DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

esponding chromospheric network defined by
bright emission in Ca K spectroheliograms.
Neupert effect Statement that the temporal
derivative of the observed soft X-ray emission
during a solar flare reproduces the observed time
development of the hard X-ray emission. This
is found in many flares and indicates that a single
energization process is responsible for the
production of both the non-thermal and thermal
radiation.
neutral equilibrium In mechanics, a configuration
in which the system experiences no net
force (equilibrium), and if the system is moved
slightly from this state, no forces arise, so a
nearby configuration is also an equilibrium. The
paradigm is a ball on a smooth horizontal surface.
See stable equilibrium, unstable equilibrium.
neutral point In electromagnetism, a point
in a magnetic configuration in which the field
intensity B drops to zero, and where therefore
the direction of the magnetic field is undefined.
Expanding the field B around an isolated neutral
point gives B = B0 + r ·∇B0 in the immediate
neighborhood of the point, with subscript zero
marking values at the point. Since B0 = 0, the
constant dyadic ∇B0 determines the character
of the field. In an “x-type neutral point,” ∇B0
has three real characteristic roots and three field
lines cross at the point, giving a configuration
like the letter “x” but usually an intricate smallscale
geometry. In an “o-type neutral point”
∇B0 has just one real root and the point is the
limiting point of nested closed field lines.
Neutral lines of x-type and o-type form
in two-dimensional geometries (extending unchanged
in the third dimension), where the number
of real roots is 2 or 0. The nearby field lines
either form an x-shaped pattern outlined by the
eigenvectors of ∇B0, or form nested o-shaped
loops.
Neutral points are of great interest in space
plasma physics because theory associates them
with magnetic reconnection. The polar cusps
of the Earth magnetosphere are also evolved (xtype)
neutral points.
In atmospheric physics, the points where the
degree of polarization of sky diffuse radiation
© 2001 by CRC Press LLC
neutrino annihilation
equals zero. Due to aerosol scattering, multiple
scattering and reflecting from surfaces, polarization
of sky light is not consistent with the
ideal status; there will be some abnormal neutral
points. According to Rayleigh scattering theory,
under ideal conditions the solar point (the direction
of solar incident ray) and anti-solar point
are neutral points. In the real atmosphere, in
general, there are three neutral points: Arago
neutral point, which locates about 15 ◦ to 25 ◦
above the anti-solar point; Babinet neutral point,
which locates about 12 ◦ to 25 ◦ above the sun;
and Brewster neutral point, which locates about
15 ◦ to 25 ◦ below the sun. All these angles and
positions may change due to the variations of the
position of sun, the atmospheric turbidity and
the reflective properties of the Earth surface.
neutral stability In meteorology, the stratification
status when the lapse rate of air temperature
(γ ) equals its adiabatic lapse rate (γd).
neutrino A stable elementary particle which
carries zero electric charge, angular momentum
of 1 2 ¯h, little or no mass, but finite momentum
and energy. Invented by Pauli in the
1930s to make energy, momentum, and angular
momentum conservation hold in weak nuclear
decays, the electron neutrino was identified in
the laboratory by Cowan and Reines in 1953.
Two additional types, called the µ and τ neutrinos
(for the other particles co-produced with
them), have been found since. Each has an antiparticle,
leading to a total of six types. In astrophysics,
electron neutrinos are produced in
stellar nuclear reactions, including the protonproton
chain and CNO cycle. All three types
are produced by Type II Supernovae, and there
should be a thermal sea of all types left from
the early universe, corresponding to the cosmic
microwave background radiation. Solar neutrinos
and those from one supernova, 1987A, have
been observed in laboratory detectors.
neutrino annihilation In core-collapse supernovae
and gamma-ray burst models, the
number density of neutrinos and anti-neutrinos
can become large, and the rate of their annihilation
and production of electron/positron pairs
and photons can be an important part of the
explosion energy. However, the annihilation
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