A note on the total mass of comets in the solar system

The assumption that the very low albedo determined for Halley's comet is typical of all short period comets, taken together with the assumption that the average sizes of long and short period comets are approximately equal, leads to an increase in the total mass of comets in the solar system by almost two orders of magnitude. If gravitational ejection from the Uranus - Neptune zone during the later phases of planet formation is indeed responsible for the classical Oort cloud between 10⁴–10¹⁵ AU, then the mass of comets in this transplanetary region during cosmogonie times has to exceed the combined masses of Uranus and Neptune by over an order of magnitude. Furthermore, if the recent arguments for as many as 10¹⁴ comets in an inner Oort cloud between ~40– 10⁴AU are valid, then the total mass of comets in the solar system approaches 2% of a solar mass.     

Note on solar hard X-ray bursts

Observationally solar X bursts fall into three different categories : soft X bursts (E < 10 keV), deka-keV bursts (10–150 keV), and very hard X bursts or deci-MeV bursts (200–1000 keV). The first kind is quasi-thermal, the last kind is non-thermal. The real existence of the third kind of burst looks probable but has not yet been proved by direct observations. The difference between deci-MeV and deka-keV bursts may mainly be a matter of geometry of the emitting plasma.     

The nature of the solar wind interaction with CO2/CO-dominated comets

The varying overall nature of the solar wind interaction with the ionospheres of CO and CO₂-dominated comets is investigated and compared with previous results for H₂O-dominated comets. It is shown that as a comet approaches the sun, it may exhibit one of two types of ionospheric transitions. (In rare circumstances, the cometary ionosphere may display a third type of transition in addition to one of the first two). For both transitions, the ionosphere turns from being hard (in other words, the ionosphere is not susceptible to compression under sudden solar wind pressure increases) to soft. However, for one type of transition, the bow shock changes from being weak (M2) to being strong (M10), whereas for the other type of transition, the bow shock remains weak. The heliocentric distance at which these transitions may occur is found to be a function of the cometary nuclear radius, the latent heat of sublimation of the surface volatiles, the surface bolometric albedo and the following ionospheric properties: the optical depth, the average ionization time scale and the amount of heat addition. Two important consequences of the strong shocks are the large solar wind velocity modulation of the energization of electrons at the bow-shock and the relatively quick formation of cometary plasma tails.These results are applied to the case of comet Humason (1962 VIII). It is shown that either a CO or CO₂ dominated surface can explain not only the strong coma and tail activity of this comet at large heliocentric distances, but it can also explain the irregular activity of this comet at such distances.     

The dissociation of OH and OD in comets by solar radiation

The radiative lifetimes of cometary OH are calculated as a function of the heliocentric velocity of the comet and the velocity distributions of the product atoms are determined. At a distance of 1 AU from the Sun, the lifetimes vary between 1.2×10⁵ and 1.9×10⁵ sec at solar minimum and between 1.0×10⁵ and 1.4×10⁵ sec at solar maximum, depending upon velocity. Continuous absorption into the repulsive 1²Σ^- state is major destruction path. The calculated lifetimes are generally consistent with the lifetimes inferred from observations, but suggest some elaboration of the models is necessary. Photodissociation of OH produces a low-velocity component of hydrogen atoms at 8 km sec^?1 relative to the parent OH molecule and a high-velocity component between 17 and 27 km sec^?1. Photodissociation of OH leads to metastable O(¹D) and O(¹S) and is an additional source of the red and green line emission of atomic oxygen. The lifetime of OD is estimated to be about 4.3× 10⁵ sec at solar minimum and 2.6×10⁵ sec at solar maximum so that the OD/OH ratio in comets is enhanced relative to the HDO/H₂O production ratio by a factor between 2 and 3. Photodissociation of OD produces only high-velocity D atoms with a mean value of 17 km sec^?1.     

Invisible comets on evolutionary track of short-period comets

We systematically surveyed the orbits of short-period (SP) comets that show a large change of perihelion distance (q) between 1–2 AU (visible comets) and 4–5 AU (invisible comets) during 4400 years. The data are taken from Cosmo-DICE (Nakamura and Yoshikawa 1991a), which is a long-term orbital evolution project for SP comets. Recognizing that q is the most critical element for observability of comets, an invisibility factor (f), defined as the ratio of unobservable time span to observable span during 4400 years, is calculated for each of the large-q-change comets. A detection limit for each comet is obtained from the heliocentric distance at discovery and/or the absolute magnitude at recent apparitions. A mean f value for 35 SP comets with 2.9 J (J is the Tisserand's invariant) is found to be 19.8. This implies that for each visible SP comet of this J-range, at every epoch of time, there exist about 20 invisible comets near the capture orbits by Jupiter, under the assumptions of steady-state flux and ergodicity for the SP-comet population.     

Note on the helium-like ion line emission in solar plasmas

An analysis is presented of the rate coefficients occurring in the Gabriel-Jordan theory on the relative intensities of the forbidden, intercombination, and resonance lines of helium-like ions in a steady-state plasma. Simple expressions are given to show the dependence on atomic number and electron temperature. The influence of proton collisions on the excitation 2³ S2³ P is estimated and deviations from the theory under non-equilibrium conditions are briefly discussed.     

The Kozai resonance in the outer solar system and the dynamics of long-period comets

We consider the secular evolution of the orbits of bodies in the Outer Solar System under the perturbations of the jovian planets assumed on coplanar and circular orbits. Through the approach used for asteroidal belt by Yoshihide Kozai in 1962, we obtain that the Kozai resonance do not affect the behavior of bodies belonging to the Kuiper belt but concerns the long-timescale evolution of long-period comets. In particular this resonance appears as a process contributing to produce Sun-grazer comets.     

Combined solar radiation pressure and drag effects on the orbits of artificial satellites

A semi analytical theory is proposed to study the joint effects of direct solar radiation pressure and atmospheric drag on the orbit of an artificial Earth satellite. Making the solar radiation pressure equal to zero the problem is reduced to one already solved by Brouwer and Hori. The solutions are not equivalent, however, since in the Brouwer and Hori theory one has spurious Poisson terms.     

Note on solar plasma irregularities and plasma instabilities

Observations of interplanetary scintillation of radio sources are used to estimate the size of plasma irregularities down to a distance of about 6 R from the Sun. This is compared with the values of the ion gyro-radius estimated for a range of distance from 1 AU to about 6 R from the Sun. The results of the calculations are discussed in the context of the hypothesis of plasma instability which is invoked to interpret the observations of the scattering of radio waves in the solar corona and of interplanetary scintillations.     

Note on the characteristics of sunspot groups which produce solar proton flares

Solar proton flares are associated with sunspot groups which show an unusual distribution of magnetic polarities. Furthermore, the gradient of the magnetic field is very large before the onset of these flares. The importance of polar cap absorptions, which is proportional to the integral flux of solar cosmic rays, tends to increase as the gradient of the magnetic field becomes greater. It is shown that the formation of such gradients is associated with the rotating motion of sunspot groups. Hence, the sunspot groups which show a reversed polarity distribution are very effective for the production of solar proton flares.NASA Associate with University of Maryland.     

Viscous relaxation on comets

Three Jupiter family comets have now been observed by spacecraft with the surprising result that these comets lack unambiguous impact craters. Large-scale topography generally appears to be softened on these comets, although sharp topography is preserved at small scales. We find that viscous relaxation of water ice may explain these observations, given reasonable assumptions about ice grain size and temperatures attained in the interiors. We suggest that both the shapes and the cratering records of Jupiter family comets may be substantially modified and no longer reflect cometary formation processes or collisional evolution in the Kuiper Belt.     

The origin of the solar system: Implications for transneptunian planets and the nature of the long-period comets

If the solar system origin is considered within the framework of the author's hypothesis on the binary stars formation as a result of rotational-exchange break-up of the rotating protostar, then difficulties involved in the usual nebular hypotheses are automatically removed (unclear aspects of the possibility of formation of the gas disc proper, the problems of the angular momentum including slow rotation of the Sun and coplanarity of the planetary orbits, of differences in planetary masses and composition, the need, for the disc remnants to be swept out, the long time of planetary formation as compared with the possible lifetime of a turbulized disc etc.).The major stages of division and evolution of the Jupiter-Sun system are described. Similarities between the massive rotating proto-Jupiter (PJ) and the classical protoplanetary discs are pointed out. The process of planetoid condensation inside PJ is discussed. The most probable site of the condensation is the region of the first Lagrangian point. The planetoids condensed were lost by PJ as a result of its fast mass decrease. A gas dynamic consideration of the motion of planetoids in PJ yields 1000–3000 yr as a time scale for the PJ's mass loss. The number of the moonlike bodies lost (the remaining Galilean satellites fixing their lower mass limit) could reach 10⁴.Evolution of such interacting bodies results in the formation beyond Neptune of a cloud (up to 10³) of moonlike (and more massive) planets.The excess concentration of the long-period comets aphelia in this area implies their genetic relation to the planets. A concept of a joint planeto-cometary cloud is introduced. A concrete hydrodynamic mechanism of ice ejection from planets into space, viz. the formation of cumulative (Monroe) jets, is pointed out.A program of further investigations is outlined and recommendations given for an experimental check on the implications of the new cosmogonic concepts.     

On the origin of comets

The paper presents a brief history of cometary cosmogony. It discusses critically the eruptive hypothesis, the hypothesis on the relict origin of comets, and the hypothesis on a genetic connection between comets and trans-Plutonian planets. Laplace’s theoretical prediction as to the capture of long-period comets by Jupiter into short-period orbits is confirmed. We conclude that the interstellar hypothesis promising is for the provenance of comets.     

On the aging of comets

This study is based primarily on the calculations of comet orbits over ~ 10⁶ years for 160 short-period comets by Harold F. Levison and Martin J. Duncan from which there are calculated ablation AGES. There are positive statistical correlations (having many deviations) with radial nongravitational forces, comet activity measures, and dust-to-gas ratios in the spectra, in the sense that comets of greater AGES tend to be less active and to show less dust in their spectra than comets of lesser AGES.Harvard-Smithsonian Center for Astrophysics     

On the origin of comets

The fact that comets are rich in volatile material shows that they were formed and kept for a long time in the outer, low-temperature regions of the solar system. In this paper we analyse the structure in the outer edge of the solar nebula and show that no formation zone of comets can exist there. Our view is that the comets evolved from the residual planetesimals in the zone between Jupiter and Neptune.     

On the aging of comets

This study is based primarily on the calculations of comet orbits over ~ 10⁶ years for 160 short-period comets by Harold F. Levison and Martin J. Duncan from which there are calculated “ablation AGES”. There are positive statistical correlations (having many deviations) with radial nongravitational forces, comet activity measures, and dust-to-gas ratios in the spectra, in the sense that comets of greater “AGES” tend to be less active and to show less dust in their spectra than comets of lesser “AGES”.     

Alfvén waves in a space plasma and its role in the solar wind interaction with comets

The importance of Alfvén wave generation in interacting plasmas is discussed in general and illustrated by the example of solar wind interaction with cometary plasma. The quasi-linear theory of Alfvén wave generation by cometary ions at distances far from the cometary nucleus is reviewed. The incorporation of a diabatic plasma compression effects into this theory modifies the spectrum of Alfvén waves and the integral intensity of magnetic field fluctuations previously published. These results are in quantitative agreement with thein situ observations near the comets Giacobini-Zinner and Halley. However, the polarization of quasi-linearly excited waves needs further detailed comparison with observations.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.