1. Russian Aviation and Space Agency Institute for Space Research NASA 2001 Mars Odyssey page 1 Workshop HEND - 2002 Radiation environment on Odyssey and on Martian surface according to HEND data Workshop “The First year of HEND operations on the NASA Odyssey Mars Orbiter” Moscow, Russia May 20-22, 2002

2. Russian Aviation and Space Agency Institute for Space Research NASA 2001 Mars Odyssey page 2 Workshop HEND - 2002 A.Sanin, D.Anfimov, A.Kozyrev, M.Litvak and I.Mitrofanov (1 W.Boynton, D.Hamara and C.Shinohara (2 R.S.Saunders (3 1) Institute for Space Research, Moscow, Russia 2) University of Arizona, Tucson, USA 3) JPL, Pasadena, USA

3. Russian Aviation and Space Agency Institute for Space Research NASA 2001 Mars Odyssey page 3 Workshop HEND - 2002 HEND has three detectors of neutrons based on proportional counters 3 He with thick (LD), medium (MD) and thin (SD) moderators, one scintillation detector for fast neutrons IN/SC and detector for anti-coincidence OUT/SC

4. Russian Aviation and Space Agency Institute for Space Research NASA 2001 Mars Odyssey page 4 Workshop HEND - 2002 PERFORMANCE: LD: 10.0 eV - 1.0 MeV MD: 0.4 eV -100.0 keV SD: 0.4 eV - 1.0 keV IN/SC/N: 850 keV – 15.0 MeV IN/SC/G:60 keV– 2.0 MeV OUT/SC:30 keV – 1.0 MeV

5. Russian Aviation and Space Agency Institute for Space Research NASA 2001 Mars Odyssey page 5 Workshop HEND - 2002 Galactic Cosmic Rays and particles of Solar Particles Events freely propagates through thin Martian atmosphere and produce neutrons, gamma-rays and X-rays in the 2-3 meters subsurface layer During the Solar Particle Event radiation background on Mars could be significantly increased

6. Russian Aviation and Space Agency Institute for Space Research NASA 2001 Mars Odyssey page 6 Workshop HEND - 2002 HEND performed measurements during the Aerobraking Stage from November 19 till December 19, 2001 HEND HV was turned off for time interval (-20; +10 minutes) around Periapsises

7. Russian Aviation and Space Agency Institute for Space Research NASA 2001 Mars Odyssey page 7 Workshop HEND - 2002 In November 23-30 HEND performed measurements during the powerful Solar Particles Event detected by NOAA GOES-8 instruments

8. Russian Aviation and Space Agency Institute for Space Research NASA 2001 Mars Odyssey page 8 Workshop HEND - 2002 This November 23 Solar Particles Event was well recorded by HEND during orbits 47 - 60 of the Aerobraking Stage. Time profiles of counts from neutron sensors SC/IN/N, MD and LD are averaged for time bins of 200 seconds. These counts are dominated by enhancement of spacecraft background.

9. Russian Aviation and Space Agency Institute for Space Research NASA 2001 Mars Odyssey page 9 Workshop HEND - 2002 During the Solar Particles Event time profiles of neutrons around periapsises were contributed by decreases background of spacecraft This component (red curve) decreases around periapsises like (4  -  ) / 4  when solid angle of Mars  increases flux of Martian neutron albedo This component (blue curve) increases around periapsises like  / 4  when solid angle of Mars  increases Time, UTC

10. Russian Aviation and Space Agency Institute for Space Research NASA 2001 Mars Odyssey page 10 Workshop HEND - 2002 There were time intervals at the outcoming phase of Periapsis flight about 2 minutes long, when HEND HV were already ON, but spacecraft still kept the permanent Mars-based orientation We use data at these time intervals of Pariapsises to estimate the enhanced neutron albedo of Mars during the Solar Particle Event

11. Russian Aviation and Space Agency Institute for Space Research NASA 2001 Mars Odyssey page 11 Workshop HEND - 2002 Estimated counts rate of high energy neutrons from Mars has been significantly increased (by a factor of  100) during the Solar Particle Event of November 23, 2001.

12. Russian Aviation and Space Agency Institute for Space Research NASA 2001 Mars Odyssey page 12 Workshop HEND - 2002 While the Hardness Ratio of neutrons (SC/MD) from the spacecraft significantly has increased during the SPE (top profile), the hardness of Martian neutrons albedo remained constant before and during the SPE of November 23, 2001 (bottom profile)

13. Russian Aviation and Space Agency Institute for Space Research NASA 2001 Mars Odyssey page 13 Workshop HEND - 2002 Conclusions: The component of high energy neutrons could be significantly increased on the surface of Mars during the powerful Solar Particles Events Energy spectra of enhanced Martian neutrons albedo remains the same during SPE as it was for the periods of quite Sun