DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

anywhere on the surface of a sphere of radius
equal to the length of the suspension string. As
with a simple pendulum, for small oscillations
the motion is simple harmonic with angular frequency
ω = (g/L) .
See simple harmonic motion.
Spica 0.98 magnitude star of spectral type
B1 at RA 13 h 25 m 11.5 s , dec −18 ◦ 18 ′ 21 ′′ .
spicule Chromospheric jet of plasma seen at
the solar limb. Spicules are ejected from the high
chromospheric part of supergranule boundaries
and reach speeds of 20 to30 kms −1 and heights
of about 11,000 km before fading. Spicules
have typical lifetimes of 5 to 10 min, diameters
of 50 to 1200 km, maximum lengths of
10,000 to 20,000 km, temperatures of 10,000
to 20,000 K and electron densities of 3×10 10 to
3×10 11 cm −3 .
spin coefficients The connection components
Ɣabcd ′ = (∇CD ′ζaA )ζ A
b ζ C c
¯ζ D′
d ′ in the SL
(2,C) spinor dyadic basis ζ A a introduced by Newman
and Penrose. They are equal to the complex
Ricci rotation coefficients in the null basis defined
by the spinor dyad, and are individually
denoted
Ɣabcd ′ =
\ ab
\ 00 01 11
cd ′ \
00 ′ κ ɛ π
10 ′ ρ α λ
01 ′ σ β µ
11 ′ τ γ ν
See affine connection.
spinel Olivine, (Fe,Mg)2SiO4, is one of the
major constituents of the Earth’s mantle, and is
expected to be an important component of the
mantles of the other terrestrial planets as well.
© 2001 by CRC Press LLC
spiral arm
Under high pressures (∼ 10 10 Pa) the olivine
crystal structure undergoes a phase change to
a more compact phase with a resulting density
increase of several percent. This more compact
polymorph of olivine is called spinel. In
the Earth, the olivine-spinel phase change is
believed to be responsible for the density discontinuity
at a depth of 400 km (pressure of
13.4 GPa).
spinor A complex 2-dimensional vector.
Spinor representations are typically used in
quantum mechanics, e.g., to represent the spin
of an electron, and in some formulations in general
relativity.
S–P interval Time difference between P- and
S-waves arriving at a point. P-wave velocity is
about √ 3 times as fast as S-wave velocity. When
underground velocity structure can be regarded
as homogeneous, S–P interval (τ sec) is proportional
to hypocentral distance (R km). Thus, for
shallow earthquakes
R =kτ (Omori’s formula for
hypocentral distance)
with
k = VpVs
Vp − Vs
where Vp and Vs are P- and S-wave velocities,
respectively. The value of k is dependent on locations
and depths of earthquakes, taking values
of about 6 to 9 km/s.
spin up The transient initial stage of certain
numerical ocean simulations when the various
modeled fields are not yet in equilibrium with
the boundary and forcing functions.
spiral arm The part of the spiral pattern of a
galaxy which is more or less continuously traceable.
Spiral arms are very evident in the blue and
visual bands (and hence were revealed since the
early days of extragalactic astronomy) or, with a
more “knotty” appearance, in narrow band images
centered on the hydrogen Balmer line Hα.
This indicates that spiral arms are sites of recent
star formation. Spiral arms accordingly
become less and less prominent at longer wavelengths,
and almost undetectable in the near infrared,
where most of the light is produced by
439