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, 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 de- flected 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 dis- ruptions 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 astron- omers are also watching for any solar activity on the east solar limb which might cause additional disruptions.
Interested observers can access the comet tail disruption watch world-wide web home page located at http://www-istp.gsfc.nasa.gov/istp/halebopp/
The SOHO CME (corona mass ejection) list is available at file://lasco6.nascom.nasa.gov/pub/lasco/status/LASCO_CME_List_1997
Miriam A. Forman
NASA Office of Space Science
Research Programs Management Division
Comet Hale-Bopp Home Page