DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

the Earth. These interactions can generate large
current sheets in the magnetosphere and ionosphere,
whose associated magnetic fields can
have direct effect on power transmission systems
because of the continental scale of those
systems.
geomagnetic elements The geomagnetic
field intensity, F, can be characterized at any
point by its geomagnetic elements. These may
be expressed as the magnitudes of three perpendicular
components, or by some other set of
three independent parameters. Several different
sets are used, depending on context. Generally,
the preferred set is (X, Y, Z) where X is the component
of the geomagnetic field measured to the
geographic north direction; Y is the component
of the geomagnetic field measured to the geographic
east direction; and Z is the component
of the geomagnetic field measured in the vertical,
with the positive sign directed downwards
(so the geomagnetic Z component is positive in
the northern hemisphere). Other elements are
H, D, and I where H is the horizontal component
of the geomagnetic field measured in the
direction of the north geomagnetic pole; D, the
magnetic declination, is the angle between the
direction of the geomagnetic field, the direction
a compass needle points, and true geographic
north, reckoned positive to the east; and the inclination,
I, or dip angle, is the angle that the geomagnetic
field dips below the horizontal. See
geomagnetic field, nanotesla.
geomagnetic field The magnetic field intensity
measured in and near the Earth. This is a
vector, with the direction defined such that the
north-seeking pole of a compass points toward
the geomagnetic north pole and in the direction
defined as positive field. Thus, the magnetic
field lines emerge from the Earth’s South Pole
and point into the North Pole. The surface intensity
of the Earth’s magnetic field is approximately
0.32× 10 −4 tesla (T) at the equator and
0.62 × 10 −4 T at the North Pole. Above the
Earth’s surface, the field has the approximate
form of a magnetic dipole with dipole moment
7.9 × 10 15 Tm −3 . Many earth-radii away, this
dipole is distorted into a teardrop with its tail
pointing anti-sunward by the magnetized solar
wind plasma flow. The principal sources of
© 2001 by CRC Press LLC
geomagnetic jerk
the geomagnetic field are convective motions
of the Earth’s electrically conducting fluid core,
magnetization of the crust, ionospheric currents,
and solar wind perturbations of the geomagnetic
neighborhood.
geomagnetic indices Geomagnetic indices
give a very useful descriptive estimate of the
extent of geomagnetic disturbances. Most common
indices of the Earth’s magnetic field are
based on direct measurements made at magnetic
observatories. Local indices are calculated
from these measurements and then global
indices are constructed using the local indices
from selected, standard locations. These global
indices are often referred to as planetary indices.
Commonly used indices are the K index (a local
index) and the Kp index (the associated planetary
K index, which depends on a specific number
of magnetic observatories); the A index (essentially
a linear local index) and Ap index (the
associated planetary index). Other important indices
are the AE index, which gives a measure
of the currents flowing in the auroral region, and
the Dst index, which gives a measure of the currents
flowing in the Earth’s magnetosphere. See
Ap, K, Kp indices.
geomagnetic jerk It has been observed that
there are on occasion impulses in the third time
derivative of the geomagnetic field (i.e., “jerks”
in the field, which appear as an abrupt change
in the gradient of the secular variation). These
appear to be global phenomena of internal origin,
with recent examples around 1969, 1978,
and 1991, and claims of several earlier examples.
The effect appears to be most notable in
the eastward component of the magnetic field,
and the phenomenon appears to take place over
a timescale of perhaps 2 years. Unless the mantle
behaves as a complicated filter causing slow
changes in the core to be revealed as relatively
rapid variation at the Earth’s surface, the sharpness
of this phenomenon places a constraint on
mantle conductivity: it must be sufficiently low
so that the magnetic diffusion timescale of the
mantle (over which sharp changes in the field at
the core’s surface would be smoothed out when
observed at the Earth’s surface) is at most the
same order of magnitude as that of the jerk. It
is not entirely clear what causes geomagnetic
199