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Theory and simulations of air shower radio emission

Theory and simulations of air shower radio emission. Contents. how we got here the general picture current models. Early history of EAS radio theory. Kahn & Lerche: geo- magnetically induced transverse currents. Jelley: (incoherent) radio Cherenkov from EAS.

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Theory and simulations of air shower radio emission

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  1. Theory and simulations of air shower radio emission

  2. Contents • how we got here • the general picture • current models ARENA conference, 20 June 2012, Erlangen

  3. Early history of EAS radio theory Kahn & Lerche: geo- magnetically inducedtransverse currents Jelley: (incoherent) radio Cherenkov from EAS Askaryan: coherent charge excess variation Allan Colgate Fuji & Nishimura Castagnoliet al. 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 Jelley et al.: experimental detection ARENA conference, 20 June 2012, Erlangen

  4. Usability of historical works • investigated the primary physics mechanisms • demonstrated dominance of geomagnetic mechanism • very simplified air shower descriptions • point sources • rings of charges • other simplifications • more sophisticated modelling efforts (e.g., Monte Carlo codes)were not sufficiently documented in the literature • all in all not detailed enough for comparisonwith concrete modern measurements • today’s researchers had to start their own modelling efforts ARENA conference, 20 June 2012, Erlangen

  5. 2003 to 2010: Many models – two classes REAS2 unipolar versus bipolar pulses? difference not easily measurable! MGMR ARENA conference, 20 June 2012, Erlangen

  6. Summary of the situation until ~2010 • models with bipolar pulses,spectra falling to zero freq. • analytical geosynchrotron • Konstantinov et al. • MGMR • models with unipolar pulses,spectra leveling off at zero • REAS1 • ReAIRES • REAS2 • Meyer-Vernet et al. • Chauvin et al. • … blue: time-domain black: frequency-domain Gousset et al. • what was the reason for the discrepancy? • the models with unipolar pulses neglected an important radiation component, the radio emission from charge variation(not the variation itself, but the emission caused by it) ARENA conference, 20 June 2012, Erlangen

  7. timederivative Illustration of the problem • here for the example of an analytic calculation (Chauvin et al.) • starting point: Liénard-Wiechert fields for single moving particle • then fold in particle motion and calculate total shower electric field as time-variabilityof charge mustbe included here additionalterms • this includes the variation of the total charge – but not the radiation caused by it! num particles asf(shower evolution) charge excess (22%) ARENA conference, 20 June 2012, Erlangen

  8. How to fix? • do time-derivative at start,sum up electric fields • „endpoint formalism“, implemented in REAS3, CoREAS t1, E1, x1: instantaneousacceleration from rest to v v R1, q1 R2, q2 t2, E2, x2: instantaneousdeceleration from v to rest Ludwig, Huege (arXiv: 1010.5343) James et al. (arXiv: 1007.4146) Alvarez-Muniz et al. (arXiv: 1107.1189) ARENA conference, 20 June 2012, Erlangen sum up vector potentials,do time-derivative at the end ZHS formalism, implemented in the ZHAireS code

  9. REAS3 • include radiation from time-varying charge excess • radiation calculated with the „endpoint formalism“ • pulses predicted by REAS3 become bipolar and spectra fall to zero at frequency zero Ludwig & Huege, Astroparticle Physics (2011) ARENA conference, 20 June 2012, Erlangen

  10. Breakthrough: agreement of REAS3 and MGMR • two very different models, REAS3 and MGMR, predict comparable pulses • field strengths matched within a factor of ~2-3 • near the shower axis, differences remained • MGMR uses a simplified air shower model Huege, Ludwig, Scholten, deVries, arXiv:1009.0346 ARENA conference, 20 June 2012, Erlangen

  11. Spectra for REAS3 and MGMR (vertical 1017) • spectra look similar, differences near the shower axis thick:REAS3 thin:MGMR Huege, Ludwig, Scholten, deVries, arXiv:1009.0346 ARENA conference, 20 June 2012, Erlangen

  12. Footprint for REAS3 and MGMR (vertical 1017) total field north-south east-west REAS3 Huege, Ludwig, Scholten, deVries, arXiv:1009.0346 MGMR 60 MHz ARENA conference, 20 June 2012, Erlangen

  13. Contents • how we got here • the general picture • current models ARENA conference, 20 June 2012, Erlangen

  14. Two important concepts • coherence – wavelengths larger than spatial dimensions of source • electric fields add up, power scales as Nparticles2 • time variation – the air shower evolves ARENA conference, 20 June 2012, Erlangen

  15. First order: geomagnetic emission • time-varying transverse currents • signal strength ~ v x B = v B sin(a) • purely linear polarisation • E-field vector aligned withv x B for all oberver locations • emission can be „compressed“ by Cherenkov effects diagrams by K.D. de Vries ARENA conference, 20 June 2012, Erlangen

  16. Second order: Askaryan emission • Askaryan: time-varying charge-excess • also occurs for n=1! • Cherenkov effects „compress“ the emission • not „classical“ Cherenkov emission ofa non-varying net charge • linearly polarized • E-field vector radially oriented, varies with observer location relative to core diagrams by K.D. de Vries ARENA conference, 20 June 2012, Erlangen

  17. Total signal – east-west asymmetry Ewest + = Enorth ARENA conference, 20 June 2012, Erlangen suporposition of transverse currents and charge excess

  18. Charge excess signature in data A. van den Berg,UHECR2012 ARENA conference, 20 June 2012, Erlangen

  19. Aside: „geosynchrotron radiation“ • original idea: acceleration of electrons and positrons in geomagnetic field directly leads to „geosynchrotron“ radiation • particles are of course accelerated in the magnetic field – but also decelerated by interactions with air molecules • the result is an equilibrium transverse current, like in a conductor • a geosynchrotron contribution must exist – but seems to be small • macroscopic models (MGMR, EVA) do not include it by construction • microscopic Monte Carlo models include it implicitly • yet both give similar results • … but see next talk! • the term should not be used for the „transverse current“ emission ARENA conference, 20 June 2012, Erlangen

  20. Lateral distance and pulse shape (n=1) near shower axis,all emission arrivessimultaneously; pulse shape governedby shower disk thickness emission:v = c particles:v ~ c particles:v ~ c far from shower axisearly emission arrivesearly, late arrives late;pulse shape governedby shower evolution emission:v = c emission:v = c ARENA conference, 20 June 2012, Erlangen

  21. Lateral distance and realistic refractive index at Cherenkov angle,all emission arrivessimultaneously; pulse shape again governed by showerdisk thickness emission:v = c/n particles:v ~ c inside Cherenkov cone, pulsessmeared out wrt n=1 case outside Cherenkov cone,similar to n=1 case emission:v = c/n ARENA conference, 20 June 2012, Erlangen

  22. Refractive index effects (unlimited bandwidth) ZHAireS MGMR REAS3.1 ARENA conference, 20 June 2012, Erlangen effects similar and drastic in several models(showers not same here!) leads to interesting high-frequency effects, see following talks

  23. Energy and Xmax sensitivity of radio emission energy determination at distancewith minimum fluctuations Xmaxdeterminationfrom lateralslope Fe – flat p – steeper, fluctuating g – steepest, fluctuating Huege, Ulrich, Engel (Astrop. Phys. 2008) ARENA conference, 20 June 2012, Erlangen original investigation for REAS2, but same qualitative behaviour for REAS3, MGMR, Konstantinov et al.

  24. Energy and Xmax sensitivity linear dependence of E-field on primary energy (coherence!) • 180 air showers simulated with REAS2, 60° zenith angle,1018 to 1020 eV, Auger location, different primaries RMS ~ 3% Energy in electromagn. cascade [GeV] RMS ~ 16 g cm-2 • shower maximum derivable from slope of radio LDF • see talk Nunzia Palmierinext session Huege, Ulrich, Engel (Astrop. Phys. 2008) ARENA conference, 20 June 2012, Erlangen

  25. Xmax sensitivity through wavefront curvature Ph.D. Thesis, F. Schröder (2010) ARENA conference, 20 June 2012, Erlangen fit electromagnetic wavefront with cone (line source) opening angle of cone correlated with Xmax

  26. Contents • how we got here • the general picture • current models ARENA conference, 20 June 2012, Erlangen

  27. An overview of current „models“ more „microscopic“ ARENA conference, 20 June 2012, Erlangen MGMR time-domain, analytic, parametrized shower, fast, free parameters, summing up „mechanisms“ Dave‘s model frequency-domain, analytic, parametrized shower, fast, free parameters, summing up „mechanisms“ EVA frequency-domain, analytic, fitted CONEX shower,summing up „mechanisms“ SELFAS2 time-domain, shower from universality, summing up vector potentials for tracks REAS3.1 time-domain, histogrammed CORSIKA showers, endpoint formalism, open source Kalmykov et al. frequency-domain, CORSIKA showers, ZHS-like formalism ZHAireS time- and frequency-domain, Aires showers, ZHS formalism CoREAS time- (later frequency-) domain, CORSIKA showers, endpoint formalism

  28. The modelers talk to each other … • workshop at OSU in 02/2012 • radio in dense mediaand air • comparing models • updated right before ARENA ARENA conference, 20 June 2012, Erlangen

  29. Pulses vertical 1017 eV shower, west field, n=1 REAS3.1 CoREAS ZHAireS EVA SELFAS2 MGMR ARENA conference, 20 June 2012, Erlangen

  30. Pulses vertical 1017 eV shower, north field, n=1 REAS3.1 CoREAS ZHAireS EVA SELFAS2 MGMR ARENA conference, 20 June 2012, Erlangen

  31. Spectra vertical 1017 eV shower, total field, n=1 REAS3.1 CoREAS ZHAireS EVA SELFAS2 MGMR ARENA conference, 20 June 2012, Erlangen

  32. Spectra vertical 1017 eV shower, total field, n=r REAS3.1 CoREAS ZHAireS EVA SELFAS2 MGMR ARENA conference, 20 June 2012, Erlangen

  33. Pulses vertical 1017 eV shower, west field, n=1 REAS3.1 CoREAS ZHAireS EVA SELFAS2 MGMR ARENA conference, 20 June 2012, Erlangen

  34. Status of radio emission modelling today ARENA conference, 20 June 2012, Erlangen • general agreement between models (qualitative and quantitative) • this is great compared to what we had 2 years ago • we need to reach a ~10% level – full Monte CarlosCoREAS and ZHAireS already very similar! • keep in mind: macroscopic models might have free parameters • even if models agree – the benchmark has to be the data • see LOPES-talk by Marianne Ludwig next session • public comparison datasets would help enormously • some things are yet to be studied …

  35. Refractive index effects – correctly treated? • numerics for the endpoint formalism break down at the Cherenkov angle • have to adopt ZHS-like treatment, making approximations • are these ZHS-like approximations correct? see Clancy‘s talk … • does „classical Cherenkov emission“ play a role – is it included in ZHS? t1, E1, x1: instantaneousacceleration from rest to v v R1, q1 R2, q2 t2, E2, x2: instantaneousdeceleration from v to rest ARENA conference, 20 June 2012, Erlangen

  36. GHz radio emission from air showers? • ANITA • EASIERin Auger • CROME • Cherenkov-boosted air shower radio emission? • see up-coming talks … R. Smida, UHECR2012workshop ARENA conference, 20 June 2012, Erlangen

  37. Conclusions • cosmic ray radio emission modelling has come a long way • breakthrough: reconciliation of macroscopic and microscopic models • disregard models with unipolar pulses before ~2010 • now several models with good agreement • need to get to the 10% level • refractive index effects are significant • caveat: is the modelling near the Cherenkov angle actually correct? • benchmark has to be the data: verify predicted features • 1st order v x B • 2nd order charge excess • energy sensitivity • Xmax sensitivity (ldf slope, conical wavefront) • it would help the modelers enormously if data sets for comparisons were made openly available! ARENA conference, 20 June 2012, Erlangen

  38. Backup slides ARENA conference, 20 June 2012, Erlangen

  39. The endpoint formalism discrete formulation for arbitrarily complex motion radiation only from endpoints ^ r antenna b1 delay: R/c b2 endpoints:decelerate from b1 to rest then accelerate from rest to b2 ARENA conference, 20 June 2012, Erlangen

  40. Radiation from a single endpoint frequency domain formulation • time domain formulation - for deceleration from b* to rest (stopping point) + for acceleration from rest to b* (starting point) ARENA conference, 20 June 2012, Erlangen

  41. Relative importance of charge-excess • importance increases with lateral distance for unlimited bandwidth • roughly constant fraction for low-frequency observations ARENA conference, 20 June 2012, Erlangen

  42. Refractive index effects Alvarez-Muniz et al. (arXiv: 1107.1189) ARENA conference, 20 June 2012, Erlangen • refractive index of atmosphere >1 and varies with height • shower can overtake radio emission • compression of shower emission near Cherenkov angle

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