The 22-Year Hale Cycle in Cosmic Ray Flux – Evidence for Direct Heliospheric Modulation
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The ability to predict times of greater galactic cosmic ray (GCR) fluxes is important for reducing the hazards caused by these particles to satellite communications, aviation, or astronauts. The 11-year solar-cycle variation in cosmic rays is highly correlated with the strength of the heliospheric magnetic field. Differences in GCR flux during alternate solar cycles yield a 22-year cycle, known as the Hale Cycle, which is thought to be due to different particle drift patterns when the northern solar pole has predominantly positive (denoted as qA>0 cycle) or negative (qA<0) polarities. This results in the onset of the peak cosmic-ray flux at Earth occurring earlier during qA>0 cycles than for qA<0 cycles, which in turn causes the peak to be more dome-shaped for qA>0 and more sharply peaked for qA<0. In this study, we demonstrate that properties of the large-scale heliospheric magnetic field are different during the declining phase of the qA<0 and qA>0 solar cycles, when the difference in GCR flux is most apparent. This suggests that particle drifts may not be the sole mechanism responsible for the Hale Cycle in GCR flux at Earth. However, we also demonstrate that these polarity-dependent heliospheric differences are evident during the space-age but are much less clear in earlier data: using geomagnetic reconstructions, we show that for the period of 1905 – 1965, alternate polarities do not give as significant a difference during the declining phase of the solar cycle. Thus we suggest that the 22-year cycle in cosmic-ray flux is at least partly the result of direct modulation by the heliospheric magnetic field and that this effect may be primarily limited to the grand solar maximum of the space-age.
Keywords22-year cycle Cosmic rays Heliospheric current sheet Solar variability Polarity reversal
We are grateful to the Space Physics Data Facility (SPDF) of NASA’s Goddard Space Flight Centre for combining the data into the OMNI 2 data set, which was obtained via the GSFC/SPDF OMNIWeb interface at omniweb.gsfc.nasa.gov and to the Marshall Space Flight Centre for the Sunspot Number data obtained from MSFC at solarscience.msfc.nasa.gov/greenwch.shtml . We also thank the Bartol Research Institute of the University of Delaware for the neutron-monitor data from McMurdo, which is supported by NSF grant ATM-0527878 and J.T. Hoeksema of Stanford University for WSO magnetograms. The work of SRT is supported by a studentship from the UK’s Natural Environment Research Council (NERC).
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