Cycle 20 solar wind modulation of galactic cosmic rays: Understanding the challenge

Details Cycle 20 solar wind modulation of galactic cosmic rays: Understanding the challenge Cycle 20 solar wind modulation of galactic cosmic rays: Understanding the challenge

Oct 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005jgra..11010106a&link_type=abstract

Journal of Geophysical Research, Volume 110, Issue A10, CiteID A10106

Astronomy and Astrophysics

Astrophysics

12

Interplanetary Physics: Cosmic Rays, Interplanetary Physics: Interplanetary Magnetic Fields, Interplanetary Physics: Solar Cycle Variations (7536), Interplanetary Physics: Solar Wind Plasma, Solar Physics, Astrophysics, And Astronomy: Coronal Mass Ejections (2101)

Scientific paper

It is well known that time variations of galactic cosmic rays for cycle 20 (1964-1976) challenge our understanding of the heliospheric dynamics and the physical processes involved in causing observed modulations. Several interesting questions have been raised over the years. We attempt to provide some plausible answers based on analyses of cosmic ray data obtained with detectors on the ground, on balloons, and on satellites; median rigidity of response of these detectors to the cosmic ray spectrum covers a range of rigidities (R) from 1 to 70 GV. In particular, we explore the observed correlation between hourly rates of neutron monitors and the time series of the product of solar wind velocity (V) and interplanetary magnetic field intensity (B) at Earth's orbit. Local convection is identified as a contributor to ``steps'' in cosmic ray time variations near Earth and in the outer heliosphere. The amplitude of 11-year modulation varies as 1/R; data do not support the concept of ``transition'' rigidity for modulation. We argue that heliosheath-related modulation effects are minimal at Earth's orbit at R > 3 GV. Heuristic considerations suggest that the space-dependent part of the diffusion coefficient should vary as 1/B (not 1/B2). We relate modulation function to the product BV, suggesting it plays a significant role in the solar modulation of cosmic rays.

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