The systematic observation of the UV nocturnal atmospheric background has been the main objective of the balloon borne experiment BaBy (Background Bypass).
BaBy was a scientific support activity to the evaluation of the sensitivity of EUSO, the Extreme Universe Space Observatory devoted to the observation of the Extreme Energy Cosmic Rays from space, detecting the faint UV fluorescence light emitted by the atmosphere as a final result of the cosmic rays interacting with the Earth’s atmosphere. For EUSO, in fact, one of the fundamental information concerns the knowledge of the UV nocturnal atmospheric background level. Sources of UV nocturnal atmospheric background looking downward straight the Earth are: light pollution from cities, planes and ships, naturally occurring bioluminescence, lightning flashes, reflected moonlight and starlight, auroral flashes, low energy cosmic ray air showers and atmospheric chemical reactions.
Since 1998, nocturnal atmospheric UV background measurements have been performed by balloon borne experiment BaBy in the wavelength interval relevant for EUSO, and over ground and over sea background radiance profiles have been obtained. BaBy flew from the Milo-Trapani base of the ASI (Italian Space Agency) in four different balloon flight campaigns:
1 – During the summer ’98 balloon flight campaign at the Milo-Trapani base, a first version of the BaBy instrument on the 30 July has flown looking down from an altitude of about 26 km of height, over the territory of the west Sicily and the Mediterranean sea for a duration time of 5 hours, in a clear night and moonless condition. The UV background level measurements were made in the integrate wavelength band of 300-400 nm and didn’t take into account the absorption of the ozone layer because of the not very high altitude reached by the balloon. The brightness value measured over the sea was 450 photons m-2sr-1ns-1, comparable with previous rocket observations. The count rate profile and the trajectory during the first BaBy flight are shown here below.
2 – In 1999 a second version of BaBy, able to measure both in the integrate wavelength band of 300-400 nm and in the narrow bands centered around 337, 357 and 391 nm (where lie the lines of emission of the atmospheric Nitrogen molecules) had, unfortunately, during the climb a failure.
3 – On 23-24 July 2001, due to a failure on the balloon, the final version of BaBy flew from an altitude of 15 km up to 30 km over the land of the west Sicily, and only for a duration time of about 3 hours. This experiment was foreseen to look downward from about 40 km of altitude (above the ozone layer) the dark nocturnal atmosphere over the sea during a transmediterranean flight. Due to the balloon failure only few good data was collect. There are only few minutes where the balloon was over a dark land at altitude of 30 km in which brightness value was comparable with the previous one (1998). Preliminary data profile and the trajectory of the flight are reported in the figure here below.
4 – On July of 2002 BaBy had a new opportunity, offered by Italian Space Agency, to repeat the measures a board of a transmediterranean balloon flight. This flight was successful: on the night of 10-11 July 2002 BaBy has flown, in a clear night and moonless condition, from an altitude of 39 km over the Mediterranean sea, providing us with valuable good data both in the integrate wavelength band of 300-400 nm and in the three narrow bands at 337, 357 and 391 nm.
BaBy had been tested and operated, before each launch, at ground. The test had been performed both at mountain and at sea level. Observations from ground of the Vega star (used as source candle), which light spectrum is known, had been made to calibrate the detectors.
The last version of BaBy carried out measures by means of a couple of collimated fast photomultipliers (PMTs), one working in photon counting mode whereas the other in charge integration mode. The entire apparatus was composed by four couple of PMTs filtered with different filters. One couple was filtered with a BG1 optical filter and covered the whole wavelength band of interest (300-400 nm). The others were coupled with a narrow bandwidth interferential filters (10 nm) centered on the lines of emission of the atmospheric Nitrogen molecules (337, 357 and 391 nm). To reduce the amount of light level seen by the PMTs (that is, the photoelectrons rate) and in order to allow the PMT to work in a linear region and in safety conditions, each PMT was collimated in order to reduce the field of view of a known geometrical factor (for example in our case, 1/100 str). Temperature and pressure sensors were positioned inside the instrument and the electronic boxes to allow a constant monitoring of the health of the instrument. To reduce the use of telemetry, limited only to the transmission of the housekeeping data and the count rates sampled every one second, all scientific data collected were stored in a hard disk by means of a dedicated on board PC.
With different commitments and tasks, the following IASF-Palermo personnel was part of the BaBy group: Salvo Giarrusso, Gaetano Agnetta, Benedetto Biondo, Osvaldo Catalano, Filippo Celi, Giancarlo Cusumano, Giacomo D’Ali Staiti, Renato Di Raffaele, Giacomo Fazio, Gino Gugliotta, Giovanni La Rosa, Maria Concetta Maccarone, Angelo Mangano, Teresa Mineo, Francesco Russo, Bruno Sacco, Andrea Santangelo, Paolo Scarsi.