DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Ballerina model
tosphere through lunar valleys. If only one bead
is apparent, a “diamond ring effect” is produced.
Ballerina model Shape of the heliospheric
current sheet as proposed by H. Alfvén in the
early 1970s. The wavy neutral line on the source
surface is carried outwards by the solar wind,
resulting in a wavy heliospheric current sheet
that resembles the skirt of a dancing ballerina.
The waviness of this current sheet is described
by the tilt angle. See heliospheric current sheet,
source surface, tilt angle.
Balmer series The series of lines in the
spectrum of the hydrogen atom which corresponds
to transitions between the state with principal
quantum number n = 2 and successive
higher states. The wavelengths are given by
1/λ =RH(1/4 − 1/n 2 ), where n = 3, 4,...
and RH is the Rydberg constant for hydrogen.
The first member of the series (n = 2 ↔ 3),
which is often called theHα line, falls at a wavelength
of 6563 Å. See Rydberg constant.
banner cloud An altocumulus lenticularis
cloud, (lenticular cloud) which forms on the lee
side of the top of a mountain and remains a stationary
feature with one edge attached to the
ridge of the mountain.
bar A unit of pressure, defined as 10 6
dyne/cm 2 (10 5 N/m 2 ). The bar is commonly
divided into 1000 mb. The pressure of atmosphere
at sea level is about 1013 hPa = 1.013 bar.
bar detectors Solid bars, made of metallic
alloys, for detecting gravitational waves. The
technology was pioneered by J. Weber. The sensitivity
of h = 10 −18 (relative strain) has been
reached in second-generation cryogenic detectors.
At this level of sensitivity, no gravitational
wave event has yet been reliably detected. See
LIGO.
barium release An experimental procedure
near-Earth space physics, in which barium is
evaporated by a thermite charge, usually in a
sunlit region above the (denser) atmosphere,
creating a greenish cloud. Barium atoms are
rapidly ionized (within 10 sec or so), and the
ions form a purple cloud, which responds to
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magnetic and electric fields and therefore often
drifts away from the neutral one.
The technique was pioneered by Gerhard
Haerendel in Germany, and many releases have
been conducted from rockets above the atmosphere,
often with the barium squirted out by a
shaped explosive charge. Typically releases are
made after sunset or before sunrise, so that the
rocket rises into sunlight but the clouds are seen
against a dark sky. Effects of electric fields have
been observed, striations like those of the aurora,
even abrupt accelerations along field lines.
Some releases have taken place in more distant
space, notably an artificial comet in the solar
wind, produced in 1984 by the AMPTE mission.
Barnard’s star Star of spectral type M3.8,
5.9ly distant with magnitude m = 9.5 and absolute
magnitude M = 13.2; located at Right
Ascension 17 h 58 m , declination +04 ◦ 41 ′ . Discovered
in 1916 by E.E. Bernard, it has the
largest known proper motion: 10.29 arcsec/year.
Measurements by van de Kamp had suggested
that there were perturbations of the motion corresponding
to an associated planet, but recent
observations using the Hubble space telescope
have not confirmed this claim.
baroclinic atmosphere or ocean An atmosphere
or ocean in which the density depends
on both the temperature and the pressure. In a
baroclinic atmosphere or ocean, the geostrophic
wind or current generally has vertical shear, and
this shear is related to the horizontal temperature
or density gradient by the thermal wind relation.
baroclinic instability A wave instability that
is associated with vertical shear of the mean flow
and that grows by converting potential energy
associated with the mean horizontal temperature
or density gradient.
baroclinic wave Wave in the baroclinic flow.
barotropic atmosphere or ocean An atmosphere
(or ocean) in which the density depends
only on the pressure. In the barotropic atmosphere
or ocean, the geostrophic wind or current
is independent of height.