DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

oring magnetic field lines to repulse each other.
Thus, an inhomogeneity in the magnetic field B
gives rise to a force density f pushing field lines
back from regions of high magnetic density into
low density areas:
f = 1
(∇×B) × B .
4π
In contrast to the gas-dynamic pressure, the
magnetic pressure is not isotropic but always
perpendicular to the magnetic field and can be
defined as pM = B2 /(8π).
magnetic reconnection The dissipation of
magnetic energy via magnetic diffusion between
closely separated regions of oppositely directed
magnetic field. The result of the dissipation is
that the oppositely directed field lines form a
continuous connection pattern across the diffusion
region resulting in a change in the magnetic
configuration.
magnetic regime (cosmic string) See electric
regime (cosmic string).
magnetic reversal In geophysics, the Earth’s
magnetic field is subject to near random reversals
on time scales of hundreds of thousands
of years. These reversals are attributed to the
chaotic behavior of the Earth’s dynamo. In solar
physics, similar but much more rapid and
local effects occur.
magnetic Reynolds number In ordinary hydrodynamics,
the Reynolds number gives the ratio
between inertial and viscous forces. If in
a flow the Reynolds number exceeds a critical
value, the flow becomes turbulent. A similar
definition can be used in magnetohydrodynamics,
only here the viscous forces do not depend
on the viscosity of the fluid but on the conductivity
of the plasma:
RM = UL
η
= 4πσUL
c 2
with U being the bulk speed, L the length scale,
σ the conductivity, and η = c 2 /(4πσ)the magnetic
viscosity. The coupling between the particles
therefore does not arise from collisions as in
ordinary fluids but due to the combined effects
of fields and particle motion.
© 2001 by CRC Press LLC
magnetism
magnetic secular variation Time variations
of the Earth’s magnetic field, usually taken to
imply the time variation of the part of the field
generated by the dynamo in the Earth’s core.
This generally includes most variation in the
magnetic field on periods of decades or longer:
external variations in the field, and their associated
induced internal counterparts, tend to be
on diurnal timescales, although some time averaging
of the field may cause measurements to
exhibit power on longer timescales such as that
of the solar cycle. On the other hand, the highest
frequency on which the core field varies is
not well known: geomagnetic jerks, for example,
appear to have timescales of around a year
or two.
magnetic shear The degree to which the direction
of a magnetic field deviates from the normal
to the magnetic neutral line, defined by the
loci of points on which the longitudinal field
component is zero. A sheared magnetic field indicates
the presence of currents since ∇×B = 0.
magnetic tension Basic concept in magnetohydrodynamics.
Graphically, magnetic tension
can be interpreted as a tendency of magnetic
field lines to shorten: if a magnetic field line
is distorted, for instance by a velocity field in
the plasma, a restoring force, the magnetic tension,
acts parallel but in opposite direction to the
distorting flow. Thus, magnetic tension can be
interpreted as a restoring force like the tension in
a string. This concept can be used, for instance,
to derive Alfvén waves in a simple way.
magnetism Magnetic fields and interactions.
In solar system physics, solar and planetary
magnetic fields are usually produced by moving
electric currents in an interior conducting
layer. In the sun, this is internal motion of the
ionized gas. In the case of the Earth, the conducting
layer is the liquid iron outer core. In the
case of Jupiter and Saturn, the magnetic fields
are produced by motions within a metallic hydrogen
layer, and for Uranus and Neptune an
interior slushy-ice layer is thought to give rise
to the magnetic fields. Mercury’s magnetic field
is produced by its very large iron core, but debate
continues as to whether this is an active
(i.e., currently produced by interior currents)
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