COMET-P HALLEY - SPATIAL DISTRIBUTIONS AND SCALE LENGTHS FOR C2, CN, NH2, AND H2O

Spatial profiles were obtained for emissions by C2, CN, NH2, and O I (1D) from comet P/Halley long-slit spectroscopic exposures on 12 dates, extending from 1985 October to 1986 May. Haser model scale lengths were fitted to these data. The extended time coverage allowed us to check for consistency between the various dates. Not unexpectedly, the time varying production rate of P/Halley severely affected the profiles after perihelion. This is demonstrated in two profile sequences on adjacent dates, March 1/2 and April 14/15. Because of the time-varying production rate, it was difficult to obtain reliable Haser model scale lengths after perihelion. Our preperihelion analysis yielded Haser model scale lengths of sufficient consistency that they can be used for production rate determinations. For C2 a slight flattening of the profiles close to the nucleus could not be fitted with a two-step Haser model. If the inner region is excluded from the fit, the daughter/parent scale length ratio changes from near 1 to about 6 which is close to previous results from other comets. However, when production rates are sought using a two-step Haser algorithm, only an equal scale-length model comes close to providing an acceptable fit. Only three observations yielded CN daughter scale lengths because our profiles did not extend sufficiently far. Additionally, the long daughter of CN makes this emission very sensitive to production rate variations causing greater scatter in the parent values. The preperihelion data where the effects of time-varying production rate were minimal yielded a parent of 24 x 10(3) km in agreement with previous values. A curious asymmetry of the scale lengths was exhibited by NH2, with the postperihelion parent being about twice the preperihelion value, but the daughter being less than half the preperihelion number. A Swings effect, as occurs for the OH lifetime, is the best explanation for this behavior. The average parent scale lengths found for NH2 are consistent with photodissociation lifetimes for NH3 and the dissociation chain NH3 --> NH2 --> NH is corroborated. Most of the O I 1D profiles, which effectively map out the comet's H2O distribution, deviated very little from a 1/r falloff so that it was not possible to obtain a reliable H2O parent scale length, although consistency with the nominal lifetime of 80 x 10(3) s is demonstrated.