We have observed cosmic-ray antiprotons at an atmospheric depth of 4 to 26 g/cm~2 with a balloon flight of the BESS Spectrometer,launched at Ft.Sumner,New Mexico,in 2001.The atmospheric antiprotons were directly detected for the first time,at balloon altitude under a geomagnetic cut-off condition.We report here the atmospheric antiproton flux in a kinetic energy region of 0.2-3.4 GeV.In a measurement of cosmic-ray antiproton flux at the top of the atmosphere,subtraction of atmospheric antiprotons is inevitable and a major source of systematic errors.Although several theoretical calculations predicted energy spectra of the atmospheric antiprotons at various altitudes [3,5,7],there have been no direct measurements to verify these predictions.We report here a measurement of the atmospheric antiproton flux by using the BESS spectrometer [1,2,4,6].
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