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Register a new userConjugate observations of Saturns northern and southern H3+ aurorae
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James O'Donoghue Mr
Publication date
2013
fields
Physics
and research's language is
English
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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.
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The magnetospheric cusps are important sites of the coupling of a magnetosphere with the solar wind. The combination of both ground- and space-based observations at Earth have enabled considerable progress to be made in understanding the terrestrial cusp and its role in the coupling of the magnetosphere to the solar wind via the polar magnetosphere. Voyager 2 fully explored Neptunes cusp in 1989 but highly inclined orbits of the Cassini spacecraft at Saturn present the most recent opportunity to repeatedly studying the polar magnetosphere of a rapidly rotating planet. In this paper we discuss observations made by Cassini during two passes through Saturns southern polar magnetosphere. Our main findings are that i) Cassini directly encounters the southern polar cusp with evidence for the entry of magnetosheath plasma into the cusp via magnetopause reconnection, ii) magnetopause reconnection and entry of plasma into the cusp can occur over a range of solar wind conditions, and iii) double cusp morphologies are consistent with the position of the cusp oscillating in phase with Saturns global magnetospheric periodicities.
From the 27th to the 28th January 2009, the Cassini spacecraft remotely acquired combined observations of Saturns southern aurorae at radio, ultraviolet and infrared wavelengths, while monitoring ion injections in the middle magnetosphere from energetic neutral atoms. Simultaneous measurements included the sampling of a full planetary rotation, a relevant timescale to investigate auroral emissions driven by processes internal to the magnetosphere. In addition, this interval coincidently matched a powerful substorm-like event in the magnetotail, which induced an overall dawnside intensification of the magnetospheric and auroral activity. We comparatively analyze this unique set of measurements to reach a comprehensive view of kronian auroral processes over the investigated timescale. We identify three source regions in atmospheric aurorae, including a main oval associated with the bulk of Saturn Kilometric Radiation (SKR), together with polar and equatorward emissions. These observations reveal the co-existence of corotational and sub-corototational dynamics of emissions associated with the main auroral oval. Precisely, we show that the atmospheric main oval hosts short-lived sub-corotating isolated features together with a bright, longitudinally extended, corotating region locked at the southern SKR phase. We assign the susbtorm-like event to a regular, internally driven, nightside ion injection possibly triggerred by a plasmoid ejection. We also investigate the total auroral energy budget, from the power input to the atmosphere, characterized by precipitating electrons up to 20 keV, to its dissipation through the various radiating processes. Finally, through simulations, we confirm the search-light nature of the SKR rotational modulation and we show that SKR arcs relate to isolated auroral spots. The resulting findings are discussed in the frame of pending questions.
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.
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.
Saturns polar stratosphere exhibits the seasonal growth and dissipation of broad, warm, vortices poleward of $sim75^circ$ latitude, which are strongest in the summer and absent in winter. The longevity of the exploration of the Saturn system by Cassini allows the use of infrared spectroscopy to trace the formation of the North Polar Stratospheric Vortex (NPSV), a region of enhanced temperatures and elevated hydrocarbon abundances at millibar pressures. We constrain the timescales of stratospheric vortex formation and dissipation in both hemispheres. Although the NPSV formed during late northern spring, by the end of Cassinis reconnaissance (shortly after northern summer solstice), it still did not display the contrasts in temperature and composition that were evident at the south pole during southern summer. The newly-formed NPSV was bounded by a strengthening stratospheric thermal gradient near $78^circ$N. The emergent boundary was hexagonal, suggesting that the Rossby wave responsible for Saturns long-lived polar hexagon - which was previously expected to be trapped in the troposphere - can influence the stratospheric temperatures some 300 km above Saturns clouds.
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