A search for posteclipse brightening of Io in 1973. I

Eight eclipse reappearances of the innermost Galilean satellite, Io, were observed during the fall of 1973. The measurements were made using photometers specially designed to cope with the problem of scattered light from Jupiter. Posteclipse brightening of Io was not detected and, if present, was less than our estimated detection threshold of about 0.02 mag.     

Two-color search for the posteclipse brightening of Io

Blue-violet photoelectric photometry of a reappearance of Io on 10 October 1974 is presented. Observations were made with a single-beam photometer on the Hvar Observatory 65-cm telescope. A brightness anomaly of about 0.15 and 0.25 mag in V and B, respectively, was detected, but owing to the relativity large estimated errors, 0.04 and 0.05 mag, respectively, detection is regarded astentative. The wavelength dependence, reported previously by some investigators, was confirmed. The urgent need of multicolor photometry is briefly stressed.     

The post-eclipse brightening of Io

We review the photometric work on eclipse reappearances of Io. New observations of eclipse reappearances of Io confirm the post-eclipse brightness anomaly reported by Binder and Cruikshank (1964) but testify to its intermittent nature. A post-eclipse anomaly of approximately 0.07 mag was observed on two occasions in 1972, while observations of Europa and Ganymede showed no brightness anomaly greater than 0.01 mag. The atmospheric condensation model for the anomaly on Io is reviewed in terms of the quantity of frost required to produce the effect and the corresponding amount of gas liberated to the atmosphere upon sublimation. The observational data and the results from a stellar occultation are in general accord with the theoretical predictions of the stability of heavy gases on Io, while both observational and theoretical criteria are satisfied by a tenuous atmosphere of a heavy gas such as methane or ammonia having a surface pressure ～10^?7 bar.     

Another possible post-eclipse brightening of Io

We observed an apparent brightening of Io after eclipse reappearance on June 25, 1971 from Kitt Peak (area scanning at 3500 Å) and Table Mountain (4000 Å) and suggest, in the future, that coordinated observations employ area scanning in the ultraviolet.     

Post-eclipse brightening and non-brightening of Io

It may be possible to understand the apparent intermittent nature of the post-eclipse brightenings and nonbrightenings of Io in terms of a nonuniform distribution of blue reflectors grouped in the hemisphere centered at 0° longitude. The dimensions required for such blue mirrors are consistent with very large craters. The high blue albedo of water frost and other ices makes these materials likely candidates for the reflectors.     

Search for color changes and brightening of Io upon eclipse reappearance

Medium-resolution spectra were made of Io as it emerged from two eclipses in December 1975. In the wavelength range 4000–5800 Å, no spectral changes greater than the standard deviations were observed when the spectrum of Io just after reappearance was divided by the spectrum of Io 20 min later. No substantial increase in total brightness was observed over the same time interval. These observations were made at a time when the sub-Earth point was in Io's northern hemisphere; therefore, prediction of positive posteclipse brightening in this circumstance is not confirmed.     

Possible correlation of Io's posteclipse brightening with major solar flares

In 6 of the 7 instances where posteclipse brightening of Io has been reported by observers using blue filters, a major solar flare occurred within 10° of the sub-Jovian longitude in the 100-day interval prior to observation. In none of the 18 instances where no posteclipse brightening was observed did such a flare occur. It is proposed that a phenomenon associated with a major solar flare causes an increase in the trapped particle flux at Io's orbit by an order of magnitude. The posteclipse brightening may be caused by thermoluminescence of Io's surface material upon emergence. Alternatively, it is possible that the increase in trapped particle flux would warm the surface, creating a temporary atmosphere which would precipitate during eclipse cooling and vaporize in the period of warming after reemergence.     

Cassini/VIMS observation of an Io post-eclipse brightening event

During the Cassini-Jupiter flyby, VIMS observed Io at different phase angles, both in full sunlight and in eclipse. By using the sunlight measurements, we were able to produce phase curves in the visual through all the near infrared wavelengths covered by the VIMS instrument (0.85-5.1 μm). The phase angle spanned from ∼2° to ∼120°. The measurements, done just after Io emerged from Jupiter's shadow, show an increase of about 15% in Io's reflectance with respect to what would be predicted by the phase curve. This behavior is observed at wavelengths >1.2 μm. Moreover, just after emergence from eclipse an increase of about 25% is observed in the depth of SO₂ frost bands at 4.07 and 4.35 μm. At 0.879<λ<1.04 μm the brightening is 10-24%. Below λ=0.879 μm the brightening, if present, should be less than the precision of our measurements (∼5%). Apparently, these observations are not explained neither by a diverse spatial distribution of SO₂ on the Io' surface nor by atmospheric SO₂ condensation on the surface during the eclipse.     

A search for new features in the visible spectrum of Io

The spectrum of Io between 4000 Å and 6000 Å has been studied at high resolution for new features. None were found. The upper limit to new emissions varies with wavelength from 13 kR to 49 kR.     

A cometary ionosphere model for Io

The ionosphere of Jupiter's satellite Io, discovered by the Pioneer 10 radio-occultation experiment, cannot easily be understood in terms of a model of a gravitationally bound, Earth-like ionosphere. Io's gravitational field is so weak that a gravitationally bound ionosphere would probably be blown away by the ram force of the Jovian magnetospheric wind — i.e., the plasma corotating in the Jovian magnetosphere. We propose here a model in which the material for Io's atmosphere and ionosphere is drawn from the ionosphere of Jupiter through a Birkeland current system that is driven by the potential induced across Io by the Jovian corotation electric field. We argue that the ionization near Io is caused by a comet-like interaction between the corotating plasma and Io's atmosphere. The initial interaction employs the critical velocity phenomenon proposed many years ago by Alfvén. Further ionization is produced by the impact of Jovian trapped energetic electrons, and the ionization thus created is swept out ahead of Io in its orbit. Thus, we suggest that what has been reported as a day-night ionospheric asymmetry is in fact an upstream-downstream asymmetry caused by the Jovian magnetospheric wind.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30th May, 1978.     

A search for microwave emission from CH in Comet Kohoutek (1973f)

A search was conducted for the three 9-cm transitions of the ground state Λ-doublet of CH in comet Kohoutek, using the CSIRO 64-m radio telescope and the Onsala Space Observatory's 25.6-m telescope. No lines were detected during the observing periods, and upper limits are given for the corresponding antenna temperatures.     

A model for the infrared brightening of a Nova shell

A model for grain formation in an expanding Nova shell is presented. It is shown that individual grains complete the growth within a very short time interval. In consequence the infrared brightening accompanied with visual darkening observed in several Novae is interpreted in terms of extension of dusty region in the ejected gas shell. Developments of infrared luminosity and visual magnitude are computed on the basis of a model, and compared with the observations of Nova Vul 1976, Nova Ser 1970, and Nova Aql 1975. The constancy and uniformity of grain temperatures for a long period after the infrared maximum observed in Nova Vul 1976 are also discussed.     

A search for solar neutrons near solar maximum,II

A plastic scintillator counter with anticoincidence screen has flown in four balloon flights at a floating altitude variable between 4 and 5 mb, during night to day, day-time and night-time flights. The analysis and comparison of the day-time and night-time parts of the flights has given for the continuous emission flux an upper limit of 5.5 × 10^–3 n/cm²s over the energy range from 10 MeV to 200 MeV. This upper limit is converted into upper limit differential solar neutron spectrum to be compared with the results obtained from other experiments.An irregular excess in counting rate observed in one of the flights, resulting in a day-night difference, is discussed.Finally the problems encountered in the operation of this type of detector at a high level of sensitivity are also discussed.This research has been sponsored in part by the Air Force Cambridge Research Laboratories through the European Office of Aerospace Research, OAR, USAF under contract F61052-68-C-0050.     

A model of the Io plasma ribbon

Using data derived from Earth-based measurements of the Io plasma torus, J.T. Trauger (1984, Science226, 337–341) has recently detected a distinct and sharply defined component of the hot inner torus which describes as “ribbon-like” in structure. We identify this plasma ribbon as the source of density fluctuations which derive outward plasma diffusion in a linearized convection model. The model yields simple expressions involving the thickness of the source ring, a linear dimension of the convection cells, and the exponent in the power law in zenocentric radius which the plasma density satisfies in the source ring. We find that the model is consistent with Trauger's data.     

A search for extremely red stars on the palomar observatory sky survey. II. Examination for light variability

The extremely red stars discovered on some Palomar Sky Survey prints in an earlier paper were investigated for variability. Four certain and one probable variables were found.     

The stability of water in Io

A variety of processes have been examined to determine their impact on water loss from Io and the formation of an anhydrous surface. Thermal escape, photolysis, and gas-phase charged particle interactions are shown to be unimportant in this regard. Recent laboratory experiments have shown that charged-particle sputtering is likely to be an effective mechanism for the removal of water ice from Io's surface. Vaporization of ice by meteoroid impacts may also be significant. The overall sputtering rate appears to be sufficiently high that the formation of a substantial regolith due to meteoroid bombardment will be prevented. However, meteoroid bombardment is probably capable of maintaining a thin (? 500 μm overturned surface layer from which all free water has been removed by sputtering. Alternatively, a thick anhydrous surface layer may have formed on Io as the result of primordial heating. The survival of such a layer to the present implies the absence of subsequent water evolution onto the surface of the satellite.     

Geologic mapping of the Amirani-Gish Bar region of Io: Implications for the global geologic mapping of Io

We produced the first geologic map of the Amirani-Gish Bar region of Io, the last of four regional maps generated from Galileo mission data. The Amirani-Gish Bar region has five primary types of geologic materials: plains, mountains, patera floors, flows, and diffuse deposits. The flows and patera floors are thought to be compositionally similar, but are subdivided based on interpretations regarding their emplacement environments and mechanisms. Our mapping shows that volcanic activity in the Amirani-Gish Bar region is dominated by the Amirani Eruptive Center (AEC), now recognized to be part of an extensive, combined Amirani-Maui flow field. A mappable flow connects Amirani and Maui, suggesting that Maui is fed from Amirani, such that the post-Voyager designation “Maui Eruptive Center” should be revised. Amirani contains at least four hot spots detected by Galileo, and is the source of widespread bright (sulfur?) flows and active dark (silicate?) flows being emplaced in the Promethean style (slowly emplaced, compound flow fields). The floor of Gish Bar Patera has been partially resurfaced by dark lava flows, although other parts of its floor are bright and appeared unchanged during the Galileo mission. This suggests that the floor did not undergo complete resurfacing as a lava lake as proposed for other ionian paterae. There are several other hot spots in the region that are the sources of both active dark flows (confined within paterae), and SO₂- and S₂-rich diffuse deposits. Mapped diffuse deposits around fractures on mountains and in the plains appear to serve as the source for gas venting without the release of magma, an association previously unrecognized in this region. The six mountains mapped in this region exhibit various states of degradation. In addition to gaining insight into this region of Io, all four maps are studied to assess the best methodology to use to produce a new global geologic map of Io based on the newly released, combined Galileo-Voyager global mosaics. To convey the complexity of ionian surface geology, we find that a new global geologic map of Io should include a map sheet displaying the global abundances and types of surface features as well as a complementary GIS database as a means to catalog the record of surface changes observed since the Voyager flybys and during the Galileo mission.     

A survey of polarization in the JVAS/CLASS flat-spectrum radio source surveys – II. A search for aligned radio polarizations

We have used the very large Jodrell Bank VLA Astrometric Survey/Cosmic Lens All-Sky Survey 8.4-GHz surveys of flat-spectrum radio sources to test the hypothesis that there is a systematic alignment of polarization position angle vectors on cosmological scales of the type claimed by Hutsemékers et al. The polarization position angles of 4290 sources with polarized flux density ≥1 mJy have been examined. They do not reveal large-scale alignments either as a whole or when split in half into high-redshift  ( z ≥ 1.24)  and low-redshift subsamples. Nor do the radio sources which lie in the specific areas covered by Hutsemékers et al. show any significant effect. We have also looked at the position angles of parsec-scale jets derived from very long baseline interferometry observations and again find no evidence for systematic alignments. Finally, we have investigated the correlation between the polarization position angle and those of the parsec-scale jets. As expected, we find that there is a tendency for the polarization angles to be perpendicular to the jet angles. However, the difference in jet and polarization position angles does not show any systematic trend in different parts of the sky.     

Mass pickup in sub-Alfvénic plasma flow: A case study for Io

Mass pickup may be an important process in the vicinity of the Jovian satellite Io and could be largely responsible for the slowing and deviation of the magnetospheric plasma flow in Io's vicinity. Theoretical treatments hitherto have treated a flow dominated by mass pickup near Io in a manner exactly analogous to the case where Io's immediate vicinity is treated as a simple electrical conductor. The analogy although instructive is not exact for it does not take account of the increased mass density in the flow downstream of Io. We point out that when correctly treated, mass pickup gives rise to a distinct wake structure behind Io. Observationally there is no way at present of distinguishing between dominant mechanisms using the Voyager 1 data; a wake pass is needed.