Physics and Astronomy Colloquium

Parts of the solar corona accelerate into space to form the supersonic solar wind that pushes interstellar material out of the solar system to distances beyond 100 AU creating the "bubble" known as the Heliosphere. The solar wind also transports part of the Sun�s dipole magnetic field into the Heliosphere filling it with magnetized plasma. The Heliospheric Magnetic Field (HMF) prevents the interstellar plasma and magnetic field from penetrating into the Heliosphere leading to a sharp boundary separating the solar and interstellar plasmas. Solar wind outflow leaves �holes� in the corona because of the depletion of the coronal density. When solar activity is low, the Sun�s magnetic poles coincide with two large coronal holes in the north and south polar caps that emit high- speed solar wind while lower speed wind originates at lower latitudes. The magnetic dipole and coronal holes are tilted relative to the Sun�s rotation axis and, as the Sun rotates, the fast and slow wind interact introducing large scale heliospheric structure and dynamics involving the HMF and consisting of alternating compression and rarefaction regions. The distinctive compression regions are bounded by collision-less shocks that accelerate solar particles to high energies. Helisopheric dynamics are further enhanced by the penetration of some interstellar constituents into the Heliosphere, namely, neutral gas, electrically- charged interstellar dust and Galactic Cosmic Rays. Both the structure and dynamics of the heliosphere are profoundly affected by the solar cycle. During maximum solar activity, the Sun�s magnetic dipole effectively rotates equator-ward and eventually reverses polarity while the polar coronal holes vanish and then reappear. An overview of these physical phenomena will be presented based principally on 17 years of 3D observations by Ulysses, the first spacecraft to follow a polar orbit carrying it over and under the Sun.