100-C 25-C 50-C 75-C C+M 50-C+M C+Y 50-C+Y M+Y 50-M+Y 100-M 25-M 50-M 75-M 100-Y 25-Y 50-Y 75-Y 100-K 25-K 25-19-19 50-K50-40-40 75-K 75-64-64

CHAPTER 11

The Sun–Earth

Connection

J. G. Luhmann

University of California, Berkeley

S. C. Solomon

National Center for Atmospheric Research, Boulder

1. The Solar and Heliospheric Roles in the Sun–Earth 5. Implications for Planetary
   Connection Astronomy and Astrophysics


2. The Geospace Role in the Sun–Earth Connection 6. Epilogue


3. Atmospheric Effects of the Sun–Earth Connection Bibliography


4. Practical Aspects of the Sun–Earth Connection

T

he Sun has profound effects on the Earth through its
primarily visible and infrared photon emissions. This
radiated energy, generated as a by-product of the nuclear
reactions in the Sun’s core [seeThe Sun]is absorbed or re-
flected at different wavelengths by the sea and land surfaces
and the atmosphere. The result is the atmospheric circu-
lation system that generates tropospheric weather through
the diurnal and seasonal cycles caused by Earth’s rotation
and axis tilt. [SeeEarth as a Planet: Atmosphere and
Oceans.]The climate of the Earth is the result of the long-
term interaction of solar radiation, weather, surface, oceans,
and human activity.
These influences are not the only ways the Sun affects
the Earth. Ultraviolet (UV) and X-ray light from the Sun are
much less intense, but more energetic and variable than the
visible emissions. The UV radiation is absorbed in the strato-
sphere where it affects the production of the ozone layer
and other atmospheric chemistry, while the extreme ultravi-
olet (EUV) photons and X-rays are absorbed in the thermo-
sphere (above∼90 km), creating the ionized component of
the upper atmosphere known as the ionosphere. Even more
variable is the emission of charged particles and magnetic
fields by the Sun. One form of this output is the magnetized
solar windplasmaand its gusty counterpart, thecoronal
mass ejectionorCME. CMEs interact with the Earth

to create majorgeomagnetic storms. These and other

forms of matter, energy, and momentum transfer couple

the physical domains of the connected Sun–Earth system,

which is illustrated in Figure 1. A brief summary of the

subject of this chapter, whose focus is this system, follows

here.

Sun–Earth connection physics begins in the solar inte-

rior where dynamo activity [seeThe Sun]generates the

solar magnetic field. The solar magnetic field, coupled with

the mechanical and radiative energy outputs from core fu-

sion reactions, ultimately determines both the variability

of the Sun’s energetic (EUV, X-ray) photon outputs and

the interplanetary conditions at the orbit of Earth. The

latter include the solar wind plasma properties, the inter-

planetary magnetic field magnitude and orientation, and

the energetic particle radiation environment. Both the en-

ergetic photon outputs and interplanetary conditions vary

with the∼11-year solar cycle, which is characterized by

changing frequencies of solar flares and CMEs, the two pri-

mary forms of solar activity. These in turn determinespace

weatherconditions in near-Earth space orgeospace, the

region comprising of the magnetosphere, the upper atmo-

sphere, and the ionosphere. Only in the 1960s was it ap-

preciated that the interplanetary magnetic field orientation

relative to Earth’s own dipolar field plays a major role in solar

Encyclopedia of the Solar System 2e©C2007 by Academic Press. All rights of reproduction in any form reserved. 213