Scientific Requirements for the A.S. device

1. Scientific Requirements for the A.S. device Two questions will be (tentatively) addressed : - What parameters are to be measured and with what precision, - How many showers will need an AS correction. • Important: • The impact of the atmosphere on the detection of Cosmic showers is a complicated 3 dimensional problem that implies the treatment of multiple scattering processes…(D.Lebrun; fluorescence echo on dense clouds…) • This has always been considered beyond the scope of the Phase A studies. • A realistic estimation of effects and scientific requirement implies realistic (and statistically representative) 3 dim cloud scenes !! • Only a very simplified picture can be presented today. E.Plagnol - Munich Nov 2003

2. WarningAll numbers should be taken with careand independently confirmed E.Plagnol - Munich Nov 2003

3. In cases where clouds are present, the cloud altitude profile and some of its optical parameters (Optical depth, albedo, transmission coefficients) are necessary. A shape analysis of the shower signal is a potential signal of cloud presence but the matter is complicated by the low statistics of photo-electrons. The simplified picture It is assumed that, at the time of shower detection, a global view of the cloud scene will be obtained: IR camera (night view camera) ? This will allow to classify the shower in three categories: 1. No clouds are present in its neighbourhood, 2. Clouds are present and homogenous, 3. The cloud scene has a complicated topology (CSF). But… a high altitude sub-visible cloud can also be present ! E.Plagnol - Munich Nov 2003

4. The lidar signal (equation) is given by : A lidar has the potential to measure the transmission coefficients. This however depends on knowledge on the scatterers : the backscattering probability (bi(x)) or,the S factor: Energy measurement and the Laser (Lidar) method The measurement of the energy is proportional to the production of the fluorescence photons (photo-electrons) : Where x represents the geographical location of the fluo+cer production. E.Plagnol - Munich Nov 2003

5. Energy Resolution The present day "End to End" simulators allow to estimate the error on the energy resolution: Nature, angle, temperature, density, fluorescence, atmospheric transmission, optic throughput, quantum efficiency , … A total (Rayleigh+Mie) uncertainty of ±5% is a reasonable requirement E.Plagnol - Munich Nov 2003

6. EUSO receptor 0.100 Joules 1 shot In the case of high altitude Cirrus clouds, the Scientific Requirement could be that the total transmission be measured (below the cloud) with a precision of ∆T ≈ ± 0.05 The signal does not reveal the cloud The case of a clear atmosphere with a Cirrus cloud (I) altitude 10-12km, Optical depth (t) = 0.1 E.Plagnol - Munich Nov 2003

7. In the case of low altitude dense clouds, the Scientific Requirement could be that the transmission coefficient be measured (across the cloud) with a precision of ∆T ≈ ± 0.05 in steps of ≤ 250 meters The case of a clear atmosphere with a low altitude dense cloud (I) E.Plagnol - Munich Nov 2003

8. Another look at vertical resolution : D.Naumov The error in the energy can be directly related to the error in the density and, hence, to the error on the measurement of the fluorescence maximum (Hmax) A vertical resolution of 250m would introduce an error of ∆E ≈ 2.5% Sergio Bottai : The angular resolution of qshower does not need a precise knowledge of the shower altitude E.Plagnol - Munich Nov 2003

9. The pointing precision of the Lidar The IR camera will give an overview of the cloud scene The lidar will act as a microscope and inspect the local properties The lidar will probably not give a reliable information for complicated cloud topologies Showers too close to "holes" in the cloud topology may be "lost" It is therefore not essential that a precision pointing be performed : a global scan of the shower region would therefore preferred. A defocusing of the laser beam could even be considered. Pointing precision : from 0.1 to 0.5° ? E.Plagnol - Munich Nov 2003

10. Evaluation of the fraction of events to be corrected (I) Euso will be measuring cosmic showers above the clouds. The Lidar will measure the cloud altitude and confirm: i) the cloud scene as observed by the IR camera, ii) the interpretation of the shower signal. But mainly, the lidar will measure the Transmission Coefficients, when necessary, around and above the fluorescence maximum. E.Plagnol - Munich Nov 2003

11. Without clouds With clouds The shower maximum is above the cloud top Evaluation of the fraction of events to be corrected (II) The criterion for the necessity of an atmosphere correction can be base on the relative distance between the fluorescence maximum and the cloud height. The case of showers with q = 60° can be taken … E.Plagnol - Munich Nov 2003

12. Showers detected in the vicinity of clouds Showers who should not need AS corrections • 17% of showers have Fluomax < Cloudalt-2 km • 22% of showers have Fluomax > Cloudalt+2 km Showers who could need correction 24 % Shower below cloud Shower above cloud Showers detected in the CSF Total 33% Showers who could need correction 12 % Shower below cloud Evaluation of the fraction of events to be corrected (III)Showers with q = 60°+ISCCP clouds Grand total 36% -> ≈ 50% of good showers E.Plagnol - Munich Nov 2003

13. A.S. Scientific Requirements : Conclusions • The A.S. will measure the cloud height with a resolution of 250 meters • It should be able to detect clouds with Optical Depths > 0.1 (TMie ≈ 0.1) • It should measure the molecular transmission (TMie) coefficient with a precision better than ∆T=±0.05 and an altitude resolution better than 250m. • The Rayleigh transmission coefficient should be also known to within ∆T =±0.05 (molecular density profile, ∆P ?) • The impact of these resolutions should induce an imprecision on the shower energy measurement no greater than ±5% (distinguish role of Rayleigh and Mie). • The percentage of showers that would benefit from the A.S. correction is estimated at ≈ 50% + 20% ( ? sub-visible clouds) (value obtained for showers with q = 60°) • Cloud top altitude is always desirable (?) • The quality of A.S. correction is still difficult to evaluate because important effects due to multiple scattering (both for Lidar and shower signal) are still to be evaluated. • A proper estimate of this quality factor should be one of the aim of Phase B. • A realistic "End to End" simulation with cloud effects is not for February… a test case could be shown. E.Plagnol - Munich Nov 2003

14.  As the shower develops, fluorescence photons are emitted isotropically along the shower path. Their arrival on EUSO depends on the "forward transmission"  As the shower develops, the Cerenkov beams builds up and is ultimately reflected by the albedo of the earth. The arrival of these photons on EUSO depends also on the "forward transmission"  On their way down to ground, some photons of the Cerenkov beam are scattered and can eventually reach EUSO . This probability depends on the angle of scattering as well as on the forward transmission towards EUSO . UHECR Energy Measurement : The scattering of photons E.Plagnol - Munich Nov 2003

15. UHECR Energy Measurement E.Plagnol - Munich Nov 2003

16. The importance of Rayleigh & Mie Scattering The transmission is the part of the flux not scattered by Rayleigh or Mie • l >> Rs • Rayleigh Scattering is: • Not dependent on the nature of the molecular scatterer. • - ≈ isotropic • l << Rs • Mie Scattering is: • - Aerosol dependent • non isotropic • dependent on the nature of the scatterer The importance of each process will depend on the density of the scatterers E.Plagnol - Munich Nov 2003

17. Lidar Operations A laser pulse can be seen as a monochromatic (355,532, 1064 nm) Cerenkov pulse From G.Fiocco & R.Viola The intensity of the return signal will depend on the transmission and on the back scattering E.Plagnol - Munich Nov 2003

18. LIDAR analysis : The Lidar equation The "return" signal The forward transmission Rayleigh Scattering Assumptions: - Only one type of Aerosol - S1 does not depend on z T1(z) and r(z) follow s(n,q) does not follow ! E.Plagnol - Munich Nov 2003

19. A possible strategy for the Energy measurement with a Lidar A C B E.Plagnol - Munich Nov 2003

20. Other usages of the Lidar • The Lidar will also be used for: • determining cloud top altitude for dense clouds, • determining the cloud albedo for these clouds, • measuring (statistically) the EUSO duty cycle. • … Conclusion  If an energy resolution of 20% is the aim of EUSO, then: A Lidar is indispensable in many "cloudy" situations… but it will NOT save all the showers !  The knowledge of cloud top altitude and albedo may salvage many Cerenkov type showers…it will lower the energy threshold (Auger).  How can we live without it ? - a poorer ∆E … is 20% necessary for the physics, - Cloud top and duty cycle by the Cerenkov peak (Y.T.) ? E.Plagnol - Munich Nov 2003