Time Dependence of Solar Modulation throughout Solar Cycle 23 as Inferred from ACE Measurements of Cosmic-Ray Energy Spectra

Abstract

Since the launch of the Advanced Composition Explorer (ACE) in August 1997, the Cosmic Ray Isotope Spectrometer (CRIS) has been providing measurements of cosmic-ray energy spectra between ∼50 and ∼500 MeV/nuc for a wide range of elements. The shape of these low-energy spectra, which are measured with good statistical precision on time scales as short as a solar rotation, are strongly influenced by modulation in the heliosphere. By combining calculated local interstellar spectra from a leaky-box propagation model with a sphericallysymmetric, steady-state model of solar modulation, we have derived the time dependence of the modulation strength throughout solar cycle 23. In addition, we have investigated hysteresis effects in the modulation by comparing time dependences at different energies in two ways. First, we have compared values of the modulation parameter, φ, calculated based on CRIS spectral measurements with published values derived from neutron monitor data. Second, we have studied effects over relatively small energy ranges by comparing CRIS intensity measurements at different energies. We report the ACE/CRIS observations, discuss the trends in the data, and compare with previous results on the variations of the solar modulation level over the solar cycle.

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