Galactic Cosmic Ray Intensities Reach Record Levels in 2009

Details Galactic Cosmic Ray Intensities Reach Record Levels in 2009 Galactic Cosmic Ray Intensities Reach Record Levels in 2009

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsh13c..08m&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #SH13C-08

Astronomy and Astrophysics

Astrophysics

[2104] Interplanetary Physics / Cosmic Rays, [2162] Interplanetary Physics / Solar Cycle Variations, [7536] Solar Physics, Astrophysics, And Astronomy / Solar Activity Cycle, [7984] Space Weather / Space Radiation Environment

Scientific paper

Recent data from ACE show that during the spring and summer of 2009 the galactic cosmic ray (GCR) intensity at ~100 to ~500 MeV/nuc (near the peak in the spectrum at 1 AU) reached the highest intensities of the space era. During mid-2007 the GCR intensities of Be to Ni (4 ≤ Z ≤ 28) had apparently leveled off at intensities comparable to those measured in the 1976-77 and 1997-1998 solar minima, and the onset of new solar activity was expected in 2008. Instead, solar-minimum conditions continued, and the GCR intensity began to increase again in early 2008. During the second quarter of 2009 the 120-470 MeV/nuc GCR Fe intensity reached 19% ± 1% above the 1997-1998 solar-minimum level, with similar increases in other species. Comparisons with earlier spacecraft and neutron-monitor data show conclusively that the GCR intensities in the spring and summer of 2009 are the highest of the space era. However, when viewed in the context of the long-term Be-10 record, the space era has until now experienced a below-average cosmic-ray intensity. The record-setting intensity levels are likely due to a combination of factors that include the weakened interplanetary magnetic field strength, the reduced solar wind speed and dynamic pressure, and the extended solar minimum conditions. In addition, during alternate solar minima, including the present one, the drift pattern of cosmic rays in the heliosphere is sensitive to the tilt of the interplanetary magnetic current sheet, which was still inclined at ~20° in mid-2009. If the current sheet tilt declines further before the onset of new solar activity, it is likely that the GCR intensity will reach even higher levels. This paper will compare the 2009 cosmic ray intensities with those from the past ~50 years and with the long-term archival record, summarize the role of the various solar-wind parameters in modulating the near-Earth cosmic ray intensity, and discuss the implications of the 2009 cosmic-ray intensities for estimates of the interplanetary radiation dose.

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