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Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari 1 for the Pierre Auger Collaboration 2 [Photo by S. Saffi, Univ. of Adelaide] 1 Kavli Institute for Cosmological Physics, University of Chicago, Chicago,

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SLIDE 1

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory

Max Malacari1 for the Pierre Auger Collaboration2

1Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL, USA 2Observatorio Pierre Auger, Av. San Martín Norte 304, 5613 Malargüe, Argentina

Email: auger_spokespersons@fnal.gov Full author list: http://www.auger.org/archive/authors_icrc_2017.html

[Photo by S. Saffi, Univ. of Adelaide] 35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea

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SLIDE 2

The Pierre Auger Observatory

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory 35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 2

Fluorescence detector (FD) Measurement of the longitudinal shower profile Ø Used to calibrate the Observatory’s energy scale. Ø Sensitive to the atmospheric aerosol transmission. 3,000 km2 array in western Argentina

[The Pierre Auger Collaboration, NIM A 798 (2015) 172-213]

Key observables:

  • Energy (integral → calorimetric energy)
  • Depth of maximum development, Xmax

Max Malacari

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SLIDE 3

Height [m a.g.l] 2000 4000 6000 8000 10000 VAOD 0.01 0.02 0.03 0.04 0.05 0.06 0.07

Aerosol attenuation measurements

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 3

Ø Quantity of interest is the Vertical Aerosol Optical Depth (VAOD). Ø Measured with an hourly time resolution via two complementary techniques. Ø Inferred from observations of the 355 nm vertically-fired laser of the Central and eXtreme Laser Facilities (CLF/XLF).

VAOD(h) = Z h αA(h0)dh0

[The Pierre Auger Collaboration, Astropart. Phys. 33 (2010) 108-129] Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

(3.5 km,355 nm) τ

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Entries

100 200 300 400 500 600 700

> = 0.040

a

τ Los Leones < > = 0.042

a

τ Los Morados < > = 0.038

a

τ Coihueco < > 90 %

a

T Quality cut for data Clear nights Dirty nights

Entries VAOD(3.5 km a.g.l, 355 nm)

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SLIDE 4

Height [m a.g.l.] 2000 4000 6000 8000 10000 12000 14000 Normalized Photons at Aperture 50 100 150 200 250 300

1 2

h FD L a s e r

~25 km

Aerosol attenuation measurements

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 4

Assumptions § Atmosphere horizontally uniform. § Laser light scattered towards the detector by molecules only (DN only). § No multiple scattering of laser light. Data Normalized (DN) analysis Laser Simulation (LS) analysis

~90% of aerosol profiles Measured light flux relative to that on a nominally aerosol free reference night

[The Pierre Auger Collaboration, JINST 8 (2013) P04009]

~10% of aerosol profiles

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

VAOD(h) = −1 1 + 1/ sin φ2 ln ✓Naer Nref ◆

1 3 2

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SLIDE 5

Height [km a.g.l] 2 4 6 8 10 12

AS

VAOD ∆ 0.002 0.004 0.006 0.008 0.01 0.012 VAOD = 0.02 VAOD = 0.04 VAOD = 0.06 VAOD = 0.1

Aerosol scattering correction

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 5

Ø Correction is iterative in nature. Ø Increases the VAOD low in the atmosphere. Ø Sensitive to the shape of the aerosol scattering phase function. Assumptions § Atmosphere horizontally uniform. § Laser light scattered towards the detector by molecules only (DN only). § No multiple scattering of laser light.

✓ e, SA = αA(h) · ✓ 1 σ dσ dΩ ◆

A

✓ 1 d ◆

Aerosol volume scattering coefficient Aerosol scattering phase function

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

∆VAOD(h) = 1 1 + 1/ sin φ2 ln(1 + SA/SM)

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SLIDE 6

azimuth [deg] 40 60 80 100 120 140 5 10 15 20 25 30

Scattering angle [deg] 20 40 60 80 100 120 140 160 180 Phase function [arb. units]

4

10

5

10

/ ndf

2

χ 5.433 / 18 A 142.6 ± 5893 B 141.7 ± 6283 g 0.01937 ± 0.52 f 0.084 ± 0.2496 / ndf

2

χ 5.433 / 18 A 142.6 ± 5893 B 141.7 ± 6283 g 0.01937 ± 0.52 f 0.084 ± 0.2496 / ndf

2

χ 5.433 / 18 A 142.6 ± 5893 B 141.7 ± 6283 g 0.01937 ± 0.52 f 0.084 ± 0.2496

Aerosol scattering phase function

✓ 1 σ dσ dΩ ◆

A

= 1 − g2

4π ✓ 1

(1 + g2 − 2g cos θ)3/2 + f 3 cos2 θ − 1

2(1 + g2)3/2 ◆

Modified Henyey-Greenstein phase function 𝒉 – asymmetry parameter 𝒈 – backscattering parameter Total PF (fit) Aerosol PF Molecular PF Ø Aerosol scattering phase function shape measured hourly. Ø APF monitor beam fired horizontally across the FD field-

  • f-view @ 350 nm.

Ø Angular distribution of measured signal fit to 4- parameter function.

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 6 [S.Y. BenZvi et al., Astropart. Phys. 28 (2007) 312-320] Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

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SLIDE 7

Aerosol scattering phase function

Entries 2151 Mean 0.5688 RMS 0.1013

Asymmetry parameter g 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Counts 50 100 150 200 250 300 350

Entries 2151 Mean 0.5688 RMS 0.1013

𝒉 = 𝟏. 𝟔𝟖 ± 𝟏. 𝟐𝟏

Month 2 4 6 8 10 12 Asymmetry parameter g 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Ø 𝑕 ~ 0.6, consistent with expectation for a dry desert atmosphere. Ø Seasonal variation in 𝑕 less than the RMS spread in a given month.

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 7

Aerosol scattering correction to the VAOD is sensitive to the asymmetry parameter. Analysis of ~2,100 hours of APF monitor data taken between 2011 and 2015.

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

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SLIDE 8

Multiple scattering correction

Height [km a.g.l] 2 4 6 8 10 12 VAOD ∆ 0.001 0.002 0.003 0.004 0.005 0.006 VAOD = 0.02 VAOD = 0.04 VAOD = 0.08 VAOD = 0.1

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 8

𝒈 - the fraction of the detected light originating from height ℎ attributed to MS

Ø Sensitive to the shape of the aerosol scattering phase function. Ø Correction is iterative in nature. Ø Increases the VAOD low in the atmosphere. Assumptions § Atmosphere horizontally uniform. § Laser light scattered towards the detector by molecules only (DN only). § No multiple scattering of laser light.

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

∆VAOD(h) = −1 1 + 1/ sin φ2 ln ✓1 − faer 1 − fref ◆

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SLIDE 9

x [km]

  • 20
  • 10

10 20 y [km]

  • 20
  • 10

10 20 Height [km a.g.l] 5 10 15 20 25 30

Azimuth [deg] 15 − 10 − 5 − 5 10 15 Elevation [deg] 5 10 15 20 25 30 Photons at aperture

2

10

3

10

4

10

Multiple scattering correction

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea

Ø Multiply scattered fraction 𝑔 parametrized from Monte Carlo raytracing simulations. Ø Simulations performed under various aerosol attenuation conditions. Beam SS MS 𝑔𝜂 ℎ = MS photons Total photons

9 [M. Malacari and B.R. Dawson, Astropart. Phys. 93 (2017) 38-45] Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

For observations of the CLF/XLF 𝜂 = 1.5°

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SLIDE 10

x [km]

  • 20
  • 10

10 20 y [km]

  • 20
  • 10

10 20 Height [km a.g.l] 5 10 15 20 25 30

Multiple scattering correction

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea

Ø Multiply scattered fraction 𝑔 parametrized from Monte Carlo raytracing simulations. Ø Simulations performed under various aerosol attenuation conditions. Beam SS MS 𝑔𝜂 ℎ = MS photons Total photons

10 [M. Malacari and B.R. Dawson, Astropart. Phys. 93 (2017) 38-45] Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

For observations of the CLF/XLF 𝜂 = 1.5°

𝜼

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SLIDE 11

Multiple scattering correction

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 11

fζ(α, τ) = k · αA · τ

Total optical depth (aerosol + molecular) Total attenuation coefficient (aerosol + molecular)

𝜼~𝟐. 𝟔° for CLF/XLF

  • bservations

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

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SLIDE 12

Height [m a.g.l] 2000 4000 6000 8000 10000 VAOD 0.01 0.02 0.03 0.04 0.05 0.06 0.07

Effect of the corrections

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 12

Original VAOD + aerosol scattering correction + multiple scattering correction Smoothed result ~50,000 hours of aerosol profiles 2004 – 2015. Example reconstructed VAOD profile

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

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SLIDE 13

[E/eV]

10

log 18 18.5 19 19.5 20 E/E [%] ∆ 0.5 1 1.5 2 2.5 3 3.5 4 4.5 [E/eV]

10

log 18 18.5 19 19.5 20 ]

2

[g/cm

max

X ∆ 1 2 3 4 5 6 7

FD reconstruction

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 13

+ 5 g/cm2 + 3% Increasing shower distance Small energy and Xmax increase relative to reconstruction using

  • ld aerosol database.

~25,000 high-quality events measured between 2004 and 2015. Aerosol systematic uncertainties (𝟒×𝟐𝟏𝟐𝟗 ÷ 𝟐𝟏𝟑𝟏 eV) Energy scale: 3% - 6% [ICRC 2013] Xmax: ±5 g/cm2 - >?

@A g/cm2 [Phys. Rev. D 90 (2014) 122005]

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

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SLIDE 14

max

Aerosol transmission to X 0.4 0.5 0.6 0.7 0.8 0.9 1

FD

/E

SD

E 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

no correction

0.036 ± slope = -0.006 After improvements

max

Aerosol transmission to X 0.4 0.5 0.6 0.7 0.8 0.9 1

FD

/E

SD

E 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 0.040 ± slope = -0.268 Before improvements

Internal consistency check

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 14

Ø Exploit the hybrid nature of the Observatory. Ø Valid description of the aerosol atmosphere → ESD/EFD no dep. on TA(Xmax). Internal consistency within reconstructed air shower data following improvements to the aerosol analysis

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

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SLIDE 15

Summary

Ø Hourly aerosol attenuation measurements made during FD data taking. Measured laser traces from the CLF/XLF are used to infer the aerosol loading above the Observatory using two independent techniques. Ø Aerosol analysis improved to remove two simplifying assumptions. Aerosol scattering out of the CLF/XLF beam and multiple scattering of the CLF/XLF beam. Ø Impact on reconstructed energy deposit profiles is within existing systematic uncertainties. 3% increase in energy and 5 g/cm2 increase in Xmax at an energy of 1019.5 eV. Ø Internal consistency check using air shower data indicates a sound understanding

  • f the vertical distribution of aerosols in the atmosphere above the array.

Ratio of the SD to FD energy is flat as a function of the aerosol transmission to shower maximum.

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 15 Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

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SLIDE 16

Backup

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 16 Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

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SLIDE 17

Treatment of laser traces

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 17

Ø 50 vertical shots every 15 minutes (200 per hour). Ø Laser energy monitored using pick-off probe. Ø 200 shots within an hour block averaged and normalized to a reference energy. Ø Clouds identified before VAOD analysis begins.

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari [The Pierre Auger Collaboration, JINST 8 (2013) P04009]

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SLIDE 18

Selection of reference nights

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 18

Ø One reference night identified each year. Ø Checked against simulations and APF monitor measurements.

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari Scattering angle [deg] 20 40 60 80 100 120 140 160 180 Phase function [arb. units]

4

10

5

10

/ ndf

2

χ 9.524 / 21 B 39.69 ± 7862 / ndf

2

χ 9.524 / 21 B 39.69 ± 7862 / ndf

2

χ 9.524 / 21 B 39.69 ± 7862

Molecular PF Real profile Simulated profile

Time bin [100 ns] Normalized photons at aperture

[The Pierre Auger Collaboration, JINST 8 (2013) P04009]

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SLIDE 19

Attenuation of the laser beam

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 19 Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

SA + SM TA1 · TM1 TA2 · TM2

Detector Laser φ1 φ2 θ h

TA = exp(−VAOD/ sin φ) Nref ∝ TM1 · SM · TM2

Naer ∝ TM1 · TA1 · (SA + SM) · TM2 · TA2

Sx = αx ✓ 1 σ dσ dΩ ◆

x

VAOD(h) = −1 1 + 1/ sin φ2 ln Naer Nref 1 1 + SA/SM

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SLIDE 20

Aerosol scattering correction

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 20 Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

[deg] θ 20 40 60 80 100 120 140 160 180 ]

  • 1

) [sr Ω /d σ )(d σ (1/

  • 4

10

  • 3

10

  • 2

10 f = 0.4, g = 0.7 f = 0.4, g = 0.6 f = 0.4, g = 0.5 Rayleigh [deg] θ 20 40 60 80 100 120 140 160 180 ]

  • 1

) [sr Ω /d σ )(d σ (1/

  • 3

10

  • 2

10 f = 0.3, g = 0.6 f = 0.4, g = 0.6 f = 0.5, g = 0.6 Rayleigh

Varying the asymmetry parameter (𝑕) Varying the backscattering parameter (𝑔) APF monitor measurement limits FD field of view limits for laser measurements

✓ 1 σ dσ dΩ ◆

A

= 1 − g2

4π ✓ 1

(1 + g2 − 2g cos θ)3/2 + f 3 cos2 θ − 1

2(1 + g2)3/2 ◆

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SLIDE 21

Aerosol scattering correction

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 21 Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

Height [km a.g.l] 2 4 6 8 10 12 VAOD

0.02 0.04 0.06 0.08 0.1

Reconstructed VAOD - corrected with g = 0.4 Reconstructed VAOD - corrected with g = 0.5 Reconstructed VAOD - corrected with g = 0.7 Reconstructed VAOD - corrected with g = 0.8 Reconstructed VAOD - no aersol scattering correction True VAOD (g = 0.6)

Height [km a.g.l] 2 4 6 8 10 12 VAOD ∆

0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02

Height [km a.g.l] 2 4 6 8 10 12 VAOD

0.02 0.04 0.06 0.08 0.1

Reconstructed VAOD - corrected with f = 0.2 Reconstructed VAOD - corrected with f = 0.3 Reconstructed VAOD - corrected with f = 0.5 Reconstructed VAOD - corrected with f = 0.6 Reconstructed VAOD - no aersol scattering correction True VAOD (f = 0.4)

Height [km a.g.l] 2 4 6 8 10 12 VAOD ∆

0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02

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SLIDE 22

Height [m a.g.l.] 2000 4000 6000 8000 10000 12000 14000

VAOD

0.05 0.1 0.15 0.2

Effect of the corrections - simulation

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 22

Original VAOD + aerosol scattering correction Smoothed result Input VAOD Example reconstructed simulated VAOD profile

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

Ø Aerosol scattering correction applied to the reconstruction of a simulated laser trace. Ø Input VAOD retrieved.

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SLIDE 23

FD reconstruction

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 23 Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

]

2

Atmospheric depth [g/cm 300 400 500 600 700 800 dE/dX [arb. units] 0.2 0.4 0.6 0.8 1 Before correction After correction Aerosol transmission ratio 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 1.1 Ø Aerosol transmission decreased close to the ground. Ø Energy and Xmax increased by elongated tail of shower profile. Toy model

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SLIDE 24

max

Aerosol transmission to X 0.4 0.5 0.6 0.7 0.8 0.9 1

FD

/E

SD

E 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

= 20.0 km)

Mie

= 0.25 km (L

Mie

) = 0.0125 h ∞ VAOD( ∆ (31)

0.035 ± slope = 0.068

Internal consistency check

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 24

Ø Additional aerosols artificially added close to the ground. Ø Positive slope indicates aerosol overcorrection. PRELIMINARY

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

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SLIDE 25

VAOD uncertainty

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 25

Ø Separated into systematic and statistical contributions.

Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

Correlated Uncorrelated Relative FD calibration 2% 4% Relative laser energy (CLF) 1 – 2.5% 2% Relative laser energy (XLF) 1% 2% Reference night 3% – Atmospheric fluctuations – 3%

[The Pierre Auger Collaboration, JINST 8 (2013) P04009]

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SLIDE 26

Data Normalized vs. Laser Simulation

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 26 Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari [The Pierre Auger Collaboration, JINST 8 (2013) P04009]

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SLIDE 27

Seasonal variation in the VAOD

35th International Cosmic Ray Conference, July 12 – 20 2017, Busan, South Korea 27 Improvements to aerosol attenuation measurements at the Pierre Auger Observatory Max Malacari

Ø Austral summer → burnt biomass originating from northern Argentina. Ø Winter → primary aerosol contaminants from Pacific Ocean. Ø Average concentrations driven by local sources: dust, pollution, etc.

[The Pierre Auger Collaboration, Atmos. Res. 149 (2014) 120-135]