Voyager 1 Observations of Galactic Cosmic Rays in the Local Interstellar Medium

Tuesday, 16 December 2014: 11:05 AM
Alan C Cummings1, Edward C Stone1, Bryant C Heikkila2, Nand Lal3 and William R Webber4, (1)California Institute of Technology, Pasadena, CA, United States, (2)NASA Goddard Space Flight Center, Greenbelt, MD, United States, (3)NASA Goddard Space Flight Center, Code 670, Greenbelt, MD, United States, (4)New Mexico State University Main Campus, Las Cruces, NM, United States
The twin Voyager spacecraft were launched in 1977 and continue to be on a remarkable journey of exploration. Both spacecraft have crossed the termination shock of the solar wind and Voyager 1 (V1) crossed into interstellar space in ~mid-2012. At that crossing of the heliopause, the particles of heliospheric origin that had dominated the energy spectrum of most cosmic ray nuclei below approximately 50 MeV/nucleon disappeared, revealing for the first time the energy spectra of galactic cosmic rays (GCRs) down to about 3 MeV/nucleon. The intensity of GCRs has not shown any significant long-term gradient since the crossing, suggesting that V1 is observing the energy spectra of GCRs in the local interstellar medium unaffected by solar modulation. The energy spectra of H, He, C, and O have rather broad peaks in the ~20-100 MeV/nucleon energy range. The H/He ratio in this energy range is ~12 and that of C/O is ~1. We are also observing the local interstellar electron spectrum and find that a power-law energy dependence with spectral index approximately -1.5 from ~5-70 MeV is consistent with the data. We will report on the latest observations at the meeting. This work was supported by NASA under contract NNN12AA012.