Low frequency radio- emission associated with UHE cosmic rays KALYANEE BORUAH Physics Department, Gauhati University

Radio Emission from Air Showers: A Brief History Oscilloscope traces of CR radio pulses Jelley et al. (1965)‏

Historical development Theory Askaryan predicted radio Cerenkov from –ve charge excess Kahn & Lerche developd geomagnetic charge separation model of dipole & transverse current through the atmospher Experiment Jelley detected 44MHz radio pulse associated with EAS => Intensive research VLF(few kHz) to VHF (hundreds of MHz) Allan found polarization depends on geomagnetic field Experimental work ceased due to technical problem, man-made interference & advent of alternative techniques

Geomagnetic charge Segmentation LF radio emission Kahn & Lerche’s Model.

Later development 1985 – Nishimura proposed Transition Radiation (TR) mechanism to explain high field strength at low frequency (LF)‏ Askaryan type charge excess mechanism plays a major role in dense media such as ice & used to detect neutrino induced shower (RICE)‏ Falcke & Gorham proposed coherent geosynchrotron radiation from highly relativistic electron positron pairs gyrating in earth’s magnetic field Huege & Falcke: analytic calculation using synchrotron theory from individual particle is applied to air showers. Detailed Monte Carlo simulation is used to study dependence on shower parameters.

Present understanding UHECRs produce particle showers in atmosphere Shower front is ~2-3 m thick ~ wavelength at 100 MHz e ± emit synchrotron in geomagnetic field ~ 0.3G (10-100MHz)-Geosynchrotron emission. Emission from all e ± (N e ) add up coherently Radio power grows quadratically with N e. The mechanisms for the highest and the lowest frequencies are found to be very different. VHF emission is well explained by geo synchrotron mechanism, but VLF (<1MHz) emission is yet unclear, may be explained by Transition radiation mechanism.

New concept 2001 Peter Biermann points out potential relevance for digital radio interferometer called LOFAR radio telescope using advanced digital signal processing, capable to simultaneously monitor the full sky for transient radio signals, even in today’s environment of high radio frequency interference.

LOFAR Prototype Station (LOPES)‏ Falcke & Gorham proposed LOFAR for observing radio emission from EAS Horneffer et al developed LOPES project, an experiment based on LOFAR. It consists of 30 antennas working as a phased array in conjunction with the particle detector array KASCADE-Grande in Germany. LOPES has the capability to measure linearly polarised emission, necessary for verification of geosynchrotron as dominant mechanism.

Mechanisms of Radio-emission Charge separation in Earth‘s magnetic field (Kahn and Lerche, 1965)‏  classical electric dipole Gyration of electrons along a small arc  emission of synchrotron radiation ( MHz)‏ Electrons (charge excess) in a shower disk of small thickness (2m < one wavelength at 100 MHz)  coherent emission, beamed into propagation direction (Askaryan, 1960)‏ Transition Radiation (charged particles moving from atmosphere to ground): VLF (proposed)‏

transition radiation emission from a charge e

Transition Radiation The existence of Transition radiation was first suggested by Frank & Ginzburg(1946)‏ emitted when a uniformly moving charged particle traverses the boundary separating two media of different dielectric properties. Later, Garibian deduced wave solutions in the radiation zone, a method used by Dooher (1971) to calculate Transition radiation from magnetic monopoles. We extend and apply TR theory to develop a prototype model for radioemission following Dooher’s approach.

Radio observation of Cosmic rays by Guwahati University Cosmic Ray (GUCR) Group (1970-present)‏ Early Work : in the frequency range MHz, using dipole & Yagi Antenna with conventional particle detector array : Correlation studies show different mechanisms for HF/LF emissions Recent Work : 30kHz loop antenna with miniarray particle detector for UHE Cosmic Rays.

BLOCK DIAGRAM OF THE EXPERIMENTAL SETUP

Photographic view of the Experimental Setup.

GU miniarray could detect UHE cosmic rays of primary energy eV. Efforts have been made to detect radio emission associated with UHE cosmic ray air showers as detected by the miniarray detector, using loop antenna, placed close to the miniarray. However, when triggered by miniarray pulse, no coincidence was observed. On the other hand when the miniarray channel was decoupled, radio-radio coincidence could be observed. New findings

The new findings may be explained by a model based on mechanism of transition radiation, which shows that when the radio antenna is inside the shower disk (i.e., when radio antenna is close to the particle detector), contribution of transition radiation from opposite sides of the shower disk cancel, resulting in very small pulse height. This means, radiation could be detected when antenna is placed outside the shower disk, i.e, at large core distances

Theoretical Model: This method involves solving Maxwell’s equations and resolving field vectors into Fourier components with respect to time as suggested by Fermi [1940]. The magnetic field component of the radiation field is effective in producing induced current in the loop antenna. The homogeneous solution in the first medium is given by-

Where, and For the vacuum to medium case,, And for extremely relativistic particles, …(1)‏

And equation (1) simplifies to-

The integral is a delta function, Hence finally

the r.m.s. field strength induced at the loop antenna, A FORTRAN program is written to calculate the arrival time of the transition radiation at the position of the loop antenna, from different elements of the shower front after striking ground. and the corresponding induced field strength

SIMULATION The excess charge for different effective radii (R ef  R) of the shower disk are estimated using CORSIKA simulation code for proton of primary energy eV. Excess charge is assumed to be distributed uniformly in the shower front. The particle output file from CORSIKA is first decoded with available FORTRAN code and the decoded output is further processed with a C++ program to get the excess of e- over e+. This negative charge excess is then put as input to the FORTRAN program which calculates the pulse profile due to Transition Radiation. To transfer the data from time domain to frequency domain FFT is done with the UNIX version of the standard package MATLAB.

Result Frequency profiles of the transition radiation from 1EeV proton induced showers for different radial distances show that the positions of the highest peaks shift to lower frequencies as the effective radius increases. A plot of effective radius versus peak frequency shows that, for large distances this frequency remains constant around 30kHz. This result may be compared with experimental results using loop antenna, where peaks are observed near 30kHz.

Future outlook Simulation to be carried out with different primary mass and higher energies to study possible dependence of shower parameters with the associated radioemission. To design detector array based on detail simulation.

Acknowledgement : The authors wish to thank the University Grants Commission, Govt. of India for financial support under Special Assistance Program, for infrastructure to carry out computational work.