DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

magnetopause
forealkalivaporinstrumentsareoftenused, generally
in conjunction with fluxgates. These are
absolute instruments, observing the intensity of
the magnetic field by the frequency at which a
swept-frequency radio signal causes enhanced
absorption of a light beam in a glass cell filled
with alkali vapor. Rubidium or strontium vapor
is generally used, and the beam is emitted from
the same element, in a narrow frequency range.
The mechanism is based on optical pumping of
energy sub-states of these atoms.
Other instruments (used mainly on the
ground) include the proton precession magnetometer,
on which the widespread technique
of nuclear magnetic resonance in medicine is
based. The Overhauser effect magnetometer is
related to this but it is more precise, and it can
be used as an alternative to the alkali vapor instrument
in geomagnetic survey satellites.
magnetopause The boundary of the magnetosphere,
separating plasma attached to the
Earth from that flowing with the solar wind.
The magnetopause is defined by the surface on
which the pressure of the solar wind is balanced
by that of the Earth’s magnetic field. The “nose”
of the magnetopause, on the sunward side of
the Earth is ∼ 15 Earth radii away, on average.
As the pressure of the solar wind changes, the
magnetopause shrinks or expands accordingly.
The idea of a magnetopause has been around
since about 1930 when Chapman and Ferraro
proposed a theory that explained geomagnetic
storms as interactions of the Earth’s magnetosphere
with plasma clouds ejected from the sun.
The magnetopause was first discovered in 1961
by NASA’s Explorer 12 spacecraft.
magnetosonic wave See magnetoacoustic
wave.
magnetosphere The planetary region of
space where a body’s magnetic field can be detected.
As the solar wind plasma embedded
with interplanetary magnetic field (IMF) flows
around the planet, it interacts with the Earth’s
magnetic field and confines it to a cavity called
the magnetosphere. Since the solar wind is supersonic,
a shock known as Bow shock is formed
on the sunward side of the magnetosphere. The
solar wind then flows across the bow shock and
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its speed is reduced from supersonic to subsonic.
The solar wind ahead is deflected at a boundary
between the magnetosphere and the solar wind
known as the magnetopause. The subsolar point
on the magnetopause is about 10 Earth radii from
the center of the Earth. The bow shock is about
3 Earth radii from the sunward side of the magnetopause
at the subsolar point. The region between
the bow shock and the magnetopause is
called the magnetosheath.
At the magnetopause, the solar wind pressure
outside is balanced by the magnetic field
pressure inside the magnetosphere. As the solar
wind sweeps past the Earth, the magnetic field
lines from the polar cap are pulled toward the
nightside to form a geomagnetic tail. This tail
can be observed as far as 1000 Earth radii, as
the combined pressure of the field and plasma
prevent its closing on the night side. Moreover,
the polar cap magnetic field lines do not
close resulting in a thin sheet in the equatorial
plane known as the neutral sheet across which an
abrupt field reversal occurs. The region where
magnetic field lines from sub-auroral latitudes
tend to close is known as plasma sheet of thickness
about 1 Earth radii. The plasma sheet consists
of energetic particles in the energy range of
about 1 to 10 kev and is the source of radiation
belt particles.
magnetospheric substorms Apart from
quiet variations in the Earth’s magnetic field like
solar quiet (S q) and lunar variations resulting
from the generation of ionospheric currents by
solar and lunar tides, other observed variations
are far from constant and without any periodicity.
Such variations result from the interaction
of solar wind with the geomagnetic field
of the Earth and are denoted by D. These are
called magnetospheric storms and have characteristic
times of minutes to days. The magnetospheric
storm consists of frequently occurring
small storms called magnetospheric substorms.
The intensity of the storm is given in terms of
the AE (auroral electrojet) index and of the substorm
by the Dst index. Each substorm is associated
with injection of energetic protons of
few tens of kev in the Van Allen belts. When
frequent injection occurs, a proton belt known
as ring current is formed whose strength is given
in terms of Dst. At first there is a sudden com-