Encyclopedia of the Solar System 2nd ed

(Marvins-Underground-K-12) #1
The Sun–Earth Connection 221

FIGURE 11 A more detailed illustration of the magnetosphere,
showing features mentioned in the text, such as the ring current
and magnetotail current sheet. The plasmasphere is a region of
denser, corotating magnetospheric plasma of ionospheric origin.
The plasma sheet is a denser region of magnetotail plasma that
participates in the physics of magnetotail reconnection and
ring-current formation. (Rice University.)


nature of these boundary conditions can introduce ad-
ditional complexity into the solar wind–magnetosphere
coupling.
Resulting geospace consequences of CMEs are numer-
ous and varied, as illustrated by Fig. 11. The associated
magnetospheric compression is accompanied by enhance-
ments of the energetic radiation belt particles trapped
in the Earth’s dipole field, due to a combination of in-
ward diffusion and energization of the existing particle
populations. [SeePlanetary Magnetospheres.]Solar
energetic particles accelerated at the CME-driven shock
or in associated solar flares can also leak into the mag-
netosphere along newly reconnected field lines at the
magnetopause or along open field lines into the polar
regions, as these particles tend to stream along field lines.
Magnetic reconnection between the stretched out, an-
tiparallel fields in the magnetotail causes currents to flow
through the high-latitude ionosphere. As magnetospheric
charged particles move toward Earth with the field lines,
they are accelerated, in some cases by electric fields par-
allel to the magnetic field. These energized particles in-
clude electrons, protons and other heavier ions. When
they reach the upper atmosphere, they collide with neu-
tral gases at altitudes of∼100 to∼200 km, causing ioniza-
tion and excitation of atoms and molecules. The ionization
enhances the flow of magnetosphere–ionosphere currents,
and when the excited atoms and molecules decay back
to their ground states, they emit the light known as the


aurora. (Further information about the aurora can be
found in Section 3.)
Magnetotail reconnection also triggers injection of parti-
cles toward Earth at low latitudes that form a ring current at
∼4to∼7 Earth radii. In the polar regions, protons and ions,
including ionospheric oxygen ions, O+, are driven upward
from the base of open field lines and flow into the magne-
tosphere, changing the composition of the magnetosphere
and ring current ion populations. The ring current notice-
ably changes the magnetic field in the magnetosphere and
at Earth’s surface. Altogether these phenomena character-
ize a geomagnetic storm, whose magnitude is characterized
by the ring current–related reduction of the field at the
ground, defined by the index Dst. (Another index, Kp, is
also used, but it is more a measure of the auroral current
systems.) Eventually, the magnetosphere and ionosphere
return to their prestorm states. Most effects are gone after
a few days, but some trapped particle populations may last
much longer. This complex geospace response to a CME
has recently been simulated by several research groups us-
ing numerical models of geospace, with solar wind measure-
ments defining the time-dependent boundary conditions on
the magnetosphere. Some results from one of these models
are illustrated in Fig. 12.
There are also weaker, more frequent geomagnetic
disturbances known as substorms. Substorms may occur
during storms, as periodic enhancements of the storm-time
geospace responses, or as standalone disturbances when
the quiet interplanetary magnetic field has a southward
component. In some cases, they appear to follow a sud-
den change in the interplanetary north–south field compo-
nent or a dynamic pressure pulse in the solar wind. Current
ideas on the reasons for substorms, which have been de-
bated for decades, include internal instabilities of the mag-
netosphere that occur in response to a variety of triggers.
However, geomagnetic storms involve the largest episodic
energy transfers from the solar wind and are thus responsi-
ble for the strongest Sun–Earth connection effects, collec-
tively referred to as space weather.

3. Atmospheric Effects of the Sun–Earth

Connection

The atmospheric responses to solar activity and its magneto-
spheric consequences are the closest counterparts of space
weather to traditional weather. They are therefore of special
interest in Sun–Earth connection research. Direct effects
are largely confined to the thermosphere and ionosphere,
above the mesopause at∼90 km. They fall into two main
categories: the effects of particles entering or “precipitat-
ing” into the atmosphere and the effects of high-latitude
ionospheric convection from magnetosphere–ionosphere
coupling.
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