Time Evolution of Galactic and Anomalous Cosmic‐Ray Spectra in a Dynamic Heliosphere

Abstract

Galactic and anomalous cosmic-ray modulation in a dynamic realistic heliosphere is studied. We present the time evolution of cosmic-ray spectra from solar minima to maxima as computed by a newly developed hybrid model. This model consists of a hydrodynamic part to model the plasma flow of protons in the solar wind and in the LISM, neutral H atoms, and heliospheric H pick-up ions. The heliospheric geometry and plasma flow are then used in a kinetic transport part to calculate cosmic-ray transport. We show that the model results in realistic cosmic-ray modulation as solar activity increases from solar minimum to maximum. In particular it is shown that, depending on energy, fewer cosmic-ray particles are accelerated at the shock for solar maximum. These particles are also more modulated with the modulation amplitude depending on distance, energy, and the polarity of the heliospheric magnetic field. There is also a much smaller dependence of cosmic-ray intensities on solar activity in the heliosheath, compared to other regions. It is interesting that, for the very low energies, e.g., <10 MeV, the computed anomalous spectrum at the shock is not sensitive to solar conditions, but in the inner heliosphere it may well disappear well below the Galactic spectra for solar maximum. Also, we show that depending on energy, there exists a large computed nose-tail asymmetry in the cosmic-ray intensities. Furthermore, the acceleration of these particles at the shock also depends on latitude; e.g., most of the accelerated anomalous cosmic rays are found in the equatorial regions.