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Abstract We present a stochastic field line mapping model where the interplanetary magnetic field lines are described by a density distribution function satisfying a Fokker–Planck equation that is solved numerically. Due to the spiral geometry of the nominal Parker field and to the evolving nature of solar wind turbulence, the heliospheric diffusion of the magnetic field lines is both heterogeneous and anisotropic, including a radial component. The longitudinal distributions of the magnetic field lines are shown to be close to circular Gaussian distributions, although they develop a noticeable skewness. The magnetic field lines emanating from the Sun are found to differ, on average, from the spirals predicted by Parker. Although the spirals remain close to Archimedean, they are here underwound, on average. Our model predicts a spiral angle that is smaller by ∼5° than the Parker spiral angle at Earth’s orbit for the same solar wind speed ofVsw= 400 km s−1. It also predicts an angular position on the solar disk of the best magnetically connected footpoint to an observer at 1 au that is shifted westward by ∼10° with respect to the Parker’s field model. This significantly changes the angle of the most probable magnetic connection between possible sources on the Sun and observers in the inner heliosphere. The results have direct implications for the heliospheric transport of “scatter-free” electrons accelerated in the aftermath of solar eruptions.
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Galactic Cosmic-Ray Propagation in the Inner Heliosphere: Improved Force-field Model
Abstract A key goal of heliophysics is to understand how cosmic rays propagate in the solar system’s complex, dynamic environment. One observable is solar modulation, i.e., how the flux and spectrum of cosmic rays change as they propagate inward. We construct an improved force-field model, taking advantage of new measurements of magnetic power spectral density by Parker Solar Probe to predict solar modulation within the Earth’s orbit. We find that modulation of cosmic rays between the Earth and Sun is modest, at least at solar minimum and in the ecliptic plane. Our results agree much better with the limited data on cosmic-ray radial gradients within Earth’s orbit than past treatments of the force-field model. Our predictions can be tested with forthcoming direct cosmic-ray measurements in the inner heliosphere by Parker Solar Probe and Solar Orbiter. They are also important for interpreting the gamma-ray emission from the Sun due to scattering of cosmic rays with solar matter and photons.
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- Award ID(s):
- 2012955
- PAR ID:
- 10372025
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 937
- Issue:
- 1
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 27
- Size(s):
- Article No. 27
- Sponsoring Org:
- National Science Foundation
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