The Sun-Earth Connection
The Sun is a variable star, meaning that its activity varies
over time. The changing degree of activity is due to two factors:
its rotation and its convection processes, which transport hot
gas from the solar interior to the surface. The interaction of
these two motions-rotation and convection-generates powerful
magnetic fields and influences the cyclic activity level demonstrated
by the ebb and flow of sunspots and solar flares.
Ulysses became the first spacecraft to explore the Sun from
polar orbit.
Additionally, activity in the Sun's corona-the white "halo"
of gas seen during total eclipses-causes ejection at very high
velocities of a hot electrified gas called the solar wind, which
courses throughout the solar system transporting energy to Earth
and all the other planets. Its interaction with Earth's magnetic
fields causes a whole range of effects, such as the aurora magnetic
storms, disruption of radio communications, and power surges
in transmission lines.
An important facet of NASA's space science program is solar-terrestrial
research, which embraces the study of the Sun as a variable star,
the origin and transmission of the solar wind, its interactions
with Earth's magnetosphere, and how all these phenomena connect
the Sun to the Earth and the heliosphere, the vaguely-bounded
region of space where the Sun's magnetic field and the solar
wind extend.
Over the years, NASA has employed a number of spacecraft to
study the processes that link Earth with the Sun. In the 1990s,
this activity has expanded under the International Solar Terrestrial
Physics (ISTP) program, a joint effort of NASA, the European
Space Agency (ESA) and the Japanese Institute of Space and Astronautics.
Not a component of ISTP but a joint NASA/ESA mission, Ulysses-launched
October 6, 1990-is the first effort to explore the heliosphere
from solar polar orbit over a full range of solar latitudes.
After a roundabout four-year flight from Earth, Ulysses reached
an area of the Sun's south pole in June 1994, then flew into
the northern hemisphere of the heliosphere in 1995 and conducted
a four-month observation of the north polar region. Late in 1995,
Ulysses completed its primary mission, having returned volumes
of invaluable data on the Sun's corona, wind, solar and non-solar
cosmic rays, solar radio bursts and plasma waves, and the heliosphere's
magnetic field. Ulysses is managed for NASA by Jet Propulsion
Laboratory.
The Solar Heliospheric Observatory (SOHO), launched December
2, 1995, surprised scientists by its initial reports, which cited
intense activity on the Sun's surface at a time-the low ebb of
the 11-year solar cycle-when it should have been relatively inactive.
A part of the ISTP program, SOHO is a joint NASA/ESA mission
designed to perform remote measurements of the Sun and in situ
measurements of the solar wind to improve knowledge of the corona
and the origin of the solar wind.
The two-ton spacecraft was launched into orbit at a point
about a million miles sunward from Earth, a point where the gravities
of the Sun and Earth cancel each other out and provide a stable
position from which to conduct long term continuous observation
of the Sun. In the early months of 1996, SOHO returned to Earth
motion pictures of the Sun's activity and a mass of data on solar
interior dynamics and the composition of the solar wind. In addition
to NASA and ESA, participants include Germany, the United Kingdom,
France, Finland and Switzerland; Goddard Space Flight Center
manages the NASA-provided elements of SOHO.
Another component of the ISTP program, launched into polar
orbit on February 24, 1996, is the Polar spacecraft, built by
Lockheed Martin Astro Space under the management of Goddard Space
Flight Center. Polar and its sister spacecraft Wind (launched
November 1, 1994) are a pair of complementary spacecraft, developed
under NASA's Global Geoscience Space Program to gain broader
understanding of the relationship between solar plasma emitted
by the Sun and its interaction with Earth's magnetosphere, ionosphere
and magnetic poles.
Polar's job is to measure the energy, energization and transport
of plasma into the magnetosphere by the solar wind. Additionally,
it is making direct measurements of global energy deposition
into Earth's atmosphere. At midyear 1996, NASA was receiving
good data from all 11 of Polar's instruments.
Two veteran spacecraft launched almost 30 years ago-Voyagers
1 and 2-are now playing a part in solar-terrestrial research.
Having completed their grand tours of the solar system and flybys
of the outer planets, they have been assigned new jobs: to observe
cosmic rays and the solar wind, and to search for the transition
boundary between the solar wind and interstellar space as they
head out of the solar system on escape trajectories. Both spacecraft
are estimated to have about 20 years of useful life remaining.
|
