Abstract
Secondary electrons and positrons of air showers emit a coherent radio electric field in a wide frequency band. The CODALEMA experiment installed at the Nançay radioastronomy observatory since 2002 detects air showers and the associated electric field in [20; 200] MHz. EXTASIS, triggered by the CODALEMA scintillators, detects since 2016 the air shower electric field in [1; 6] MHz. We also expect an additional signal at low frequency: the sudden death pulse, corresponding to the sudden disappearance of the shower front particles when they reach the ground level. We present the instrumental setups, their performances and the first results of EXTASIS.
Highlights
The detection of the electric field emitted by the secondary charged particles – with the 57 autonomous stations covering the full area of 1 km2, measuring the electric field in both north/south and east/west polarizations with a Butterfly antenna. – with the 10 cross-polarized antennas covering a very small area to study the electric-field at short distances. – a single 3D antenna, triggered by the scintillator array, in order to measure the three components of the electric-field vector, allowing to check the validity of the far-field approximation
We did not detect the sudden death signal so far. This could be due to the fact that the Nancay observatory has an altitude of 180 m: the atmospheric depth at this altitude is of the order of 1000 g/cm2 for vertical showers and the number of e± reaching the ground level is not sufficient to emit a measurable sudden death electric field
The 6 EXTASIS antennas involved in the event are indicated by the colored triangles; the arrival direction estimated using the low-frequency data only is in very good agreement with the direction obtained by the CODALEMA data
Summary
The detection of the electric field at low frequency (1 − 6 MHz), with the 7 radio stations spread over the area of 1 km2. As the CODALEMA radio stations are able to measure the electric field up to 200 MHz, we can improve the 20-80 MHz reconstruction by taking into account the high frequency data.
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