First Experimental Characterization of Microwave Emission from Cosmic Ray Air Showers

We report the first direct measurement of the overall characteristics of microwave radio emission from extensive air showers. Using a trigger provided by the KASCADE-Grande air shower array, the signals of the microwave antennas of the Cosmic-Ray Observation via Microwave Emission experiment have been read out and searched for signatures of radio emission by high-energy air showers in the GHz frequency range. Microwave signals have been detected for more than 30 showers with energies above $3\times {10}^{16}\mathrm{eV}$. The observations presented in this Letter are consistent with a mainly forward-directed and polarized emission process in the GHz frequency range. The measurements show that microwave radiation offers a new means of studying air showers at $E\ge {10}^{17}\mathrm{eV}$.

Figure 1

Power (linear scale) as a function of time relative to the KASCADE-Grande trigger for the event with the highest signal-to-noise ratio (top panel) and a stereo event (bottom panel). The 8 dB thresholds for the signal search are shown as dashed lines, with the horizontal extents indicating the time windows in which signals are expected.

Figure 2

Distribution of viewing angles for showers passing the KASCADE-Grande and geometry selection criteria for the 37 receivers with microwave signals above the search threshold (red line) and about 15000 receivers without (black line).

Figure 3

Positions at which a microwave signal has been detected relative to the shower core at (0, 0). The color contours indicate the maximum total field strength at ground level predicted by CoREAS for a typical vertical ($X_{\max }=658\mathrm{g}{\mathrm{cm}}^{-2}$, upper panel) and a very deep vertical shower ($X_{\max }=895\mathrm{g}{\mathrm{cm}}^{-2}$, lower panel) of ${10}^{17}\mathrm{eV}$.

Figure 4

Polarization directions of receivers in which a microwave signal was detected (black lines). In addition, the predicted polarization ellipses simulated with CoREAS for iron showers are shown in red. For dual-polarized receivers, the polarization direction in which no signal was detected is shown as a dashed line. The shower core is always at (0, 0).