Abstract
We present an analysis of recent high spatial and spectral resolution ground-based infrared observations of H3+ obtained with the 10-m Keck II telescope in April 2011. We observed H3+ emission from Saturn’s northern and southern auroral regions, simultaneously, over the course of more than 2h, obtaining spectral images along the central meridian as Saturn rotated. 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±18K in northern Spring and 583±13K 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.
Highlights
Saturn’s upper atmosphere is defined as the region at which the neutral molecular species within cease to mix convectively and are able to separate according to their scale heights; it is dominated by H, H2, and He and the boundary between the mixing and non-mixing regions is known as the homopause (Nagy et al, 2009)
We view the aurorae as they rotate past the spectrometer slit and so variability is a combination of temporal changes occurring at local noon and longitudinal variations rotating into view
The total emission is ∼1.5 times higher in the south, 0.98±0.02 Wm−2str−1, compared with 0.65±0.03 Wm−2str−1 in the north. This result is similar to previous work based on Cassini Visual and Infrared Mapping Spectrometer (VIMS) observations in which they examined IR wavelengths associated with H+3 emission at ∼3.6 μm, which showed the pre-equinox southern main region to be on average ∼1.3 times more intense than the northern main auroral region (Badman et al, 2011)
Summary
Saturn’s upper atmosphere is defined as the region at which the neutral molecular species within cease to mix convectively and are able to separate according to their scale heights; it is dominated by H, H2, and He and the boundary between the mixing and non-mixing regions is known as the homopause (Nagy et al, 2009). Study of the Saturnian aurorae is generally divided into two wavelengths, the UV and infrared (IR) The former reveals the impact of highly energetic electrons on the polar ionosphere through the excitation of H and H2 (Shemansky and Ajello, 1983; Gustin et al, 2009), whilst the latter is the focus of this study and is observed primarily via the discrete ro-vibrational emission lines of the molecular ion H+3. We study the main auroral region H+3 temperature, column density and total emission in the northern and southern hemispheres of Saturn at the same time with a temporal resolution of 15 minutes, and explore the implications of these measurements
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