Encyclopedia of the Solar System 2nd ed

570 Encyclopedia of the Solar System

FIGURE 19 The spectacular 1996 disconnection event in comet
Hyakutake. The 24.82 March, 25.74 March, and 26.66 March
images appeared on the July 1996 cover ofSky and Telescope
and are courtesy ofSky and Telescopeand S. Numazawa, Japan.
The 25.09 March image is courtesy of P. Keller and G.
Schmidbauer, Ulysses Comet Watch. (Image sequence courtesy
of the Ulysses Comet Watch.)

solar wind flow is shown at approximately 700 eV/Q, and
it steadily decreases toward closest approach (CA) as the
solar wind flow is loaded by the addition of cometary ions.
The situation reverses as the spacecraft passes through the
comet. The higher energy ions are the pick-up molecules
from the comet.
The exception to the picture of plasma tails usually being
attached to the comet’s head is when disconnection events
(DEs) occur. Here, the entire plasma tail disconnects from
the head and drifts away. The comet forms a new plasma tail.
Many DEs have been observed over the last century, and
Fig. 19 shows a spectacular example in comet Hyakutake.
DEs occur when a comet crosses the heliospheric current
sheet (HCS). The HCS is an important feature in the so-
lar wind. It separates “hemispheres” of opposite magnetic
polarity and is, in essence, the magnetic equator of the helio-
sphere. When a comet crosses the HCS, the field lines being
captured by the comet (as described earlier) are of opposite
polarity. Thus, field lines of opposite polarity are pressed to-
gether in the comet causing the field lines to be severed by
the process of magnetic reconnection. The old plasma is no
longer attached to the head and moves away. Meanwhile,
the comet develops a new plasma tail. The sequence is a
regular process and repeats at each HCS crossing.

The HCS separates the heliosphere into regions of oppo-

site magnetic polarity and defines the latitudinal structure

of the solar wind. Well away from solar maximum, the so-

lar wind is organized into a dense, gusty, slow equatorial

region and a less dense, steady, fast polar region. These so-

lar wind properties are clearly reflected in plasma tails. In

the polar region, plasma tails have a smooth appearance,

show aberration angles corresponding to a fast solar wind,

and do not exhibit DEs. In the equatorial region, plasma

tails have a disturbed appearance, show aberration angles

corresponding to a slow solar wind, and exhibit DEs.

Although cometary X rays properly belong in the coma

discussion, they are included here because they are pro-

duced by a solar wind interaction. X rays in the energy

range 0.09–2.0 keV were unexpectedly discovered in comet

Hyakutake; see Fig. 20 for a false-color X-ray image of

comet LINEAR. When databases were searched, several

more comets were seen as X-ray sources. X-ray emission is

an expected phenomenon of all comets.

The principal mechanism is charge exchange between

heavy minor species in the solar wind and neutral molecules

in the coma. The heavy species in the solar wind are mul-

tiply ionized. For example, six-time ionized oxygen can

FIGURE 20 False-color rendering of an X-ray image of comet

LINEAR obtained on 14 July 2000 by the Chandra X-Ray

Observatory. (Courtesy of C. M. Lisse, University of Maryland,

College Park; D. J. Christian, Queens University, Belfast, United

Kingdom: K. Dennerl, Max-Planck-Institut f ̈ur

Extraterrestrische Physik, Garching, Germany; and S. J. Wolk,

Chandra X-Ray Center, Harvard-Smithsonian Center for

Astrophysics.)