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EUSO Extreme Universe Space Observatory Light of the Night Sky in EUSO – Duty Cycle and Background- Presented by D. Lebrun E-mail : didier.lebrun@lpsc.in2p3.fr http://www.euso-mission.org Artificial light Transient Luminous Phenomena in atmosphere Lightnings Sprites, elves and Blue Jets Nadir observation : Backward diffusion of the night sky light by Atmosphere and Ground (or clouds) Direct view of Airglow layer at 100 km 1) Sea reflectance : 0.08 2 ) Cloud top height 3km, reflectance 0.4 3 ) M =22/arcsec2 4 ) Balloon flight at 40km The brightness of the night sky from above Atmosphere is strongly dependent on atmospheric conditions within the FOV. The diffused and reflected light by atmosphere will vary with the presence of Aerosols in the lower layer of atmosphere, and mainly with the presence of clouds. EUSO Duty Cycle: Moon and Sun light effects The major limitation on the EUSO duty cycle comes from the fraction of the time the field of view will be exposed to the sun light or to the moon light. A further limitation could come from the fact that the telescope shutter might have to be closed also when the ISS itself is exposed to either light source. We compute the year averaged duty cycle due to the light-shadow effect under different conditions, taking into account the exact position of the ISS, the sun and the moon minute per minute. This fine time step prediction allows also computing the distribution of the shutter opening times which can be as small as 1 minute. The duty cycle for different background conditions. Conclusions In conclusion, we have shown that the average duty cycle for EUSO due to the sun and moonlight is 12.86% if both the sun and the moon are requested to be safely below the horizon from the ISS. Accounting for the variation of the moonlight flux with the moon phase angle, and computing the moonlight fraction reflected by the earth atmosphere, one can estimate the additional background induced by moonlight. Accepting even a quasi negligible additional background allows to improve the duty cycle to more than 18 %. With <100 photons/m2/ns/sr, the duty cycle reaches 27%. The other positive effect of such a decision is that the active time consists of much fewer short periods of exposure and much less time lost in manoeuvring the shutter. Night duration at 400 km Distribution of the exposure durations in minutes for each day in one year if the sun only is taken into account. The periods when the exposures are dropping to zero correspond to the days when the orbit axis is almost aligned with the sun direction. The same distribution but this time taking only the moon into account by requesting it to out of sight. Again, there are periods when the orbit is facing the moon and when data taking wont be possible. Lunar absolute irradiance at 555 nm as a function of the moon phase from the ROLO photometric measurements [4]. The strong phase dependence, the opposition effect, the east-west asymmetry are clear seen. The simple parametric phase curve from [3] is overlaid in red for comparison. Moon Phases At each observation location, its brightness depends on the solar radiation exposure, the molecular oxygen density at this altitude and the relaxation time of atomic collisions. One expects year variations (solar cycle), latitude variations with respect to ecliptic (seasonal), longitude or time variations (local time). C.Berat,1 D.Lebrun,1 F.Montanet,1 and J.Adams2 on behalf of EUSO Collaboration 1 LPSC Grenoble 2 MSFC & NSSTC,NASA Huntsville Lightnings can mimic cosmic ray EAS But they occur below clouds. Observed from above, they will appear highly diffused hitting around 10 pixels; their mean duration time is 600 mseconds longer than an EAS Frequency : 40 Hz World wide Mosltof them occur over land in equatorial regions;moreover 60% occur between 0 and 8pm. The mean rate of lightnings above ocean at night time will be around 10 Hz worldwide. Converted to Euso detection system this will correspond to 15 hit/pixel for the entire 3 years mission Night Sky brightness from Ch.Leinert et al. In Astron.Astrophys.Suppl.Ser.127(1998) Euso is concerned by the 0.3-0.4 m band. Night sky brightness at Nadir from orbit Luminous phenomena occuring in mesosphere at an Altitude between 10 and 100 km. They occur mainly above thunderstorm clouds associated to cloud to ground posivite lightnings. Their freauency is estimated to Less than 3% of lightings. Their brightness can reach 50kJoules in optical channel governed by Nitrogen Fluorescence spectrum (blue for 2P transition and red for 1P) They show various shapes in space and time, and they cannot be confused with EAS. They can be considered as Background fake events as far as the main scientific objective is concerned. However they can be an object of study within the EUSO scientific program. Some models predict that they are triggered by cosmic rays whose EAS act as a seed for their formation.Looking for such correlation with cosmic rays is in under study in EUSO collaboration The survey of regions of important human activity in clear sky conditions, the level of Background light in the EUSO Field of View will raise due mainly to city lights (and to a less extend to gas flares or fishing fleet lamps). The scale of the map below is expressed in natural background unit in V band. Green correspond to an increase of background by a factor 0.33 to 1 unit (yellow to a factor 1 to 3 and so on)*. T he total covered area is rather small and will not affect the effective Duty cycle by a significant amount. City downtowns with high brightness (>B*10) will appear as spots on few pixels and moving at the ISS velocity i.e. 7km/sec *Assuming the same variation in U band, this will depend on the nature of the lamps used in city lightnings.

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