DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Now the heliospheric field is not perfectly
aligned with the solar poles and equator (equivalently,
the underlying solar dipole is tilted with
respect to the rotation axis), so that the heliospheric
current sheet is tilted with respect to
the heliographic equator. Moreover, the solar
surface field is always far from being a perfect
dipole, and the heliospheric current sheet itself
is warped.
The sun rotates, and the heliospheric magnetic
field configuration must rotate as well.
Hence, a fixed observer near the equatorial plane
will be immersed first in field of one polarity
(inward, say), then in field of outward polarity,
then inward again, etc. The net result is
that the magnetic record shows two or more sectors
of opposite polarity, and this pattern is usually
approximately repeated in the next solar rotation.
The phenomenon is ubiquitous at low
heliographic latitudes, and in particular at each
planet the magnetic field in the solar wind will
exhibit alternating polarity. This organization of
the low-latitude heliospheric field into sectors of
opposite polarity is known as the interplanetary
magnetic sector structure. The heliospheric current
continues to exist at the largest heliocentric
distances where measurements have been made,
and presumably extends out to the heliospheric
current shock.
The heliospheric magnetic field, like all
quantities observed in the solar wind, also exhibits
a rich variety of transitory variations. The
large-scale morphology described above is a
background on which waves, turbulence and
other transient structures are superposed.
heliospheric stream structure Longitudinal
organization of the solar wind into faster
and slower streams (also called interplanetary
stream structure). Apart from the occasional
violent outbursts associated with coronal mass
ejections, the fastest solar wind comes from
coronal holes. Coronal holes most often are
found at relatively high solar latitudes, so that on
average the high-latitude wind is faster than the
wind at equatorial latitudes (velocity 800 km/s
as against 400 km/s). However, the polar coronal
holes often have equatorward extensions for
a substantial range of longitudes, so that there is
a substantial longitudinal variation in the equatorial
solar wind speed.
© 2001 by CRC Press LLC
helium
For much of the solar activity cycle this longitudinal
structure is long-lived enough that the
wind forms stream patterns that approximately
corotate with the sun. It should be stressed
that this corotation is wave-like in the sense
that although the pattern may corotate out to
large heliocentric distances, the plasma itself,
which is subject only to relatively weak magnetic
torques, does not corotate. In fact, beyond
a few solar radii the flow must be essentially radial,
so the rotation of the sun sets up a situation
in which fast wind will overtake slow wind from
below.
The interaction between fast and slow wind
occurs over several astronomical units in heliocentric
distance. By 1 a.u. there is substantial
compression near the stream interfaces. By
5 a.u. the interaction has proceeded to the point
that regions of compressed plasma bounded by
shocks, the so-called corotating interaction regions
or CIRs, are common; the amplitude of
the stream-structure velocity variation at 5 a.u.
is substantially smaller than at 1 a.u. This erosion
of the velocity structure continues as the wind
flows farther out, and beyond about 10 a.u. the
CIRs are no longer apparent.
heliospheric termination shock The shock
wave associated with the transition from supersonictosubsonicflowinthesolarwind;
seesolar
wind, heliosphere. The location of the termination
shock is estimated to be at 100 astronomical
units from the sun. Outbound heliospheric
spacecraft have passed 60 astronomical units heliocentric
distance, but have not yet encountered
the termination shock. There is optimism that
Voyagers 1 and 2 will encounter the shock within
the next decade or two.
helium (From Greek. helios, the sun.) An
odorles colorless gas; the second lightest (after
hydrogen) gas, atomic number: 2. Consisting of
two isotopes: 3 He, and 4 He. 4 He is the second
most abundant element in the universe (presumably
formed early on in the Big Bang), but is very
rare on Earth (partial pressure ≈ 10 −5 atm) at
the surface of the Earth. The helium content of
the atmosphere is about 1 part in 200,000.
Helium was first detected in 1868 in the solar
spectrum and in 1895 in uranium-containing
minerals. In 1907, alpha particles were demon-
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