Collisional processes among interplanetary dust grains: An unlikely origin for the β meteoroids

The question of the collisional production of the β meteoroids is reexamined incorporating recent experimental results (A. Fugiwara, G. Kamimoto, A. Tsukamoto, 1977, Icarus31, 277–288). The collisional model yields a flux of fragments supported by the conservation of mass flux which does not account by far for the observed flux of submicron grains. Particles larger than about 100 μm will be destroyed by collisions inside 1 AU, well before they can get near the Sun. The existence of two independent populations of interplanetary dust grains as proposed by L. B. Le Sergeant and Ph. L. Lamy (1978, Nature266, 822–824; 1980, Icarus43, 350–372) appears reinforced. It is proposed that the bulk of submicron grains does not necessarily travel in hyperbolic orbits and that β meteoroids may be a phenomenon—possibly transitory—of limited importance.     

Spatially resolved absolute spectrophotometry of Saturn: 3390 to 8080 Å

Absolute spectrophotometry of four regions on the visible disk of Saturn (north and south polar regions, equatorial band, south “temperate” region) from 3390 to 8080 Å is reported. Spectral resolution is 10 Å in the interval 3390–6055 Å, and 20 Å; aperture size is 1.92 arcsec. The explicit purpose of our observations was to provide ground-based photometric calibration for the Pioneer Saturn Imaging Photopolarimeter (IPP). We also compare our data with earlier spectrophotometric measurements of Saturn (R.L. Younkin and G. Munch, 1963,Mem. Soc. Roy. Sci. Liege7, 123–136; W.M. Irvine and A.P. Lane, 1971,Icarus16, 10–26; T.B. McCord, T.V. Johnson, and J.H. Elias, 1971,Astrophys. J.165, 413–424) and with the M. Podolak and R.EE. Danielson (1977)Icarus30, 479–492) parameterization of “Axel Dust.” The latter reproduces the broad features but not the details of the observed spectral reflectivity (I/F). We find that large depths of clear molecular hydrogen (>14 km-am in the temperate regions) are needed to match the observed upturn in reflectivity shortward of 3800 Å.     

A possible rotation period for the minor planet 164 Eva

The minor planet 164 Eva passed through opposition on December 1, 1975 with a magnitude B_opp = 11.3 mag. Photoelectric observations at the Observatory of Torino, Italy, were carried out in two nights on Oct. 27/28 and Nov. 11, each with a run of about 3 hr. Two further successful photoelectric observations were carried out at the OHP, France, each with a run of about 6 hr. From all observed parts of the lightcurve a resulting synodic period of rotation of about 27.3 hr can be deduced, with a range of the total amplitude of at least Δm = 0.07 mag. With this period of 27.3 hr the minor planet 164 Eva is one more long period object, falling now between 654 Zelinda (H. J. Schober, 1975, Astron. Astrophys.44, 85–89) and 139 Juewa (J. Goguen et al., 1976, Icarus29, 137–142), at the high end in the histogram of the distribution of minor planet rotation periods.     

Infrared spectrum of Io, 2.8–5.2 μm

The reflectance spectrum of Io is presented from 2.8 to 5.2 μm, extending the earlier results of D. P. Cruikshank, T. J. Jones, and C. B. Pilcher (1978, Astrophys. J. 225, L89–L92), and demonstrating the full extent of the broad and deep spectral absorption between 3.5 and 4.8 μm. Laboratory spectra of nitrates and carborates diluted with sulfur do not satisfactorily reproduce the Io spectrum, but new information based on the recently discovered volcanic activity on the satellite lead to consideration of other classes of compounds as reported in a companion paper (F. P. Fanale, R. H. Brown, D. P. Cruikshank, and R. N. Clark, 1979, Nature280, 761–763).     

Variability of carbon monoxide in the mars atmosphere

In January of 1982 we measured a microwave spectrum of CO in the Martian atmosphere utilizing the rotational J = 1 → 2 transition of CO. We have analyzed data and reanalyzed the microwave spectra of R. K. Kakar, J. W. Waters, and W. J. Wilson, (Science196, 1090–1091, 1977, measured in 1975) and J. C. Good and F. P. Schloerb, (Icarus47, 166–172, 1981 measured in 1980) in order to constrain estimates of the temporal variability of CO abundance in the Martian atmosphere. Our values of CO column density from the data of Karar et al., Good and Schloerb, and our own are 1.7 ± 0.9 × 10²⁰, 3.0 ± 1.0 × 10²⁰, and 4.6 ± 2.0 × 10²⁰cm^?2, respectively. The most recent estimate of CO column density from the 1967 infrared spectra of J. Connes, P. Connes, and J.P. Maillard, (Atlas de Spectres Infarouges de Venus, Mars, Jupiter, et Saturne, Editions due Centre National de la Recherche Scientifique, Paris, 1969), is 2.0 ± 0.8 × 10²⁰ cm^?2 (L.D.G. Young and A.T. Young, Icarus30, 75–79, 1977). The large uncertainties given for the microwave measurements are due primarily to uncertainty in the difference between the continuum brightness temperature and atmospheric temperatures of Mars. We have accurately calculated the variation among the observations of the continuum (surface) brightness temperature of Mars, which is primaroly a function of the observed aspect of Mars. A more difficult problem to consider is variability of global atmospheric temperatures among the observations, particularly the effects of global dust storms and the ellipticity of the orbit of Mars. The large bars accompanying our estimates of CO column density from the three sets of microwave measurements are primarily caused by an assumed uncertainty of ±10°K in our atmospheric temperature model due to possible dust in the atmosphere. A qualitative consideration of seasonal variability of global atmospheric temperatures among the measurements suggests that there is not strong evidence for variability of the column abundance of CO on Mars, although variability of 0–100% over a time scale of several years is allowed by the data set. The implication for the variability of Mars O₂ is, crudely, a factor of two less. We found that the altitude distribution of CO in the atmosphere of Mars was not well constrained by any of the spectra, although our spectrum was marginally better fitted by an altitude increasing profile of CO mixing ratios.     

Candidates for astrometric or direct-imaging detection of extra-solar planets,and comments on the photometric method

Knowledge of a star's rotation period and ν sin i can be used to select stars that are seen pole-on, and thus are well suited to planetary searches by astrometric or direct-imaging means. A table of such stars is presented. This method is not suitable for discriminating equator-on systems and so cannot be used to select candidates for the photometric method of W. J. Borucki and A. L. Summers (1984, Icarus58, 121–134).     

The pole orientation of asteroid 433 Eros determined by photometric astrometry

Refinements to the pole-determination method photometric astrometry (PA) were completed in 1983 (R. C. Taylor and E. F. Tedesco, 1983, Icarus54, 13–22). A goal is to redo the pole analysis for every asteroid whose pole had been determined from earlier versions of PA: Previous PA poles are reviewed in this paper. Asteroid 433 Eros is in that collection and has redone. The result are prograde rotation; a sidereal period of 0.219588 ± 0.000005 day; and a north pole at 22° longitude, +9° latitude. The uncertainty of the pole is 10°. The pole position of Eros determined by C.D. Vesely (1971, In Physical Studies of Minor Planets (T. Gehrels, Ed.), pp. 133–140, NASA SP-267) and Dunlap (1976, Icarus28, 69–78), using earlier versions of photometric astrometry, were within 21 and 7°, respectively, of the present result.     

Structures far from the head of comet Kohoutek: II. A discussion of the swan cloud of January 11 and of the general morphology of cometary plasma tails

Photographs taken in New Mexico and in Japan during 1974 January 9–11 show that the “Swan Cloud” observed in comet Kohoutek on January 11 (C. L. Hyder, J. C. Brandt, and R. G. Roosen, 1974, Icarus23, 601–610: Paper I) was an advanced stage of a plasma tail disconnection event (DE). The event commenced and evolved with the development of strong tail ray activity, the actual tail disconnection, the merging of the disconnected tail with the new tail to form the Swan, and a new feature not discussed in recent disconnection models—the formation of arcade loops in the space between closing tail rays. An unusual aspect of the event was the deceleration of the rejected tail as it receded from the head.The observed morphological sequence is easily understood in the sector boundary model (M. B. Niedner, Jr., and J. C. Brandt, 1978, Astrophys. J.223, 655–670; 1979, Astrophys. J.234 723–732), in which dayside reconnection of captured interplanetary fields at sector boundary crossings strips away the existing tail, and field capture in the new sector builds a new tail. The associated sector boundary for the event passed Earth of 1974 January 14.The arcade loops are proposed to be reconnected flux tubes between oppositely polarized tail rays in the incipient new tail which followed the disconnection. This interpretation is not only consistent with the magnetic field topology advocated in Alfvén's original model (H. Alfvén, 1957 Tellus9 92–96), but it may also provide a physical model for some of the features known historically as “condensation,” at least some of which appear to have formed in the same manner.     

Nonequilibrium chemical analysis of the coma of comet P/Stephan-Oterma

A computer code to calculate the time-dependent nonequilibrium chemistry taking place within the coma of a comet has been developed. This code incorporates 1249 chemical reactions involving 128 species. Models were fit to data on Comet P/Stephan-Oterma (A. L. Cochran and E. S. Barker, 1985, Icarus62, 72–81). It was shown that (1) HCN is the parent for CN; (2) C₂H₂ is a parent for C₂; (3) pure gas-phase chemistry with known species cannot adequately reproduce the observed C₃ but a single step process can; and (4) at least prior to perihelion, the vaporization rate seems to have been controlled by water vaporization.     

Saturn's rings: 3-mm low-inclination observations and derived properties

Recent 3-mm observations of Saturn at low ring inclinations are combined with previous observations of E. E. Epstein, M. A. Janssen, J. N. Cuzzi, W. G. Fogarty, and J. Mottmann (Icarus41, 103–118) to determine a much more precise brightness temperature for Saturn's rings. Allowing for uncertainties in the optical depth and uniformity of the A and B rings and for ambiguities due to the C ring, but assuming the ring brightness to remain approximately constant with inclination, a mean brightness temperature for the A and B rings of 17 ± 4°K was determined. The portion of this brightness attributed to ring particle thermal emission is 11 ± 5°K. The disk temperature of Saturn without the rings would be 156 ± 6°K, relative to B. L. Ulich, J. H. Davis, P. J. Rhodes, and J. M. Hollis' (1980, IEEE Trans. Antennas Propag.AP-28, 367–376) absolutely calibrated disk temperature for Jupiter. Assuming that the ring particles are pure water ice, a simple slab emission model leads to an estimate of typical particle sizes of ≈0.3 m. A multiple-scattering model gives a ring particle effective isotropic single-scattering albedo of 0.85 ± 0.05. This albedo has been compared with theoretical Mie calculations of average albedo for various combinations of particle size distribution and refractive indices. If the maximum particle radius (≈5 m) deduced from Voyager bistatic radar observations (E. A. Marouf, G. L. Tyler, H. A. Zebker, V. R. Eshleman, 1983, Icarus54, 189–211) is correct, our results indicate either (a) a particle distribution between 1 cm and several meters radius of the form r^?s with 3.3 ? s ? 3.6, or (b) a material absorption coefficient between 3 and 10 times lower than that of pure water ice Ih at 85°K, or both. Merely decreasing the density of the ice Ih particles by increasing their porosity will not produce the observed particle albedo. The low ring brightness temperature allows an upper limit on the ring particle silicate content of ≈10% by mass if the rocky material is uniformly distributed; however, there could be considerably more silicate material if it is segregated from the icy material.     

Martian CO abundance from the J = 1 → 0 rotational transition: Evidence for temporal variations

Mars was observed in the CO (J = 1 → 0) 2.6-mm wavelength line between 29 March and 1 April, 1980. The data were analyzed using a model atmosphere based on Viking measurements. A least-squares fit of the model to the observed line profile yielded an average CO mixing ratio of (3.2 ± 1.1) × 10^?3. This value is four times larger than that obtained by L. D. Kaplan, J. Connes, and P. Connes, 1969 (Astrophys. J.157, L187-L195) from analysis of an infrared spectrum obtained in 1967 by J. Connes, P. Connes, and J. P. Maillard, 1969 (Atlas of Near Infrared Spectra of Venus, Mars, Jupiter, and Saturn, Centre National de la Recherche Scientifique, Paris). Models of the Martian atmospheric chemistry indicate that this implied temporal variation could easily exist and that it would be due primarily to variations in the abundance of H₂O.     

Observations of Saturn's outer ring and new satellites during the 1980 edge-on presentation

Observations of Saturn's satellites and external rings during the 1980 edge-on presentation were obtained with a focal coronograph. A faint satellite traveling in the orbit of Dione and leading it by 72° has been detected, together with the two inner satellites already suspected (cf. J. W. Fountain and S. M. Larson, 1978,Icarus36, 92–106). The external ring has been observed on both east and west sides; it may extend up to $\text{?8.3}$ Saturn radii, and appears structured.     

The reflection spectrum of liquid sulfur: Implications for Io

The spectral reflectance from 0.38 to 0.75 μm of a column of liquid sulfur has been measured at several temperatures between the melting point (～118°C) and 173°C. Below 160°C the spectral reflectance was observed to vary reversibly as a function of temperature, independent of the previous thermal history of the column. Once the temperature exceeded 160°C, the spectrum would not change given a subsequent decrease in temperature. The spectral reflectance of the liquid-sulfur column at all temperatures was very low (10–19%). Combining this information with Voyager spectrophotometry of Jupiter's satellite Io, it is concluded that liquid sulfur at any temperature on Io's surface would be classified as a “black area” according to the standards used by the Voyager imaging team in their spectrophotometric analysis (L. Soderblom, T. V. Johnson, D. Morrison, E. Danielson, B. L. Smith, J. Veverka, A. Cook, C. Sagan, P. Kupferman, D. Pieri, J. Mosher, C. Avis, J. Gradie, and T. Clancy (1980). Geophys. Res. Lett.7, 963–966).     

Properties of the 15 February 2011 Flare Seismic Sources

The first near-side X-class flare of Solar Cycle 24 occurred in February 2011 (SOL2011-02-05T01:55) and produced a very strong seismic response in the photosphere. One sunquake was reported by Kosovichev (Astrophys. J. Lett. 734, L15, 2011), followed by the discovery of a second sunquake by Zharkov, Green, Matthews et al. (Astrophys. J. Lett. 741, L35, 2011). The flare had a two-ribbon structure and was associated with a flux-rope eruption and a halo coronal mass ejection (CME) as reported in the CACTus catalogue. Following the discovery of the second sunquake and the spatial association of both sources with the locations of the feet of the erupting flux rope (Zharkov, Green, Matthews et al., Astrophys. J. Lett. 741, L35, 2011), we present here a more detailed analysis of the observed photospheric changes in and around the seismic sources. These sunquakes are quite unusual, taking place early in the impulsive stage of the flare, with the seismic sources showing little hard X-ray (HXR) emission, and strongest X-ray emission sources located in the flare ribbons. We present a directional time–distance diagram computed for the second source, which clearly shows a ridge corresponding to the travelling acoustic-wave packet and find that the sunquake at the second source happened about 45 seconds to one minute earlier than the first source. Using acoustic holography we report different frequency responses of the two sources. We find strong downflows at both seismic locations and a supersonic horizontal motion at the second site of acoustic-wave excitation.     

Interferometry of Saturn and its rings at 1.30-cm wavelength

We present interferometric observations of Saturn and its ring system made at the Hat Creek Radio Astronomy Observatory at a wavelength of 1.30 cm. The data have been analyzed by both model-fitting and aperture synthesis techniques to determine the brightness temperature and optical thickness of the ring system and estimate the amount of planetary limb darkening. We find that the ring optical depth is close to that observed at visible wavelenghts, while the ring brightness temperature is only 7 ± 1°K. These observational constraints require the ring particles to be nearly conservative scatterers at this wavelength. A conservative lower limit to the single-scattering albedo of the particles at 1.30-cm wavelength is 0.95, and if their composition is assumed to be water ice, then this lower limit implies an upper limit of 2.4 m for the radius of a typical ring particle. The aperture synthesis maps show evidence for a small offset in the position of Saturn from that given in the American Ephemeris and Nautical Almanac. The direction and magnitude of this offset are consistent with that found from a similar analysis of 3.71-cm interferometric data which we have previously presented (F.P. Schloerb, D.O. Muhleman, and G.L. Berge, 1979b, Icarus39, 232–250). Limb darkening of the planetary disk has been estimated by solving for the best-fitting disk radius in the models. The best-fitting radius is 0.998 ± 0.004 times the nominal Saturn radius and indicates that the planet is not appreciably limb dark at 1.30 cm. Since our previous 3.71-cm data also indicated that the planet was not strongly limb dark (F.P. Schloerb, D. O. Muhleman, and G.L. Berge, 1979a, Icarus39, 214–230), we feel that the limb darkening is not strongly wavelength dependent between 1.30 and 3.71 cm. The difference between the best-fitting disk radii at 3.71 and 1.30 cm is +0.007 ± 0.007 times the nominal Saturn radius and suggests that the planet is more limb dark at 1.30 cm than at 3.71 cm. Models of the atmosphere which have NH₃ as the principal source of microwave opacity predict that the planet will be less limb dark at 1.30 cm. However, the magnitude of the effect predicted by the NH₃ models is ?0.009 and only marginally different from the observed value.     

VLA Measurements of Faraday Rotation through Coronal Mass Ejections

Coronal mass ejections (CMEs) are large-scale eruptions of plasma from the Sun, which play an important role in space weather. Faraday rotation is the rotation of the plane of polarization that results when a linearly polarized signal passes through a magnetized plasma such as a CME. Faraday rotation is proportional to the path integral through the plasma of the electron density and the line-of-sight component of the magnetic field. Faraday-rotation observations of a source near the Sun can provide information on the plasma structure of a CME shortly after launch. We report on simultaneous white-light and radio observations made of three CMEs in August 2012. We made sensitive Very Large Array (VLA) full-polarization observations using 1?–?2 GHz frequencies of a constellation of radio sources through the solar corona at heliocentric distances that ranged from 6?–?\(15~\mathrm{R}_{\odot}\). Two sources (0842+1835 and 0900+1832) were occulted by a single CME, and one source (0843+1547) was occulted by two CMEs. In addition to our radioastronomical observations, which represent one of the first active hunts for CME Faraday rotation since Bird et al. (Solar Phys., 98, 341, 1985) and the first active hunt using the VLA, we obtained white-light coronagraph images from the Large Angle and Spectrometric Coronagraph (LASCO) C3 instrument to determine the Thomson-scattering brightness [\(\mathrm{B}_{\mathrm{T}}\)], providing a means to independently estimate the plasma density and determine its contribution to the observed Faraday rotation. A constant-density force-free flux rope embedded in the background corona was used to model the effects of the CMEs on \(\mathrm{B}_{\mathrm{T}}\) and Faraday rotation. The plasma densities (\(6\,\mbox{--}\,22\times10^{3}~\mbox{cm}^{-3}\)) and axial magnetic-field strengths (2?–?12 mG) inferred from our models are consistent with the modeling work of Liu et al. (Astrophys. J., 665, 1439, 2007) and Jensen and Russell (Geophys. Res. Lett., 35, L02103, 2008), as well as previous CME Faraday-rotation observations by Bird et al. (1985).     

Net thermal radiation in the atmosphere of Venus

The four entry probes of the Pioneer Venus mission measured the radiative net flux in the atmosphere of Venus at latitudes of 60°N, 31°S, 27°S, and 4°N. The three higher latitude probes carried instruments (small probe net flux radiometers; SNFR) with external sensors. The measured SNFR net fluxes are too large below the clouds, but an error source and correction scheme have been found (H. E. Revercomb, L. A. Sromovsky, and V. E. Suomi, 1982, Icarus52, 279–300). The near-equatorial probe carried an infrared radiometer (LIR) which viewed the atmosphere through a window in the probe. The LIR measurements are reasonable in the clouds, but increase to physically unreasonable levels shortly below the clouds. The probable error source and a correction procedure are identified. Three main conclusions can be drawn from comparisons of the four corrected flux profiles with radiative transfer calculations: (1) thermal net fluxes for the sounder probe do not require a reduction in the Mode 3 number density as has been suggested by O. B. Toon, B. Ragent, D. Colburn, J. Blamont, and C. Cot (1984, Icarus57, 143–160), but the probe measurements as a whole are most consistent with a significantly reduced mode 3 contribution to the cloud opacity; (2) at all probe sites, the fluxes imply that the upper cloud contains a yet undetected source of IR opacity; and (3) beneath the clouds the fluxes at a given altitude increase with latitude, suggesting greater IR cooling below the clouds at high latitudes and water vapor mixing ratios of about 2–5 × 10^?5 near 60°, 2–5 × 10^?4 near 30°, and 5 × 10^?4 near the equator. The suggested latitudinal variation of IR cooling is consistent with descending motions at high latitudes, and it is speculated that it could provide an important additional drive for the general circulation.     

Cometary impacts with the Sun: Physical and dynamical considerations

D. J. Michels, N. R. Sheeley, Jr., R. A. Howard, and M. J. Koomen (Science215, 1097–1102, 1982) observed a comet which appears to have impacted the Sun. Z. Sekanina (Astron. J..87, 1059–1072, 1982) showed that the comet, 1979XI, was probably a member of the Kreutz group of sungrazing comets. The sungrazers typically have perihelia of 1.2–1.9 solar radii but Sekanina found q = 0.35 R_⊙ for 1979XI. It is interesting to speculate how the perihelion may have been reduced to this small value. The change in perihelion can not be explained by planetary, stellar, or nongravitational perturbations. Tidal splitting of the nucleus on a previous perihelion passage is also ruled out, through a random splitting event near aphelion of the comet's orbit is a remote possibility. The most plausible explanation is collision with another body, most likely a comet, at large heliocentric distance. However, the expected probability of such an event is exceedingly small. Another aspect of the problem is whether the nucleus of 1979XI sublimated completely before impacting the Sun. Assuming a water ice nucleus, it is shown that a surface layer of only 5–15 m thickness would be sublimated prior to impact. Although it is likely that the nucleus tidally disrupted after crossing the solar Roche limit, the ultimate destruction of the nucleus probably resulted from the shock of hitting the denser regions of the solar atmosphere, just above the photosphere.     

Models of the Io Torus

Three-dimensional models of the Io torus are employed to analyze the spectroscopic data reported by J.S. Morgan (1985, Icarus62, 389–414). These models are used to compare Morgan's ground-based spectroscopic data with R.J. Oliversen's (1983, The Io Plasma Torus: Its Structure and Sulfur Emission Spectra. Ph.D. thesis, University of Wisconsin-Madison) nearly simultaneous [SII] images and with the in situ measurements made by Voyager 1. The models are also used to investigate whether the observed [SII] longitudinal intensity variations were caused by intrinsic or geometric effects, and to test the hypothesis that the observed optical east-west variations are consistent with the convective motions suggested by D.D. Barbosa and M.G. Kivelson (1983, Geophys. Res. Lett.10, 210–213) and W.-H. Ip and C. K. Goertz (1983, Nature302, 232–233). Oliversen's images are found to be in good agreement with Morgan's spectroscopic measurements. Three significant differences exist between these data and the torus described in the Voyager 1 experiments: (1) the torus beyond ～5.7R_J was found to be at least 1.5 to 2 times denser in 1981 than at the time of the Voyager 1 measurements in 1979, (2) the outer torus SII ion temperatures were approximately two times cooler than those measured by Voyager 1, and (3) in 1981, the outer torus OII mixing ratios were lower than were suggested by the Voyager 1 experiments. The 1981 ground-based OII/SII intensity ratios are found to be consistent with a radial peak near 6.0R_J in the ratio of oxygen to sulfur. At its maximum this ratio is ～2, and it falls to ～1 within ～0.5R_J inside and outside of this radius. Viewing geometry variations were found to be inadequate to account for the longitudinal variations observed by Morgan (1984). Intrinsic longitudinal intensity changes of about a factor of 2 are required to match the 1981 observations. Convective motions were found to adequately explain the observed optical east-west intensity asymmetry, but problems in interpreting the [OII] doublet line ratios still remain. It is suggested that systematic errors are present in the measurements of the [OII] line ratios.