The Heliospheric Magnetic Field

L.A. Fisk

University of Michigan, USA

During previous solar cycles the heliospheric magnetic field, often referred to as the open magnetic flux of the Sun, appeared to evolve in a very simple manner. There was a single current sheet separating two regions of opposite polarity that appeared to rotate over during the solar cycle, accomplishing the field reversal. There was a background level of open magnetic flux that remains relatively constant, indicating little disconnection of open flux at the current sheet. Superimposed on this background level is the magnetic flux of CMEs, which is most evident at solar maximum. During the current solar minimum, the background level of open magnetic flux appears to have decreased relative to its level in previous minima. Other features of the solar wind, including its mass flux and ionic charge states, have changed. The implications of these observations for our understanding of the evolution of the heliospheric magnetic field and for the solar dynamo will be discussed.