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Register a new userDisruption of Saturns Quasi-Periodic Equatorial Oscillation by the Great Northern Storm
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Observations of planets throughout our Solar System have revealed that the Earth is not alone in possessing natural, inter-annual atmospheric cycles. The equatorial middle atmospheres of the Earth, Jupiter and Saturn all exhibit a remarkably similar phenomenon - a vertical, cyclic pattern of alternating temperatures and zonal (east-west) wind regimes that propagate slowly downwards with a well-defined multi-Earth-year period. Earths Quasi-Biennial Oscillation (QBO, observed in the lower stratospheres with an average period of 28 months) is one of the most regular, repeatable cycles exhibited by our climate system, and yet recent work has shown that this regularity can be disrupted by events occurring far away from the equatorial region, an example of a phenomenon known as atmospheric teleconnection. Here we reveal that Saturns equatorial Quasi-Periodic Oscillation (QPO, with a ~15-year period) can also be dramatically perturbed. An intense springtime storm erupted at Saturns northern mid-latitudes in December 2010, spawning a gigantic hot vortex in the stratosphere at $40^circ$N that persisted for 3 years. Far from the storm, the Cassini temperature measurements showed a dramatic $sim10$-K cooling in the 0.5-5 mbar range across the entire equatorial region, disrupting the regular QPO pattern and significantly altering the middle-atmospheric wind structure, suggesting an injection of westward momentum into the equatorial wind system from waves generated by the northern storm. Hence, as on Earth, meteorological activity at mid-latitudes can have a profound effect on the regular atmospheric cycles in the tropics, demonstrating that waves can provide horizontal teleconnections between the phenomena shaping the middle atmospheres of giant planets.
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Saturns Great Storm of 2010 - 2011 produced a planet-encircling wake that slowly transitioned from a region that was mainly dark at 5 microns in February 2011 to a region that was almost entirely bright and remarkably uniform by December of 2012. The uniformity and high emission levels suggested that the entire wake region had been cleared not only of the ammonia clouds that the storm had generated and exposed, but also of any other aerosols that might provide significant blocking of the thermal emission from Saturns deeper and warmer atmospheric layers. Our analysis of VIMS wake spectra from December 2012 provides no evidence of ammonia ice absorption, but shows that at least one significant cloud layer remained behind: a non-absorbing layer of 3 - 4 optical depths (at 2 microns) extending from 150 to ~400 mbar. A second layer of absorbing and scattering particles, with less than 1 optical depth and located near 1 bar, is also suggested, but its existence as a model requirement depends on what value of the He/H2 ratio is assumed. The observations can be fit well with just a single (upper) cloud layer for a He/H2 ratio of 0.064 in combination with a PH3 deep volume mixing ratio of 5 ppm. At lower He/H2 ratios, the observed spectra can be modeled without particles in this region. At higher ratios, in order to fit the brightest wake spectrum, models must include either significant cloud opacity in this region, or significantly increased absorption by PH3, NH3, and AsH3. As the exceptional horizontal uniformity in the late wake is most easily understood as a complete removal of a deep cloud layer, and after considering independent constraints on trace gas mixing ratios, we conclude that the existence of this remarkable wake uniformity is most consistent with a He/H2 mixing ratio of 0.055 (+0.010, -0.015), which is on the low side of the 0.038 - 0.135 range of previous estimates.
We report the discovery of a large ($sim$8500 km diameter) infrared-bright storm at Neptunes equator in June 2017. We tracked the storm over a period of 7 months with high-cadence infrared snapshot imaging, carried out on 14 nights at the 10 meter Keck II telescope and 17 nights at the Shane 120 inch reflector at Lick Observatory. The cloud feature was larger and more persistent than any equatorial clouds seen before on Neptune, remaining intermittently active from at least 10 June to 31 December 2017. Our Keck and Lick observations were augmented by very high-cadence images from the amateur community, which permitted the determination of accurate drift rates for the cloud feature. Its zonal drift speed was variable from 10 June to at least 25 July, but remained a constant $237.4 pm 0.2$ m s$^{-1}$ from 30 September until at least 15 November. The pressure of the cloud top was determined from radiative transfer calculations to be 0.3-0.6 bar; this value remained constant over the course of the observations. Multiple cloud break-up events, in which a bright cloud band wrapped around Neptunes equator, were observed over the course of our observations. No dark spot vortices were seen near the equator in HST imaging on 6 and 7 October. The size and pressure of the storm are consistent with moist convection or a planetary-scale wave as the energy source of convective upwelling, but more modeling is required to determine the driver of this equatorial disturbance as well as the triggers for and dynamics of the observed cloud break-up events.
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.
We present an analysis of recent high spatial and spectral resolution ground-based infrared observations of H3+ spectra obtained with the 10-metre Keck II telescope in April 2011. We observed H3+ emission from Saturns northern and southern auroral regions, simultaneously, over the course of more than two hours, obtaining spectral images along the central meridian as Saturn rotates. Previous ground-based work has derived only an average temperature of an individual polar region, summing an entire night of observations. Here we analyse 20 H3+ spectra, 10 for each hemisphere, providing H3+ temperature, column density and total emission in both the northern and southern polar regions simultaneously, improving on past results in temporal cadence and simultaneity. We find that: 1) the average thermospheric temperatures are 527+/-18 K in northern Spring and 583+/-13 K in southern Autumn, respectively; 2) this asymmetry in temperature is likely to be the result of an inversely proportional relationship between the total thermospheric heating rate (Joule heating and ion drag) and magnetic field strength - i.e. the larger northern field strength leads to reduced total heating rate and a reduced temperature, irrespective of season, and 3) this implies that thermospheric heating and temperatures are relatively insensitive to seasonal effects.
Since the 1950s, quasi-periodic oscillations have been studied in the terrestrial equatorial stratosphere. Other planets of the solar system present (or are expected to present) such oscillations, like the Jupiter Equatorial Oscillation(JEO) and the Saturn Semi-Annual Oscillation (SSAO). In Jupiters stratosphere, the equatorial oscillation of its relative temperature structure about the equator, is characterized by a quasi-period of 4.4 years. The stratospheric wind field in Jupiters equatorial zone has never been directly observed. In this paper, we aim at mapping the absolute wind speeds in Jupiters equatorial stratosphere to quantify vertical and horizontal wind and temperature shear. Assuming geostrophic equilibrium, we apply the thermal wind balance using nearly simultaneous stratospheric temperature measurements between 0.1 and 30 mbar performed with Gemini/TEXES and direct zonal wind measurements derived at 1 mbar from ALMA observations, all carried out between March 14th and 22nd, 2017. We are thus able to calculate self-consistently the zonal wind field in Jupiters stratosphere where the JEO occurs. We obtain stratospheric map of the zonal wind speeds as a function of latitude and pressure about Jupiters equator for the first time. The winds are vertically layered with successive eastward and westward jets. We find a 200 m/s westward jet at 4 mbar at the equator, with a typical longitudinal variability on the order of ~50 m/s. By extending our wind calculations to the upper troposphere, we find a wind structure qualitatively close to the wind observed using cloud-tracking techniques. Nearly simultaneous temperature and wind measurements, both in the stratosphere, are a powerful tool for future investigations of the JEO (and other planetary equatorial oscillations) and its temporal evolution.
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