Low frequency radio- emission from UHE cosmic ray air showers 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. t

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. Recent findings report that this mechanism is not really applicable Huege & Falcke: extended synchrotron theory 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 Geosynchrotron emission - still in doubt. 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 magnetic and coherent mechanisms, 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 (LOw Frequency ARray) 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 to be combined with existing EAS array 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 radio- emission 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 negative correlation indicating different mechanisms for HF & LF emissions. Measured high field strength at LF, explained by TR mechamism. Recent Work : 30kHz loop antenna with miniarray particle detector for UHE Cosmic Rays and detailed calculation using TR model.

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 the radio antenna picks up signal emitted by excess charged particles after striking ground. As such there is a time delay of about 10μsec between the particle pulses from miniarray and the radio signal. Therefore trigger circuit operating at 2.5 μsec window gives no coincidence between miniarray and radiopulse. However, radio-radio coincidence between pair of antennas is possible.

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 magnetic field component perpendicular to the plane of the loop antenna is effective in producing induced voltage and current. A FORTRAN program is written to evaluate the inducing electric field at the loop antenna due to the kth element on the shower front and the corresponding arrival time (counted from shower striking ground at A) t(k), to get the pulse profile. This information is transformed to the frequency domain by using DFFT in MATLAB.

SIMULATION The excess charge distribution at the shower front are estimated using CORSIKA simulation code for a vertical showers of proton, Iron and gamma primary of energy eV & eV. 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 DFFT is done with the UNIX version of the standard package MATLAB.

Comparison of density of negative charge excess for proton induced near vertical showers : (a) at different energies and (b) for different masses at eV. Comparison of density of negative charge excess for proton induced near vertical showers of primary energy eV. : (a) at different energies and (b) for different masses. (a)(b)

The radio pulses obtained from a eV proton induced near vertical shower at different lateral distances.

Absolute filed strength and corresponding frequency spectra from a eV proton induced near vertical shower at different lateral distances

.(a) Absolute field strength, (b) frequency spectra, (c) lateral dependence for different orientations of the loop antenna and (d) lateral dependence of 30 kHz and 40 kHz component of bandwidth limited field strength from a eV proton induced near vertical shower at 300 m. (a) (b) (d) (c)

(a) Radio pulse profiles and (b) dependence of peak field strength on primary energy at 300 m lateral distance from the shower centre. (a)(b)

Comparison with REAS 3 and experimental observation due to Hough et al. at eV (left) and Comparison with earlier GUCR model at eV.

Result We have used a simple geometrical model for production of TR from cosmic ray EAS. The model helps to establish the observed higher field strength at lower frequency. Also information about primary energy and mass composition may be obtained from measurement of radio frequency and field strength. But looking at the complexity of the phenomena there is scope for further improvement of this model. Further, the model is based on the assumption that the shower front is plane and the ground is also plane e

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 study polarization of radio-emission. 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.