Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published November 1, 2010 | Published
Journal Article Open

Record-setting Cosmic-ray Intensities in 2009 and 2010


We report measurements of record-setting intensities of cosmic-ray nuclei from C to Fe, made with the Cosmic Ray Isotope Spectrometer carried on the Advanced Composition Explorer in orbit about the inner Sun-Earth Lagrangian point. In the energy interval from ~70 to ~450 MeV nucleon^(–1), near the peak in the near-Earth cosmic-ray spectrum, the measured intensities of major species from C to Fe were each 20%-26% greater in late 2009 than in the 1997-1998 minimum and previous solar minima of the space age (1957-1997). The elevated intensities reported here and also at neutron monitor energies were undoubtedly due to several unusual aspects of the solar cycle 23/24 minimum, including record-low interplanetary magnetic field (IMF) intensities, an extended period of reduced IMF turbulence, reduced solar-wind dynamic pressure, and extremely low solar activity during an extended solar minimum. The estimated parallel diffusion coefficient for cosmic-ray transport based on measured solar-wind properties was 44% greater in 2009 than in the 1997-1998 solar-minimum period. In addition, the weaker IMF should result in higher cosmic-ray drift velocities. Cosmic-ray intensity variations at 1 AU are found to lag IMF variations by 2-3 solar rotations, indicating that significant solar modulation occurs inside ~20 AU, consistent with earlier galactic cosmic-ray radial-gradient measurements. In 2010, the intensities suddenly decreased to 1997 levels following increases in solar activity and in the inclination of the heliospheric current sheet. We describe the conditions that gave cosmic rays greater access to the inner solar system and discuss some of their implications.

Additional Information

© 2010 American Astronomical Society. Received 2010 June 5; accepted 2010 September 13; published 2010 October 7. This work was supported by NASA at Caltech (under grants NNX08AI11G and NNX10AE45G), the Jet Propulsion Laboratory, the Goddard Space Flight Center, and Washington University in St. Louis. We thank Wilcox Solar Observatory for making HCS data available, Robert McGuire for providing IMP-8/ GME data, and NASA's OmniWeb for providing solar wind data. We also appreciate the availability of neutron monitor data from the University of New Hampshire (under NSF grant ATM-0339257), sunspot data from the Royal Observatory of Belgium, and CME data from the SOHO/LASCO CME Catalog. Finally, we thank the MAG, SWEPAM, and SWICS teams for providing data through the ACE Science Center.

Attached Files

Published - Mewaldt2010p12070Astrophys_J_Lett.pdf


Files (481.4 kB)
Name Size Download all
481.4 kB Preview Download

Additional details

August 22, 2023
October 20, 2023