Interaction of Comets with the Interplanetary Medium

Abstract

The interaction of the solar wind and solar radiation with a comet, as the comet moves around the sun, is discussed. It is shown that the nature of this interaction is highly variable. It is also shown that the cometary neutrals play a dominant role in standing-off the solar wind, particularly during “quiet” conditions in the solar wind. Consequently the cometary ionosphere is generally highly incompressible, although it can be highly inflated subsequent to its interaction with a high speed solar wind stream.

The generally prevailing view, that the solar wind is gradually decelerated ahead of the outer shock front due to its contamination with heavy cometary ions, is shown to be not universally true. In certain circumstances the cometary neutrals cannot penetrate the ionopause to interact with the solar wind ahead of it. Then the solar wind is decelerated via a strong shock (M ≈ 2) present at other times.

At all times the outflowing cometary ions are decelerated and diverted into the tail by a strong “inner” shock which is also of a variable nature. When the cometary neutrals cannot penetrate the ionopause, this inner shock is of a special type. It becomes a hybrid “ion-neutral” shock, wherein not only the ions but also the neutrals are decelerated and diverted into the tail.

Strong outer shocks and ion-neutral inner shocks are present only when the comet is sufficiently close to the sun (d ≤ d_c). For an “average” (R_n ≈1 km) comet dominated by H₂O, d_c ≈ 0.7 5 AU, whereas for such a comet dominated by CO₂ or CO, d_c ≥ 23 AU.

Finally, the direct interaction of the solar wind and the ultraviolet solar radiation with the cometary nucleus, when it is sufficiently distant from the sun (d≥ 5 AU) is also discussed. It is shown that, while much of the sunlit hemisphere attains significant positive electrostatic potentials (≈ 5 V) as a result, the dark hemisphere attains numerically large negative potentials ≈ -1 kV). As a result of these surface potentials and associated electric fields, any loose, fine dust that may exist on the surface will levitate above the surface and the smallest grains (R_g ≤ 0.1 μ) will be completely “blown off.” Consequently at large heliocentric distances a comet could lose a portion of any dusty mantle it may possess without the assistance of any outflowing gases.