more-phenomenological models that use observations at the solar surface to make short-term predictions must be relied on.


The phenomenon of the solar modulation of cosmic rays is the result of the solar wind and its magnetic field inhibiting the interstellar cosmic rays from entering the inner solar system. Since the effect is at a maximum during high sunspot activity, the cosmic ray intensity is a minimum at Earth during the period around sunspot maximum. Conversely, it reaches a maximum at Earth during sunspot minimum. This reflects a general, heliospheric depression of the intensity of galactic cosmic rays, which are entering the solar system from the outside.

The heliosphere, the bubble with the Sun at its center, is carved out of the interstellar gas by the solar wind, which blows radially outward, carrying with it the solar magnetic field, producing a classical Archimedean spiral magnetic field. (See Figure 2.1.) At a radius of about 100 AU, because of the resistance of the interstellar gas, the wind undergoes a shock transition to subsonic flow. This shock is called the termination shock. Some distance (probably about 30 to 50 AU) beyond the shock is the contact surface that separates the ionized part of the interstellar gas from the solar wind gas. A possible second, outer shock in the interstellar plasma is also present. The outer portions of this picture, beyond the inner shock, are not well understood. Fortunately, the general properties of the modulated intensity of approximately

FIGURE 2.1 The heliosphere. SOURCE: Courtesy of National Aeronautics and Space Administration.

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