D. Asoka Mendis

International Cometary Explorer encounter with Giacobini-Zinner - Magnetic field observations

The vector helium magnetometer on the International Cometary Explorer observed the magnetic fields induced by the interaction of comet Giacobini-Zinner with the solar wind. A magnetic tail was penetrated ∼7800 kilometers downstream from the comet and was found to be 104 kilometers wide. It consisted of two lobes, containing oppositely directed fields with strengths up to 60 nanoteslas, separated by a plasma sheet ∼103kilometers thick containing a thin current sheet. The magnetotail was enclosed in an extended ionosheath characterized by intense hydromagnetic turbulene and interplanetary fields draped around the comet. A distant bow wave, which may or may not have been a bow shock, was observed at both edges of the ionosheath. Weak turbulence was observed well upstream of the bow wave.

Special Issue Dedicated to Professor Hannes Alfvén on the Occasion of his 80th Birthday, 30 May 1988

Ten years ago, Astrophysics and Space Science dedicated its May issue to Professor Hannes Alfven on the occasion of his seventieth birthday. Time has now come to honour him on his eightieth birthday by publishing this special issue of Astrophysics and Space Science.

LUKEWARM MODELS OF THE INTERGALACTIC MEDIUM

The physical state of a moderately dense, uniform, pure hydrogen medium, pervading the metagalaxy and partaking in the general cosmic expansion, is considered. The cosmic X-ray photon spectrum, which is now observed down to about 0.25 keV, together with various plausible extrapolations down to the Lyman limit, is invoked as the source of ionization and heating of this medium. Isothermal and adiabatic modes of expansion of the intergalactic medium are studied as limiting cases of the present mode of expansion. The isothermal mode generates highly ionized but ‘lukewarm’ (7700 K≤T≤25000 K) models of the medium, with the, so far unknown, turnover point in the cosmic X-ray spectrum as the parameter required to fix the temperature and degree of ionization uniquely, for any assumed value of the present density. The adiabatic mode, while giving rise to high degrees of ionization, also produces significantly higher temperatures which are independent of density and always greater than about 28000 K. In conclusion, a possible explanation for the anomalous 3C 9 result is adduced.