Solar Wind and Coronal Mass Ejections In addition to its emissions of electromagnetic radiation, the Sun also emits material (mostly in the form of electrons, protons, and helium nuclei) which flows outward into the solar system (some of it reaching Earth’s vicinity). The major part of this mass ejection, especially in times of low solar activity, is the solar wind, a steady flow of ionized gas outward through the solar system, having low enough energy as to not have major effects on the planets and their local environments. At Earth’s distance from the Sun, the solar wind has a typical density of about 7 atoms/cm3 and typical velocity of 300-700 km/sec. Because of the angular momentum induced by the Sun’s rotation on its axis, the solar wind travels outward in a spiral fashion, along solar magnetic field lines. More significant, in terms of its effects, are coronal mass ejections, mostly associated with active regions on the solar surface, which are most frequent and energetic during times of high solar activity. Coronal mass ejections can result from solar flares and are often associated with sunspots and their local surroundings.

The Solar Wind

Solar Wind Variations Due To Solar Rotation Note, the Sun’s axis of rotation is not perpendicular to to the plane of Earth’s orbit around the Sun. Also, the solar wind source is not confined to the Sun’s equator, but is variable over a range of solar latitudes.

THE SOLAR WIND Because of the Sun’s magnetic field and its rotation on its axis, solar wind particles travel outward at much higher speeds along the magnetic polar directions than along its magnetic equatorial plane.

Coronal Mass Ejections Coronal mass ejections, sometimes associated with solar flares, travel outward much more rapidly than the normal solar wind, and can create a “shock wave” within the interplanetary medium. Coronal mass ejections are the “hurricanes” of the solar wind and space weather! Shock Wave

Coronal Mass Ejection Observed with LASCO in Visible Light The image on the left is with a narrow-field coronagraph (dark occulting disk blocks direct view of the Sun; white circle indicates size of the Sun image without disk). The image on the right is with a wide-field coronagraph, taken nearly 6 hours later. (Red and blue are false colors.)

Comparison of Solar Mass Ejections with Chromospheric XUV Emissions

Effects of Solar Activity on the Near-Earth Space Environment In addition to the heat and light that the Sun provides to us on the surface of Earth (which is very stable over long periods of time), it also has much more variable effects on Earth’s upper atmosphere and the near-Earth space environment. These latter effects are due to (1) far-ultraviolet and X-ray radiations from the Sun, and (2) energetic solar particle (proton and electron) emissions, traveling along interplanetary magnetic field lines and interacting with Earth’s magnetic field in near-Earth space. Although the far-UV and X-ray emissions of the Sun are only a small percentage of its total output, they are responsible for creating the Earth’s ionosphere by ionization of the upper atmosphere. Solar flares greatly increase the X-ray radiation and high-energy particle components of the Sun’s emissions. The solar wind, and much more energetic coronal mass ejections, create the Earth’s magnetosphere and (indirectly) Earth’s polar auroras. In times of high solar activity, these energetic radiation and solar particles can cause problems with communications and electronic equipment, on the ground as well as in space, and can be hazardous to astronauts in near-Earth and interplanetary space.

THE HELIOSPHERE The heliosphere is defined as the entire region of space in which the Sun’s mass ejections (including solar wind) and magnetic field predominate over those of the Galaxy (the interstellar medium and the galactic magnetic field). By this definition, the heliosphere extends well beyond the outer planets of our solar system. Studies of the heliosphere and its boundary with the interstellar medium have been made by the two Voyager spacecraft, which flew by the outer planets (Jupiter, Saturn, Uranus, and Neptune) as their primary missions in the 1979-1989 time periods. Voyager 1 recently (about December 16, 2004) crossed the boundary known as the “termination shock”, where the outgoing solar wind transitions from supersonic to subsonic velocity, about 93 AU from the Sun. This, in turn, resulted in an abrupt increase in the density, and count rate, of solar wind particles.

Voyager 1 Crosses Heliospheric Terminal Shock, 2005 Note, the increased count rate of solar wind particles is due to the crossing of the boundary between supersonic (closer to the Sun) and subsonic (further from the Sun) velocities, resulting in higher density.

Solar Wind Termination Shock Analogy

Voyager 1 Crossing of the Heliospheric Termination Shock

THE HELIOSPHERE It is believed that the boundary between the solar wind and interstellar medium, called the heliopause, is still further distant from the Sun than is the termination shock. The region between the termination shock and the heliopause is called the heliosheath. Between the heliopause and the general interstellar medium is a region in which both solar wind and interstellar gas are combined, and travel at subsonic velocity. The outermost feature is a “bow shock” in which incoming supersonic interstellar gas impacts, and mixes with, the outgoing solar wind particles. Outside of the bow shock is the interstellar medium, mostly hydrogen and helium, which fills our entire Galaxy.

Voyager Trajectories and the Heliosphere Helio-sheath