Hubble observations of Jupiter’s north–south conjugate ultraviolet aurora

Comparisons of the northern and southern far ultraviolet (UV) auroral emissions of Jupiter from the Hubble Space Telescope (HST) or any other ultraviolet imager have mostly been made so far on a statistical basis or were not obtained with high sensitivity and resolution. Such observations are important to discriminate between different mechanisms responsible for the electron acceleration of the different components of the aurora such as the satellite footprints, the «main oval» or the polar emissions. The field of view of the ACS and STIS cameras on board HST is not wide enough to provide images of the full jovian disk. We thus compare the morphology of the north and south aurora observed 55 min apart and we point out similarities and differences. On one occasion HST pointed successively the two polar regions and auroral images were seen separated by only 3 min. This makes it possible to compare the emission structure and the emitted FUV power of corresponding regions. We find that most morphological features identified in one hemisphere have a conjugate counterpart in the other hemisphere. However, the power associated with conjugate regions of the main oval, diffuse or discrete equatoward emission observed quasi-simultaneously may be different in the two hemispheres. It is not directly nor inversely proportional to the strength of the B-field as one might expect for diffuse precipitation or field-aligned acceleration with equal ionospheric electron density in both hemispheres. Finally, the lack of symmetry of some polar emissions suggests that some of them could be located on open magnetic field lines.     

Mercury resolved spectroscopy from NTT

Mercury has been observed in June 2006 with the SofI instrument at NTT. Resolved spectroscopy was performed in the range. The observations were acquired close to a favorable quadrature with maximum elongation, and span mostly the southern hemisphere in the 235–315°E longitude area.In contrast to recent observations from IRTF, the present study does not indicate any pyroxene absorption in the near-infrared range, neither in the region nor at . Detailed spectral analysis allows to derive an upper limit of 1.5% type-A pyroxene (hedenbergite) or 1.1% enstatite (assuming minimum lunar iron content) in the regions observed, at the 700 km scale. Assuming lunar-like maturity effects on band depth, this corresponds to an upper limit of 0.15 and 0.6 wt.% FeO content in silicates, an order of magnitude less than previous estimates.     

Titan haze distribution and optical properties retrieved from recent observations

The VIMS instrument onboard Cassini observed the north polar region of Titan at 113° phase angle, 28 December 2006. On this spectral image, a vast polar cloud can be seen northward to 62°N, and elsewhere, the haze appears as the dominant source of scattering. Because the surface does not appear in the wavelength range between 0.3 and , this spectro-image is ideal to study airborn scatterers both in methane bands and windows. In this work, we study this image, along with another image taken at 13° phase angle. This image probe both the atmosphere and the surface from pole to pole. First, we characterise the spatial distribution of the haze layer above 100 km between 80°S and 70°N. We find a north south asymmetry with a haze opacity increasing by a factor 3 from the south pole to the equator, then a constant value up to about 30°N and a decrease of a factor 2 between 30°N and about 60°N. Beyond 60°N, we can see the influence of the north polar cloud, even in the band, but no polar haze accumulation. The fact that the north polar region is still in the polar night is a possible explanation. No haze accumulation is observed in the southern polar region. Secondly, we partly identify the origin of spectral features in the 2.8-μm methane window, which are found to be due to deuterated methane (CH₃D). This allows the analyse of this window and to retrieve the opacity of scatterers layer below 80 km (presumably made of aerosols and condensate droplets) between 35°N and 60°N. Finally, we constrained the values and the spectral behaviour of the imaginary part of the aerosol refractive index in the range between 0.3 and . To do so, we studied the 2.8-μm window with the image taken at 113° phase angle. To complete the analysis, we studied the transmission through the haze layer in the 3.4-μm band observed in solar occultation mode with VIMS, and we analysed the single scattering albedo retrieved with DISR instrument between 0.4 and . The imaginary part of the refractive index that we find for Titan aerosols follows Khare et al. (Khare, B.N. et al. [1984]. Icarus 60, 127-137) optical constant up to and becomes constant beyond this wavelength at least up to . It also has a prominent peak at and a secondary peak at , which indicates material rich in C-H bonds, with much less N-H bonds than in Khare et al. (1984) tholins.     

A strong vortex in Saturn's South Pole

Saturn's southern pole was observed at high resolution by the Cassini Imaging Science Subsystem (ISS) during the spacecraft insertion orbit in July 2004. Cloud tracking of individual features on images taken at a wavelength of 938 nm reveal the existence of a strong polar vortex enclosed by a jet with maximum speed of relative to System III rotation frame, and peak at 87 °S planetographic latitude. Radiative transfer models of the reflected light, based on the Cassini images complemented by Hubble Space Telescope images from March 2004, indicate that the aerosol particles in the vortex are structured vertically in three detached layers. We find two hazes and one dense cloud distributed in altitude between ∼500 mbar (top of the dense cloud) and few mbar (top of the stratospheric haze), spanning a vertical altitude range of ∼200 km. The vortex area coincides with a thermal hot spot recently reported, indicating that winds decrease with altitude above polar clouds.     

Extension of atmospheric dust loading to high altitudes during the 2001 Mars dust storm: MGS TES limb observations

Mars Global Surveyor (MGS) visible (solarband bolometer) and thermal infrared (IR) spectral limb observations from the Thermal Emission Spectrometer (TES) support quantitative profile retrievals for dust opacity and particle sizes during the 2001 global dust event on Mars. The current analysis considers the behavior of dust lifted to altitudes above 30 km during the course of this storm; in terms of dust vertical mixing, particle sizes, and global distribution. TES global maps of visible (solarband) limb brightness at 60 km altitude indicate a global-scale, seasonally evolving (over 190-240° solar longitudes, L_S) longitudinal corridor of vertically extended dust loading (which may be associated with a retrograde propagating, wavenumber 1 Rossby wave). Spherical radiative transfer analysis of selected limb profiles for TES visible and thermal IR radiances provide quantitative vertical profiles of dust opacity, indicating regional conditions of altitude-increasing dust mixing ratios. Observed infrared spectral dependences and visible-to-infrared opacity ratios of dust scattering over 30-60 km altitudes indicate particle sizes characteristic of lower altitudes (cross-section weighted effective radius, ), during conditions of significant dust transport to these altitudes. Conditions of reduced dust loading at 30-60 km altitudes present smaller dust particle sizes . These observations suggest rapid meridional transport at 30-80 km altitudes, with substantial longitudinal variation, of dust lifted to these altitudes over southern hemisphere atmospheric regions characterized by extraordinary (m/s) vertical advection velocities. By L_S=230° dust loading above 50 km altitudes decreased markedly at southern latitudes, with a high altitude (60-80 km) haze of fine (likely) water ice particles appearing over 10°S-40°N latitudes.     

Periodicity and Hemispheric Phase Relationship in High-Latitude Solar Activity

The counts of the monthly averaged polar faculae, from observations of the National Astronomical Observatory of Japan (NAOJ), are examined by using linear and nonlinear approaches to find the periodicity characteristics of the polar faculae in the northern and southern hemispheres and the phase relationship between them. Both the cross-wavelet transform (XWT) and wavelet coherence (WTC) indicate the prominent period with 95% confidence level, namely the Schwabe cycle of about 11 years. The Schwabe cycle is in phase in the two hemispheres. Within the 11-year frequency band, there is a small phase difference during the period of 1966 – 1975 when the activity of polar faculae in the northern hemisphere slightly leads the one in the southern hemisphere. A cross-recurrence plot analysis and the line of synchronization (LOS) extracted from the cross-recurrence plot show further the phase difference between the two hemispheres. The LOS deviates significantly from the main diagonal during the period of 1965 – 1970 and LOS >0, showing that the activity of polar faculae in the northern hemisphere leads in phase, which is in accordance with XWT and WTC analyses. Moreover, asynchronization is highest (about 30 months) during this period.     

Seasonal and inter-annual changes of volume density of martian CO2 snow from time-variable elevation and gravity

The martian atmosphere seasonally exchanges CO₂ with the surface by repeating condensation and sublimation, causing seasonal growth and decay of the polar CO₂ snowcaps. These processes leave two kinds of geodetic signatures, i.e. seasonal changes of the martian gravity field and of surface elevation of the snow-covered regions. Here we study gradual increase of the volume density of the martian snow due to compaction, by combining these two data sets during 1999-2001 covering three martian winters. We found that light fresh snow of slowly becomes denser reaching or more immediately before it thaws. The maximum snow density varies slightly from year to year, and between hemispheres. In the second southern winter, the density became as high as . This might have been caused by a dust storm activity, e.g. increased mixing of silicate particles and/or enhancement of sintering.     

Compositional heterogeneity of Asteroid 4 Vesta’s southern hemisphere: Implications for the Dawn mission

High signal-to-noise, rotationally-resolved spectra of Asteroid 4 Vesta’s southern hemisphere from the 2007 opposition were used to constrain its compositional and mineralogical variations. The spectra were rotationally-phased using closely timed HST observations of Vesta by Li et al. (Li, J.-Y., McFadden, L.A., Thomas, P.C., Mutchler, M.J., Parker, J.Wm., Young, E.F., Russell, C.T., Sykes, M.V., Schmidt, B.E. [2010]. Icarus 208, 238–251). The average surface of Vesta’s southern hemisphere is analogous to a howardite or polymict eucrite assemblage similar to the northern hemisphere, although the band parameters are distinctly shifted towards the diogenite zone on the Band–Band plot. A few distinct compositional units were detected and they might be related to albedo features detected by Hubble Space Telescope (Li et al., 2010). We have identified two compositionally distinct regions overlaying the background surface. The first unit is a polymict eucrite and/or low-Ca eucrite compositional unit at 143° longitude that border the eucrite zone on the Band–Band plot and the second is a diogenite unit at 159°. While we did not detect any distinct olivine units as suggested by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157), we cannot rule out the possibility of smaller olivine-rich units that are below the detection limit of the instrumentation we used. Based on the analysis and the limitations of the data, we do not suggest that Vesta’s surface is olivine-free. Mean pyroxene chemistry estimates for both hemispheres broadly agree with one another (to within one-sigma) with the northern hemisphere ferrosilite (Fs) and wollastonite (Wo) values being slightly higher than southern hemisphere.     

Saturn eddy momentum fluxes and convection: First estimates from Cassini images

We apply an automated cloud feature tracking algorithm to estimate eddy momentum fluxes in Saturn's southern hemisphere from Cassini Imaging Science Subsystem near-infrared continuum image sequences. Voyager Saturn manually tracked images had suggested no conversion of eddy to mean flow kinetic energy, but this was based on a small sample of <1000 wind vectors. The automated procedure we use for the Cassini data produces an order of magnitude more usable wind vectors with relatively unbiased sampling. Automated tracking is successful in and around the westward jet latitudes on Saturn but not in the vicinity of most eastward jets, where the linearity and non-discrete nature of cloud features produces ambiguous results. For the regions we are able to track, we find peak eddy fluxes and a clear positive correlation between eddy momentum fluxes and meridional shear of the mean zonal wind, implying that eddies supply momentum to eastward jets and remove momentum from westward jets at a rate . The behavior we observe is similar to that seen on Jupiter, though with smaller eddy-mean kinetic energy conversion rates per unit mass of atmosphere (). We also use the appearance and rapid evolution of small bright features at continuum wavelengths, in combination with evidence from weak methane band images where possible, to diagnose the occurrence of moist convective storms on Saturn. Areal expansion rates imply updraft speeds of over the convective anvil cloud area. As on Jupiter, convection preferentially occurs in cyclonic shear regions on Saturn, but unlike Jupiter, convection is also observed in eastward jet regions. With one possible exception, the large eddy fluxes seen in the cyclonic shear latitudes do not seem to be associated with convective events.     

Curvature and surface distribution of the polar rays in the solar corona on 12 November 1966

Following a technique developed by Saito, the positions and orientations of the polar rays visible in a photograph of the total solar eclipse of 12 November 1966 were examined to infer the length of an equivalent bar magnet representing the Sun's external magnetic field and the true distribution of polar rays from the two-dimensional projection seen in the photograph. In spite of the strong asymmetry in solar activity between the northern and southern solar hemispheres at the time of the eclipse, it was found that the magnetic field as delineated by the polar rays could be well represented by an extended dipole whose poles were each 0.43 R from the center of the Sun and on the axis of rotation. The polar rays are distributed mainly in a circular zone 9° away from the pole and about 8° wide, with this zone bounded by empty regions at the pole and 15° to 20° away from it, beyond which there are again some rays, but not so many. These results are in good accord with expectation from Saito's analysis.Operated by the Association of Universities for Research in Astronomy Inc. under contract with the National Science Foundation.     

Giant longitude zones on the sun

Regularities in the longitudinal distribution of high-flare-activity active regions during 1964–1979 have been examined. The Fourier spectrum for each 30°-wide longitude strip in northern and southern hemispheres has been obtained. It is seen that there exist two giant longitude zones in each hemisphere and solar activity remains concentrated in one of the giant zones. It also appears that the leading edge of a giant zone is more active as compared to its following edge.     

North–South Asymmetry in the Distribution of Solar Background Magnetic Field

The aim of this article is to investigate how the background magnetic field of the Sun behaves in different hemispheres. We used SOHO/MDI data obtained during a period of eight years from 2003 to 2011 to analyze the intensity distribution of the background magnetic field over the solar surface. We find that the background fields of both polarities (signs) are more intense in the southern than in the northern hemisphere. Mixed polarities are observed in the vicinity of the equator. In addition to the main field, a weaker field of opposite polarity is always present in the polar regions. In the declining phase of the cycle, the main field dominates, but at the minimum and in the rising phase of the cycle, it is gradually replaced by the growing stronger secondary field.     

Estimating Mercury's 88-day libration amplitude from orbit

In this paper it is derived that the libration of Mercury can be described by where Φ₀ is the unknown libration amplitude, M is Mercury's mean anomaly and K=−9.483. Φ₀ can be determined by comparing pairs of images of the same landmarks taken by an orbiter at different positions of Mercury. If the angle between the orbit plane of a polar orbiter and Mercury's line of periapsis is between −60° and 60° and if one landmark at the equator is imaged per day with a relative precision of , then the libration amplitude can be determined in two Mercury years (176 days) with an accuracy of or better, which is sufficient to answer the question whether Mercury has a solid or fluid core.     

Mars magnetic field: Sources and models for a quarter of the southern hemisphere

The heavily-cratered southern hemisphere of Mars encompasses the planet’s strongest, most widespread magnetization. Our study concentrates on this magnetized region in the southern hemisphere within 40° of latitude 40°S, longitude 180°W. First we rotate the coordinates to position the center at −40°, 180° and treat these new latitudes and longitudes as if they were Cartesian coordinates. Then, using an ordinary two-dimensional Fourier analysis for downward continuation, the MGS (MAG/ER) magnetic field data at satellite mapping elevation of ∼400 km are extrapolated to 100 km, sources are estimated and used to model the fields. Quantitative comparison of the downward continued field with the aerobraking field for bins having angular deviation within ±30° gives correlation of .947, .868, and .769 for the components, respectively. This agreement of the fields may result from most of the power in the magnetization resting in wavelengths ∼400 km, with comparatively little at ∼100 km. Over this region, covering nearly an octant of the planet, just a dozen sources can account for 94% of the variance of the magnetic field at the surface. In these models for the field an obvious asymmetry in polarity exists, with majority of the sources being positive. The locations of strongest surface magnetization appear to be near - but not actually within - ancient multi-ringed basins. We test the likelihood of this association by comparing the observed sources found within and near basins for two alternative basin location scenarios with random distributions. For both alternatives we find the observed distributions to be low-probability occurrences. If contemporaneous, this would establish that Mars’ magnetic field extended to the time of impacts causing these basins.     

Interferometric millimeter observations of water vapor on Mars and comparison with Mars Express measurements

We present interferometric mapping of the 225.9-GHz HDO and 203.4-GHz lines on Mars obtained with the IRAM Plateau de Bure facility (PdBI). The observations were performed during martian year 28 (MY28), at L_s=320.3° for the HDO line, and at L_s=324.3° for the line. The HDO line is detected at the eastern (morning) and western (evening) limbs of the northern hemisphere, corresponding to a water column density in the range 3-6 pr.-μm. The line is not detected, which is compatible with the column densities derived from the HDO line. Quasi-simultaneous far infrared measurements obtained by the Planetary Fourier Spectrometer (PFS) onboard the Mars Express spacecraft confirm our PdBI results, yielding a 5±1 pr.-μm meridionally constant water column abundance.Such a low water abundance during the southern mid-autumn of MY28 does not correspond to the standard martian climatology as observed during the previous years. It was however already retrieved from near-infrared observations performed by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter spacecraft [Smith, M.D., Wolff, M.J., Clancy, R.T., Murchie, S.L. 2009. CRISM observations of water vapor and carbon monoxide. J. Geophys. Res. 114, doi: 10.1029/2008JE003288]. Our observations thus confirm that the planet-encircling dust storm that occurred during MY28 significantly affected the martian water cycle. Our observations also demonstrate the usefulness of interferometric submillimeter observations to survey the martian water cycle from ground-based facilities.     

A high-amplitude thermal inertia anomaly of probable magnetospheric origin on Saturn’s moon Mimas

Spectral maps of Mimas’ daytime thermal emission show a previously unobserved thermal anomaly on Mimas’ surface. A sharp V-shaped boundary, centered at 0°N and 180°W, separates relatively warm daytime temperatures from a cooler anomalous region occupying low- to mid-latitudes on the leading hemisphere. Subsequent observations show the anomalous region is also warmer than its surroundings at night, indicating high thermal inertia. Thermal inertia in the anomalous region is , compared to < outside the anomaly. Bolometric Bond albedos are similar between the two regions, in the range 0.49-0.70. The mapped portion of the thermally anomalous region coincides in shape and location to a region of high-energy electron deposition from Saturn’s magnetosphere, which also has unusually high near-UV reflectance. It is therefore likely that high-energy electrons, which penetrate Mimas’ surface to the centimeter depths probed by diurnal temperature variations, also alter the surface texture, dramatically increasing its thermal inertia.     

The asymmetry of solar activity in the years 1959–1969

One of the most outstanding feature of solar activity in the decade 1959–1969 was a very strong asymmetry on the two hemispheres. On the northern hemisphere spots, faculae and prominences were more numerous and the white light corona was brighter than on the southern hemisphere. This happened as well in the main zone as in the polar zone. The green coronal line too was brighter on the northern hemisphere, but the intensity of the red line was asymmetric in the opposite sense. From this behaviour it follows that over the more active hemisphere the corona is denser and hotter. Between density N _e and temperature T holds the relation: N _e = 10^–10 T ³. The real asymmetry was strengthened by a phase difference of the two hemispheres. This phase shift is subject to a long period that contains 8 eleven-year cycles. The intensity of the individual cycles follows the same long period. With low maxima of solar activity the northern hemisphere precedes, with high maxima the southern hemisphere (Figure 3).Astronomische Mitteilungen der Eidgenössischen Sternwarte Zürich, No. 302.     

Meridional variations of temperature, C2H2 and C2H6 abundances in Saturn's stratosphere at southern summer solstice

Measurements of the vertical and latitudinal variations of temperature and C₂H₂ and C₂H₆ abundances in the stratosphere of Saturn can be used as stringent constraints on seasonal climate models, photochemical models, and dynamics. The summertime photochemical loss timescale for C₂H₆ in Saturn's middle and lower stratosphere (∼40-10,000 years, depending on altitude and latitude) is much greater than the atmospheric transport timescale; ethane observations may therefore be used to trace stratospheric dynamics. The shorter chemical lifetime for C₂H₂ (∼1-7 years depending on altitude and latitude) makes the acetylene abundance less sensitive to transport effects and more sensitive to insolation and seasonal effects. To obtain information on the temperature and hydrocarbon abundance distributions in Saturn's stratosphere, high-resolution spectral observations were obtained on September 13-14, 2002 UT at NASA's IRTF using the mid-infrared TEXES grating spectrograph. At the time of the observations, Saturn was at a LS≈270°, corresponding to Saturn's southern summer solstice. The observed spectra exhibit a strong increase in the strength of methane emission at 1230 cm⁻¹ with increasing southern latitude. Line-by-line radiative transfer calculations indicate that a temperature increase in the stratosphere of ≈10 K from the equator to the south pole between 10 and 0.01 mbar is implied. Similar observations of acetylene and ethane were also recorded. We find the 1.16 mbar mixing ratio of C₂H₂ at −1° and −83° planetocentric latitude to be and , respectively. The C₂H₂ mixing ratio at 0.12 mbar is found to be at −1° planetocentric latitude and at −83° planetocentric latitude. The 2.3 mbar mixing ratio of C₂H₆ inferred from the data is and at −1° and −83° planetocentric latitude, respectively. Further observations, creating a time baseline, will be required to completely resolve the question of how much the latitudinal variations of C₂H₂ and C₂H₆ are affected by seasonal forcing and/or stratospheric circulation.     

The Solar Corona 530.3 nm in the 20th cycle

The paper presents an analysis of the coronal data of emission line 530.3 nm. The analysis was carried out with data recorded at the coronal station of Lomnický tít for the period 1965–1976. Up till now the observed time and latitudal changes of the shape of the corona as well as the existence of two maxima have been fully proved. The development of intensities has, in many cases, different characters in the southern and northern hemispheres. Apart from this, the third increase of intensities in the course of the cycle was observed nearly at all latitudes in the coincidence with sunspot numbers in 1974–1975 years. The maximum of intensities of this impulse of activity was observed at latitudes around 60° in both hemispheres.