The following paper I co-authored is now online on slideshare! I participated in the Codalema Experiment operations and data analysis since July 2009 until February 2014. 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 data Methods. The data exhibits a shift in the ground position in the shower cores seen from the radio data and the particle data. 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 the secondary particle and detected by the CODALEMA experiment. Conclusions. The systematic shift between the shower core estimation using separately the particle array data and the radio array data of the CODALEMA experiment is discussed. Using the simulation code SELFAS2 we show that the consideration of the charge-excess contribution in the total radio emission of air showers generates a shift of the apparent ground radio core along the east-west axis in good agreement with the observations. This radio core shift is then characterized for the CODALEMA setup and compared with the data. The observation of this systematic shift can be considered as an experimental signature of the charge excess contribution.

1. Astronomy & Astrophysics manuscript no. Core c ESO 2011
December 8, 2011
Evidence for the charge-excess contribution in air shower radio
emission observed by the CODALEMA experiment
A. Bell´etoile1, D. Charrier1, R. Dallier1, L. Denis2, P. Lautridou1, A. Lecacheux2, V. Marin*1, L. Martin1, O.
Ravel1, A. Rebai1, B. Revenu1, and D. Torres1
1
SUBATECH, Universit´e de Nantes ´Ecole des Mines de Nantes IN2P3-CNRS, Nantes France
2
LESIA, 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 data
Methods. The data exhibits a shift in the ground position in the shower cores seen from the radio data and the particle
data. 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 the
secondary particle and detected by the CODALEMA experiment.
Conclusions. The systematic shift between the shower core estimation using separately the particle array data and
the radio array data of the CODALEMA experiment is discussed. Using the simulation code SELFAS2 we show that
the consideration of the charge-excess contribution in the total radio emission of air showers generates a shift of the
apparent ground radio core along the east-west axis in good agreement with the observations. This radio core shift is
then characterized for the CODALEMA setup and compared with the data. The observation of this systematic shift
can 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 that
air showers initiated by high-energy cosmic rays pro-
duce strong radio pulses that can be detected between 0
and 300 MHz. Different mechanisms were proposed to in-
terpret this phenomena. Kahn & Lerche (1966) suggested
two different main processes responsible for air shower radio
emission: 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 the
shower front. It is also pointed out in Askaryan (1962) that
the extensive air shower (EAS) radio emission is favored by
the coherence of the signal for wavelengths larger than the
characteristic dimensions of the emissive zone. This coher-
ence condition is verified below 100 MHz which corresponds
to wavelengths larger than few meters, comparable to the
longitudinal extension of the particles of the shower front.
Kahn and Lerche predicted that the dominant contribution
is 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 MHz
range (Marin & Revenu 2011; Huege et al. 2011; de Vries
et 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, implying
a strong asymmetry in the counting rates as a function of
the arrival direction, at energies smaller than the energy
corresponding to the full acceptance of the considered ex-
periment. However, recent models predict also an additional
contribution due to the EAS charge-excess variation which
should 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 computed
using the electrons and positrons present in the shower.
We do not take into account the radiation coming from the
ions 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 for
1017
eV vertical air showers simulated with SELFAS2 at
the CODALEMA site. In section 3, we characterize with
SELFAS2 the new observable introduced in section 2 for
CODALEMA, predicting its behavior for the same number
of events than that we have in the data. Finally, we com-
pare this result to the experimental data giving for the first
time the interpretation of the observed shift between the
positions of the particle cores and the corresponding radio
cores observed in the CODALEMA data. In this paper, we
will call ”radio core” (respectively ”particle core”) the core
position of the shower on the ground estimated with the
radio (respectively particles) data only.
1

2. A. Bell´etoile et al.: Charge-excess contribution in CODALEMA
2. SELFAS2 : 1017 eV EAS at CODALEMA
SELFAS2 (see Marin & Revenu 2010, 2011) is a code which
computes the radio emission of extensive air showers using a
microscopic description based on universal distributions of
the secondary particles of the shower (Lafebre et al. 2009).
SELFAS2 shows that the total radio emission is mainly due
to two phenomena: the transverse current and the charge
excess variation (see de Vries et al. 2010; Ludwig & Huege
2010).
As a first example, we simulated a 1017
eV vertical air
shower for the CODALEMA site. The ground altitude is
fixed to 140 m. The geomagnetic field characteristics at the
Nancay site are |B| = 50 µT, θB = 27◦
and φB = 270◦
where θB and φB are the zenith angle and the azimuthal
angle (with orientation so that φ = 0◦
corresponds to the
east and φ = 90◦
corresponds to the north). All EAS simu-
lated with SELFAS2 used in this paper are performed with
this 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) of
the 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 EW
polarization deposited by a 1017
eV vertical EAS simulated with
SELFAS2. The origin of the frame corresponds to the simulated
particle cores. The contour lines are in µV m−1
, a 23-83 MHz
numerical passband filter is applied on the signal. We see that
the position of the radio core is shifted toward the east with
respect 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 location
where the value of the interpolated electric-field strength
is 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 radio
signal: the charge-excess and the transverse current con-
tributions. The superposition of their different polarization
patterns (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 transverse
current contributions in the plane perpendicular to the shower
axis. The polarization vectors of these two contributions are not
always oriented in the same direction: their superposition can be
constructive or destructive depending on the antenna position
with respect to the particle core.
shower, the electric-field strength in the EW polarization
is higher on the east side of the plane containing the parti-
cle core and the geomagnetic field, resulting in an apparent
radio core position shifted to the east with respect to the
position of the particle core. This shift can be considered
as a signature of the charge-excess contribution in the total
radio signal emitted by EAS.
In Fig.3 we show different ground footprints for different
EAS 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 contribution
depends on the arrival direction of the shower, contrarily
to 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 particle
core position, depends also on the EAS arrival direction as
it 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 same
number of selected events in CODALEMA.
3. Predicted radio core density map for
CODALEMA
We simulated two sets of 3151
showers following the arrival
directions 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 energy
value). 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 the
complete SELFAS2 algorithm: both transverse current and
charge-excess contributions are computed. The second set
1
It is the number of events passing the quality cuts in the
CODALEMA data set, see section 4.
2

3. A. Bell´etoile et al.: Charge-excess contribution in CODALEMA
0
10
10 20
30
40
50
60
70
80
90
100
110
Θ 40°
Φ 90°
3
6 9
13
16
19
23
2629
33
36
39
Θ 15°
Φ 270°
1
1
3
3
3
4
4 6
6
8
8
9
9
11
11
12
14
Θ 25°
Φ 270°
3
6
9
9
12
15
18
21
24
27
30
33
36
Θ 45°
Φ 270°
Fig. 3. Evolution of the ground footprint of the electric-field
strength in the EW polarization for different EAS arrival di-
rections in the plane containing the geomagnetic field and the
south-north axis. The contours and axis scales are the same as in
Fig.1 and the origin of the frame corresponds to the position of
the particle core. Top left: the event is coming from the north. In
this case the transverse current contribution is large compared
to the charge-excess contribution, the shift of the position of the
radio core is smaller than that presented in Fig.1. For an event
coming from the south (top right), the radio core is strongly
shifted because the relative intensity of the transverse current
contribution decreases. In the case of an EAS parallel to the geo-
magnetic field (bottom left), only the charge excess contributes
to the radio emission. For air showers coming from the south
with zenith angles larger than the geomagnetic field (bottom
right), 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 in
the 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, the
on-axis extrapolated electric-field strength in the EW po-
larization, d0 the lateral slope and d the antenna distance
to the shower axis. The parameter d contains the actual
fitted parameters xr
c, yr
c which are the ground coordinates
of the radio core in the CODALEMA frame.
In Fig.4, for the first set of 315 events (with transverse
current and charge-excess contributions) we compare the
reconstructed radio core positions relatively to the parti-
cle core positions. The reconstructed radio core positions
are represented by the black crosses and a 10 m Gaussian
smoothed map is superimposed where the red lines repre-
sent the contour levels. A global shift toward the east is
clearly visible.
In order to exhibit the charge-excess contribution effect
in the simulation code, the same procedure is applied on
the second set of 315 events (with the transverse current
contribution 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 is
presented in Fig.5. We do not observe any shift as compared
to Fig.4.
Fig. 4. Reconstructed radio core positions (black crosses) of 315
events simulated with SELFAS2 (following the observed core
distribution), relatively to the particle core positions of each
simulated event. The red lines represent contour levels of a 10 m
Gaussian smoothed map. A global shift toward the east is clearly
visible.
Fig. 5. Same legend as in Fig.4 except that the charge-excess
contribution has been switched off in SELFAS2. The average
position of the radio core positions is centered on the origin,
corresponding to the particle core positions.
3

4. A. Bell´etoile 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 this
effect does not depend on the primary energy.
In the next section we compare the result derived from
SELFAS2 to the data.
4. Reconstructed radio cores of the events
detected by CODALEMA
The CODALEMA setup used in this article is described
in Ardouin et al. (2006, 2009). The radio detector array is
composed of 24 EW-polarized antennas and is triggered
by a particle detectors array of 17 scintillators covering
an area of 340 × 340 m2
. In order to identify EAS in the
full data set and using the data from both arrays, we can
compare 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 particles
data must agree within 3◦
maximum and must be in the
same time window of ±100 ns. We also require that the
event core position estimated by the particle array is fully
contained inside the particle array. There are 315 events
passing these cuts (see Ardouin et al. 2009; O.Ravel & the
CODALEMA Collaboration 2010). The radio core positions
of these 315 events relatively to the particle cores positions
are presented in Fig.6. Possible experimental biases coming
Fig. 6. Reconstructed radio core positions (blue crosses) of the
315 selected events detected by the CODALEMA radio array,
relatively to the reconstructed particle core positions. The red
lines are contour levels of a 10 m Gaussian smoothed map. The
radio cores distribution is shifted toward the east as observed in
Fig.4 with SELFAS2.
from trigger effect or from the array geometry have been
checked but the EW radio core shift is robust (see also
Lecacheux & the CODALEMA Collaboration 2009). The
similarity of this general behavior with what we obtained
in Fig.4 suggests that this radio core shift along the EW
direction in the CODALEMA data is an experimental sig-
nature of the charge-excess contribution in the total EAS
radio emission.
We can compute the core shift amplitude as a func-
tion of the arrival direction. This dependence is presented
in Fig.7. In this figure, the expected behavior of the core
0.5 0.0 0.5 1.0
200
100
0
100
200
n B EW
cr
EW
m
— SELFAS2
SELFAS2 Σ
CODALEMA
Fig. 7. Shift of the ground radio core positions relatively to the
positions of the ground particles shower core as a function of
the EW component of (v × B) (where n is the shower axis di-
rection). The simulated values are represented by the black line
and the ±σ limits (gray dashed lines) expected with SELFAS2
for the CODALEMA experiment. Data are superimposed (cir-
cles, see text for details). Monte-Carlo simulation were done on
each event reconstruction to obtain error bar on ∆c.
shift as a function of the EW component of n × B (where
n and B are respectively the shower axis and the geomag-
netic field directions) is superimposed to the data. The lack
of events coming with arrival directions close to B and
from the south at the CODALEMA site, does not allow to
state on a clear correlation for values of |n×B|EW smaller
than 0.3, corresponding to an angle between the shower axis
and the geomagnetic field smaller that 16◦
.
5. Conclusion
The CODALEMA data presents a statistically significant
disagreement between the reconstructed radio core posi-
tions and the particle core positions. No instrumental bi-
ases can explain this disagreement which appears to be due
to physical reasons. Using simulations, we argue that this
observation can be interpreted as the contribution of the
charge-excess mechanism in the generation of the electric
field emitted by EAS. Our results also show that the charge-
excess description used in the simulation code SELFAS2 is
in good agreement with the observation, and consequently
that 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 and
on 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 of
the shift between the radio core positions and the particle
core positions with the arrival direction is confirmed with
the new generation of radio-detection experiments (AERA,
CODALEMA3, TREND), this signature will mark a new
step in the understanding of the radio emission process.
4

5. A. Bell´etoile et al.: Charge-excess contribution in CODALEMA
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