Coherent radio-wave emission from extensive air showers.

1. Coherent radio-wave emission from extensive air showers. Olaf Scholten KVI, University of Groningen +Krijn de Vries + Klaus Werner: Astropart. Phys. 29, 94 (2008). Astropart. Phys. 29, 393 (2008), arXiv:0712.2517 [astro-ph] Proceedings Arrena ’08 conference ARENA 2010, Nantes

2. Multiple Approaches • Microscopic: • Follow orbit of each electron & positron • Add fields of individual particles • Macroscopic, Emphasize collective aspect • Average motion of individual particles • Add charges & currents and calculate fields Talk: Marianne Ludwig Easier to understand basic structure radio signal • Competing length scales Talk: Tim Huege ARENA 2010, Nantes

3. General features coherent signal Observation: • Currents and charges confined to limited space-time volume • Typical coherent length scale: L • Pancake thickness • Shower length Consequence: • Response at large wavelength vanishes • E=0 @ n=0 • Response vanishes for wavelength < L • E=0 @ n> c/L Physics determined by length & time scales ARENA 2010, Nantes

4. General features II Frequency Time =0  cancelation L Issues for models • competing length scales • magnitude (distance) ARENA 2010, Nantes

5. Coherence • Charges within wavelength (λ) cannot be distinguished  contribute coherently L<λ : I = E2 ≈ Nc2 where Nc ≈ 106 • Far-apart charges add incoherently E ≈ √Nc L>λ : I = E2 ≈ Nc • Difference is factor ≈ 106 ARENA 2010, Nantes

6. D e+ e- vd vd j BE e- D c c Coherent GeoMagnetic Radiation  Geomagnetic Current Lorentz force pulls charges apart induces net current Shower core photons  Moving Dipole net separation of charges • Static Dipole Separated charges slowly diffuse • Charge excess Coherent radiation Stopped electrons Stopped positrons Efficient model: Assume all charges concentrated near shower core ARENA 2010, Nantes

7. Induced Electric Current • Lorentz force: accelerated • Air density: deceleration. • Net result: equilibrium, almost constant perp. drift velocity. Induced electric current Jx=e vdNc = J0ρ(z) strongly varies 3 km Classical E&M: • Electric field = derivative (vector) potential • Potential = sum charge (current) / distance ARENA 2010, Nantes

8. Air shower Macroscopic GeoMagnetic Radiation > The Basic picture < Horizontal drift velocity of electrons creates current Pancake = e+ e- plasma Electric current develops when plasma moves through magnetic field of the Earth Radiation emitted by time varying electric current Pulse sensitive to shower development prior to maximum ARENA 2010, Nantes

9. D >25 % e- D Complete study Dominant Transverse current Usually Moving Dipole ~15 % Static Dipole ~2 % Charge Excess Harm Schoorlemmer Krijn de Vries Conclusion: Interesting Physics in Radio signal Pulse shape reflects shower profile before maximum ARENA 2010, Nantes

10. Recent developments • Chemical composition • Azimuth dependence • Close proximity to shower core • Talk Krijn de Vries: • Coherent Charge excess radiation = Askaryan • Cherenkov effects ARENA 2010, Nantes

11. Competing scales long. shower structure: Zero X-ing <-> shower Max pulse shape: tmax shifts as d2 Emax prop to d-4 d=300 m d=700 m Pulse broadens with increasing distance Pancake thickness important at short distances ARENA 2010, Nantes

12. pancake at small d At short distance pulse shape is stable Result independent on Convergence parameter Only top part shower is sampled by pulse ARENA 2010, Nantes

13. Pulse shape in frequency Low frequencies dominate at large distances ARENA 2010, Nantes

14. Lateral dependence Ratio Power above 25 MHz ARENA 2010, Nantes

15. Chemical composition Longitudinal profile E=1017 eV proton & iron 40 showers each Pancake distribution p Peaks lower than Fe ARENA 2010, Nantes

16. Chemical composition II power ratio v.s. cut-off frequency 40 proton and 40 Fe showers Why have protons a faster drop? Peak closer to ground  lower frequency  sooner out of juice with increasing d ARENA 2010, Nantes

17. Polarizationsat different observer positions Geomagnetic polarization ~ β x B Charge excess polarization: Depending on observer position. Pointing inwards Moving dipole polarization: Depending on observer position. ARENA 2010, Nantes

18. Disentangle contributions Azimuth dependence of signal strength I Vertical shower, 00 = East 900 = North ARENA 2010, Nantes

19. Conclusions • Pulse shape reflects shower profile before maximum • Competing shower thickness & profile • Sensitive to chemical composition • Signal is Phi-dependent Pulse can be understood based on simple geometrical arguments Interesting Physics in Radio signal Disentangle contributions ARENA 2010, Nantes