Near-infrared polarization studies of Saturn and Jupiter

Polarization measurements of Jupiter, Saturn, and Saturn's rings from 1 to 3.5 μm are presented. At 1.6 μm on the discs of the two planets, the radially directed limb polarizations observed in the visible undergo, in some cases, a surprising 90° rotation to a tangential direction, particularly on the poles. The only immediate explanation for this effect is double Mie scattering, due to aerosols which must be of the order of a micrometer in size. On Jupiter the patterns are not uniform and are not stable, reflecting variable aerosol concentrations on the two poles. The ring polarization is uniformly negative (E vector parallel to the ecliptic plane) from the visible through 3.5 μm, and is inversely proportional to the albedo. This is as expected from Wolff's model for scattering from rough solid surfaces; but the degree of polarization seems uncommonly high, exceeding 2% at 3.5 μm.     

The north-south asymmetry of polarization of Jupiter: The causes of seasonal variations

We present new results obtained from the analysis of the seasonal variations in the asymmetry of polarization of light reflected by Jupiter. From the 23-year set of observations, the anticorrelation between the asymmetries of polarization and insolation has been revealed. The mechanism explaining the observed seasonal variations of polarization has been proposed. The core of this mechanism is the effect of temperature changes in the planetary stratosphere on the processes of the stratospheric aerosol haze formation. Additional irregular factors that may influence the observed polarization asymmetry are considered.     

The circular polarization of sources of synchrotron radiation

The degree of circular polarizationp _c is calculated for two models of a source of synchrotron radiation:

1. A source with an inhomogeneous magnetic field and isotropic angular distribution of the electrons with respect to the magnetic field;
2. A source with a homogeneous magnetic field and anisotropic angular distribution of the electrons in which the anisotropy of angular distribution substantially increases with the electron energy.

The first model can be used to describe extended radio-sources; and the second, to describe compact radio-sources. For those sources, whose observed polarization properties correspond to the first model, we obtain an integral equation which connects the observed distribution of the sources with the extent of their linear and circular polarization (p _l andp _c ) and the unknown distribution of the sources over the strengthB and the degree of homogeneity ?=(B ₀/B)² of the magnetic field;B ₀ is a homogenous field,B ₀?B. A solution of the integral equation obtained is found for a particular case. This solution makes it possible to determine the distribution of different types of sources over ? if the distribution of these sources in the extent of linear polarization is known. The formulae obtained make it possible to indicate which sources with a known degree of linear polarization should be expected to exhibit highest circular polarization. In the discussion of the first model the question is raised as to the information one can get about the magnetic field by using observations of both linear and circular polarization for a separate source, and for a number of sources. It is shown that the determination of the most probable values ofB and ? in a separate source based on the known values ofp _l andp _c for the source, is possible only if one knows the distribution overB and ? of the sources of the type to which the source in question belongs. The observational data now available make it possible to find the distribution of the sources only over ?. Since the distribution overB and ? is at present unknown, even a very strong upper limit forp _c in the case of a separate source does not enable us to give an exact upper limit for the strength of the magnetic field in this source. In the first model the upper limit for the magnetic field can be obtained only if the upper limit ofp _c is known for a certain number of sourcesN, withN?1. This limit allows for much stronger fields than are usually admitted. This last fact should be taken into consideration when one deals with the results of observations of circular polarization in sources with strong magnetic fields. The first model presents some difficulties when we compare it with observations of some compact sources. The second model can explain why one observes in these sources a violation of the lawp _c ～v ^?1/2 and a change of sign inp _c when the frequency of the observationsv changes.     

Radio circular polarization of active galaxies

We present the first results of a circular polarization survey conducted with the Australia Telescope Compact Array (ATCA). We demonstrate the ability to make circular polarization measurements with a standard error of only 0.01 per cent, and have detections from both blazar and non-blazar active galactic nuclei (AGN). Our results show that, as a group, BL Lac sources and quasars have systematically higher circular polarization than radio galaxies. We demonstrate the association of high levels of circular polarization with total-intensity variability and flat/inverted spectral index as further evidence that circular polarization is associated with blazar activity. We also include preliminary circular polarization monitoring data and the detection of circular polarization from the GHz Peaked Spectrum (GPS) source PKS 1934–638, and discuss possible implications.     

On the degree of circular polarization of synchrotron radiation

Formulas describing synchrotron radiation are extended to include the effect of the presence of an ambient medium and the effect of reabsorption and Faraday rotation on the degree of circular polarization. Results are: (1) The onset of Razin-Tsytovich suppression has no significant effect on the degree of circular polarization. (2) The onset of self-absorption in a source subject to weak Faraday rotation (negligible Faraday rotation in an absorption length) causes the degree of circular polarization to reverse its sign and to decrease (by between one half and one quarter for typical parameters) in magnitude. (3) As in (2) but for strong Faraday rotation (many rotations of the plane of polarization in an absorption length) the degree of circular polarization also reverses its sign and becomes slightly smaller in magnitude than for weak Faraday rotation.The transfer equation including the effects of the polarization is discussed in detail.     

An interpretation of the broad-band circular polarization of sunspots

The broad-band circular polarization of sunspots is discussed on the basis of the observations made in the Okayama Astrophysical Observatory. The observation with the spectrograph proves that it is the integrated polarization of spectral lines in the observed spectral range. A velocity gradient in the line-of-sight can produce this integrated polarization due to the differential saturation between Zeeman components of magnetically sensitive lines. The observed degree of polarization and its spatial distribution in sunspots is explained when we introduce a differentially twisted magnetic field in addition to the velocity gradient. The differential twist has the azimuth rotation of the magnetic field along the line-of-sight and generates the circular polarization from the linear polarization due to the magneto-optical effect. The required azimuth rotation is reasonable and amounts at most to 30°. The required velocity gradient is compatible with the line asymmetry and its spatial distribution observed in sunspots. The observed polarity rule leads to the conclusion that the sunspot magnetic field has the differential twist with the right-handed azimuth rotation relative to the direction of the main magnetic field, without regard to the magnetic polarity and to the solar cycle. The twist itself is left-handed under the photosphere, when the sunspot is assumed to be a unwinding emerging magnetic field.     

Comments on light and dark spots in the equatorial regions of Jupiter

The analysis of light and dark spots in the equatorial regions of Jupiter by Browne and Meadows (1978) is shown to have major inconsistencies in the interpretation of the colours of these features. In order to prevent similar errors with subsequent analyses of multispectral Jovian images, it is essential to use the data in digital form and analyse it using an interactive computing facility.     

The planet Jupiter

Summary. The exploration of Jupiter, the closest and biggest giant planet, has provided key information about the origin and evolution of the outer Solar system. Our knowledge has strongly benefited from the Voyager and Galileo space missions. We now have a good understanding of Jupiter's thermal structure, chemical composition and magnetospheric environment. There is still debate about the nature of the heating source responsible for the high thermospheric temperatures (precipitating particles and/or gravity waves). The measurement of elemental abundance ratios (C/H, N/H, S/H) gives strong support to the “nucleation” formation model, according to which giant planets formed from the accretion of an initial core and the collapse of the surrounding gaseous protosolar nebula. The D/H and He/He ratios are found to be representative of their protosolar value. The helium abundance, in contrast, appears to be slightly depleted in the outer envelope with respect to the protosolar value; this departure is interpreted as an evolutionary effect, due to the condensation of helium droplets in the liquid hydrogen ocean inside Jupiter's interior. The cloud structure of Jupiter, characterized by the belt-zone system, is globally understood; also present are specific features like regions of strong infrared radiation (“hot spots”), colder regions (“white ovals”) and the Great Red Spot (GRS). Clouds were surprisingly absent at the hot spot corresponding to the Galileo probe entry site, and the water abundance measured there was strongly depleted with respect to the solar O/H value. This probably implies that hot spots are dry, cloud-free regions of subsidence, while “normal” air, rich in condensibles, is transported upward by convective motions. As a result, the Jovian meteorology, still based on Halley-type cells, seems to be much more complex than a simple zone-belt system. The nature of the GRS, a giant anticyclonic storm, colder and higher than its environment, has been confirmed by the Galileo observations, but its internal structure appears to be very complex. Strong winds, probably driven by the Jovian internal source, were measured at deep tropospheric levels. The troposphere might be statically stable at pressures higher than 18 bars, but the extent of this putative radiative layer is still unknown. Received 23 November 1998     

Variable circular polarization associated with relativistic ejections from GRS 1915 + 105

We describe H α , SCUBA and MERLIN imaging of the interacting galaxy pair NGC 4490 and 4485. We detect an H α filament emerging from the disc of NGC 4490 to a projected distance of 3 kpc which has counterparts in both the radio continuum and H  i . The H  i counterpart extends to a projected distance of ∼30 kpc from NGC 4490 and we argue that this is evidence that the giant H  i envelope in this system has its origins in star formation. We use SCUBA and radio continuum data to attempt to place constraints on the distribution of dust with respect to the star forming regions. This analysis is limited by the lack of an independent estimate of the dust temperature, something that both 'SIRTF' and 'SOFIA' will be able to provide, however we find some evidence that most obscuring dust is not located within H  ii regions themselves.     

The simplest solar microbursts flux and circular polarization at 22 GHz

The simplest solar microwave microbursts detected with high sensitivity may be the response to the simpler energetic burst injections. Seventeen events from this category were identified in a series of more than 150 bursts recorded in 21–26 November, 1982. This first systematic study suggest that microbursts e-folding rise times concentrate into two classes of time scales, 0.05 s < t 1 s and 0.5 s t 2 s. Microbursts circular polarization present a dominant steady or slowly varying component that sets in before maximum emission. In some cases a faster component of polarization was found superimposed, which is not always well correlated in time with flux.     

Gas Dissipation from the Protosatellite Disk of Jupiter

We consider the dissipation of the gaseous component from the gas–dust accretion disk of Jupiter in which the Galilean satellites were formed. The thermal dissipation of hydrogen and helium is shown to be ineffective. It could ensure the loss of gas only for a low-mass disk and only if the rarefied outer layers of the disk are heated to 10⁴ K. Such a high disk temperature is not reached through Jupiter's radiation in existing models of its formation, but it could be provided by UV radiation of the early Sun after the dissipation of the protoplanetary disk. The viscous dissipation (with a viscosity parameter 10^–3 in the -disk model) related to disk accretion onto Jupiter could disperse a low-mass disk in 10⁷ years. A magnetocentrifugal mechanism, which produced a disk wind during accretion capable of carrying away 0.1 of the accreted gas mass, was probably also involved in the dispersal of the Jovian disk. Differential dispersion, with the loss of only hydrogen and helium and the retention of water vapor and heavier gases in the disk, is possible only in a low-mass disk model. We conclude that the water contained in the Galilean satellites was brought in mainly by solid planetesimals captured into the disk during mutual inelastic collisions in Jupiter's sphere of influence.     

Examining the feasibility of air shower core-location from polarization properties of associated Cherenkov light

Based on this exploratory investigation involving CORSIKA simulation code generated Cherenkov photons and a linearly polarized, hypothetical photon beam, we make a case here for exploiting polarization properties of atmospheric Cherenkov events for providing an independent method for locating air-shower cores by a TACTIC-like array of atmospheric Cherenkov telescopes. Preliminary results based on simulations indicate that for a 3 TeV γ-ray having ∼30% degree of polarization for its associated Cherenkov light at a core distance of ∼100 m, core location can be found with an error of ∼27 m. Deceased This revised version was published online in July 2006 with corrections to the Cover Date.     

On the long-term behaviour of the circular polarization from coronal condensation radio emission at 4.3 cm wavelength

The circular polarization from coronal condensations at = 4.3 cm correspond to the extraordinary mode of propagation, due to the contribution of preceding spots' polarities, being usually left-handed. The fewer cases of right-handed polarization are normally associated to an excess of sunspot plages in the southern hemisphere, thus making it difficult to give evidence for magnetoionic coupling phenomena as a general rule.     

Phase function and polarization curve of interplanetary scatterers from zodiacal light photopolarimetry

Wide-angle ecliptic measurements of zodiacal light brightness (Z) and polarization (P) lead to fundamental results about optical properties of interplanetary scatterers, under a few reasonable assumptions (that they depend upon heliocentric distance by a r^?n law, and suffer no significant distortion of their scattering indicatrix between 0.5 and 2 a.u.): 1. The phase function σ(θ) is expressed (Equation 6) as a function of n and of (Z) data. 2. At the elongation ? = 90°, the derivative $\text{dZ}\text{d?}$ yields an absolute determination of the intensity $\text{T}$ scattered at right angles from the Sun by a single unit-volume of interplanetary medium (Equation 7). 3. The polarization degree $\text{P}$(θ) of the sunlight scattered by a single volume is derived (Equation 12) from n and from (Z + P) data. For two special values of the scattering angle θ, n vanishes in Equation (12), so that a fair knowledge of the polarization curve (Fig. 2) is reached prior to any assumption, or any forthcoming Jupiter-probe measure, about the value of n.Should n be provided by the Pioneers, then a thorough treatment of the whole problem of phase function and polarization curve can be performed by means of Equations (6) and (12) supplied with available zodiacal light photopolarimetric observations.