- Email: [email protected]
Atmospheric monitoring and its impact on air shower observables at the Pierre Auger Observatory
Nuclear Instruments and Methods in Physics Research A 630 (2011) 87–90
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Atmospheric monitoring and its impact on air shower observables at the Pierre Auger Observatory Aurelio S. Tonachini Universita degli Studi di Torino and INFN Sezione di Torino, 10125 Torino, Italy
For the Pierre Auger Collaboration
1
a r t i c l e in fo
abstract
Available online 15 June 2010
The Pierre Auger Observatory, completed in September 2008, detects cosmic rays by observing and studying the development of extensive air showers in atmosphere. The atmosphere itself, with its unpredictable changes of conditions, directly influences the estimation of the air shower characteristics. In this paper, all the instruments and techniques adopted by the Auger Observatory are explained, and the impact of their results on the most important shower parameters are shown. & 2010 Elsevier B.V. All rights reserved.
Keywords: Lidar Atmospheric monitoring Cosmic rays Cloud detection Optical depth
1. Introduction The Pierre Auger Observatory, sited in the province of Mendoza, Argentina, covers a detection area as large as 3000 km2 with an array of 1600 ground-based Cherenkov water tanks (SD), surrounded by 24 fluorescence detectors (FD) located in four sites. The huge atmospheric volume, where the detected showers develop, is far from being constant in properties. For instance, variations of pressure, temperature, and even humidity can influence the production of fluorescence and Cherenkov light induced by charged particles of an extensive air shower (EAS). Another important issue is the atmospheric opacity to fluorescence light: while the light is propagating from the cascade to a fluorescence detector, it is scattered and/or absorbed by molecules and particulate matter composing the atmosphere. The presence of clouds may also compromise the observations by dimming or deflecting the light, and thus by deforming or obscuring part of the development of a shower as seen by an FD. FD telescopes perform nearly calorimetric measurements of EAS energies, thus providing a reference for the indirect energy measurements done by the SD. The observation with the fluorescence detectors of the atmospheric slant depth at which showers reach their maximum development (Xmax) gives, on average, an estimation of the chemical composition of cosmic rays (CR). With the combination of simultaneous detections by FD and SD (known as hybrid detection) a very high precision in the arrival direction calculation is gained. Taking into account all
atmospheric effects is thus of main importance for guaranteeing the quality of FD measurements. In order to characterize the atmosphere and monitor its changes during the data acquisition, the Pierre Auger Observatory deployed a large amount of monitoring devices shown in Fig. 1. Atmospheric variables, such as temperature, pressure, humidity, and wind speed, are constantly monitored at ground by five weather stations installed near the Central Laser Facility (CLF) and at each FD site. Regular measurements with radiosondes are performed from the Balloon Launching Station (BLS) to study the atmospheric state variables as a function of altitude, and thus give a local characterization of the Auger site [1]. Aerosol characteristics, which typically show strong temporal variations, are measured by several units, such as two laser facilities (CLF and XLF), four elastic lidars [2], a Raman lidar, two optical telescopes (FRAM and HAM), and aerosol phase function (APF) monitors [4]. The presence of clouds is detected by both lidars and infrared cloud cameras [3]. Atmospheric characteristics retrieved by this instrumentation are stored in a database, which is in turn used for all reconstructions of cosmic ray events. In the following sections the influence of atmospheric data on shower light production and UV light transmission are discussed. The last section is dedicated to a presentation of the systematic uncertainties that affect shower reconstructions at different energies.
2. Fluorescence and Cherenkov light production E-mail address: [email protected] ¨ Pierre Auger Observatory, av. San Martin Norte 304, (5613) Malargue, Argentina. 1
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.06.034
The variations of air temperature, pressure, and humidity with altitude are measured by using balloon-borne radio soundings.
88
A.S. Tonachini / Nuclear Instruments and Methods in Physics Research A 630 (2011) 87–90
Fig. 1. Atmospheric monitoring devices of the Pierre Auger Observatory include a balloon launching station (BLS), two laser steerable facilities (CLF and XLF), four infrared cameras (IR Cam), four elastic lidars, one Raman Lidar, two aerosol phase function monitors (APF), two optical telescopes (HAM and FRAM), and five weather stations (WS).
Balloons are launched approximately every five days. The data collected are used to define average monthly profiles for the Auger area. The use of these profiles instead of more generic atmospheric models (such as the 1976 US Standard Atmosphere) significantly pulls down the systematic uncertainties on the energy and Xmax estimations. A prominent contribution to systematic uncertainties comes from the dependence of fluorescence and Cherenkov light yield on temperature T, pressure p, and vapor pressure e. In particular, the main source of systematic shifts is the weather dependence of the fluorescence light production: the radiative transitions of N2 molecules, excited by charged particles of a shower, is quenched by nearby N2 and O2 molecules and even by water vapor. The effects due to the variability of the collisional cross-sections sNN ðTÞ, sNO ðTÞ, and the water vapor cross section sðeÞ can be evaluated by using the fluorescence model published by Keilhauer et al. [5]. A comparison with the previous model by AIRFLY [6], in which cross-sections were not T-dependent and water vapor effects were not considered, shows a systematic increase of the reconstructed energy of 5:5% and a decrease of Xmax of 2 g cm 2 [7]. Monthly models do not take into account daily variations of the atmospheric variables. These changes have a rather small effect on systematics. The size of this contribution has been evaluated by reconstructing simulated proton and iron showers with energies in the range between 1017.7 and 1020 eV. These showers have been reconstructed with both monthly models and cloud-free radio soundings. The comparison of the two reconstructions shows that monthly models introduce, on average, systematic shifts in energy and Xmax as small as DE=E ¼ 0:5% and DXmax ¼ 2 g cm2 . On the other hand, shower by shower fluctuations are larger, and tend to increase with energy. The combination of the temperature-dependent quenching effects and the atmospheric variability is shown in Fig. 2: the spread of the uncertainties reflects the daily variability of the atmospheric parameters.
Fig. 2. Systematic uncertainties related to the ultraviolet light production. Systematic effects shift the reconstructed energies of about 5%. Systematic shifts on Xmax, caused by both uncertainties on quenching effects and daily variations of atmospheric state variables, almost compensate each other. These shifts do not show any energy dependence. On the contrary, the error bars of the points, which are related to shower by shower fluctuations, tend to increase with energy.
its source to a fluorescence detector. The transmittance of light is a function of the wavelength l and the distance of the source. In a horizontally homogeneous atmosphere, it is expressed by T ðh, l, fÞ ¼ exp½tðh, lÞ=sinf ð1 þH:O:Þ
ð1Þ
where tðh, lÞ is the so-called optical depth, f is the elevation angle of the arrival direction of the light with respect to the FD, and H.O. represents higher-order single and multiple scattering effects. The optical depth can be written as the sum of the molecular optical depth tmol and the aerosol optical depth taer . This allows the two terms to be treated separately. 3.1. Molecular attenuation Molecular extinction of near-UV light is dominated by elastic scattering, and can be described by the Rayleigh theory of scattering. Hence the vertical molecular optical depth can be calculated by the following equation: Z h tmol ðh, lÞ ¼ NðhuÞsR ðl,huÞ dhu ð2Þ hFD
where N is the number density of scatterers and sR is the Rayleigh cross-section. sR varies inversely with the fourth power of the wavelength. Monthly averages of atmospheric state variables permit to estimate the molecular optical depth. Daily variations of pressure and temperature thus affect molecular transmission uncertainties.
3. Light transmission
3.2. Aerosol attenuation
The ultraviolet light produced by the passage of a shower is absorbed and scattered by the atmosphere while traveling from
Since aerosols have a size which is comparable with the wavelengths of interest, they are not treatable with the Rayleigh
A.S. Tonachini / Nuclear Instruments and Methods in Physics Research A 630 (2011) 87–90
theory. Aerosol scattering depends on the particulate composition, size, and shape. The dependence on the wavelength varies with the aerosol type. Furthermore, the atmospheric aerosol content shows sharp variations due to wind and weather conditions in general. During data acquisition, laser shots generated by CLF and XLF are recorded by fluorescence detectors; at the same time four elastic lidars perform independent measurements of aerosols in proximity of each FD. Both instruments provide hourly vertical profiles of taer ðhÞ. For aerosols the dependence on wavelength is parametrized as
taer ðlÞ=taer ðl0 Þ ¼ ðl0 =lÞg
ð3Þ
where taer ðh, l0 Þ is the aerosol optical depth at a reference wavelength l0 , and the shift to different wavelengths is governed ˚ ¨ by the Angstr om exponent g [8]. Measurements of g have been performed using HAM and FRAM devices [9]. When aerosol measurement uncertainties are propagated into the reconstruction of hybrid events recorded since 2004, the 3:6 systematic shifts in energy increase from DE=E ¼ þ 3:0 % at 20 7:9 E¼1017.7 eV to DE=E ¼ þ % at E¼10 eV. The same happens 7:0 to Xmax, where systematic uncertainties change from þ 3:3 2 7:3 2 DXmax ¼ 1:3 g cm to DXmax ¼ þ . Uncertainties in the 4:8 g cm aerosol measurements are dominated by the aerosol optical depth ˚ ¨ with minor contributions from the Angstr om exponent and the shape of the aerosol cross-sections. Atmospheric horizontal uniformity is studied by comparing shower reconstructions with aerosol profiles characterized by CLF measurements at different sites. Non-uniformities above the Auger Observatory affect the estimation of E and Xmax, such that in the range of energies RMSðDE=EÞ ¼ 3:627:4% and RMSðDXmax Þ ¼ 5:727:6 g cm2 . Aerosol systematic effects as a function of CR energy is shown in Fig. 3. The use of hourly measurements of the atmospheric opacity due to aerosols instead of using an average aerosol model decreases the systematic uncertainties by about a factor 2.
89
3.3. Molecular and aerosol multiple scattering Light scattered by molecules and aerosols may also be directed towards the FD, thus increasing the recorded signal. This phenomenon causes a systematic overestimation of the shower signal, especially near the surface layer, where aerosols concentrate. This reflects in an overestimation of the shower energy and Xmax. Different studies adopting Monte Carlo simulations parametrized multiple-scattered light as a function of the total optical depth tðhÞ [10,11]. Models show that, in the range of energies between 1017.7 and 1020 eV, taking into account multiple-scattering corrects overestimations of 2–5% in energy and 1–3 g cm 2 in Xmax.
3.4. Light attenuation by clouds Shower light may be strongly attenuated by clouds along the light path from the source to the FD. Clouds may deform the longitudinal profile recorded by an FD, or obscure part of the field of view, thus favoring the detection of deeper showers. The presence of clouds in the field of view of fluorescence detectors can also modify the effective FD aperture. Clouds are monitored by both lidars and IR cloud cameras. Lidars scan the sky outside the FD field of view measuring the cloud coverage and the height of cloud layers up to 12 km; cloud cameras take pictures of the FD field of view every 5 min thus distinguishing cloud-free zones from zones potentially obscured by clouds. The combination of the information coming from both instruments gives a good characterization of sky cloudiness for all the events used in Auger analyses. The data collected by lidars show that 50% of hours of FD acquisition were cloud-free. Sky was moderately clear (cloud coverage o 25%) in 60% measured hours. During 20% of the hours lidar detected a thick cloud coverage exceeding 80% of the sky. In Fig. 4 it is shown that beside the 60% of cloud-free hours, the remaining hours can be divided into groups depending on their altitude. Very low clouds may prevent measurement of reconstructable events: the number of events is thus reduced by quality cuts on the reconstructed profiles. During overcast conditions, optical depth measurements may fail: hybrid events without a valid optical depth profile are discarded from the main analyses. With the remaining events, the application of a cut of less than 25% cloud coverage reduces the observed flux by 4:6% 100 90 Percentage of hours [%]
80 70 60 50 40 30 20 10 0
Fig. 3. Systematic shifts on the reconstructed E and Xmax due to aerosol uncertainties as a function of energy. Error bars represent the effect of atmospheric horizontal non-uniformity.
CLEAN
h < 4 km
h < 8 km
h > 8 km
Fig. 4. Cloud coverage measured by lidars during FD data acquisition. More than 60% of the hours are cloud-free. When cloud coverage exceeds 25% of the sky, clouds can be divided into groups depending on their altitude. Optical depth measurements may fail when clouds are low.
90
A.S. Tonachini / Nuclear Instruments and Methods in Physics Research A 630 (2011) 87–90
Table 1 Summary of systematic uncertainties in the hybrid reconstruction catalogued by source. Source of systematics
log(E/eV)
DE=E ð%Þ
RMS(DE=E) (%)
DXmax ðg cm2 Þ
RMS(DXmax Þ ðg cm2 Þ
Light production (quenching effects and local models)
17.7–20.0
+ 5.5, 0.5
1.5–3.0
2.0 + 2.0
7.2–8.4
Aerosol optical depth taer ðh, lÞ
o 18:0 18.0–19.0 19.0–20.0
+ 3.6, 3.0 + 5.1, 4.4 + 7.9, 7.0
1.67 1.6 1.87 1.8 2.57 2.5
+ 3.3, 1.3 + 4.9, 2.8 + 7.3, 4.8
3.0 73.0 3.7 73.7 4.7 74.7
˚ ¨ exponent g Angstr om
17.7–20.0
0.5
2.0
0.5
2.0
Phase function
17.7–20.0
1.0
2.0
2.0
2.5
Molecular horizontal uniformity
17.7–20.0
1.0
1.0
1.0
2.0
Aerosol horizontal uniformity
o 18:0 18.0–19.0 19.0–20.0
0.3 0.4 0.2
3.6 5.4 7.4
0.1 0.1 0.4
5.7 7.0 7.6
Higher order effects
o 18:0 18.0–19.0 19.0–20.0
0.4 0.5 1.0
0.6 0.7 0.8
1.0 1.0 1.2
0.8 0.9 1.1
in the energies between 1017.7 and 1020 eV. Comparing the results with and without cloud cuts, a systematic shift of Xmax of 3 g cm 2 is found at all energies.
4. Summary The Pierre Auger Observatory is carrying out an extensive atmospheric monitoring program, which allows quantification of systematics on fluorescence detector measurements. Regular measurements of balloon launches have defined local monthly models for a better characterization of the ultraviolet light yield and the molecular scattering properties. The Observatory has designed many instruments for studying aerosols as well. Aerosol content and its properties can change very quickly even during the course of a night. For this reason aerosol monitoring devices provide information every hour during fluorescence detector data taking. The data collected are stored in a multi-gigabyte dedicated database accessed by all the analyses on hybrid events. Crosscorrelations between different instruments help understanding the horizontal homogeneity of the atmosphere and the position of
the clouds in the FD field of view. The most prominent sources of systematic shifts are related to the dependence of quenching cross-sections on the atmospheric state variables, and the horizontal uniformity of the atmospheric opacity over the whole site. All systematic uncertainties discussed in this paper are summarized in Table 1.
References [1] B. Keilhauer, for the Pierre Auger Collaboration, in: Proceedings of the 31st ICRC, Lodz, Poland, 2009. [2] S.Y. BenZvi, et al., Nucl. Instr. and Meth. A 574 (2007) 171. [3] B. Fick, et al., JINST 1 (2006) P11003. [4] S.Y. BenZvi, et al., Astropart. Phys. 28 (2007) 312. [5] B. Keilhauer, et al., Nucl. Instr. and Meth. A 597 (2008) 99. [6] M. Ave, et al., Astropart. Phys. 28 (2007) 41. [7] B. Keilhauer, M. Unger, in: Proceedings of the 31st ICRC, Lodz, Poland, 2009. ˚ ¨ [8] A. Angstr om, Geographical Anal. 12 (1929) 130. [9] S.Y. BenZvi, et al., in: Proceedings of the 30th ICRC, vol. 4, Me´rida, Me´xico, 2007, pp. 355–358. [10] M.D. Roberts, J. Phys. G 31 (2005) 1291. [11] J. Pe¸kala, et al., in: Proceeding of the 30th ICRC, vol. 4, Me´rida, Me´xico, 2007, pp. 515–518.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Atmospheric monitoring and its impact on air shower observables at the Pierre Auger Observatory Aurelio S. Tonachini Universita degli Studi di Torino and INFN Sezione di Torino, 10125 Torino, Italy
For the Pierre Auger Collaboration
1
a r t i c l e in fo
abstract
Available online 15 June 2010
The Pierre Auger Observatory, completed in September 2008, detects cosmic rays by observing and studying the development of extensive air showers in atmosphere. The atmosphere itself, with its unpredictable changes of conditions, directly influences the estimation of the air shower characteristics. In this paper, all the instruments and techniques adopted by the Auger Observatory are explained, and the impact of their results on the most important shower parameters are shown. & 2010 Elsevier B.V. All rights reserved.
Keywords: Lidar Atmospheric monitoring Cosmic rays Cloud detection Optical depth
1. Introduction The Pierre Auger Observatory, sited in the province of Mendoza, Argentina, covers a detection area as large as 3000 km2 with an array of 1600 ground-based Cherenkov water tanks (SD), surrounded by 24 fluorescence detectors (FD) located in four sites. The huge atmospheric volume, where the detected showers develop, is far from being constant in properties. For instance, variations of pressure, temperature, and even humidity can influence the production of fluorescence and Cherenkov light induced by charged particles of an extensive air shower (EAS). Another important issue is the atmospheric opacity to fluorescence light: while the light is propagating from the cascade to a fluorescence detector, it is scattered and/or absorbed by molecules and particulate matter composing the atmosphere. The presence of clouds may also compromise the observations by dimming or deflecting the light, and thus by deforming or obscuring part of the development of a shower as seen by an FD. FD telescopes perform nearly calorimetric measurements of EAS energies, thus providing a reference for the indirect energy measurements done by the SD. The observation with the fluorescence detectors of the atmospheric slant depth at which showers reach their maximum development (Xmax) gives, on average, an estimation of the chemical composition of cosmic rays (CR). With the combination of simultaneous detections by FD and SD (known as hybrid detection) a very high precision in the arrival direction calculation is gained. Taking into account all
atmospheric effects is thus of main importance for guaranteeing the quality of FD measurements. In order to characterize the atmosphere and monitor its changes during the data acquisition, the Pierre Auger Observatory deployed a large amount of monitoring devices shown in Fig. 1. Atmospheric variables, such as temperature, pressure, humidity, and wind speed, are constantly monitored at ground by five weather stations installed near the Central Laser Facility (CLF) and at each FD site. Regular measurements with radiosondes are performed from the Balloon Launching Station (BLS) to study the atmospheric state variables as a function of altitude, and thus give a local characterization of the Auger site [1]. Aerosol characteristics, which typically show strong temporal variations, are measured by several units, such as two laser facilities (CLF and XLF), four elastic lidars [2], a Raman lidar, two optical telescopes (FRAM and HAM), and aerosol phase function (APF) monitors [4]. The presence of clouds is detected by both lidars and infrared cloud cameras [3]. Atmospheric characteristics retrieved by this instrumentation are stored in a database, which is in turn used for all reconstructions of cosmic ray events. In the following sections the influence of atmospheric data on shower light production and UV light transmission are discussed. The last section is dedicated to a presentation of the systematic uncertainties that affect shower reconstructions at different energies.
2. Fluorescence and Cherenkov light production E-mail address: [email protected] ¨ Pierre Auger Observatory, av. San Martin Norte 304, (5613) Malargue, Argentina. 1
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.06.034
The variations of air temperature, pressure, and humidity with altitude are measured by using balloon-borne radio soundings.
88
A.S. Tonachini / Nuclear Instruments and Methods in Physics Research A 630 (2011) 87–90
Fig. 1. Atmospheric monitoring devices of the Pierre Auger Observatory include a balloon launching station (BLS), two laser steerable facilities (CLF and XLF), four infrared cameras (IR Cam), four elastic lidars, one Raman Lidar, two aerosol phase function monitors (APF), two optical telescopes (HAM and FRAM), and five weather stations (WS).
Balloons are launched approximately every five days. The data collected are used to define average monthly profiles for the Auger area. The use of these profiles instead of more generic atmospheric models (such as the 1976 US Standard Atmosphere) significantly pulls down the systematic uncertainties on the energy and Xmax estimations. A prominent contribution to systematic uncertainties comes from the dependence of fluorescence and Cherenkov light yield on temperature T, pressure p, and vapor pressure e. In particular, the main source of systematic shifts is the weather dependence of the fluorescence light production: the radiative transitions of N2 molecules, excited by charged particles of a shower, is quenched by nearby N2 and O2 molecules and even by water vapor. The effects due to the variability of the collisional cross-sections sNN ðTÞ, sNO ðTÞ, and the water vapor cross section sðeÞ can be evaluated by using the fluorescence model published by Keilhauer et al. [5]. A comparison with the previous model by AIRFLY [6], in which cross-sections were not T-dependent and water vapor effects were not considered, shows a systematic increase of the reconstructed energy of 5:5% and a decrease of Xmax of 2 g cm 2 [7]. Monthly models do not take into account daily variations of the atmospheric variables. These changes have a rather small effect on systematics. The size of this contribution has been evaluated by reconstructing simulated proton and iron showers with energies in the range between 1017.7 and 1020 eV. These showers have been reconstructed with both monthly models and cloud-free radio soundings. The comparison of the two reconstructions shows that monthly models introduce, on average, systematic shifts in energy and Xmax as small as DE=E ¼ 0:5% and DXmax ¼ 2 g cm2 . On the other hand, shower by shower fluctuations are larger, and tend to increase with energy. The combination of the temperature-dependent quenching effects and the atmospheric variability is shown in Fig. 2: the spread of the uncertainties reflects the daily variability of the atmospheric parameters.
Fig. 2. Systematic uncertainties related to the ultraviolet light production. Systematic effects shift the reconstructed energies of about 5%. Systematic shifts on Xmax, caused by both uncertainties on quenching effects and daily variations of atmospheric state variables, almost compensate each other. These shifts do not show any energy dependence. On the contrary, the error bars of the points, which are related to shower by shower fluctuations, tend to increase with energy.
its source to a fluorescence detector. The transmittance of light is a function of the wavelength l and the distance of the source. In a horizontally homogeneous atmosphere, it is expressed by T ðh, l, fÞ ¼ exp½tðh, lÞ=sinf ð1 þH:O:Þ
ð1Þ
where tðh, lÞ is the so-called optical depth, f is the elevation angle of the arrival direction of the light with respect to the FD, and H.O. represents higher-order single and multiple scattering effects. The optical depth can be written as the sum of the molecular optical depth tmol and the aerosol optical depth taer . This allows the two terms to be treated separately. 3.1. Molecular attenuation Molecular extinction of near-UV light is dominated by elastic scattering, and can be described by the Rayleigh theory of scattering. Hence the vertical molecular optical depth can be calculated by the following equation: Z h tmol ðh, lÞ ¼ NðhuÞsR ðl,huÞ dhu ð2Þ hFD
where N is the number density of scatterers and sR is the Rayleigh cross-section. sR varies inversely with the fourth power of the wavelength. Monthly averages of atmospheric state variables permit to estimate the molecular optical depth. Daily variations of pressure and temperature thus affect molecular transmission uncertainties.
3. Light transmission
3.2. Aerosol attenuation
The ultraviolet light produced by the passage of a shower is absorbed and scattered by the atmosphere while traveling from
Since aerosols have a size which is comparable with the wavelengths of interest, they are not treatable with the Rayleigh
A.S. Tonachini / Nuclear Instruments and Methods in Physics Research A 630 (2011) 87–90
theory. Aerosol scattering depends on the particulate composition, size, and shape. The dependence on the wavelength varies with the aerosol type. Furthermore, the atmospheric aerosol content shows sharp variations due to wind and weather conditions in general. During data acquisition, laser shots generated by CLF and XLF are recorded by fluorescence detectors; at the same time four elastic lidars perform independent measurements of aerosols in proximity of each FD. Both instruments provide hourly vertical profiles of taer ðhÞ. For aerosols the dependence on wavelength is parametrized as
taer ðlÞ=taer ðl0 Þ ¼ ðl0 =lÞg
ð3Þ
where taer ðh, l0 Þ is the aerosol optical depth at a reference wavelength l0 , and the shift to different wavelengths is governed ˚ ¨ by the Angstr om exponent g [8]. Measurements of g have been performed using HAM and FRAM devices [9]. When aerosol measurement uncertainties are propagated into the reconstruction of hybrid events recorded since 2004, the 3:6 systematic shifts in energy increase from DE=E ¼ þ 3:0 % at 20 7:9 E¼1017.7 eV to DE=E ¼ þ % at E¼10 eV. The same happens 7:0 to Xmax, where systematic uncertainties change from þ 3:3 2 7:3 2 DXmax ¼ 1:3 g cm to DXmax ¼ þ . Uncertainties in the 4:8 g cm aerosol measurements are dominated by the aerosol optical depth ˚ ¨ with minor contributions from the Angstr om exponent and the shape of the aerosol cross-sections. Atmospheric horizontal uniformity is studied by comparing shower reconstructions with aerosol profiles characterized by CLF measurements at different sites. Non-uniformities above the Auger Observatory affect the estimation of E and Xmax, such that in the range of energies RMSðDE=EÞ ¼ 3:627:4% and RMSðDXmax Þ ¼ 5:727:6 g cm2 . Aerosol systematic effects as a function of CR energy is shown in Fig. 3. The use of hourly measurements of the atmospheric opacity due to aerosols instead of using an average aerosol model decreases the systematic uncertainties by about a factor 2.
89
3.3. Molecular and aerosol multiple scattering Light scattered by molecules and aerosols may also be directed towards the FD, thus increasing the recorded signal. This phenomenon causes a systematic overestimation of the shower signal, especially near the surface layer, where aerosols concentrate. This reflects in an overestimation of the shower energy and Xmax. Different studies adopting Monte Carlo simulations parametrized multiple-scattered light as a function of the total optical depth tðhÞ [10,11]. Models show that, in the range of energies between 1017.7 and 1020 eV, taking into account multiple-scattering corrects overestimations of 2–5% in energy and 1–3 g cm 2 in Xmax.
3.4. Light attenuation by clouds Shower light may be strongly attenuated by clouds along the light path from the source to the FD. Clouds may deform the longitudinal profile recorded by an FD, or obscure part of the field of view, thus favoring the detection of deeper showers. The presence of clouds in the field of view of fluorescence detectors can also modify the effective FD aperture. Clouds are monitored by both lidars and IR cloud cameras. Lidars scan the sky outside the FD field of view measuring the cloud coverage and the height of cloud layers up to 12 km; cloud cameras take pictures of the FD field of view every 5 min thus distinguishing cloud-free zones from zones potentially obscured by clouds. The combination of the information coming from both instruments gives a good characterization of sky cloudiness for all the events used in Auger analyses. The data collected by lidars show that 50% of hours of FD acquisition were cloud-free. Sky was moderately clear (cloud coverage o 25%) in 60% measured hours. During 20% of the hours lidar detected a thick cloud coverage exceeding 80% of the sky. In Fig. 4 it is shown that beside the 60% of cloud-free hours, the remaining hours can be divided into groups depending on their altitude. Very low clouds may prevent measurement of reconstructable events: the number of events is thus reduced by quality cuts on the reconstructed profiles. During overcast conditions, optical depth measurements may fail: hybrid events without a valid optical depth profile are discarded from the main analyses. With the remaining events, the application of a cut of less than 25% cloud coverage reduces the observed flux by 4:6% 100 90 Percentage of hours [%]
80 70 60 50 40 30 20 10 0
Fig. 3. Systematic shifts on the reconstructed E and Xmax due to aerosol uncertainties as a function of energy. Error bars represent the effect of atmospheric horizontal non-uniformity.
CLEAN
h < 4 km
h < 8 km
h > 8 km
Fig. 4. Cloud coverage measured by lidars during FD data acquisition. More than 60% of the hours are cloud-free. When cloud coverage exceeds 25% of the sky, clouds can be divided into groups depending on their altitude. Optical depth measurements may fail when clouds are low.
90
A.S. Tonachini / Nuclear Instruments and Methods in Physics Research A 630 (2011) 87–90
Table 1 Summary of systematic uncertainties in the hybrid reconstruction catalogued by source. Source of systematics
log(E/eV)
DE=E ð%Þ
RMS(DE=E) (%)
DXmax ðg cm2 Þ
RMS(DXmax Þ ðg cm2 Þ
Light production (quenching effects and local models)
17.7–20.0
+ 5.5, 0.5
1.5–3.0
2.0 + 2.0
7.2–8.4
Aerosol optical depth taer ðh, lÞ
o 18:0 18.0–19.0 19.0–20.0
+ 3.6, 3.0 + 5.1, 4.4 + 7.9, 7.0
1.67 1.6 1.87 1.8 2.57 2.5
+ 3.3, 1.3 + 4.9, 2.8 + 7.3, 4.8
3.0 73.0 3.7 73.7 4.7 74.7
˚ ¨ exponent g Angstr om
17.7–20.0
0.5
2.0
0.5
2.0
Phase function
17.7–20.0
1.0
2.0
2.0
2.5
Molecular horizontal uniformity
17.7–20.0
1.0
1.0
1.0
2.0
Aerosol horizontal uniformity
o 18:0 18.0–19.0 19.0–20.0
0.3 0.4 0.2
3.6 5.4 7.4
0.1 0.1 0.4
5.7 7.0 7.6
Higher order effects
o 18:0 18.0–19.0 19.0–20.0
0.4 0.5 1.0
0.6 0.7 0.8
1.0 1.0 1.2
0.8 0.9 1.1
in the energies between 1017.7 and 1020 eV. Comparing the results with and without cloud cuts, a systematic shift of Xmax of 3 g cm 2 is found at all energies.
4. Summary The Pierre Auger Observatory is carrying out an extensive atmospheric monitoring program, which allows quantification of systematics on fluorescence detector measurements. Regular measurements of balloon launches have defined local monthly models for a better characterization of the ultraviolet light yield and the molecular scattering properties. The Observatory has designed many instruments for studying aerosols as well. Aerosol content and its properties can change very quickly even during the course of a night. For this reason aerosol monitoring devices provide information every hour during fluorescence detector data taking. The data collected are stored in a multi-gigabyte dedicated database accessed by all the analyses on hybrid events. Crosscorrelations between different instruments help understanding the horizontal homogeneity of the atmosphere and the position of
the clouds in the FD field of view. The most prominent sources of systematic shifts are related to the dependence of quenching cross-sections on the atmospheric state variables, and the horizontal uniformity of the atmospheric opacity over the whole site. All systematic uncertainties discussed in this paper are summarized in Table 1.
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