Recent Results from the Cosmic Ray Isotope Spectrometer on NASA's Advanced Composition Explorer

Abstract

The Advanced Composition Explorer (ACE), which carries instrumentation for making high-precision measurements of the charge and mass of energetic nuclei between ∼1 keV/nuc and ∼1 GeV/nuc, was launched in August 1997 into an orbit about the L1 Lagrangian point 1.5 million km sunward of the Earth. From this vantage point, ACE collects data that are used for a wide range of studies in galactic and heliospheric physics. The ACE spacecraft and instruments are, for the most part, continuing to perform very well and to extend their data sets. The long mission duration, which is already almost an order of magnitude greater than the two years that was nominally required of ACE, has made it possible to address science topics beyond those envisioned when the mission was proposed. In this paper we address several topics to which ACE’s Cosmic Ray Isotope Spectrometer (CRIS) is contributing in the area of galactic cosmic radiation. CRIS has been able to make measurements of extremely rare species, including the primary radioactive nuclide 60Fe and a number of stable “ultraheavy” elements with atomic numbers in the range Z=30 to 40. In addition, elemental energy spectra have now been measured over a significant fraction of two solar cycles, thereby enabling studies of the time dependence of the solar modulation of galactic cosmic rays.