DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

This implies that the tangent vectors to the congruence
form a vector field and, further, every
smooth vector field generates a congruence. See
vector field.
conical spacetime See deficit angle (cosmic
string).
conics Pitch angle distributions whose peak
intensity is neither along the field line (“cigar”)
nor perpendicular to it (“pancake”) but at some
intermediate angle. Ion conics (generally of O +
ions) are often observed on magnetic field lines
above the auroral zone, at altitudes of the order
of 5000 km, associated with the aurora. It is believed
that they arise from wave-ion interactions
which energize the ions at an altitude below the
one where the conic is observed. Such an interaction
preferentially increases the ion’s velocity
components perpendicular to the magnetic
field, producing a pancake distribution. By the
conservation of magnetic moment, the pancake
transforms into a cone at higher altitudes.
conic section Any of the plane figures obtained
by intersecting a circular cone with a
plane. If the plane is parallel to the base of the
cone, the figure is a circle. As the tilt angle β
of the plane increases, 0 < βα, the figure is a hyperbola.
In Newtonian physics, gravitational motion
is an orbit that is a conic section with the sun at
one focus.
conjugate depths The two water depths that
appear on either side of a hydraulic jump in open
channel flow. Also referred to as sequent depths.
They differ from alternate depths in that the specific
energy is not the same for conjugate depths
due to energy loss in the hydraulic jump.
conjunction Orientationofplanetssothatthe
angle planet-Earth-sun equals zero. For outer
planets, the planets are as far from Earth as possible
in their orbits, because they are then on
opposite sides of the sun. For inner planets,
conjunction is either a closest approach to Earth
(both planets on the same side of the sun), or the
© 2001 by CRC Press LLC
Conrad discontinuity
most distant (the inner planet on the opposite
side of the sun from Earth).
connection See affine connection.
connection longitude Heliographic longitude
to which a spacecraft is connected magnetically.
The connection longitudeφconn can be determined
from the “offset” φ of the footpoint
of the archimedian magnetic field spiral through
the observer with respect to the observer’s heliolongitude
φo according to
φ conn = φ o +φ =φ o + rω⊙
vsowi
with r being the radial distance of the observer
from the sun, ω⊙ the sun’s angular speed, and
v sowi the solar wind speed. The connection longitude
is important in the study of energetic particles
because, in contrast to the electromagnetic
radiation, particles do not propagate radially but
along the magnetic field line. For average solar
wind conditions, the Earth, or an earth-bound
spacecraft, magnetically is connected to a position
around W 58 on the sun. Connection longitudes
also can be established to shocks. See
cobpoint.
Heliolongitude and connection longitude.
Conrad discontinuity Seismic discontinuity
at a depth of around 20 km between the upper
and the lower crusts beneath a continent and an
island arc. The Conrad discontinuity is not a
sharp discontinuity like Mohorovičić’s discontinuity.
The Conrad discontinuity was originally
thought to represent a boundary between the
lower crust consisting of basaltic rocks and the
upper crust consisting of granitic rocks, deducing
from velocity of Conrad head waves (P *).