Probing Atmospheric Electric Fields in Thunderstorms through Radio Emission from Cosmic-Ray-Induced Air Showers

We present measurements of radio emission from cosmic ray air showers that took place during thunderstorms. The intensity and polarization patterns of these air showers are radically different from those measured during fair-weather conditions. With the use of a simple two-layer model for the atmospheric electric field, these patterns can be well reproduced by state-of-the-art simulation codes. This in turn provides a novel way to study atmospheric electric fields.

Figure 1

Polarization as measured with individual LOFAR antennas (arrows) in the shower plane for three measured air showers. LOFAR antennas are grouped into circular stations, of which seven are depicted. The expected polarization direction for fair-weather air showers is indicated with “normal.” The position of the shower axis, orthogonal to the shower plane, is indicated by the intersection of the dashed lines.

Figure 2

Radio intensity pattern during a thunderstorm. Top: the circles represent antenna positions. Their color reflects measured pulse power. The best-fitting coreas simulation is shown in color scale in the background. Where the colors of the circles match the background, a good fit is achieved. Bottom: measured (circles) and simulated pulse power (squares) as a function of distance to the shower axis.

Figure 3

Sensitivity of the fit quality to variations in the atmospheric depth of shower maximum $X_{\max }$ (left panel), the relative field strength (middle panel), and the field reversal altitude $h_2$ (right panel). The optimal proton simulation is the same for all plots. The electric field strength, in the upper layer, for all simulations is $|{\mathbf{E}}_U|=50\mathrm{kV}{\mathrm{m}}^{-1}$.