Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSeasonal structures in Saturns dusty Roche Division correspond to periodicities of the planets magnetosphere
435
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We identify multiple periodic dusty structures in Saturns Roche Division, a faint region spanning the $sim3000$ km between the A and F rings. The locations and extent of these features vary over Cassinis tour of the Saturn system, being visible in 2006 and 2016-2017, but not in 2012-2014. These changes can be correlated with variations in Saturns magnetospheric periods. In 2006 and 2016-2017, one of the drifting magnetospheric periods would produce a 3:4 resonance within the Roche Division, but in 2012-2014 these resonances would move into the A ring as the magnetospheric periods converged. A simple model of magnetic perturbations indicates that the magnetic field oscillations responsible for these structures have amplitudes of a few nanotesla, comparable to the magnetic field oscillation amplitudes of planetary period oscillations measured by the magnetometer onboard Cassini. However, some previously unnoticed features at higher radii have expected pattern speeds that are much slower than the magnetospheric periodicities. These structures may reflect an unexpectedly long-range propagation of resonant perturbations within dusty rings.
rate research
Read More
The seasonal evolution of Saturns polar atmospheric temperatures and hydrocarbon composition is derived from a decade of Cassini Composite Infrared Spectrometer (CIRS) 7-16 $mu$m thermal infrared spectroscopy. We construct a near-continuous record of atmospheric variability poleward of 60$^circ$ from northern winter/southern summer (2004, $L_s=293^circ$) through the equinox (2009, $L_s=0^circ$) to northern spring/southern autumn (2014, $L_s=56^circ$). The hot tropospheric polar cyclones and the hexagonal shape of the north polar belt are both persistent features throughout the decade of observations. The hexagon vertices rotated westward by $approx30^circ$ longitude between March 2007 and April 2013, confirming that they are not stationary in the Voyager-defined System III longitude system as previously thought. The extended region of south polar stratospheric emission has cooled dramatically poleward of the sharp temperature gradient near 75$^circ$S, coinciding with a depletion in the abundances of acetylene and ethane, and suggestive of stratospheric upwelling with vertical wind speeds of $wapprox+0.1$ mm/s. This is mirrored by a general warming of the northern polar stratosphere and an enhancement in acetylene and ethane abundances that appears to be most intense poleward of 75$^circ$N, suggesting subsidence at $wapprox-0.15$ mm/s. However, the sharp gradient in stratospheric emission expected to form near 75$^circ$N by northern summer solstice (2017, $L_s=90^circ$) has not yet been observed, so we continue to await the development of a northern summer stratospheric vortex. North polar minima in tropospheric and stratospheric temperatures were detected in 2008-2010 (lagging one season, or 6-8 years, behind winter solstice); south polar maxima appear to have occurred before the start of the Cassini observations (1-2 years after summer solstice). [Abridged]
The incredible longevity of Cassinis orbital mission at Saturn has provided the most comprehensive exploration of a seasonal giant planet to date. This review explores Saturns changing global temperatures, composition, and aerosol properties between northern spring and summer solstice (2015-2017), extending our previous review of Cassinis remote sensing investigations (2004-14, Fletcher et al., 2018) to the grand finale. The result is an unprecedented record of Saturns climate that spans almost half a Saturnian year, which can be used to test the seasonal predictions of radiative climate models, neutral and ion photochemistry models, and atmospheric circulation models. Hemispheric asymmetries in tropospheric and stratospheric temperatures were observed to reverse from northern winter to northern summer; spatial distributions of hydrocarbons and para-hydrogen shifted in response to atmospheric dynamics (e.g., seasonally-reversing Hadley cells, polar stratospheric vortex formation, equatorial stratospheric oscillations, and inter-hemispheric transport); and upper tropospheric and stratospheric aerosols exhibited changes in optical thickness that modulated Saturns visible colours (from blue hues to a golden appearance in the north near solstice), reflectivity, and near-infrared emission. Numerical simulations of radiative balance and photochemistry do a good job in reproducing the observed seasonal change and phase lags, but discrepancies between models and observations still persist, indicating a crucial role for atmospheric dynamics and the need to couple chemical and radiative schemes to the next generation of circulation models. With Cassinis demise, an extended study of Saturns seasons, from northern summer to autumn, will require the capabilities of ground- and space-based observatories, as we eagerly await the next orbital explorer at Saturn.
Using measurements from the Cassini spacecraft in Saturns magnetosphere, we propose a 3D physical picture of a corotating reconnection site, which can only be driven by an internally generated source. Our results demonstrate that the corotating magnetic reconnection can drive an expansion of the current sheet in Saturns magnetosphere and, consequently, can produce Fermi acceleration of electrons. This reconnection site lasted for longer than one of Saturns rotation period. The long-lasting and corotating natures of the magnetic reconnection site at Saturn suggest fundamentally different roles of magnetic reconnection in driving magnetospheric dynamics (e.g., the auroral precipitation) from the Earth. Our corotating reconnection picture could also potentially shed light on the fast rotating magnetized plasma environments in the solar system and beyond.
Nowadays, astronomers want to observe gaps in exozodiacal disks to confirm the presence of exoplanets, or even make actual images of these companions. Four hundred and fifty years ago, Jean-Dominique Cassini did a similar study on a closer object: Saturn. After joining the newly created Observatoire de Paris in 1671, he discovered 4 of Saturns satellites (Iapetus, Rhea, Tethys and Dione), and also the gap in its rings. He made these discoveries observing through the best optics at the time, made in Italy by famous opticians like Giuseppe Campani or Eustachio Divini. But was he really able to observe this black line in Saturns rings? That is what a team of optical scientists from Observatoire de Paris - LESIA with the help of Onera and Institut dOptique tried to find out, analyzing the lenses used by Cassini, and still preserved in the collection of the observatory. The main difficulty was that even if the lenses have diameters between 84 and 239 mm, the focal lengths are between 6 and 50 m, more than the focal lengths of the primary mirrors of future ELTs. The analysis shows that the lenses have an exceptionally good quality, with a wavefront error of approximately 50 nm rms and 200 nm peak-to-valley, leading to Strehl ratios higher than 0.8. Taking into account the chromaticity of the glass, the wavefront quality and atmospheric turbulence, reconstructions of his observations tend to show that he was actually able to see the division named after him.
Disc-driven planet migration is integral to the formation of planetary systems. In standard, gas-dominated protoplanetary discs, low-mass planets or planetary cores undergo rapid inwards migration and are lost to the central star. However, several recent studies indicate that the solid component in protoplanetary discs can have a significant dynamical effect on disc-planet interaction, especially when the solid-to-gas mass ratio approaches unity or larger and the dust-on-gas drag forces become significant. As there are several ways to raise the solid abundance in protoplanetary discs, for example through disc winds and dust-trapping in pressure bumps, it is important to understand how planets migrate through a dusty environment. To this end, we study planet migration in dust-rich discs via a systematic set of high-resolution, two-dimensional numerical simulations. We show that the inwards migration of low-mass planets can be slowed down by dusty dynamical corotation torques. We also identify a new regime of stochastic migration applicable to discs with dust-to-gas mass ratios $gtrsim 0.3$ and particle Stokes numbers $gtrsim 0.03$. In these cases, disc-planet interaction leads to the continuous development of small-scale, intense dust vortices that scatter the planet, which can potentially halt or even reverse the inwards planet migration. We briefly discuss the observational implications of our results and highlight directions for future work.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
