Simulations of radio emission from cosmic ray air showers
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Simulations of radio emission from cosmic ray air showers
Tim Huege &Heino Falcke
ARENA-WorkshopZeuthen, 17-03-2005
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A new modelling approach
historical works were not detailed enough for application to LOPES
refractive index of atmosphere ~1.0geomagnetic 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
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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 distributionsparticle track length & energy distributionsair shower and magnetic field geometryshower evolution as a whole
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Monte Carlo, Analytics & Data:Radial dependence
vertical 1017 eV shower
good agree-ment in spite of very different methods
Data fromAllan et al. (1971)
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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
55 MHz
incoherent regime:need better air shower model(inhomogeneities, pitch angles)
incoherent regime:need better air shower model(inhomogeneities, pitch angles)
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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 east-west [m]
dist
ance
nor
th-s
outh
[m
]
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Results: Emission patternfor inclined showers at 10 MHz
projection effects along the shower axis, but: pattern gets broader as a whole (distance to shower max)!
Vertical 1017 eV shower 45° inclined 1017 eV shower
distance east-west [m]
dist
ance
nor
th-s
outh
[m
]
dist
ance
nor
th-s
outh
[m
]
distance east-west [m]
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Results: Coherence as a function of zenith angle
peak emission levels slightly lower, but:coherence up to much higher distances
20 m
500 m
Vertical 1017 eV shower 45° inclined 1017 eV shower
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Results:Zenith angle dependence
1017 eV, 10 MHz
flattening with zenith angle
weak B-field de-pendence
not simple projection effect
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Results: Linear polarisation
most power in polarisation directionperpendicular to B-field and shower axes
distance EW [m]
dist
ance
NS
[m
]
distance EW [m]
dist
ance
NS
[m
]
45° zenith angle, shower alongnorth-south direction
45° zenith angle, shower alongeast-west direction
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Results: Scaling with Ep
for 10 MHz
nearly linear scaling
flatter at higher distan-ces
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Parametrisation formula
derived formula parametri-sing dependence on:frequencyobserving positionair shower geometryprimary particle energydepth of shower maximum
very useful for the interpretation of experimental data
accuracy better than~ 15 - 20% in thecoherent regime
available as an online calculator
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The next steps
work in progresssubstitution 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
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Conclusions
detailed emission model for the first time model thoroughly verified (analytics/MC) important results so far:
emission pattern intrinsically symmetricpolarisation characteristics (verify geomagnetic origin)radial dependencespectral dependence (low frequencies!)scaling with primary particle energyzenith angle dependence (inclined showers!)all incorporated in a useful parametrisation formula
overall result: signal well interpretable and explainable by geosynchrotron emission