Mars odyssey measurements of galactic cosmic rays and solar particles in Mars orbit, 2002-2008

Abstract

The instrument payload aboard the 2001 Mars Odyssey orbiter includes several instruments that are sensitive to energetic charged particles from the galactic cosmic rays (GCR) and solar particle events (SPE). The Martian Radiation Environment Experiment (MARIE) was a dedicated energetic charged particle spectrometer, but it ceased functioning during the large solar storm of October/November 2003. Data from two other Odyssey instruments are used here: the Gamma Ray Spectrometer and the scintillator component of the High Energy Neutron Detector. Though not primarily designed to measure energetic charged particles, both systems are sensitive to them, and several years of data are available from both. Using the MARIE data for calibration of the other systems, count rates can be normalized (with significant uncertainties) to absolute fluxes of both GCR and solar energetic particles (SEP). The data, which cover the time span from early 2002 through the end of 2007, clearly show the solar cycle-dependent modulation of the GCR starting in 2004. Many SPEs were recorded as well and are cataloged here. Threshold energies were relatively high, ranging from 16 MeV in the most sensitive channel to 42 MeV. These thresholds are not optimal for detailed studies of SEPs, but this is the range of interest for calculations of dose and dose equivalent, pertinent to human flight, and covering that range was the original motivation for MARIE. The data are available on request and are potentially of use for the Earth-Moon-Mars Radiation Environment Module collaboration and other heliospheric modeling projects. Copyright 2010 by the American Geophysical Union.