DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

or more km where the magma resides before an
eruption occurs and it ascends to the surface.
magmatic water Water that forms in phase
separation from magma. This water carries
large amounts of dissolved minerals which it deposits
in veins as it travels to the surface of the
Earth. Also called juvenile water.
magnetic anomaly The local deviation of
the geomagnetic field from the global field generated
by the dynamo in the core and the external
field, due to the magnetization of nearby
crust. This is usually obtained by measuring
the local magnetic field and then subtracting the
prediction of a global field model such as an
IGRF. Crustal rock may be magnetized in situ
after a heating event by cooling down through
its Curie temperature in the ambient magnetic
field of the Earth, or else magnetized particles
may be oriented on or after deposition in a sedimentary
environment by the Earth’s field so that
the rock is left with net magnetization. Magnetic
anomalies in oceanic plate show reversals
in magnetization as one travels perpendicular to
the mid-ocean ridge that produced them, due to
the occasional reversal of the Earth’s magnetic
fieldsothatrockthatcoolsatdifferenttimesmay
have oppositely oriented magnetization. This
provided evidence for the hypothesis of sea floor
spreading. Continental anomalies may be associated
with geological structures such as subductionorcollisionzonesorvolcanos,
wherethe
crust has been heated. Many anomalies, including
one at Bangui in Africa, are large enough to
be detected through satellite magnetic measurements.
magnetic bay A dip in the trace of a highlatitude
magnetogram, resembling a bay in a
shoreline. Magnetic bays observed this way
in the 1950s and earlier were later identified as
magnetic substorms.
magnetic carpet The distribution of magnetic
field in small scales over the quiet sun
which is observed to be recycled over a period
of 40 h. A discovery of the Michelson Doppler
Imager on the SOHO spacecraft.
© 2001 by CRC Press LLC
magnetic field
magnetic cloud The interplanetary counterparts
of coronal mass ejections (CMEs) that exhibit
the topology of helical magnetic flux ropes.
Magnetic clouds make up about one third of all
solar wind streams that are identified as interplanetary
consequences of CMEs.
magnetic crochet Accompanying a solar
flare, dayside magnetic fields may show a sharp
change, called a crochet, starting with the flare,
but shorter-lived, due to electric currents set up
in the lower ionosphere as a result of the flare induced
ionization and conductivity changes. See
short wave fadeout.
magnetic declination The azimuth, measured
from geographic north, of the geomagnetic
field vector at a given location.
magnetic diffusivity A measure of the ability
of a magnetic field to diffuse out of a volume
of plasma. A perfectly conducting plasma has
zero magnetic diffusivity. The solar atmosphere
has a magnetic diffusivity comparable to that
of copper wire. However, because of the large
scales involved on the sun, compared to the typical
length of a copper wire, the diffusion times
are extremely long.
magnetic dipole The simplest source of a
magnetic field envisaged to consist of two magnetic
poles of equal strengths but opposite signs
a small distance apart. The first-order geomagnetic
field is a dipole field.
magnetic field In general, the conceptualization
due to Faraday, and rigorized by Maxwell
of the laws relating the magnetic forces between
current loops (or fictitious magnetic monopoles)
as embodied in a space-filling collection of vectors
(a field) which collectively define magnetic
flux tubes. In geophysics, the magnetic of the
Earth or of other planets. On Earth, the field is
closely approximated by that of a dipole. The
north and south magnetic poles are relatively
close to the geographic poles. The magnetic
field is generated by dynamo processes in the
Earth’s liquid, iron-rich core. The rotation of
the Earth plays an important role in the generation
of the magnetic field and this is the reason
for the close proximity of the magnetic and ge-
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