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: [astro-ph]
Proceedings Arrena ’08 conference
ARENA 2010, Nantes
June 29 - July 2, 2010

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
June 29 - July 2, 2010

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
June 29 - July 2, 2010

4. General features II L =0  cancelation Frequency Time
Issues for models
competing length scales
magnitude (distance)
ARENA 2010, Nantes
June 29 - July 2, 2010

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
June 29 - July 2, 2010

6. 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 D
Stopped positrons
Stopped electrons
Efficient model:
Assume all charges concentrated near shower core e- e+ vd c e- vd c j BE D
ARENA 2010, Nantes
June 29 - July 2, 2010

7. Induced Electric Current
Lorentz force: accelerated
Air density: deceleration.
Net result:
equilibrium, almost constant perp. drift velocity.
Induced electric current
Jx=e vd Nc = J0 ρ(z)
strongly varies
Classical E&M:
Electric field = derivative (vector) potential
Potential = sum charge (current) / distance
3 km
ARENA 2010, Nantes
June 29 - July 2, 2010

8. Macroscopic GeoMagnetic Radiation > The Basic picture <
Air shower
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
June 29 - July 2, 2010

9. Complete study Conclusion: Interesting Physics in Radio signal
Dominant
Transverse current D
Usually
Moving Dipole
~15 % e-
Static Dipole D
~2 %
>25 %
Charge Excess
Harm Schoorlemmer
Krijn de Vries
Conclusion: Interesting Physics in Radio signal
Pulse shape reflects shower profile before maximum
ARENA 2010, Nantes
June 29 - July 2, 2010

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
June 29 - July 2, 2010

11. Competing scales pulse shape: tmax shifts as d2 Emax prop to d-4
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
June 29 - July 2, 2010

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
June 29 - July 2, 2010

13. Pulse shape in frequency
Low frequencies dominate at large distances
ARENA 2010, Nantes
June 29 - July 2, 2010

14. Lateral dependence Ratio Power above 25 MHz ARENA 2010, Nantes
June 29 - July 2, 2010

15. Chemical composition p Peaks lower than Fe E=1017 eV proton & iron
Longitudinal profile
E=1017 eV
proton & iron
40 showers each
Pancake distribution
p Peaks lower than Fe
ARENA 2010, Nantes
June 29 - July 2, 2010

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
C:\Documents and Settings\Olaf Scholten\My Documents\Attachments\2009\ldf.jpg
ARENA 2010, Nantes
June 29 - July 2, 2010

17. Polarizations at 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
June 29 - July 2, 2010 17

18. Azimuth dependence of signal strength I
Vertical shower,
00 = East
900 = North
Disentangle contributions
ARENA 2010, Nantes
June 29 - July 2, 2010

19. Conclusions Pulse can be understood based on
simple geometrical arguments
Interesting Physics in Radio signal
Pulse shape reflects shower profile before maximum
Competing shower thickness & profile
Sensitive to chemical composition
Signal is Phi-dependent
Disentangle contributions
ARENA 2010, Nantes
June 29 - July 2, 2010