Solar Drivers of 11-yr and Long-Term Cosmic Ray Modulation

Abstract

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 rv2 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. rv 14°), while solar wind B was significantly lower (rv 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 postshock 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 rv 1900-present interval and can be used to reliably estimate GCR intensity over this period. For earlier epochs, however, a recent lOBe-based reconstruction covering the past"" 10 4 years shows nine abrupt and relatively short-lived drops of B to ;SO nT, with the first of these corresponding to the Sporer 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. E.W. Cliver (12Sl) Space Vehicles

An empirical model of heliospheric cosmic ray modulation on long-term time scale

“…For this model, the ascending and maximum phases are considered to be shorter than the descending phase, reflecting the asymmetry of the tilt angle cyclic shape (Hathaway 2010). Although observational limitations restrict the maximum angle to be 70° (Suess et al 1993), this does not affect the use of this model's reconstructions for the study of GCRs heliospheric modulation, since convective-diffusive processes are the main contributors to cosmic rays modulation for tilt angle values greater than 50° (Lopate & Simpson 1991;Potgieter & Le Roux 1992;Cliver et al 2013). The cyclic behaviour of the model is consistent with the cyclic shape followed by the WSO estimated values (Fig.…”

The solar wind during current and past solar minima and maxima

“…If this trend holds, we will observe lower dynamic pressures later this solar cycle. Low magnetic fields have lead to record high cosmic ray fluxes [ Cane et al , ; Mewaldt et al , ; Cliver et al , ]. Low pressures lead to a contraction of the heliosphere [ Richardson and Wang , ].…”

Near‐Earth heliospheric magnetic field intensity since 1750: 2. Cosmogenic radionuclide reconstructions

“…Similarly, the sunspot number is a more global measure than just the HMF sampled near Earth. Nevertheless, the anticorrelation between annual averages of solar wind B near Earth and cosmic ray intensity [ Cliver et al , , Figure 10] is strong enough that we can reasonably expect to use radionuclides as a tool to infer B , at least to within some level of uncertainty. Extracting HMF data from these radionuclides, however, is a much more indirect process than inferring them from sunspot or geomagnetic data.…”