X-ray and extreme ultraviolet emission from comet P/Encke 1997

In an effort to understand the newly discovered phenomenon of cometary X-ra y emission, we have obtained observations of the short period (3.3 years), well-studied comet 2P/Encke (Encke) during its July 1997 close approach to Earth. Extended, variable emission on the sunward side of the nucleus was f ound in the Rontgen X-ray satellite High Resolution Imager (HRI) at 0.090-0 .75 keV and in the Extreme Ultraviolet Explorer (EUVE) scanner telescopes' Lexan B 0.090- to 0.28-keV and Al/Ti/C 0.050- to 0.16-keV bandpasses; usefu l upper limits were found in the Ti/Sb/Al 0.020- to 0.040-eV bandpass. Simi lar to our results for C/Hyakutake. the emission morphology was roughly sym metric with respect to a vector from the comet's nucleus toward the Sun, wi th a light curve consisting of a slowly varying baseline emission and a lar ge impulsive event on 7 July 1997 with a time scale of similar to 3 h and a n amplitude of similar to 3 times the baseline. A count rate of similar to 0.17 counts s(-1) in the HRI was measured for the slowly varying emission, corresponding to a total luminosity L-x of 4 x 10(14) erg s(-1). Unlike Hya hutake, however the bulk of the emission clearly originates outside the com et's bow-shock, arguing against the magnetic reconnection and plasma dust e mission models. The comet's lightcurve does not correlate with the solar X- ray lightcurve, ruling out scattering of solar X-rays as the emission mecha nism. The multiwavelength HRI/EUVE photometry is inconsistent with the Habe rli et al. (1997, Science 276, 939-942) charge transfer, plasma dust, and a ttogram dust models of cometary X-ray emission and is consistent with the W egmann et al. (1988, Planet. Space Sci. 46, 603-612) charge exchange, 0.15- to 0.45-keV thermal bremsstrahlung, and photon index 1.6-2.0 power law mod els. While the impulsive event correlates very well with the passage of a s olar magnetic field boundary at the Earth and an increase in the solar wind particle flux, it is not coincident, according to current models of the so lar wind magnetic current sheet, with the passage of the sector boundary by the comet, suggesting that new models of the current sheet are necessary. (C) 1999 Academic Press.