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Simulations of radio emission from cosmic ray air showers

Simulations of radio emission from cosmic ray air showers. Tim Huege & Heino Falcke. ARENA-Workshop Zeuthen, 17-03-2005. A new modelling approach. historical works were not detailed enough for application to LOPES refractive index of atmosphere ~1.0

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Simulations of radio emission from cosmic ray air showers

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  1. Simulations of radio emission from cosmic ray air showers Tim Huege & Heino Falcke ARENA-WorkshopZeuthen, 17-03-2005

  2. A new modelling approach • historical works were not detailed enough for application to LOPES • refractive index of atmosphere ~1.0 • geomagnetic mechanism favoured over Askaryan-type Cherenkov radiation • approach: geosynchrotron emission • electron-positron pairs gyrating in the earth’s magnetic field: radio pulses • coherent emission at low frequencies • earlier: analytic calculations • presented here: Monte Carlo simulations based on analytic parametrisations of air shower properties • in progress: Monte Carlo simulations based on CORSIKA-simulated air showers

  3. Monte Carlo simulations of geosychrotron radiation • time-domain MC • no far-field approximations • full polarisation info • thoroughly tested • compared with analytics and data • takes into account: • longitudinal & lateral particle distributions • particle track length & energy distributions • air shower and magnetic field geometry • shower evolution as a whole

  4. Monte Carlo, Analytics & Data:Radial dependence • vertical 1017 eV shower • good agree-ment in spite of very different methods Data from Allan et al. (1971)

  5. 55 MHz incoherent regime: need better air shower model (inhomogeneities, pitch angles) Results:Spectra for a vertical shower • spectra steeper at higher distances • 55 MHz: coherence only up to ~ 300 m (vertical showers) • 10 MHz: very coherent

  6. Results:A vertical shower at 10 MHz Total field strength pattern of a vertical 1017 eV shower: • total field strength emission pattern highly symmetrical in spite of intrinsic asymmetry introduced by geomagnetic field • only small effect of geomagnetic field inclination on total emission distance north-south [m] distance east-west [m]

  7. Results: Emission patternfor inclined showers at 10 MHz Vertical 1017 eV shower 45° inclined 1017 eV shower distance north-south [m] distance north-south [m] • projection effects along the shower axis, but: • pattern gets broader as a whole (distance to shower max)! distance east-west [m] distance east-west [m]

  8. 20 m 500 m Results: Coherence as a function of zenith angle Vertical 1017 eV shower 45° inclined 1017 eV shower • peak emission levels slightly lower, but: • coherence up to much higher distances

  9. Results:Zenith angle dependence • 1017 eV, 10 MHz • flattening with zenith angle • weak B-field de-pendence • not simple projection effect

  10. distance NS [m] distance NS [m] distance EW [m] distance EW [m] Results: Linear polarisation 45° zenith angle, shower alongnorth-south direction 45° zenith angle, shower alongeast-west direction • most power in polarisation directionperpendicular to B-field and shower axes

  11. Results: Scaling with Ep • for 10 MHz • nearly linear scaling • flatter at higher distan-ces

  12. Parametrisation formula • derived formula parametri-sing dependence on: • frequency • observing position • air shower geometry • primary particle energy • depth of shower maximum • very useful for the interpretation of experimental data • accuracy better than~ 15 - 20% in thecoherent regime • available as an online calculator

  13. The next steps • work in progress • substitution of analytically parametrised air shower with CORSIKA-based model, including a realistic pitch-angle distribution and intrinsic inhomogeneities • inclusion of atmospheric electric fields • in the near future • inclusion of Askaryan-type Cherenkov effects (time-domain) • detailed comparison of simulated events with events measured by LOPES

  14. Conclusions • detailed emission model for the first time • model thoroughly verified (analytics/MC) • important results so far: • emission pattern intrinsically symmetric • polarisation characteristics (verify geomagnetic origin) • radial dependence • spectral dependence (low frequencies!) • scaling with primary particle energy • zenith angle dependence (inclined showers!) all incorporated in a useful parametrisation formula • overall result: signal well interpretable and explainable by geosynchrotron emission

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