observed photon showers. Already a very small numberof UHE photons may help to test and fix the absolute energy scale.

A method to calibrate the absolute energy scale of air showers with ultra-high energy photons