Radio detection of extensive air showers

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by  Tim Huege

Radio detection of extensive air showers
TimHuegea
aInstitutfu¨rKernphysik,KarlsruherInstitutfu¨rTechnologie-CampusNord,Postfach3640,76021Karlsruhe,Germany
Abstract
Radio detection of extensive air showers initiated in the Earth’s atmosphere has made tremendous progress in the last decade.
Today, radio detection is routinely used in several cosmic-ray observatories. The physics of the radio emission in air showers is
well-understood,andanalysistechniqueshavebeendevelopedtodeterminethearrivaldirection,theenergyandanestimateforthe
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massoftheprimaryparticlefromtheradiomeasurements. Theachievedresolutionsarecompetitivewiththoseofmoretraditional
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techniques. Inthisarticle,Ishortlyreviewthemostimportantachievementsanddiscussthepotentialforfutureapplications.
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Keywords:

nhigh-energycosmicrays,radioemission,extensiveairshowers
a
J

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1. Introduction particle number, then reaches a maximum and then dies
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out, these transverse currents undergo a time-variation.

Whiletheunderstandingofcosmicrayshasprogressedvery
] Thetime-variationofthecurrentsleadstoradioemission.
Msignificantlyinthelastdecades,manyquestionsabouttheirori- Thisisthedominanteffectresponsibleforapproximately
gin and the physics of their acceleration are still unanswered
I 90%oftheelectricfieldamplitude, usuallyreferredtoas
.[1]. New detection techniques help to maximize the informa-
h “geomagneticemission”[4,5].
ption gathered about each detected cosmic-ray particle. This is
-especiallyimportantatthehighestenergieswherefluxesareex- • Duringtheair-showerevolution,anegativechargeexcess
o
tremelylowandmeasurementsthroughairshowersyieldonly builds up in the shower front. This arises mostly be-
r
tratherindirectinformationontheprimaryparticles. Inthelast cause ionization electrons from the ambient medium are
s
ayears,radiodetectionofairshowersintheveryhighfrequency swamped with the shower, while positive ions stay be-
[(VHF)band,typicallyaround30–80MHz,hasbeenresearched hind. Again,astheshowerevolves,thenetchargegrows,

withgreateffort. Today,thetechniquehasmaturedfromapro- reachesamaximumandthendeclines. Thetime-variation
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vtotypestagetoawell-establisheddetectiontechniquethatben- ofthenetchargeexcessleadstoradioemissionwhichcon-
5efitsanycosmic-raydetectorinwhichitisemployed. Theen- tributesapproximately10%oftheelectricfieldamplitude.
9ergy range accessible with the so-far developed approaches is Thisistheso-called“Askaryaneffect”whichisthedomi-
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illustratedinFig.1 nantmechanismforradioemissionfromparticleshowers
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In the following, I give a concise overview of the most im- indensemedia[6,7].
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.portant achievements made with the technique to date. For a
1 • At VHF frequencies the radio emission is generally co-
detailed discussion of the state of the field, I kindly refer the
0 herent. Thismeansthatelectricfieldamplitudesfromin-
7readertoapreviously-publishedextensivereview[3].
dividual particles add up constructively. The total elec-
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tric field amplitude thus scales linearly with the number
:
v2. Emissionphysics of particles in the air shower, which in turn scales ap-
i
X proximately linearly with the energy of the primary cos-
The most important breakthrough of the past few years has
r mic ray. Consequently, the radiated power (and energy)
abeenthedetailedunderstandingoftheradioemissionprocesses scales quadratically with the cosmic-ray energy. Coher-
inextensiveairshowers. Threeeffectsareimportant: enceisgovernedbydifferentscalesintheairshower: the
• Electronsandpositronsintheextensiveairshowerareac- thickness of the shower pancake, the lateral width of the
shower,andthetime-delaysarisingfromthegeometryof
celeratedbytheLorentzforceinthegeomagneticfield.At
theshowerdiskpropagatingwiththespeedoflightasseen
thesametimetheyaredeceleratedbyinteractionswithair
from a specific observer location. The latter is strongly
molecules. Anequilibriumarises, andthenetdriftofthe
influenced by the refractive index of the air which is ap-
particlesindirectionsperpendiculartotheair-showeraxis
proximately 1.0003 at sea level and scales with the den-
leads to transverse currents. As the shower first grows in
sitygradientoftheatmosphere. Thisleadsto“Cherenkov
rings”intheradio-emissionfootprints;observersonthese
∗Email:[email protected] rings see time-compressed radio signals for which coher-
PreprintsubmittedtoNIMA January12,2017
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Energy (eV/particle)
Figure1:Cosmic-rayenergyspectrumoverlaidwiththereachoftheVHFradiodetectiontechnique.AtlowenergiestheradiosignalsarehiddeninGalacticnoise,
atveryhighenergiesconceptshaveyettobedevisedtocoverextremelylargedetectionareas.Diagramupdatedandadaptedfrom[2],reprintedfrom[3].
encereachesuptoGHzfrequencies[8,9]. 3. Reconstructionofcosmic-rayparameters
Any detection technique for extensive air showers is only a
meanstotheendofdeterminingtheparametersoftheprimary
The geomagnetic and Askaryan mechanisms have different cosmic-rayparticle:thearrivaldirection,theenergy,andamea-
polarization characteristics, their superposition leads to con- sureforthemass.Andinfact,alloftheseparametershavebeen
structive interference or destructive interference depending on demonstrated to be reconstructable from radio measurements
thelocationoftheobserverwithrespecttotheshoweraxis.The withresolutionscompetitivewiththoseofmoretraditionalde-
resultingradio-emission“footprint”isthusasymmetric. Anil- tectiontechniques.
lustrationofthemechanismsandtheirpolarizationcharacteris- The arrival direction of the air shower can be determined
ticsisshowninFig.2. fromthearrival-timedistributionoftheradiopulsesinindivid-
ual antennas. It is important to note that the wavefront of the
The interpretation discussed so far is based on macroscopic radiosignalisnotaplanewavebuthasacomplexstructure: It
modelswhichemployconceptssuchascurrentsandnetcharges ishyperbolical[17,18],i.e.,sphericalclosetotheshoweraxis
intheairshower. Correctincorporationoftheenormouscom- andconicalfurtherawayfromtheshoweraxis. Thearrivaldi-
plexityoftheairshowerandtheresultingcoherenceeffectsin rectionhasbeendemonstratedtobereconstructablewithin0.5◦
macroscopicmodelsis,however,verydifficult,andtherequired andpossiblyaswellas0.1◦withradiotechniques.
simplifications degrade the quality of the modelled radio sig- Duetothecoherentnatureoftheradioemission,theampli-
nals. This is why microscopic simulations in which the radio tude of the radio signal scales approximately linearly with the
emission from the particle shower is calculated by adding up energy of the primary particle. The depth of the shower max-
theemissionfromeachindividualelectronandpositron[8,13] imum (related to the mass of the primary particle) influences
are most widely used in the community. These are based on the steepness of the lateral distribution of the radio signal, so
first principle calculations, applying discretized formalisms of a measurement at a given position will exhibit intrinsic fluc-
classicalelectrodynamics[14,15]totheindividualmovingpar- tuations of the measured amplitude. Two concepts have been
ticles in the air shower. Consequently, they predict the radio devised to minimize these intrinsic fluctuations for a precise
signal on an absolute scale without any free parameters in the measurement of the energy of the cosmic ray. First, a charac-
simulation. All measurements to date have been described by teristic lateral distance exists [21] at which these fluctuations
such simulations within errors. An example for a comparison are minimized. Experiments such as LOPES and Tunka-Rex
of amplitudes measured with LOPES [16] and simulated with [22]havethususedamplitudemeasurementsatthischaracter-
CoREAS [13] at a distance of 100m from the shower axis is istic lateral distance as an energy estimator, and have reached
giveninFig.3. Similarresultshavebeenreportedbyotherex- resolutionsasgoodas15%usingthisapproach,cf.Fig.4. Sec-
periments. ond, instead of measuring the amplitude at a specific lateral
2
v x v x B v x v x B
v x B v x B
Figure2: Left: Thegeomagneticradiationmechanism. Thearrowsindicatethedirectionoflinearpolarizationintheplaneperpendiculartotheairshoweraxis.
TheemissionislinearlypolarizedalongthedirectiongivenbytheLorentzforce,(cid:126)v×B(cid:126)(east-westforverticalairshowers). Right: Thechargeexcess(Askaryan)
emission. Thearrowsindicatethepolarizationwhichislinearwithelectricfieldvectorsorientedradiallywithrespecttotheshoweraxis. Diagramshavebeen
adaptedfrom[10]and[11]andreprintedfrom[3].
]
z
H
M102 proton V]
m/ Ee 1
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V or
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00 et
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S- nk
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Correlationwithuncert.
1 1:1correlation(x=y)
1 10 102
0.1 1
LOPES ε [μV/m/MHz] Energyfromradiodetector[EeV]
100
Figure3: Comparisonoftheamplitudeatalateraldistanceof100masmea- Figure 4: Cosmic-ray energy determined with the Tunka-Rex radio anten-
suredwithLOPESandsimulatedwithCoREASforairshowersinducedby nasincomparisonwiththeenergyreconstructedwiththeTunka-133optical
protons. Thefewoutliereventsinthelower-rightpartsofthediagramsare Cherenkovdetectors.Adaptedfrom[19],reprintedfrom[3].
notunderstood, butconstitutelessthan2%ofthedata. Adaptedfrom[12],
reprintedfrom[3].
of the primary particle is radiated in the form of radio signals
intheVHFband: Fora1018eVprimaryparticle,theradiation
distance, an area integral can be performed over the complete energy amounts to approximately 15.8MeV [24]. Neverthe-
radio-emissionfootprint. Thisapproachhasbeenpioneeredby less,photonstatisticsneednotbeconsideredastheenergyofa
theAugerEngineeringRadioArray(AERA)[23]. Theenergy 55MHzphotonisoforder10−7eV.
fluence (in units of eV/m2) as measured at individual antenna Themostdifficultchallengeisthereconstructionofthedepth
locationsisfittedwithamodelofthetwo-dimensionalemission of shower maximum (in g/cm2, usually called X ), which is
max
footprinttothenintegrateoverareaanddeterminethetotal“ra- theprimaryestimatorforparticlemassusedinair-showermea-
diationenergy”(inunitsofeV)depositedintheformofradio surements.FluorescenceandCherenkovlightdetectorsareable
signals on the ground. This radiation energy scales quadrati- tomeasureX witharesolutionof20–25g/cm2. Radiosig-
max
cally with the energy of the primary cosmic ray, the achieved nalsfromairshowerscarryinformationonthedistanceofthe
energyresolutionamountsto17%,cf.Fig.5. Theradiationen- sourceoftheemissionandthusonthedepthoftheshowermax-
ergy has the benefit of being a well-defined physical quantity imum,encodedinthesteepnessofthelateralamplitudedistri-
thatisindependentoftheobservationaltitudeandzenithangle butionaswellasinthewavefrontstructureandtheradiopulse
oftheairshower[24]. Itisthusconceptuallyveryattractiveas shapes. Sofar,mostanalysesonlyexploitthelateralsignaldis-
ameanstocross-calibratetheabsoluteenergyscalesofexperi- tribution. TheTunka-Rexexperimenthasdemonstratedaclear
mentsagainsteachotheroragainstfirst-principlecalculations. correlationbetweenX measuredwiththeTunkaCherenkov
max
Aninterestingasideisthatonlyaminutefractionoftheenergy light detectors and the Tunka-Rex radio antennas, cf. Fig. 6.
3
determine the true X value from the best-fitting simulation
max
109 (see Fig. 7), X resolutions better than 20g/cm2 are achiev-
max
able also with radio measurements. With such a good mea-
surement precision, even small data sets can be used to begin
constrainthemasscompositionofcosmicrays[26].
]V 108
e
[
α 4. Futureapplications
2
n
si
/ Therearenumerousapplicationsinwhichradiodetectorscan
Hz107 benefitexistingornewlybuiltcosmic-raydetectors. Inparticu-
M
ger−80 lar:
AuE30 • Anyparticledetectorwillprofitfromadditionalradioan-
106 tennaswhichallowmeasurementsofthepureelectromag-
neticcascadewithaverygoodenergyresolutionandrea-
sonableX resolution. Thisisattractiveinparticular if
max
much of the infrastructure (cabling, power, communica-
105 tions) can be shared between the detectors — the radio
1017 1018 1019
antenna and readout electronics themselves are fairly in-
EAuger Surface Detector[eV]
expensive with prices below 1,000 USD per antenna cer-
tainlyachievable.
Figure5: Correlationbetweentheradiationenergy(normalizedforincidence
perpendiculartothegeomagneticfield)andthecosmic-rayenergymeasured • Radio detectors can be used for a precise calibration of
withtheAugersurfacedetector. Opencirclesrepresentairshowerswithradio
theabsoluteenergyscaleofacosmic-raydetector. Thisis
signalsmeasuredinthreeorfourAERAdetectors,filledcirclescorrespondto
showerswithfiveormoremeasuredradiosignals.Adaptedfrom[20],reprinted becausetheradiosignalisnotinfluencedbyatmospheric
from[3]. conditions (no scattering, no absorption) and gives direct
access to the calorimetric energy in the electromagnetic
1000 cascadeoftheairshower[28]. Cross-calibrationbetween
2m] different experiments, e.g., via the radiation energy, or
g/c even calibration of experiments using first-principle cal-
[ culationsarethusveryattractive.
m 800
u
m • Horizontalairshowersilluminateareasofseveralkm2[29]
xi
ma and can thus be detected with antenna arrays using grid
er spacingsofakilometerorsparser,meaningthatmeasure-
w 600
o mentsofinclinedairshowerscanbeusedtomeasurethe
h
s electromagnetic component of ultra-high-energy cosmic
o
t rays.
e
c
n 400
a • Very dense antenna arrays might allow cosmic-ray mea-
st
di surements with unprecedented reconstruction quality for
ov individualair-showerevents. TheupcomingSquareKilo-
k
en 200 Correlationwithuncert. metre Array is expected to reach Xmax resolutions below
er 1:1correlation(x=y) 10g/cm2 [30]andcouldinvestigatethemasscomposition
h
C in the region of transition from Galactic to extragalactic
200 400 600 800 1000 cosmicrayswithunprecedentedmassresolution.
Radiodistancetoshowermaximum[g/cm2]
5. Conclusions
Figure 6: Atmospheric depth between shower maximum and observer alti-
tudeasdeterminedwiththeTunka-RexradiomeasurementandtheTunka-133
Overthepastdecade,radiodetectionofextensiveairshow-
Cherenkovdetectors.Adaptedfrom[19],reprintedfrom[3].
ershasmaturedfromaprototypephasewithsmallinstallations
andanunclearpictureoftheradioemissionmechanismstofull-
TheestimatedX resolutionoftheradioreconstructionisof fledgeddetectorarraysandadetailedunderstandingoftheradio
max
order 40g/cm2 and thus not yet competitive with established emission physics. Radio detection has by now become a rou-
techniques. However,thereisstillroomforimprovement,e.g., tinepartofseveralcosmic-raydetectioneffortsandcontributes
by exploiting additional signal information. Using the dense valuable information for the analysis of cosmic-ray data. Par-
antennaarrayofLOFAR[25]andatop-downapproachwhere ticularpotentialliesinaprecisecalibrationoftheabsoluteen-
individual events are compared with dozens of simulations to ergyscaleofcosmic-raydetectors,inthelarge-scaledetection
4
400 1.2 18
LOFARdata 2.0
axis [m] 230000 units] 1.0 CoREASsimulation 1146 1.5
on along v x v x B 1100000 al power [arbitrary 000...468 2χfit quality [ /ndf] 116802 1.0 600 700 800
siti 200 Tot 4
o
P 0.2
300 2 CoREAS p
CoREAS Fe
400400 300 200 100 0 100 200 300 400 0.00 50 100 150 200 250 300 350 400 450 5050 600 650 700 750 800 850 900 950
Position along v x B axis [m] Axis distance [m] Xmax [g/cm2]
Figure7:Left:EnergyfluencemeasuredatindividualLOFARantennas(coloredcircles)incomparisonwiththesignaldistributionpredictedbythebest-fittingof
asetofCoREASsimulations(background-color)foraparticularairshowerevent.Middle:One-dimensionalprojectionofthetwo-dimensionalsignaldistribution.
Right:QualityoftheagreementbetweentheenergyfluencedistributionmeasuredwithLOFARandthedistributionpredictedbydifferentCoREASsimulationsof
theairshowerevent.AclearcorrelationbetweenthevalueofXmaxandthequalityofthefitisobvious.Alldiagramsadaptedfrom[27],reprintedfrom[3].
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