Sensitivity Modelling of a 12.5-25 μm Filter Channel for an Ice Giants Net Flux Radiometer

Abstract:

The ice giants, Uranus and Neptune, hydrogen rich atmospheres are active and complex. It is important to comprehend more fully the atmospheric transport processes by determining the global heat balance and intrinsic heat flow. In order to improve model constraints on the interior structure and evolution, in situ probe measurements, of energy balance and interior heat flux are needed and would provide a reference profile to lift ambiguities inherent to remote observations. An Ice-Giants Net Flux Radiometer (IG-NFR), onboard a probe descending deep into the atmosphere, Fig. 1, will address these science objectives by measuring in-situ the upward and downward radiation flux in seven spectral filter channels as a function of altitude/pressure.

One of the long-wavelength filter channels proposed with bandwidth of 12.5 to 25 μm, is prone to large variations in radiation flux since it lies in the valley between the radiance contributions of downwelling solar insolation and upwelling thermal energy from the planet’s interior. In this abstract we present Signal-to-Noise Ratio (SNR) modelling results for this channel for varying atmospheric temperature profiles of Uranus, using the Planetary Spectrum Generator (PSG), a radiative transfer modelling simulator.

Four temperature variance scenarios in the standard atmospheric model of Uranus, Fig. 2, are considered. PSG employs forty-four atmospheric layers in the atmospheric model of Uranus, with each computation taking place at a specific atmospheric layer. Once a calculation is done for every layer in an atmosphere, the resulting radiation flux can be used to calculate the SNR. The upward and downward radiation fluxes are calculated separately by PSG, through the Looking Up and Nadir viewing geometry features. The ‘Hot Global’ approach, adds +50 K to the temperature profile of every atmospheric layer, while the ‘Hot Local’ approach, adds +50 K to the “nth” atmospheric layer being calculated, as well as the layers above and below the “nth” layer. The ‘Warm Global’ and ‘Warm Local’ approach follows the same method but adds only +5 K to the specified parameters.

The results of the sensitivity study, Fig. 3 and Fig. 4, help to define the useful SNR range of the 12.5-25 μm channel as a function of pressure and will help in optimizing the filter channel selections for a future IG-NFR.