DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Peru current
case). Perturbative solutions are ubiquitous in
astrophysics. Planetary orbits and the positions
of planets on the orbits are calculated by such
perturbative methods where the small parameter
is the eccentricity of the orbit. In cosmology, the
most commonly used description of the formation
of galaxies is by solving Einstein’s equations
perturbatively, with the small parameter
being the size of the matter deviation from the
spatially homogeneous background. Perturbative
solutions are often possible where an exact
solution seems prohibitively difficult, but they
have their own difficulties. In applications in
general relativity care must be taken to distinguish
between genuine perturbations and spurious
perturbations.
Peru current A cold ocean current running
up the west coast of the South American continent,
feeding into the South Equatorial current.
Petchek reconnection Fast reconnection at
an X-point or in a small localized region. About
three-fifths of the inflowing magnetic energy is
converted into kinetic energy behind the shock
waves, the remaining two-fifths heat the plasma.
WithvA,in being the Alfvén speed in the inflowing
plasma, L being the length scale of the reconnection
region,σ being the conductivity, and
c being the speed of light, the reconnection rate
Rp can be written as
Rp = π
8 ln
c
LσvA,in4π
.
Petchek reconnection varies only weakly with
the conductivityσ and is very efficient in mixing
fields and plasmas. It probably does not play
a role in magnetospheric plasmas but might be
important in solar flares.
Petchek reconnection is stationary reconnection:
the onset of reconnection does not destroy
the general field configuration. Stationary reconnection
results in an equilibrium between inflowing
mass and magnetic flux, magnetic diffusion,
and outflowing mass and magnetic flux.
Petrovtypes Typesinaclassificationofgravitational
fields according to the algebraic structure
of the Weyl tensor (a tensor algebraically
constructed from the Riemann tensor to be completely
traceless), introduced by Petrov (1966).
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In general relativity the traces of the Riemann
tensor are pointwise determined by the stressenergy
tensor. The trace-free part, the Weyl tensorCabcd
is symmetric in the skew pairsab and
cd of indices. There are six independent components
of each skew pair; hence the Weyl tensor
can be viewed as a symmetric 6× 6 matrix.
The eigenvalues of this matrix sum to zero. An
alternate (calculationally simpler) development
can be given in terms of spinors. The eigenvectors
determine null vectors; for the general type
I there are the four different principal spinors
αA ,βA ,γA and δA which determine different
principal null directions. For types II, III, and N,
respectively, two, three, and all four of the principal
directions coincide. Type D arises when
there are two pairs of coincident principal null
directions. When the Weyl curvature vanishes,
the type is O. See principal spinor, Weyl tensor.
Petzold data A widely used data set containing
volume scattering functions for various
waters ranging from very clear to very turbid;
the scattering phase function is highly peaked at
small scattering angles and is somewhat independent
of water type.
Pfund series The set of spectral lines in the
extreme infrared region of the hydrogen spec-
trum with frequency obeying
ν = cR
1/n 2 f − 1/n2
i
where c is the speed of light, R is the Rydberg
constant, and nf and ni are the final and initial
quantum numbers of the electron orbits, with
nf = 5 defining the frequencies of the spectral
lines in the Pfund series. This frequency is
associated with the energy differences of states
in the hydrogen atom with different quantum
numbers via ν = E/h, where h is Planck’s
constant, and where the energy levels of the hydrogen
atom are:
En = hcR/n 2 .
phase In planetary astronomy; variance in
the total amount of the visible disk of the moon,
or of the inner planets, which appears illuminated
at any one time. Phases arise because our
vantage point sometimes allows the unlit side of
,