Measuring Radio Emission from Cosmic Ray Air Showers with a Digital Radio Telescope

Abstract

The earth is continously bombarded by highly energetic particles, the cosmic radiation. Even 90 years after their discovery the origin and acceleration of the highest energy particles is still unknown. Air showers from cosmic rays are known to emit radio pulses since these pulses were detected in 1965. In the 1970ties the measurement of these radio pulses ceased almost completely mostly due to difficulties with radio interference, uncertainty about the interpretation of the results and the success of other methods.<br /> The measurement of radio pulses from air showers has a number of advantages: It gives a much higher duty cycle than measuring optical light, it is complementary to measuring the particles that reach the ground level, and with radio one can get a good direction estimate for the air shower. The advent of fast digital computers and high bandwidth, high dynamic range ADCs enables us to use digital filtering and beam forming to suppress the radio interference.<br /> To test this technology we set up LOPES a prototype digital radio telescope in the scope of this thesis. It consists of 30 single polarisation antennas (10 in the first phase) that are set up at the site of the KASCADE-Grande experiment, an existing air shower array. It directly samples the radio signal in the frequency range from 40 to 80 MHz and stores 0.82 ms of raw data every time it was triggered by KASCADE-Grande. For the analysis the data is offline correlated with data from the KASCADE array, radio interference is digitally filtered, and a beam in the direction given by the KASCADE array is formed.<br /> With this we have been able to reliably pick out radio pulses from air showers. The data from the first phase of LOPES are also analysed. By making cuts on the shower size (e.g. on the number of muons measured by KASCADE-Grande) and on the angle to the geomagnetic field one can select events so that each has a detected air shower radio pulse. The height of the radio pulse shows a nearly linear dependence on the shower size (with a power law index slightly smaller than one), an exponential decline with the distance of the antennas to the shower axis, and a monotonic rise with the angle of the air shower to the geomagnetic field. The latter dependence is a discrepancy to the current theoretical predictions. This has to be studied with better theoretical models and further measurements, e.g. of the polarisation characteristics.<br /> We have shown that digital radio telescopes can measure radio pulses from air showers and give reproducible results. This is the first step to establish radio measurements of air showers as an additional method of cosmic ray research.