Solar Drivers of 11-yr and Long-Term Cosmic Ray Modulation
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In the current paradigm for the modulation of galactic cosmic rays (GCRs), diffusion is taken to be the dominant process during solar maxima while drift dominates at minima. Observations during the recent solar minimum challenge the pre-eminence of drift at such times. In 2009, the ∼2 GV GCR intensity measured by the Newark neutron monitor increased by ∼5% relative to its maximum value two cycles earlier even though the average tilt angle in 2009 was slightly larger than that in 1986 (∼20° vs. ∼14°), while solar wind B was significantly lower (∼3.9 nT vs. ∼5.4 nT). A decomposition of the solar wind into high-speed streams, slow solar wind, and coronal mass ejections (CMEs; including post-shock flows) reveals that the Sun transmits its message of changing magnetic field (diffusion coefficient) to the heliosphere primarily through CMEs at solar maximum and high-speed streams at solar minimum. Long-term reconstructions of solar wind B are in general agreement for the ∼1900-present interval and can be used to reliably estimate GCR intensity over this period. For earlier epochs, however, a recent 10Be-based reconstruction covering the past ∼104 years shows nine abrupt and relatively short-lived drops of B to ≲0 nT, with the first of these corresponding to the Spörer minimum. Such dips are at variance with the recent suggestion that B has a minimum or floor value of ∼2.8 nT. A floor in solar wind B implies a ceiling in the GCR intensity (a permanent modulation of the local interstellar spectrum) at a given energy/rigidity. The 30–40% increase in the intensity of 2.5 GV electrons observed by Ulysses during the recent solar minimum raises an interesting paradox that will need to be resolved.
KeywordsSun Solar Cycle Solar Wind Coronal Mass Ejections High-speed Streams Galactic Cosmic Rays Cosmic Ray Modulation
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