First upper limits on the radar cross section of cosmic-ray induced extensive air showers

Abstract TARA ( TelescopeArray Radar) is a cosmic ray radardetection experiment colocated with TelescopeArray, the conventional surface scintillation detector (SD) and fluorescence telescopedetector (FD) near Delta, Utah, U.S.A. The TARA detector combines a 40 kW, 54.1 MHz VHF transmitter and high-gain transmitting antenna which broadcasts the radarcarrier over the SD array and within the FD field of view, towards a 250 MS/s DAQ receiver. TARA has been collecting data since 2013 with the primary goal of observing the radarsignatures of extensive air showers(EAS). Simulations indicate that echoes are expected to be short in duration (∼ 10 µs) and exhibit rapidly changing frequency, with rates on the order 1 MHz/µs. The EAS radar cross-section(RCS) is currently unknown although it is the subject of over 70 years of speculation. A novel signal search technique is described in which the expected radarecho of a particular air showeris used as a matched filter template and compared to waveforms obtained by triggering the radarDAQ using the TelescopeArray fluorescence detector. No evidence for the scattering of radio frequency radiation by EAS is obtained to date. We report the first quantitative RCS upper limits using EAS that triggered the TelescopeArray Fluorescence Detector.