The Sun–Earth Connection 225
significant that the Earth’s field still maintains an impor-
tant higher order harmonic component (e.g., a quadrupole
moment) during reversals.
5. Implications for Planetary Astronomy
and Astrophysics
The Sun–Earth connection scenario, and the physics it en-
compasses, is often invoked in the planetary sciences in con-
nection with solar wind interaction issues. Our knowledge
of the solar wind coupling to the Earth’s magnetosphere
and upper atmosphere is far greater than our comparable
knowledge for any other planet due to both the wealth of
available observations and the efforts that have been put
into their interpretation. Planetary spacecraft found that
there are magnetospheres around Mercury, Jupiter, Saturn,
Uranus, and Neptune. Mars does not have a strong dipo-
lar internal field, but it has patchy crustal magnetism that
makes a rather unique obstacle to the solar wind. Venus
has essentially no planetary field and thus represents an-
other extreme contrast in solar wind interaction styles. One
of the main goals of theMessengermission to Mercury is
to better observe the magnetosphere there in terms of its
response to solar wind conditions, and its particle content
and dynamics. Mercury has no substantial atmosphere or
ionosphere so it represents an interesting contrast to Earth
that may tell us more about the atmosphere’s role in the
Sun–Earth connection. Jupiter’s giant magnetosphere was
found by theGalileospacecraft to be dominated by the in-
ternal mass content contributed by the volcanic satellite Io,
while Saturn’s magnetosphere is currently under scrutiny by
theCassini Orbiter. Saturn, like Jupiter, has an aurora that
was observed by theHubble Space Telescopeprior to the
recent missions, but these auroras, and Earth’s, may have
different reasons for occurring where and when they do.
The comparative analysis of planet–solar wind interactions
and the related atmospheric effects is extremely valuable for
achieving maximum understanding from necessarily limited
planetary data.
In the world of astrophysics, extrasolar planetary sys-
tem research strives to infer from poorly resolved observa-
tions the details of individual planets. One possibility for
remote sensing is provided by the stellar wind interaction
with the planets, which may produce detectable emissions
from the planetary atmospheres. To be useful, these emis-
sions must be interpretable in terms of familiar examples in
our own solar system. Of particular interest is the detection
of Earth-like planets. The Sun–Earth connection suggests
a range of possible remote signatures for applications to
these “origins” investigations. Similarly, the identification
of the effects of stellar winds around other stars is enabled
in part by our own heliospheric experience. The interaction
of the stellar wind and the surrounding interstellar medium
produces a feature like the magnetosheath that is remotely
detectable in Lyman-alpha emission. Some stellar outbursts
suggest the occurrence of CMEs, and the associated space
weather around remote worlds.
Finally, fundamental astrophysical processes are in-
volved in energetic particle acceleration as well as in mag-
netic reconnection in the Sun–Earth Connection system.
Much of what has been learned about particle acceleration
at shocks in plasmas has come from the analysis of the ob-
servations from the region around the Earth’s bow shock.
Similarly, reconnection processes at the magnetopause and
in Earth’s magnetotail have been examined using spacecraft
data from both single spacecraft and small constellations.
These difficult observations of a dynamical and nonuni-
form space plasma system with many scales are slowly
yielding information about the process that suggests it oc-
curs when and where electrons are no longer controlled
by the magnetic field. In addition, numerical simulations
have been carried out using both fluid, kinetic (particle),
and hybrid (mixed particle and fluid) codes to shed light
on the microphysics of how oppositely directed magnetic
fields in space plasmas undergo major topological changes.
Laboratory work has also contributed to these investiga-
tions, all under the umbrella of Sun–Earth connection
research.6. Epilogue
The term “Sun–Earth connection” is used to describe the
physically rich and dynamic system by which processes at
the Sun affect near-Earth space via other than the solar
constant radiative emissions. The subject has most recently
been given a new label at NASA, which perhaps better com-
municates its impact. The “Living with a Star” program
seeks to investigate, through sponsored research and space
missions, the ways in which the Sun controls the Earth’s
past, present, and future. To do this, it is necessary to use
a combination of theory, measurements, and modeling to
study the system components—the heliosphere, the mag-
netosphere, and the upper atmosphere and ionosphere—to
learn how they are coupled. As described above, the cou-
plings are numerous and diverse. They are sometimes sub-
tle like cosmic ray effects on clouds and sometimes overt
like CMEs and the related topological changes reconnec-
tion imposes on the magnetosphere in response to their as-
sociated large southward-oriented interplanetary magnetic
fields. The consequences of these couplings are only partly
understood. Practical applications of space environment
knowledge are in the meantime growing in popularity and
demand. Other fields are beneficiaries of the Sun–Earth
connection planetary and astrophysical “laboratory.” And in
an era of new human exploration initiatives, space weather
may one day become part of the weather report on your
local news.