DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

tidal period The time between two points of
equal phase in a tidal curve. Commonly measured
between two high or two low tides.
tidal prism The volume of water that is exchanged
between a tidal estuary and the sea in
one tidal period. Equivalent to the average tide
range within the estuary times the area of the
estuary. Since the range will vary in space and
time, the volume is not simply a horizontal slice,
but a complicated function.
tidal radius The radius within which all the
luminous matter of a cluster or a galaxy is contained.
The tidal radius can be measured for
globular clusters and for galaxies belonging to
clusters, which are found to have well-definite
outer limits (in contrast with brightness profiles
of isolated elliptical galaxies, described by de
Vaucouleurs or Hubble’s–Reynolds law). The
name arises from the understanding that in the
case of a cluster galaxy, repeated encounters
with nearby galaxies can lead to tidal stripping
of the outer stars, which are loosely gravitationally
bound, and to the evaporation of the outer
envelope, leaving only stars which are inside the
tidal radius.
tidal stripping The escape of gas and stars
gravitationally bound to a system, such as a
galaxy or a globular cluster, due to tidal forces
exerted by an object external to the system. For
example, in a cluster of galaxies, tidal stripping
may remove loosely bound stars from the galaxy
outer envelope; in a close encounter between
galaxies, stars and gas can be transferred from
one galaxy to the other.
tidal tail A highly elongated feature produced
by tidal forces exerted on a spiral galaxy
by a companion galaxy. A most notable example
of tidal tails is observed in the “Antennae”
pair of galaxies (NGC 4038 and NGC 4039),
where the tidal tail extends for a projected linear
size of ≈100 kpc, much larger than the size of
the galaxies themselves. Computational models
show that very extended tails, like the ones
in the Antennae, are produced by a prograde encounter
between galaxies, i.e., an encounter between
a spiral galaxy and an approaching com-
© 2001 by CRC Press LLC
time dilatation
panion galaxy which moves in the same sense
of the spiral rotation.
tide The response of the solid or fluid components
of a planet or other astronomical body,
under the influence of tidal forces.
tide range The vertical distance between
high tide and low tide at a point. Will vary in
time because of temporal variation in the tide
signal.
tilt angle In solar magnetohydrodynamics,
on the source surface, a neutral line separates
the two hemispheres of the sun with opposing
magnetic polarity. The maximum excursion of
this neutral line with respect to the heliographic
equator is called tilt angle. Since this neutral
line is carried outwards as the heliospheric current
sheet, the tilt angle also defines the waviness
of the current sheet and, therefore, is an important
parameter in the modulation of the galactic
cosmic radiation.
In terrestrial magnetospheric research, it is
the angle between the z-axis in GSM coordinates
and the dipole axis of the Earth. The GSM
x-y plane can be viewed as providing an approximate
north–south symmetry plane of the magnetosphere
(see equatorial surface). A tilt angle
ψ = 0 0 , therefore, signifies a magnetosphere
with the dipole axis perpendicular to the equator,
and the larger ψ, the more the axis departs
from the perpendicular. Sunward inclination of
the dipole gives ψ > 0, tailward inclinations
ψ