Astronomy & Astrophysics manuscript no. Core c© ESO 2011December 8, 2011

Evidence for the charge-excess contribution in air shower radioemission observed by the CODALEMA experiment

A. Belletoile1, D. Charrier1, R. Dallier1, L. Denis2, P. Lautridou1, A. Lecacheux2, V. Marin*1, L. Martin1, O.Ravel1, A. Rebai1, B. Revenu1, and D. Torres1

1SUBATECH, Universite de Nantes Ecole des Mines de Nantes IN2P3-CNRS, Nantes France2LESIA, USN de Nancay, Observatoire de Paris-Meudon INSU-CNRS, Meudon France

December 8, 2011

ABSTRACT

Context. Observation of the charge-excess mechanism in the emission of the electric field from cosmic ray air showers.Aims. It is shown that the signature of the charge-excess mechanism is present in the CODALEMA dataMethods. The data exhibits a shift in the ground position in the shower cores seen from the radio data and the particledata. This shift is explained when using a simulation code taking into account or not the charge-excess mechanism.Results. Evidence for the charge-excess in the atmospheric shower has been found via the electric field emitted by thesecondary particle and detected by the CODALEMA experiment.Conclusions. The systematic shift between the shower core estimation using separately the particle array data andthe radio array data of the CODALEMA experiment is discussed. Using the simulation code SELFAS2 we show thatthe consideration of the charge-excess contribution in the total radio emission of air showers generates a shift of theapparent ground radio core along the east-west axis in good agreement with the observations. This radio core shift isthen characterized for the CODALEMA setup and compared with the data. The observation of this systematic shiftcan be considered as an experimental signature of the charge excess contribution.

Key words. radiodetection,charge-excess mechanism,cosmic rays

1. Introduction

Jelley et al. (1965) experimentally demonstrated thatair showers initiated by high-energy cosmic rays pro-duce strong radio pulses that can be detected between 0and 300 MHz. Different mechanisms were proposed to in-terpret this phenomena. Kahn & Lerche (1966) suggestedtwo different main processes responsible for air shower radioemission: the radiation from the net charge excess of elec-trons in the shower, initially predicted by Askaryan (1962),and a geomagnetically induced transverse current in theshower front. It is also pointed out in Askaryan (1962) thatthe extensive air shower (EAS) radio emission is favored bythe coherence of the signal for wavelengths larger than thecharacteristic dimensions of the emissive zone. This coher-ence condition is verified below 100 MHz which correspondsto wavelengths larger than few meters, comparable to thelongitudinal extension of the particles of the shower front.Kahn and Lerche predicted that the dominant contributionis that of the transverse current.

These last years, important efforts made on EAS ra-dio emission modeling permitted to converge toward a con-sensus about the expected EAS radio signal in the MHzrange (Marin & Revenu 2011; Huege et al. 2011; de Vrieset al. 2010). As it was recently confirmed by recent exper-iments such as CODALEMA and LOPES (Ardouin et al.2006, 2009; Falcke et al. 2005; Apel et al. 2006), the dom-inant mechanism is due to the geomagnetic field, implyinga strong asymmetry in the counting rates as a function ofthe arrival direction, at energies smaller than the energy

corresponding to the full acceptance of the considered ex-periment. However, recent models predict also an additionalcontribution due to the EAS charge-excess variation whichshould be detectable (Marin & Revenu 2011; de Vries et al.2010; Ludwig & Huege 2010). A first hint of this charge-excess contribution in experimental data has been found re-cently in Revenu & the Pierre Auger Collaboration (2011).In SELFAS2, the charge-excess contribution is computedusing the electrons and positrons present in the shower.We do not take into account the radiation coming from theions of the atmosphere since it is negligible.

In this work, we use a new observable due to the contri-bution of the charge excess in the total EAS radio signal.In section 2, we discuss the behavior of this observable for1017 eV vertical air showers simulated with SELFAS2 atthe CODALEMA site. In section 3, we characterize withSELFAS2 the new observable introduced in section 2 forCODALEMA, predicting its behavior for the same numberof events than that we have in the data. Finally, we com-pare this result to the experimental data giving for the firsttime the interpretation of the observed shift between thepositions of the particle cores and the corresponding radiocores observed in the CODALEMA data. In this paper, wewill call ”radio core” (respectively ”particle core”) the coreposition of the shower on the ground estimated with theradio (respectively particles) data only.

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A. Belletoile et al.: Charge-excess contribution in CODALEMA

2. SELFAS2 : 1017 eV EAS at CODALEMA

SELFAS2 (see Marin & Revenu 2010, 2011) is a code whichcomputes the radio emission of extensive air showers using amicroscopic description based on universal distributions ofthe secondary particles of the shower (Lafebre et al. 2009).SELFAS2 shows that the total radio emission is mainly dueto two phenomena: the transverse current and the chargeexcess variation (see de Vries et al. 2010; Ludwig & Huege2010).

As a first example, we simulated a 1017 eV vertical airshower for the CODALEMA site. The ground altitude isfixed to 140 m. The geomagnetic field characteristics at theNancay site are |B| = 50 µT, θB = 27◦ and φB = 270◦

where θB and φB are the zenith angle and the azimuthalangle (with orientation so that φ = 0◦ corresponds to theeast and φ = 90◦ corresponds to the north). All EAS simu-lated with SELFAS2 used in this paper are performed withthis site configuration.

In Fig.1, we present the strength (defined as the maxi-mum value of the electric field in the band 23-83 MHz) ofthe electric field on the ground in the east-west (EW) po-larization. The simulated particle core is located in (0, 0).

Fig. 1. Ground footprint of the electric-field strength in the EWpolarization deposited by a 1017 eV vertical EAS simulated withSELFAS2. The origin of the frame corresponds to the simulatedparticle cores. The contour lines are in µV m−1, a 23-83 MHznumerical passband filter is applied on the signal. We see thatthe position of the radio core is shifted toward the east withrespect to the position of the particle core.

As it is done experimentally, a 23-83 MHz numerical pass-band filter is applied on the signal of each of the 145 sim-ulated antennas used to obtain this figure. The result re-veals an EW asymmetry of the electric-field strength, sug-gesting a shift of the apparent radio core (ground locationwhere the value of the interpolated electric-field strengthis maximum) toward the east with respect to the parti-cle core. This behavior predicted by modern simulations(see Marin & Revenu 2011; de Vries et al. 2010; Ludwig& Huege 2010) is a direct consequence of the superposi-

tion of two different contributions in the total EAS radiosignal: the charge-excess and the transverse current con-tributions. The superposition of their different polarizationpatterns (see Fig.2) generates a loss of the cylindrical sym-metry around the EAS axis. In this case study of a vertical

Fig. 2. Polarization vectors of the charge excess and transversecurrent contributions in the plane perpendicular to the showeraxis. The polarization vectors of these two contributions are notalways oriented in the same direction: their superposition can beconstructive or destructive depending on the antenna positionwith respect to the particle core.

shower, the electric-field strength in the EW polarizationis higher on the east side of the plane containing the parti-cle core and the geomagnetic field, resulting in an apparentradio core position shifted to the east with respect to theposition of the particle core. This shift can be consideredas a signature of the charge-excess contribution in the totalradio signal emitted by EAS.

In Fig.3 we show different ground footprints for differentEAS arrival directions in the plane containing the geomag-netic field and the south-north axis.

We see the evolution of the apparent radio core posi-tion as a function of the angular distance to the geomag-netic field (see the picture caption for more informations).The electric field due to the transverse current contributiondepends on the arrival direction of the shower, contrarilyto the electric field due to the charge-excess contribution.This implies that the distance ∆c along the EW axis be-tween the reconstructed radio core position and the particlecore position, depends also on the EAS arrival direction asit is qualitatively shown in Fig.3.

In the next section we describe the method which per-mits to characterize the behavior of ∆c applied to the samenumber of selected events in CODALEMA.

3. Predicted radio core density map forCODALEMA

We simulated two sets of 3151 showers following the arrivaldirections and the particle core positions distributions ob-served in CODALEMA. The energy of the simulated show-ers is set to 1017 eV (which is the most represented energyvalue). We reconstruct the input simulated shower geome-try using the same antenna array setup than CODALEMA.The first set of 315 showers has been obtained with thecomplete SELFAS2 algorithm: both transverse current andcharge-excess contributions are computed. The second set

1 It is the number of events passing the quality cuts in theCODALEMA data set, see section 4.

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A. Belletoile et al.: Charge-excess contribution in CODALEMA

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Fig. 3. Evolution of the ground footprint of the electric-fieldstrength in the EW polarization for different EAS arrival di-rections in the plane containing the geomagnetic field and thesouth-north axis. The contours and axis scales are the same as inFig.1 and the origin of the frame corresponds to the position ofthe particle core. Top left: the event is coming from the north. Inthis case the transverse current contribution is large comparedto the charge-excess contribution, the shift of the position of theradio core is smaller than that presented in Fig.1. For an eventcoming from the south (top right), the radio core is stronglyshifted because the relative intensity of the transverse currentcontribution decreases. In the case of an EAS parallel to the geo-magnetic field (bottom left), only the charge excess contributesto the radio emission. For air showers coming from the southwith zenith angles larger than the geomagnetic field (bottomright), the radio core position is shifted toward the west.

contains showers simulated with the transverse current con-tribution only. We restrict the analysis on simulated show-ers to the EW polarization only because we have data inthe EW polarization only (see section 4).

The lateral profile of each simulated event is ana-lyzed using the same procedure used in the data analy-sis chain. The profile is modeled with an exponential func-tion E(d) = E0 e

−d/d0 with four free parameters: E0, theon-axis extrapolated electric-field strength in the EW po-larization, d0 the lateral slope and d the antenna distanceto the shower axis. The parameter d contains the actualfitted parameters xrc , yrc which are the ground coordinatesof the radio core in the CODALEMA frame.

In Fig.4, for the first set of 315 events (with transversecurrent and charge-excess contributions) we compare thereconstructed radio core positions relatively to the parti-cle core positions. The reconstructed radio core positionsare represented by the black crosses and a 10 m Gaussiansmoothed map is superimposed where the red lines repre-sent the contour levels. A global shift toward the east isclearly visible.

In order to exhibit the charge-excess contribution effectin the simulation code, the same procedure is applied onthe second set of 315 events (with the transverse currentcontribution only). For these events, the same number of

electrons and positrons was generated during the simula-tion. The density map of the reconstructed radio core po-sitions with respect to the positions of the particle core ispresented in Fig.5. We do not observe any shift as comparedto Fig.4.

Fig. 4. Reconstructed radio core positions (black crosses) of 315events simulated with SELFAS2 (following the observed coredistribution), relatively to the particle core positions of eachsimulated event. The red lines represent contour levels of a 10 mGaussian smoothed map. A global shift toward the east is clearlyvisible.

Fig. 5. Same legend as in Fig.4 except that the charge-excesscontribution has been switched off in SELFAS2. The averageposition of the radio core positions is centered on the origin,corresponding to the particle core positions.

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A. Belletoile et al.: Charge-excess contribution in CODALEMA

The charge excess contribution has therefore a clear sig-nature on the core location. It is important to note that thiseffect does not depend on the primary energy.

In the next section we compare the result derived fromSELFAS2 to the data.

4. Reconstructed radio cores of the eventsdetected by CODALEMA

The CODALEMA setup used in this article is describedin Ardouin et al. (2006, 2009). The radio detector array iscomposed of 24 EW-polarized antennas and is triggeredby a particle detectors array of 17 scintillators coveringan area of 340 × 340 m2. In order to identify EAS in thefull data set and using the data from both arrays, we cancompare the two independent estimations of the shower ge-ometry. We first select a subset of events: the arrival di-rections estimated with the radio data and the particlesdata must agree within 3◦ maximum and must be in thesame time window of ±100 ns. We also require that theevent core position estimated by the particle array is fullycontained inside the particle array. There are 315 eventspassing these cuts (see Ardouin et al. 2009; O.Ravel & theCODALEMA Collaboration 2010). The radio core positionsof these 315 events relatively to the particle cores positionsare presented in Fig.6. Possible experimental biases coming

Fig. 6. Reconstructed radio core positions (blue crosses) of the315 selected events detected by the CODALEMA radio array,relatively to the reconstructed particle core positions. The redlines are contour levels of a 10 m Gaussian smoothed map. Theradio cores distribution is shifted toward the east as observed inFig.4 with SELFAS2.

from trigger effect or from the array geometry have beenchecked but the EW radio core shift is robust (see alsoLecacheux & the CODALEMA Collaboration 2009). Thesimilarity of this general behavior with what we obtainedin Fig.4 suggests that this radio core shift along the EWdirection in the CODALEMA data is an experimental sig-nature of the charge-excess contribution in the total EASradio emission.

We can compute the core shift amplitude as a func-tion of the arrival direction. This dependence is presentedin Fig.7. In this figure, the expected behavior of the core

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Fig. 7. Shift of the ground radio core positions relatively to thepositions of the ground particles shower core as a function ofthe EW component of (v ×B) (where n is the shower axis di-rection). The simulated values are represented by the black lineand the ±σ limits (gray dashed lines) expected with SELFAS2for the CODALEMA experiment. Data are superimposed (cir-cles, see text for details). Monte-Carlo simulation were done oneach event reconstruction to obtain error bar on ∆c.

shift as a function of the EW component of n×B (wheren and B are respectively the shower axis and the geomag-netic field directions) is superimposed to the data. The lackof events coming with arrival directions close to B andfrom the south at the CODALEMA site, does not allow tostate on a clear correlation for values of |n×B|EW smallerthan 0.3, corresponding to an angle between the shower axisand the geomagnetic field smaller that 16◦.

5. Conclusion

The CODALEMA data presents a statistically significantdisagreement between the reconstructed radio core posi-tions and the particle core positions. No instrumental bi-ases can explain this disagreement which appears to be dueto physical reasons. Using simulations, we argue that thisobservation can be interpreted as the contribution of thecharge-excess mechanism in the generation of the electricfield emitted by EAS. Our results also show that the charge-excess description used in the simulation code SELFAS2 isin good agreement with the observation, and consequentlythat the charge-excess level deduced by Lafebre et al. (2009)is supported by the data.

The fraction of the charge excess contribution to the to-tal EAS radio emission depends on the arrival direction andon the observation point. In the case of a vertical shower,it can contribute from few percents to almost 30% depend-ing on the observation point. If such a clear correlation ofthe shift between the radio core positions and the particlecore positions with the arrival direction is confirmed withthe new generation of radio-detection experiments (AERA,CODALEMA3, TREND), this signature will mark a newstep in the understanding of the radio emission process.

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A. Belletoile et al.: Charge-excess contribution in CODALEMA

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