Helium in near Earth orbit

Abstract The helium spectrum from 0.1 to 100 GeV/nucleon was measured by the Alpha Magnetic Spectrometer (AMS) during space shuttle flight STS-91 at altitudes near 380 km. Above the geomagnetic cutoff the spectrum is parameterized by a power law. Below the geomagnetic cutoff a second helium spectrum was observed. In the second helium spectra over the energy range 0.1 to 1.2 GeV/nucleon the flux was measured to be (6.3±0.9)×10 −3 ( m 2 sec sr ) −1 and more than ninety percent of the helium was determined to be 3 He (at the 90% CL). Tracing helium from the second spectrum shows that about half of the 3 He travel for an extended period of time in the geomagnetic field and that they originate from restricted geographic regions similar to protons and positrons.