Solar cycle variations of solar wind dynamics and structures

This paper reviews solar cycle variations in the heliosphere. We show that some solar cycle changes, such as the correlation between speed and density, result from the evolution of the solar structure from dipolar with strong latitudinal gradients at solar minimum to a disordered field with little latitudinal variation at solar maximum. Other changes, such as the variation of the solar wind dynamic pressure, occur at all heliolatitudes independent of the solar wind structure. The solar cycle dependence of interplanetary coronal mass ejections (ICMEs) results in changes in the solar wind structure in the outer heliosphere, with large merged interaction regions (MIRs) driven by ICMEs near solar maximum. The helium/ proton (He/H) ratio also changes over the solar cycle and suggests that the slow solar wind has two sources, one poor in He and the other with larger He/H ratios. r 2007 Elsevier Ltd. All rights reserved.