Transcript Slide 1

Low frequency radioemission 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
1960- Askaryan predicted
radio Cerenkov from
–ve charge excess
1966- Kahn & Lerche
developd geomagnetic
charge separation
model of dipole &
transverse current
through the atmospher
Experiment
1965- Jelley detected
44MHz radio pulse
associated with EAS
=> Intensive research
VLF(few kHz) to VHF
(hundreds of MHz).
1967- Allan found
polarization depends
on geomagnetic field
1970 - Experimental work ceased due to technical problem,
man-made interference & advent of alternative techniques
Geomagnetic charge Segmentation
t
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)
• 2001- Askaryan type charge excess mechanism
plays a major role in dense media such as ice &
used to detect neutrino induced shower (RICE)
• 2003- 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.
• 2004- 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± (Ne) add up coherently
• Radio power grows quadratically with Ne.
• 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)
• 2003- Falcke & Gorham proposed LOFAR
to be combined with existing EAS array for
observing radio emission from EAS.
• 2004- 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 ??
(10-100 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 2-220 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.
FFT
of
received
radio
signal
when
the
installed in the rooftop of Physics building
antenna
was
New findings
• GU miniarray could detect UHE cosmic
rays of primary energy 1017-1018 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.
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-
…(1)
Where,
and
For the vacuum to medium case,
,
And for extremely relativistic particles,
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 1017 eV & 1018 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
(a)excess for proton induced near vertical(b)
showers : (a) at different energies and (b)
for different masses at 1017 eV.
Comparison of density of negative charge excess for
proton induced near vertical showers of primary energy
1017 eV. : (a) at different energies and
(b) for different masses .
The radio pulses obtained from a 1017 eV proton induced
near vertical shower at different lateral distances.
Absolute filed strength and corresponding frequency
spectra from a 1017 eV proton induced near vertical
shower at different lateral distances
(a)
(c)
(b)
(d)
.(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 1017eV proton induced near vertical shower at 300 m .
(a)
(b)
(a) Radio pulse profiles and (b) dependence of peak
field strength on primary energy at 300 m lateral
distance from the shower centre.
Comparison with REAS 3 and experimental observation
due to Hough et al. at 1017 eV (left) and Comparison with
earlier GUCR model at 1018 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.