Hydrogen Coma Research Articles

A model for the hydrogen coma of a comet

A model for the hydrogen coma of a comet on the basis of the Monte Carlo method is presented. In this model isotropic ejections of H atoms produced by photodissociation of H 2O and OH, thermalization of the H atoms due to collisions with ambient H 2O molecules, and the solar radiation pressure have been taken into account. A production spectrum of H atoms from OH is evaluated by using the predissociation rates and the level populations of OH, confirming that the spectrum has a sharp peak around 8.0 km sec −1 with the standard deviation of 0.1 km sec −1. Including the above effects, velocity distribution functions of the H atoms at various positions in the coma for the first time, as well as their density and outflow velocity profiles, have been calculated. It is pointed out that the collisional thermalization process in the inner coma is an important factor at small heliocentric distances in determining the density profiles and the velocity distributions. It is shown that thermalization leads to an increase in the H density in the inner coma larger than those expected from other models such as the vectorial model, in which collision is not taken into account. Lyman α isophotes and its line profiles in the optically thin region are computed by using the velocity distribution function.

Hydrogen coma of comet P/Halley

The main molecular processes to produce the hydrogen comae of comets are now well known: Water, the main constituent of cometary atmospheres, is photodissociated by the solar ultraviolet radiation to form the high (20 km s −1) and low (8 km s −1) velocity components of the atomic hydrogen. The hydrogen clouds of various fresh comets have been observed in 1216Å by a number of spacecrafts. Ultraviolet observations of short period comets are, however, rather rare. Consequently Comet P/Halley in this apparition is a good object to obtain new physics of the hydrogen coma. Strong breathing of the hydrogen coma of this comet found by “Suisei” provides just such an example. The rotational period of Comet Halley's nucleus, its activity in the form of outbursts alone, and the position of jet sources etc. are determined from the breathing phenomena. Atomic hydrogen from organic compounds with a velocity of 11 km s −1 play an important role in that analysis. The time variations of the water production rate of Comet Halley during this apparition observed by various spacecrafts appear to be in agreement with each other and are about 1.5–2 times larger than the standard model. The difficulty of the calibration problem was emphasized.

Nucleus studies of comet Halley

Early results from, and research initiatives warranted by, the Earth-based observations of Halley's near-nucleus and related phenomena are reviewed. Where appropriate, this information is combined with spacecraft data obtained by the various flight projects. The basic objective is to gain a greater insight into the nature of the comet's nucleus and its environment. Among topics are the brightness variations at large heliocentric distances along the inbound leg of the orbit; the bulk and rotational properties of the nucleus, including possible precession; the surface morphology and the formation of dust jets; subfemtogram dust particles and their presence in a sunward spike and relation to CN jets; comparison of the hydrogen coma's “pulsation” pattern with the surface distribution of major dust vents; the events causing Giotto's wobbling near its closest approach to the comet; and the recent developments in theoretical modeling of the icy-conglomerate nucleus.

Planet-A mission to Halley

Looking at the chance of the next apparition of the Halley comet in 1986, ISAS decided to send a first Japasanese interplanetary spacecraft for the study of cometary hydrogen coma and solar wind. The Planet-A spacecraft which carries VUV imaging camera and solar wind plasma analyser will be launched in August 1985 and flyby the Halley comet in early March 1986 with the distance of several million kilometers from the comet nucleus. This mission is not only self-consistent but collaborative with other space mission as well as earth-bound observations. In the present paper, the Planet-A mission to Halley is described with brief explanation of the spacecraft.

Wide field ultraviolet observations of Comet Halley with the FAUST Spacelab I instrument

The wide field (7.5°), arc minute imaging, and spectroscopic capabilities of the Far Ultraviolet FAUST telescope which will be flown on Spacelab I can provide valuable information on Comet Halley. The use of the FAUST instrument in obtaining images of the hydrogen coma at 1216 Å, and in obtaining objective grating spectroscopy from 1300–3300 Å of the comet and tail, are described. The FAUST images would provide large field of view data that are required for model calculations of gas production rates and the determination of scale lengths and lifetimes of ion species.

Scientific instrumentation of PLANET-A VUV imaging of the hydrogen coma of Halley

The PLANET-A spacecraft to fly by Comet Halley is equipped with a VUV imaging camera which will take pictures of the hydrogen coma of the comet. The camera is composed of a telescopic mirror lens, a VUV image intensifier, two dimensional CCD, and controlling electronic circuits with a microprocessor. In order to eliminate the blur in the image due to the spinning motion of the spacecraft, a special technique called “spinsynchronized charge swift” is used in the CCD driving.

A High-Velocity Component of Atomic Hydrogen in Comet Bennett (1970 II)

The Lyman alpha emission from Comet Bennett (1970II) was measured near perihelion (March 1970) by the University of Colorado ultraviolet photometer experiment on OGO-5 The spectrometer field of view of about 3° crossed the cometary hydrogen coma four times. The hydrogen coma was observed to extend more than 30 x 106 km in the antisolar direction.A model for the hydrogen density was developed which took the actual cometary motion and the gradients of the forces of gravitation and radiation pressure into account Exact trajectories of atoms in the orbital plane representing the column densities perpendicular to the plane were calculated. The variation of the hydrogen lifetime along the trajectory as well as the solar Lα profile were considered.

On the Cometary Hydrogen Coma and Far UV Emission

Comet Tago-Sato-Kosaka (1969 IX, hereafter TSK) was the first medium bright comet passing by in the era of ultraviolet satellites. The Orbiting Astronomical Observatory (OAO-2), observed in January 1970 the strong Lyman alpha signal at 1216 A which led to the first detection of cometary hydrogen. The peak signal in the photometer field-of-view (FOV) of 10′ diameter was about 70 kR (Code et. al , 1970) The resonance scattering emission of hydrogen was optically thick in the central part of the coma. A rocket experiment of Jenkins and Wingert (1972) using an objective grating spectrograph confirmed the OAO-2 observations Later in Spring 1970 comet Bennett (1970 II) was observed in Lα by OAO-2 and by two photometers onboard the Orbiting Heophysical Observatory (0G0-5) (Bertaux and Blamont, 1970; Keller and Thomas, 1973).

Lyman-α imagery of Comet Kohoutek

Electrographic imagery of Comet Kohoutek in the 1100–1500 Å wavelength range was obtained from a sounding rocket on January 8, 1974, and from the Skylab space station on 13 occasions between November 26, 1973 and February 2, 1974. These images are predominantly due to Lyman-α (1216 Å) emission from the hydrogen coma of the comet. The rocket pictures have been calibrated for absolute sensitivity and a hydrogen production rate has been determined. However, the Skylab camera suffered degradation of its sensitivity during the mission, and its absolute sensiti vity fbservation ofn only be estimated by comparison of the comet images with those taken by the rocket camera, with imagery of the geocoronal Lyman-α glow, of the moon in reflected Lyman-α, and of ultraviolet-bright stars. The rocket and geocoronal comparisons are used to derive a preliminary, qualitative history of the development of the cometary hydrogen coma and the associated hydrogen production rate.