This paper summarises recent results on the cosmic ray energy spectrum, composition and anisotropy from the knee region to the GZK cutoff [1,2] of the spectrum by means of ground-based experiments. Most of the information reported in this contribution is taken from [3, 4]

Bertaina, M.

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Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2015-15029-xColloquia: IFAE 2014IL NUOVO CIMENTO 38 C (2015) 29A review on ground-based measurements of cosmic raysM. BertainaDipartimento di Fisica, Universita` di Torino and INFN, Sezione di Torino - Torino, Italyreceived 7 January 2015Summary. — This paper summarises recent results on the cosmic ray energyspectrum, composition and anisotropy from the knee region to the GZK cutoff [1,2]of the spectrum by means of ground-based experiments. Most of the informationreported in this contribution is taken from [3,4].PACS 95.55-Vj – Neutrino, muon, pion, and other elementary particle detectors;cosmic ray detectors.PACS 96.50-sd – Extensive air showers.1. – IntroductionThe cosmic ray energy spectrum above 1014 eV has a power-law like behaviour(∝ E−γ , with γ ∼ 2.7) with features which are known as the “knee” at 3–4 × 1015 eV,where the spectrum steepens to γ ∼ 3.0, the “ankle” at 2–8× 1018 eV, which is charac-terised by a flattening of the spectrum by roughly the same change of the spectral index,i.e. back to γ ∼ 2.7, and the GZK cut-off around 5× 1019 eV.The shape and composition of the primary spectrum as well as the large-scaleanisotropy in the arrival direction of cosmic rays are key elements to understand theorigin, acceleration and propagation of the Galactic radiation. The paradigm of the ori-gin of Galactic cosmic rays (CR) are supernovae, as their shock waves can provide therequired power to explain the intensity of the CR radiation at least up to 1015 eV [5].This paradigm has been recently confirmed well below the knee by the observations ofAGILE [6] and FERMI satellites [7]. However, the possibility for supernovae to acceler-ate CRs at energies above 1015 eV is quite challenging, therefore, different populations ofsources have been envisaged as responsible for the radiation in galactic and extragalacticenergy ranges [8, 9]. Those sources would be subject to a rigidity cutoff in the max-imum energy at which the various elements are accelerated, as proposed originally byPeters [10]. In this approach, the knee at ∼ 4 × 1015 eV would represent the end of thespectrum of CRs accelerated by supernova remnants in the Milky Way and the ankle at∼ 4× 1018 eV the transition to particles from extragalactic sources. However, the anklestructure could be explained also in a completely different way, such as a consequence ofthe physical process of pair production by protons during propagation through the cosmicCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 M. BERTAINAmicrowave background radiation [11]. In this case, the Galactic-extragalactic transitionoccurs below 1018 eV.A refined study of the CR primary spectrum and composition is, therefore, extremelyimportant to address the above questions. As acceleration and propagation mechanismsin magnetic fields would lead to the same rigidity dependence, the study of large scaleanisotropies in the arrival direction could provide relevant information to distinguishsource and propagation effects.The direct study of CRs by means of satellites or balloon-borne detectors is performedonly at energies below 1015 eV. Close to the knee, the flux becomes of the order of 1particle m−2 sr−1 yr−1. This fact prevents the possibility of a direct observation ofits structure by currently planned satellites or balloon experiments. Indeed, at leasthundred of events above the knee are necessary to determine its existence with enoughsignificance. Around the knee and at higher energies, CRs are studied by means of largearrays located at ground that measure the secondary particles produced by the primaryCR cascading in the atmosphere, the so-called Extensive Air Showers (EAS). Typically,the energy is proportional to the total number of secondaries sampled at ground, whilethe composition is inferred either through a multi-component measurement, such as theelectromagnetic and muonic components, or through the measurement of the emittedlight (Cherenkov or fluorescence lights) along the longitudinal development of the shower.Despite the fact that shower arrays allow one to collect high statistics, the interpretationof the results is based on the comparison with expectations from simulation describingthe EAS development in atmosphere, which are at some level inaccurate. This introducesa systematic uncertainty on the results, especially on the mass composition.Interestingly, the TeV region allows some partial overlap between direct and indirectmeasurements. Several techniques have been employed recently on ground detectors thatare sensitive to specific components of the CR radiation to overcome those uncertain-ties [12]. Among them, it is worth mentioning the measurement of the light component(p alone, or p+He) using hadron calorimeters [13], or Cherenkov light measurements incoincidence with TeV muons [14], and RPC counters at high altitude [15]. Those resultsare in quite good agreement with measurements by CREAM [16] balloon. In particu-lar, the ARGO results allow one to cross-check the fluxes on an extended energy range(5–250TeV). These results show that, when indirect measurements have the opportunityof selecting almost pure beams, their findings are in reasonable agreement with directones and confirm a fair representation of the EAS development in the atmosphere bysimulation codes such as CORSIKA [17].A fundamental ingredient of the CORSIKA simulation is the hadronic interactionmodel which generates the hadronic cascade at the origin of the electromagnetic cascade.Since 2009, the Large Hadron Collider (LHC) provides a lot of very precise data whichhave been used to improve two of the hadronic models used for air shower simulations,EPOS [18] and QGSJetII [19], giving birth to the most updated versions: EPOS-LHC andQGSJetII-04. Remarkably, interaction models employed in air shower simulations pro-vided a somewhat better prediction of global observables (multiplicities, pT -distributions,forward and transverse energy flow, etc.) than typical tunes of HEP models, such asPYTHIA or PHOJET [20]. The cross-section is particularly important for the EASdevelopment and the depth of shower maximum. The proton-proton scattering totalcross-section measured by TOTEM [21] at√s = 7TeV allowed to reduce to 20 gr/cm2the difference in Xmax position of the two models, which is comparable to the systematicuncertainties in the measurement by experiments. The muon number depends on theratio between particles producing hadronic sub-showers and the total number of particles.A REVIEW ON GROUND-BASED MEASUREMENTS OF COSMIC RAYS 3LHC data allow constraining the (anti)baryon and strangeness production at mid rapid-ity as well as the forward production of π0 in fixed target experiments [22]. Taking intoaccount these data, EPOS-LHC and QGSJetII-04 provide now quite similar results [23].2. – The knee region up to the ankleThe “knee” is a distinct feature of the all-particle CR energy spectrum at ∼ 4 ×1015 eV, where the power index suddenly changes from γ ∼ −2.7 to γ ∼ −3.1. Since itsdiscovery the origin of this feature is still under debate. From the experimental point ofview, measurements indicate that such a break is observed in the hadronic, muonic, andelectromagnetic components [24-27], as well as in Cherenkov light [28]. These results givea clear indication that the knee is a peculiarity of the primary spectrum, disfavouring ahypothesis based on changes of the interaction characteristics of the primaries with airnuclei. This conclusion has been reinforced by the first comparisons of the predictionsfrom hadronic models and LHC data [29].Several experimental results associate the knee with the bending of the light compo-nent, and are compatible with a rigidity dependent cut-off [26, 25]. Unfortunately, theflux of the different components vary significantly depending on the interaction modelused to interpret the data [26]. However, if this interpretation is correct, the heavy com-ponent should show a similar bending in the energy range 5× 1016–1017 eV. This is in-deed the experimental finding of KASCADE-Grande [30] confirmed by TUNKA-133 [31]and IceTop [32] experiments. In detail, the measured all-particle energy spectrum byKASCADE-Grande exhibits a less pronounced but still clear deviation from a singlepower law between the knee and the ankle, with a spectral hardening at ∼ 2 × 1016 eVand a steepening at ∼ 1017 eV [33]. The average mass composition gets heavier afterthe knee till ∼ 1017 eV, where a bending of the heavy component is observed [34]. Anindication of a hardening of the light component just above 1017 eV has been measuredas well [35]. The flux of heavy and light components depends on the hadronic interactionmodel used to interpret the experimental data [36] as previously mentioned. The aboveresults are summarised on the left side of fig. 1.The right side of fig. 1 compares the results of several experiments in terms of 〈lnA〉,as it is often reported in literature to describe the evolution of the composition as afunction of energy. Only QGSJet-II model is considered. A more detailed descriptionincluding the role of interaction models in composition is reported in [37]. Use of differentmodels introduces a shift in the average mass comparable to the dispersion of the datain fig. 1. Despite the large uncertainty in the absolute composition, a common generaltrend is visible. Composition gets heavier through the knee region and becomes lighterapproaching the ankle. The solid line in the plot is used as a guidance line to show howthe average 〈lnA〉 from the data in the plot evolves with energy. This result is compatiblewith the concept that the galactic component of the cosmic ray radiation reaches an endfollowing a rigidity cut-off at ∼ 1017 eV and that the ankle indicates a transition to apopulation dominated by extra-galactic sources.The search for anisotropies in the arrival direction of CR around the knee re-gion can provide relevant information to distinguish source and propagation effects.The anisotropy varies with energy but the topological structure remains the same till∼ 1014 eV where it has an abrupt change as pointed out by IceCube [38]. Such a change isconfirmed till knee energies. This result seems to be inconsistent with the amplitude andphase expected according to the Compton-Getting prediction due to the relative motionof the Earth in the Galaxy. KASCADE-Grande published recently [39] an update on the4 M. BERTAINAE/eV1610 1710 1810 1910 2010)1.5 eV-1 s-1 sr-2 (m2.5dI/dE x E13101410151016101710KASCADE-Grande heavy QGSJet-II (2011)KASCADE-Grande light QGSJet-II (2013)AUGER (2013)HiResII (2008)KASCADE-Grande all QGSJet-II (2012)TUNKA-133 (2013)IceTop (2013)TA (2013)TIBET-III (2008)KASCADE all QGSJet-II (2013)KASCADE H-Si QGSJet-II (2013)KASCADE Fe QGSJet_II (2013)Fig. 1. – Left: The measurement of the CR spectrum by EAS experiments from the knee till theend of the spectrum. The main knee is explained as the bending of the light, followed by themedium, component. The fainter knee around 1017 eV is attributed to the bending of the heavyone. An ankle-like feature is observed in the light component just above 1017 eV that might berelated to the lightening of the composition approaching the ankle. Right: Average logarithmicmass of CRs as a function of energy derived from Xmax and particle detector measurementsusing QGSJet-II interaction model. Most of the data are taken from [37] and references therein.The solid line is drawn as a guidance and is obtained by averaging, in each energy bin, thevalues of 〈lnA〉 reported in the figure. The plots are taken from [3].anisotropy study based on the East-West method [40]. By investigating the variation ofthe amplitude as a function of energy it was found that the amplitudes were not signifi-cant, however, the phases were in almost all energy bins centred around 250 ± 25 degrees.This is interesting in itself because it points towards the Galactic Center. Moreover, itagrees inside the statistical uncertainties with the results of the anisotropy studies of thePierre Auger Observatory [41] in the energy range 3× 1017–1018 eV.3. – From the ankle to the end of the spectrumFigure 1 shows that the features at the Ultra High Energies (UHE), the ankle andthe cutoff, have been established beyond doubt. The spectral slopes before and afterthe ankle have been measured and agree between Pierre Auger Observatory (PAO) [42]and Telescope Array (TA) [43]. The positions of the ankle also agree within the quotederrors, and are compatible with the existing model(s). The parameters of the breakat the highest energies are known less accurately. There seems to be some discrepancyconcerning the shape of the spectrum around the break; however more statistics is neededfor a firm conclusion. The position of the break is compatible with the GZK cutoff forprotons in TA, but the PAO spectrum fits better the case of a limit in the UHECRsacceleration by the sources.As far as the mass composition is concerned, the situation is less definite, and aconsistent picture has not yet emerged [37]. While PAO sees a change in the compositiontowards a heavier one at the highest energies, the TA observes no such a trend and iscompatible with a pure proton composition. This difference in the data has profoundconsequences: the Auger data suggest that we see the maximum energy of sources,similarly to what is observed at the knee in the cosmic ray spectrum, while the TAdata suggest we observe the GZK effect. Moreover, the different compositions in theGZK- and maximum-energy scenario will affect the level of anisotropies expected to beseen in the data.A REVIEW ON GROUND-BASED MEASUREMENTS OF COSMIC RAYS 5Despite the major advances in the understanding of UHECRs nature, the currentexperiments face the limit in the statistics they can accumulate due to the extremelylow flux of 1 particle km−2 year−1 above 5× 1019 eV. For this reason future space-basedobservatories, such as JEM-EUSO, that reach much higher and uniform exposures [44],could help in sheding light on the mystery of the most energetic radiation of the Universe.REFERENCES[1] Greisen K., Phys. Lett., 16 (1966) 148.[2] Zatsepin G. T. and Kuz’min V. A., JETP, 4 (1966) 78.[3] Bertaina M., Comptes Rendus Phys., 15/4 (2014) 300.[4] Kampert K.-H. and Tinyakov P., Comptes Rendus Phys., 15/4 (2014) 318.[5] Blasi P., Astron. Astrophys. Rev., 21 (2013) 1174.[6] Giuliani A. et al. (AGILE Collaboration), Astrophys. J., 742 (2011) L30.[7] Ackermann M. et al. (FERMI-LAT Collaboration), Science, 339 (2013) 807.[8] Hillas M., J. Phys. G, Nucl. Part. Phys., 31 (2005) R95.[9] Gaisser T., Stanev T. and Tilav S., arXiv:1303.3365v1 (2013).[10] Peters B., Il Nuovo Cimento, 22 (1961) 800.[11] Berezinsky V., Gazizov A. and Grigorieva S., Phys. Rev. D, 74 (2006) 043005.[12] Bertaina M. et al., J. Phys. Soc. Jpn. Suppl. A, 78 (2009) 2010.[13] Aglietta M. et al. (EAS-TOP Collaboration), Astropart. Phys., 19 (2003) 329.[14] Aglietta M. et al. (EAS-TOP and MACRO Collaboration), Astropart. Phys., 21(2004) 223.[15] Bartoli B. et al. (ARGO Collaboration), Phys. Rev. D, 85 (2012) 092005.[16] Yoon Y. S. et al. (CREAM Collaboration), Astrophys. J., 728 (2011) 122.[17] Heck D., Report FZKA 6019 (1998).[18] Werner K., Liu F.-M. and Pierog T., Phys. Rev. C, 74 (2006) 044902.[19] Ostapchenko S., Phys. Rev. D, 74 (2006) 014026.[20] d’Denterria D. et al., Astropart. Phys., 35 (2011) 98.[21] Antchev G. et al. (TOTEM Collaboration), Europhys. Lett., 96 (2011) 21002.[22] Adriani O. et al. (LHCf Collaboration), Phys. Rev. C, 89 (2014) 065209.[23] Pierog T. and Heck D., Proc. 33rd ICRC, #0163 (2013).[24] Aglietta M. et al. (EAS-TOP Collaboration), Astropart. Phys., 10 (1999) 1.[25] Aglietta M. et al. (EAS-TOP Collaboration), Astropart. Phys., 21 (2004) 583.[26] Antoni T. et al. (KASCADE Collaboration), Astropart. Phys., 24 (2005) 1.[27] Aglietta M. et al. (EAS-TOP and MACRO Collaboration), Astropart. Phys., 20(2004) 641.[28] Budnev N. et al. (TUNKA Collaboration), Astropart. Phys., 50-52 (2013) 18.[29] Ostapchenko S., Prog. Theor. Phys. Suppl., 193 (2012) 204.[30] Apel W. D. et al. (KASCADE-Grande Collaboration), Nucl. Instr. Meth. A, 620(2010) 202.[31] Prosin V. V. et al. (TUNKA-133 Collaboration), Nucl. Instr. Meth. A, 756 (2014)94.[32] Aartsen M. G. et al. (IceTop Collaboration), Phys. Rev. D, 88 (2013) 042004.[33] Apel W. D. et al. (KASCADE-Grande Collaboration), Astropart. Phys., 36 (2012)183.[34] Apel W. D. et al. (KASCADE-Grande Collaboration), Phys. Rev. Lett., 107 (2011)171104.[35] Apel W. D. et al. (KASCADE-Grande Collaboration), Phys. Rev. D, 87 (2013)081101(R).[36] Apel W. D. et al. (KASCADE-Grande Collaboration), Adv. Space Res., 53 (2014)1456.[37] Kampert K.-H. and Unger M., Astropart. Phys., 35 (2012) 660.6 M. BERTAINA[38] Abbasi R. et al. (ICECUBE Collaboration), Astrophys. J. Lett., 718 (2010) L194.[39] Chiavassa A. et al. (KASCADE-Grande Collaboration), Proc. 33rd ICRC, #0093(2013).[40] Bonino R. et al., Astrophys. J., 738 (2011) 67.[41] Abreu P. et al. (Pierre Auger Collaboration), Astroartp. Phys., 34 (2011) 627.[42] Abraham J. et al. (Pierre Auger Collaboration), Phys. Rev. Lett., 101 (2008)061101.[43] Abu-Zayyad T. et al. (Telescope Array Collaboration), Astrophys. J., 768 (2013)L1.[44] Adams J. H. jr. et al. (JEM-EUSO Collaboration), Astropart. Phys., 44 (2013) 76.

A review on ground-based measurements of cosmic rays

Institutional Research Information System University of Turin

IL NUOVO CIMENTO Vol. ?, N. ? ?A review on ground-based measurements of cosmic raysM. BertainaDipartimento di Fisica, Universita` di Torino and INFN, Sezione di Torino - Torino, ItalySummary. — This paper summarises recent results on the cosmic ray energyspectrum, composition and anisotropy from the knee region to the GZK cutoff [1, 2]of the spectrum by means of ground-based experiments. Most of the informationreported in this contribution is taken from [3, 4].PACS 95.55 – Vj.PACS 96.50 – sd.1. – IntroductionThe cosmic ray energy spectrum above 1014 eV has a power-law like behaviour (∝E−γ , with γ ∼ 2.7) with features which are known as the ‘knee’ at 3−4×1015 eV, wherethe spectrum steepens to γ ∼ 3.0, the ‘ankle’ at 2− 8× 1018 eV, which is characterisedby a flattening of the spectrum by roughly the same change of the spectral index, i.e.back to γ ∼ 2.7, and the GZK cut-off around 5× 1019 eV.The shape and composition of the primary spectrum as well as the large-scale anisotropyin the arrival direction of cosmic rays are key elements to understand the origin, acceler-ation and propagation of the Galactic radiation. The paradigm of the origin of Galacticcosmic rays (CR) are supernovae, as their shock waves can provide the required power toexplain the intensity of the CR radiation at least up to 1015 eV [5]. This paradigm hasbeen recently confirmed well below the knee by the observations of AGILE [6] and FERMIsatellites [7]. However, the possibility for supernovae to accelerate CRs at energies above1015 eV is quite challenging, therefore, different populations of sources have been envis-aged as responsible for the radiation in galactic and extragalactic energy ranges [8, 9].Those sources would be subject to a rigidity cutoff in the maximum energy at which thevarious elements are accelerated, as proposed originally by Peters [10]. In this approach,the knee at ∼ 4 × 1015 eV would represent the end of the spectrum of CRs acceleratedby supernova remnants in the Milky Way and the ankle at ∼ 4× 1018 eV the transitionto particles from extragalactic sources. However, the ankle structure could be explainedalso in a completely different way, such as a consequence of the physical process of pairproduction by protons during propagation through the cosmic microwave backgroundradiation [11]. In this case, the Galactic-extragalactic transition occurs below 1018 eV.c© Societa` Italiana di Fisica 12 M. BERTAINAA refined study of the CR primary spectrum and composition is, therefore, extremelyimportant to address the above questions. As acceleration and propagation mechanismsin magnetic fields would lead to the same rigidity dependence, the study of large scaleanisotropies in the arrival direction could provide relevant information to distinguishsource and propagation effects.The direct study of CRs by means of satellites or balloon-borne detectors is performedonly at energies below 1015 eV. Close to the knee, the flux becomes of the order of 1particle m−2 sr−1 yr−1. This fact prevents the possibility of a direct observation ofits structure by currently planned satellites or balloon experiments. Indeed, at leasthundred of events above the knee are necessary to determine its existence with enoughsignificance. Around the knee and at higher energies, CRs are studied by means of largearrays located at ground that measure the secondary particles produced by the primaryCR cascading in the atmosphere, the so-called Extensive Air Showers (EAS). Typically,the energy is proportional to the total number of secondaries sampled at ground, whilethe composition is inferred either through a multi-component measurement, such as theelectromagnetic and muonic components, or through the measurement of the emittedlight (Cherenkov or fluorescence lights) along the longitudinal development of the shower.Despite the fact that shower arrays allow one to collect high statistics, the interpretationof the results is based on the comparison with expectations from simulation describingthe EAS development in atmosphere, which are at some level inaccurate. This introducesa systematic uncertainty on the results, especially on the mass composition.Interestingly, the TeV region allows some partial overlap between direct and indirectmeasurements. Several techniques have been employed recently on ground detectors thatare sensitive to specific components of the CR radiation to overcome those uncertain-ties [12]. Among them, it is worth mentioning the measurement of the light component(p alone, or p+He) using hadron calorimeters [13], or Cherenkov light measurements incoincidence with TeV muons [14], and RPC counters at high altitude [15]. Those resultsare in quite good agreement with measurements by CREAM [16] balloon. In particular,the ARGO results allow one to cross-check the fluxes on an extended energy range (5-250 TeV). These results show that, when indirect measurements have the opportunityof selecting almost pure beams, their findings are in reasonable agreement with directones and confirm a fair representation of the EAS development in the atmosphere bysimulation codes such as CORSIKA [17].A fundamental ingredient of the CORSIKA simulation is the hadronic interactionmodel which generates the hadronic cascade at the origin of the electromagnetic cascade.Since 2009, the Large Hadron Collider (LHC) provides a lot of very precise data whichhave been used to improve two of the hadronic models used for air shower simulations,EPOS [18] and QGSJetII [19], giving birth to the most updated versions: EPOS-LHC andQGSJetII-04. Remarkably, interaction models employed in air shower simulations pro-vided a somewhat better prediction of global observables (multiplicities, pT -distributions,forward and transverse energy flow, etc.) than typical tunes of HEP models, such asPYTHIA or PHOJET [20]. The cross-section is particularly important for the EASdevelopment and the depth of shower maximum. The proton-proton scattering totalcross-section measured by TOTEM [21] at√s = 7 TeV allowed to reduce to 20 gr/cm2the difference in Xmax position of the two models, which is comparable to the systematicuncertainties in the measurement by experiments. The muon number depends on the ra-tio between particles producing hadronic sub-showers and the total number of particles.LHC data allow constraining the (anti)baryon and strangeness production at mid rapid-ity as well as the forward production of pi0 in fixed target experiments [22]. Taking intoA REVIEW ON GROUND-BASED MEASUREMENTS OF COSMIC RAYS 3account these data, EPOS-LHC and QGSJetII-04 provide now quite similar results [23].2. – The knee region up to the ankleThe ‘knee’ is a distinct feature of the all-particle CR energy spectrum at ∼ 4× 1015eV, where the power index suddenly changes from γ ∼ −2.7 to γ ∼ −3.1. Since itsdiscovery the origin of this feature is still under debate. From the experimental point ofview, measurements indicate that such a break is observed in the hadronic, muonic, andelectromagnetic components [24, 25, 26, 27], as well as in Cherenkov light [28]. Theseresults give a clear indication that the knee is a peculiarity of the primary spectrum,disfavouring a hypothesis based on changes of the interaction characteristics of the pri-maries with air nuclei. This conclusion has been reinforced by the first comparisons ofthe predictions from hadronic models and LHC data [29].Several experimental results associate the knee with the bending of the light compo-nent, and are compatible with a rigidity dependent cut-off [26, 25]. Unfortunately, theflux of the different components vary significantly depending on the interaction modelused to interpret the data [26]. However, if this interpretation is correct, the heavy com-ponent should show a similar bending in the energy range 5× 1016− 1017 eV. This is in-deed the experimental finding of KASCADE-Grande [30] confirmed by TUNKA-133 [31]and IceTop [32] experiments. In detail, the measured all-particle energy spectrum byKASCADE-Grande exhibits a less pronounced but still clear deviation from a singlepower law between the knee and the ankle, with a spectral hardening at ∼ 2 × 1016 eVand a steepening at ∼ 1017 eV [33]. The average mass composition gets heavier afterthe knee till ∼ 1017 eV, where a bending of the heavy component is observed [34] . Anindication of a hardening of the light component just above 1017 eV has been measuredas well [35]. The flux of heavy and light components depends on the hadronic interactionmodel used to interpret the experimental data [36] as previously mentioned. The aboveresults are summarised on the left side of Fig. 1.The right side of Fig. 1 compares the results of several experiments in terms of <lnA >, as it is often reported in literature to describe the evolution of the composition asa function of energy. Only QGSJet-II model is considered. A more detailed descriptionincluding the role of interaction models in composition is reported in [37]. Use of differentmodels introduces a shift in the average mass comparable to the dispersion of the datain Fig. 1. Despite the large uncertainty in the absolute composition, a common generaltrend is visible. Composition gets heavier through the knee region and becomes lighterapproaching the ankle. The solid line in the plot is used as a guidance line to showhow the average < lnA > from the data in the plot evolves with energy. This resultis compatible with the concept that the galactic component of the cosmic ray radiationreaches an end following a rigidity cut-off at ∼ 1017 eV and that the ankle indicates atransition to a population dominated by extra-galactic sources.The search for anisotropies in the arrival direction of CR around the knee regioncan provide relevant information to distinguish source and propagation effects. Theanisotropy varies with energy but the topological structure remains the same till ∼ 1014eV where it has an abrupt change as pointed out by IceCube [38]. Such a change isconfirmed till knee energies. This result seems to be inconsistent with the amplitude andphase expected according to the Compton-Getting prediction due to the relative motionof the Earth in the Galaxy. KASCADE-Grande published recently [39] an update on theanisotropy study based on the East-West method [40]. By investigating the variationof the amplitude as a function of energy it was found that the amplitudes were not4 M. BERTAINAE/eV1610 1710 1810 1910 2010)1.5 eV-1 s-1 sr-2 (m2.5dI/dE x E13101410151016101710KASCADE-Grande heavy QGSJet-II (2011)KASCADE-Grande light QGSJet-II (2013)AUGER (2013)HiResII (2008)KASCADE-Grande all QGSJet-II (2012)TUNKA-133 (2013)IceTop (2013)TA (2013)TIBET-III (2008)KASCADE all QGSJet-II (2013)KASCADE H-Si QGSJet-II (2013)KASCADE Fe QGSJet_II (2013)Fig. 1. – Left: The measurement of the CR spectrum by EAS experiments from the knee till theend of the spectrum. The main knee is explained as the bending of the light, followed by themedium, component. The fainter knee around 1017 eV is attributed to the bending of the heavyone. An ankle-like feature is observed in the light component just above 1017 eV that might berelated to the lightening of the composition approaching the ankle. Right: Average logarithmicmass of CRs as a function of energy derived from Xmax and particle detector measurementsusing QGSJet-II interaction model. Most of the data are taken from [37] and references therein.The solid line is drawn as a guidance and is obtained by averaging, in each energy bin, the valuesof < lnA > reported in the figure. The plots are taken from [3].significant, however, the phases were in almost all energy bins centred around 250 ±25 degrees. This is interesting in itself because it points towards the Galactic Center.Moreover, it agrees inside the statistical uncertainties with the results of the anisotropystudies of the Pierre Auger Observatory [41] in the energy range 3×1017 – 1018 eV.3. – From the ankle to the end of the spectrumFig. 1 shows that the features at the Ultra High Energies (UHE), the ankle and thecutoff, have been established beyond doubt. The spectral slopes before and after theankle have been measured and agree between Pierre Auger Observatory (PAO) [42] andTelescope Array (TA) [43]. The positions of the ankle also agree within the quotederrors, and are compatible with the existing model(s). The parameters of the breakat the highest energies are known less accurately. There seems to be some discrepancyconcerning the shape of the spectrum around the break; however more statistics is neededfor a firm conclusion. The position of the break is compatible with the GZK cutoff forprotons in TA, but the PAO spectrum fits better the case of a limit in the UHECRsacceleration by the sources.As far as the mass composition is concerned, the situation is less definite, and aconsistent picture has not yet emerged [37]. While PAO sees a change in the compositiontowards a heavier one at the highest energies, the TA observes no such a trend and iscompatible with a pure proton composition. This difference in the data has profoundconsequences: the Auger data suggest that we see the maximum energy of sources,similarly to what is observed at the knee in the cosmic ray spectrum, while the TA datasuggest we observe the GZK effect. Moreover, the different compositions in the GZK-and maximum-energy scenario will affect the level of anisotropies expected to be seen inA REVIEW ON GROUND-BASED MEASUREMENTS OF COSMIC RAYS 5the data.Despite the major advances in the understanding of UHECRs nature, the currentexperiments face the limit in the statistics they can accumulate due to the extremelylow flux of 1 particle km−2 year−1 above 5× 1019 eV. For this reason future space-basedobservatories, such as JEM-EUSO, that reach much higher and uniform exposures [44],could help in sheding light on the mystery of the most energetic radiation of the Universe.REFERENCES[1] Greisen K., Phys. Lett., 16 (1966) 148.[2] Zatsepin G.T. and Kuz’min V.A., JETP, 4 (1966) 78.[3] Bertaina M., Comptes Rendus Phys., 15/4 (2014) 300.[4] Kampert K.-H. and Tinyakov P., Comptes Rendus Phys., 15/4 (2014) 318.[5] Blasi P., Astron. Astrophys. Rev., 21 (2013) 1174.[6] Giuliani A. and al. (AGILE Coll.), Astrophys. J., 742 (2011) L30.[7] Ackermann M. and al. (FERMI-LAT Coll.), Science, 339 (2013) 807.[8] Hillas M., J. Phys. G, Nucl. Part. Phys., 31 (2005) R95.[9] Gaisser T, Stanev T. and Tilav S., arXiv: 1303.3365v1 (2013).[10] Peters B., Il Nuovo Cimento, 22 (1961) 800.[11] Berezinsky V., Gazizov A. and Grigorieva S., Phys. Rev. D, 74 (2006) 043005.[12] Bertaina M. and al., J. Phys. Soc. Jpn. Suppl. A, 78 (2009) 2010.[13] Aglietta M. and al. (EAS-TOP Coll.), Astrop. Phys., 19 (2003) 329.[14] Aglietta M. and al. (EAS-TOP and MACRO Coll.), Astrop. Phys., 21 (2004) 223.[15] Bartoli B. and al. (ARGO Coll.), Phys. Rev. D, 85 (2012) 092005.[16] Yoon Y.S. and al. (CREAM Coll.), Astrophys. 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A review on ground-based measurements of cosmic rays

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