DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Helios mission
mination shock, and on the outside by the heliopause.
See heliosphere, solar wind.
Helios mission German–US satellite mission
to study the inner heliosphere. The instrumentation
includes plasma, field, particle, and dust
instruments. Two identical satellites, Helios 1
and 2, were launched into highly elliptical orbits
with a perihelion at 0.3 AU and an aphelion
at 0.98 AU. The combination of the two satellites
allowed the study of radial and azimuthal
variations; the mission lasted from 1974 to 1986
(Helios 1) and 1976 to 1980 (Helios 2).
heliosphere The cavity or bubble in the local
interstellar medium due to the presence of the
solar wind. The size of the heliosphere is not
yet established, but typical length scales must
be of order one to several hundred astronomical
units. Heliospheric plasma and magnetic field
are of solar origin, although galactic cosmic rays
and neutral interstellar atoms do penetrate into
the heliosphere.
The heliosphere is thought to comprise two
large regions; the interior region is the hypersonic
solar wind, separated from the exterior
shocked-plasma (heliosheath) region by the heliospheric
termination shock. The boundary
between the heliosheath and local interstellar
medium is called the heliopause.
If the flow of the local interstellar medium
is supersonic with respect to the heliosphere, a
termination shock will be formed in the interstellar
gas as it is deflected by the heliosphere.
Because of substantial temporal variations in
the solar wind (and, possibly, in the local interstellar
medium) it is likely that the termination
shock and heliopause are never static, but undergo
some sort of irregular inward and outward
motions.
heliospheric current sheet (HCS) The current
sheet that separates magnetic field lines
of opposite polarity which fill the northern and
southern halves of interplanetary space (“the heliosphere”).
The sun’s rotation, combined with
the stretching action of the solar wind, gives the
HCS the appearance of a sheet with spiral waves
spreading from its middle. The spiral structures
are responsible for the interplanetary sectors ob-
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served near the Earth’s orbit. See interplanetary
magnetic sector.
heliospheric magnetic field The magnetic
field that fills the heliosphere. Because coronal
and heliospheric plasmas are excellent electrical
conductors, the magnetic field is “frozen into”
the expanding gas. Solar wind gas, once it has
accelerated away from its coronal-hole origin,
is hypersonic and hyper-Alfvénic, so its kinetic
energy exceeds its magnetic energy and the field
is passively carried along by the wind. The field
lines are anchored in a rotating solar source, but
carried along by a wind flowing in the outward
radial direction, and may be idealized as lying
on cones of constant heliographic latitude within
which they are twisted to form a global spiral
pattern.
The angle ψ between the field and the radial
direction (called the Parker spiral angle) is given
by tanψ = rcosλ/V , where r is heliocentric
distance, is the angular velocity of rotation of
the sun, λ is heliographic latitude, and V is the
solar wind flow speed. In the ecliptic plane at
1AU,ψ is of order 45 ◦ , and as r→∞ the field is
transverse to the flow direction. In situ observations
of the heliospheric field are in accord with
this idealized global picture, although modest
quantitative deviations have been reported.
Although the magnetic field at the sun’s surface
is very complicated, there is an underlying
dipole pattern except perhaps for brief periods
near the time of maximum sunspot activity,
when the solar magnetic dynamo reverses
its polarity. When the underlying dipole component
is present, the polarity of the heliospheric
magnetic field at high northern or southern heliographic
latitudes coincides with the polarity
of the magnetic field in the corresponding highlatitude
regions of the solar surface. For example,
the Ulysses spacecraft’s mid-1990s observations
of high-latitude fields show outward
polarity at northern latitudes and inward polarity
at southern latitudes; these polarities will be
reversed in the next sunspot cycle.
Thus, the heliospheric magnetic field may
be characterized in the first approximation as
consisting of two hemispheres consisting of oppositely
directed spiraling field lines. These
two hemispheres are separated by a thin current
sheet, called the heliospheric current sheet.