Solar cycle variation in the heliosphere

Abstract

Variations in interplanetary plasma and magnetic field parameters over the course of a solar cycle affect a broad range of physical phenomena, such as planetary magnetospheres, cosmic ray modulation, and the interaction of the solar wind with the local interstellar medium. For more than 3 decades a succession of spacecraft have provided in situ measurements of these parameters. These measurements span a range of heliocentric distances from 0.3 to 61 AU and provide an explicit picture of the three‐dimensional structure of the inner and outer heliosphere in the vicinity of the ecliptic plane. In addition, ground‐based interplanetary scintillation measurements and observations from the Ulysses spacecraft have extended our knowledge to smaller heliocentric distances and higher heliographic latitudes. The structure of the heliosphere varies dramatically over the course of a solar cycle. Much of this variation can be related to the changes in the structure and inclination of the coronal magnetic field. The coronal neutral line, which separates the Sun's magnetic hemispheres, is associated with a belt of low‐speed and high‐density solar wind. Its inclination varies over the course of a solar cycle, from a low value near solar minimum to a high value near solar maximum. When the inclination of the coronal neutral line is low, its signature is evident in observations near the solar equator. When the inclination of the coronal neutral line is high, the heliosphere at moderate heliographic latitudes is swept by a succession of high‐ and low‐speed streams and is shaped by their interactions.