Abstract
The mass composition of ultra-high-energy cosmic rays plays a key role in the understanding of the origins ofthese rare particles. A composition-sensitive observable is the atmospheric depth at which the air shower reaches the maximum number of particles (Xmax). The Auger Engineering Radio Array (AERA) detects the radio emission inthe 30-80 MHz frequency band from extensive air showers with energies larger than 1017 eV. It consists of more than 150 autonomous radio stations covering an area of about 17 km2. From the distribution of signals measured by the antennas, it is possible to estimate Xmax. In this contribution three independent methods for the estimation of Xmax will be presented.
Highlights
The dominant contribution to the radio emission from extensive air shower is the geomagnetic radiation. It is produced by the deflection of electrons and positrons in the Earth’s magnetic field (B) and it is polarized in the direction of the Lorentz force (v × B, where v is the shower axis)
A second order contribution comes from a negative charge excess in the shower front: it is called charge excess or Askaryan effect and its radiation is radially polarized towards the shower axis
Telescopes and the spectral index measured by each radio antenna
Summary
The dominant contribution to the radio emission from extensive air shower is the geomagnetic radiation. It is produced by the deflection of electrons and positrons in the Earth’s magnetic field (B) and it is polarized in the direction of the Lorentz force (v × B, where v is the shower axis). The superposition of the geomagnetic and charge-excess radiation results in an asymmetric energy density footprint in the shower plane (v × B, v × v × B). Several features of the radio signal are used to estimate Xmax: the energy density distribution at ground level; the shape of the frequency spectrum of each individual antenna; the arrival time distribution of the signal at each antenna.
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