Physics and Chemistry of Comets 569
FIGURE 17 Images of comet Hale–Bopp in April 1997. The
left-hand image records the fluorescence emission from sodium
atoms and clearly shows the thin, straight sodium tail. Compare
to the right-hand image, which shows the traditional plasma and
dust tails. (Courtesy of Gabriele Cremonese, INAF-Astronomical
Observatory Padova, and the Isaac Newton Team.)
near perihelion are as high as 10^31 molecules s−^1. Visibility
of the sodium tail was enhanced by the sodium atom’s high
oscillator strength (one of the highest in nature), but the ex-
ceptional brightness of comet Hale–Bopp greatly increased
the likelihood of observing the sodium tails.
The plasma tails of comets are long and generally straight
and show a great deal of fine structure that constantly
changes. They are typically 10^5 –10^6 km wide, and the
lengths recorded optically are routinely several tenths of AU
(or several times 1.5× 107 km). The structure of the plasma
tail may extend much farther. Measurements of magnetic
fields and ions made on board theUlyssesspacecraft have
detected the signature of comet Hyakutake’s plasma tail
550 million km (or 3.7 AU) from the head.
These tails are composed of electron-molecular ion plas-
mas. As the neutral molecules in the coma flow outward,
they are ionized. Photoionization is the traditional process
and easiest to include in models. Impact ionization by solar
wind and cometary electrons and ionization by charge ex-
change also need to be considered. The result is to produce
the molecular ions H 2 O+,OH+,CO+,CO 2 +,CH+, and
N 2 +. Images of plasma tails, particularly those taken with
photographic emulsion, usually show the plasma tail a bright
blue because of strong bands of CO+(e.g., see Fig. 1).
These molecular ions cannot continue their simple out-
ward flow because they encounter the solar wind magnetic
field. The Larmor radius gives the radius for an ion spiraling
around the magnetic field lines, and a typical value is∼ 100
km. Thus, the solar wind and the cometary ions are joined
together. The magnetic field lines are said to be loaded with
the addition of the pickup ions and their motion slows down.
This effect is strong near the comet and weak well away
from the comet. The effect causes the field lines loaded with
ions to wrap around the comet like a folding umbrella. This
behavior is observed. These bundles of field lines loaded
with molecular ions form the plasma tail. The central, dense
part of the plasma tail contains a current sheet separating
the field lines of opposite magnetic polarity. Because the tail
is formed by an interaction with the solar wind flow, the tail
points approximately antisunward but makes an angle of
a few degrees with the prolonged radius vector opposite
to the comet’s orbital motion. The flow direction is given
by the aberration angle produced by the solar wind speed
and the comet’s motion perpendicular to the radius vector.
This aberration effect was used by L. Biermann to discover
the solar wind in 1951 and to estimate its speed. H. Alfv ́en
introduced the magnetic field into the interaction and gave
the basic view of plasma tails presented here. Spacecraft
measurements have verified this view. Note that plasma
tails usually should be considered as attached to the head
of the comet. This contrasts with dust tails where the tail
emanates from the head region but the dust particles are
on independent orbits. Additional complications from the
interaction with the solar wind are a bow shock and plasma
waves, which are present over very large volumes of space.
The interaction between the solar wind and a comet is
clearly shown in Fig. 18, which is a plot of results from the
ion analyzer on theDeep Space 1mission. The undisturbedFIGURE 18 Plasma results from comet
Borrelly measured by the ion analyzer on the
Deep Space 1mission. The times refer to
22–23 September 2001. The bar at lower
right was produced by xenon ions from the
spacecraft thruster. See text for discussion.
(Courtesy of Los Alamos National
Laboratory.)