Download Solar Wind Heliosphere

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 widefield 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 NearEarth 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
Heliosheath