NASA Press Release 97-89
NASA/ESA scientists say that between 28 April and 15 May, comet Hale-Bopp
and especially its blue "ion" tail will be most susceptible to influences
from the Sun's solar wind and magnetic field. Its ion tail is expected to
begin showing fluctuations, kinks , and perhaps moving structures. Also,
it is possible that a disconnection event might occur, when the ion tail
breaks off from the comet's head and reforms a few hours later.
The Sun has a permanent abrupt, and well-known change in magnetic field
direction in the solar wind located close to the solar equator, which the
comet will intersect. The Sun also releases a large amount of ionized
gases from quick-acting disturbances, like that recently occurring on April 7. Both types of solar
phenomenon can dramatically alter the comet's tail and, on rare occasions,
can "disconnect" the comet ion tail from the comet head.
Comet Hale-Bopp has two tails (actually, a third
been discovered!). The two tails result from the two
types of material evaporating off the "dirty snowball" nucleus as it warms
in the sunlight. There is a curved dust tail of fine gritty hard material
that brightly reflects sunlight and is easily observable. The other,
dimmer tail is an ion tail, distinguished by its bluish color in
photographs. This tail consists of gases of water, carbon monoxide and
other simple atoms and molecules evaporated off the comet that have become
electrically charged by interacting with sunlight. The charged atoms and
molecules, called ions, are then pushed along by the interplanetary
magnetic field embedded in the solar wind. (The solar wind is a stream of
charged particles that comes from the sun and drags the solar magnetic
field out into interplanetary space). It flows out from the Sun at
tremendous speed (about a million miles per hour), more than a hundred
times as fast as the comet.
Thus, the comet ion tail responds as a sort of "wind sock", that gets
deflected in the direction of the outflowing solar wind. If the
interplanetary magnetic field and solar wind are disturbed, the effect
will become quite visible, most likely as a rippling in the ion tail.
Hale-Bopp is now rapidly approaching the solar equatorial region, where
the wind is gusty and the magnetic field irregular. There it will
encounter well-known changes in the main direction of the interplanetary
magnetic field. On April 21, Hale-Bopp will be down to 15 degrees from the
solar equator, and on May 3, it passes through the plane of the solar
equator into the southern solar hemisphere.
The region of interplanetary magnetic field reversal is shaped like a wavy
disk which rotates with the sun once in 25 days. The complex solar wind is
now confined to about 25 degrees from this wavy disk. Also, the tilt of
the wavy disk is itself less than 15 degrees. A much more precise estimate
can be made with a current sheet map, based on measurements from
spacecraft, including WIND, SOHO (Solar Heliospheric Observatory), and
Ulysses. (The first two are part of NASA/ESA's International Solar
Terrestrial Program (ISTP) which analyzes data in correlative studies of
the Sun and solar wind.) The map shows that the region of complex wind
will be sinking to the south as Hale-Bopp is approaching 15 degrees, so
that this comet's ion tail will not likely be disturbed until it gets
down to less than 10 degrees, soon after April 28.
The first half of May should be the optimal time of disruptions in
Hale-Bopp's ion tail. Scientists anticipate that the major
characteristics of the map of solar wind will remain stable, but they
also say that the Sun could eject an unexpected quick-acting disturbance
at any time, similar to one on April 7.
Comet observers all over the world will be looking at Hale-Bopp for
disruptions all through this period. A person viewing with the naked eye
might see a "lumpy object" or luminous mass in the comet tail, this
traveling outward along the tail for a number of days. With photographic
equipment, this mass should be resolvable as an anomaly in the ion tail.
Solar astronomers are also watching for any solar activity on the east
solar limb which might cause additional disruptions.
Interested observers can access the ISTP page on comet Hale Bopp observations page located at http://www-istp.gsfc.nasa.gov/istp/halebopp/.
The SOHO CME (corona mass ejection) list is available at