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摘要
The Limb Darkening (LD) effect consists in an attenuation of the measured radiance at increasing emergence angles, due to the larger portion of the atmosphere traversed by the radiation observed at higher emergence angles. The behaviour of radiance emerging with atmosphere as function of the emergence angle (i.e., the Limb Darkening Function) is related to several atmosphere and cloud properties, e.g., atmospheric opacity, temperature lapse rate, aerosol single scattering albedo, optical depth, cloud particles size, cloud scale height. In addition, the retrieval of the Limb Darkening Function allows the computation of radiances as they would be acquired in Nadir observations, which is an important process of data reduction. %26lt;br%26gt;In this work Limb Darkening is studied on infrared images of the Venus nightside. The images are provided by the VIRTIS instrument (Visible and InfraRed Thermal Imaging Spectrometer) onboard the Venus Express mission. Analysis is performed at wavelengths characterised by emission coming from below the clouds (i.e., 1.03 mu m, 1.31 mu m, 1.74 mu m, 2.30 mu m) and from upper clouds (i.e., 3.72 mu m, 4.00 mu m), respectively. %26lt;br%26gt;In the first case, the Limb Darkening Function is retrieved at different latitude ranges (-40 degrees to 0 degrees, -50 degrees to -40 degrees, -60 degrees to -50 degrees, -70 degrees to -60 degrees) and different optical depths. To this end, a novel approach is developed, based on a statistical analysis of observed data. This statistical analysis is possible thanks to the very extended dataset provided by the VIRTIS instrument. These empirical Limb Darkening Functions are then compared and interpreted with the aid of radiative transfer models. %26lt;br%26gt;An agreement between our results and results of synthetic models is found between -60 degrees and 0 degrees, allowing us to consider our approach well validated. Furthermore, by means of a detailed analysis of the LDFs obtained at different latitudes, it is possible to infer a slight increase of the mode3/mode2 density ratio between -60 degrees and -50 degrees, that becomes larger between -70 degrees and -60 degrees: the mode3/mode2 ratio inferred at these latitudes is about two times its equatorial value. %26lt;br%26gt;The study of Limb Darkening from upper clouds region allows to obtain the upper cloud scale height, its variation with the atmospheric height, and the brightness temperature at different latitudes (the latitude intervals used are the same of the low atmosphere study). %26lt;br%26gt;All the latitudes are characterised by a thin layer at the upper clouds base having a short scale height (i.e., %26lt;1 km). Above that layer, scale height is calculated for every latitude interval. %26lt;br%26gt;Between -50 degrees and 0 degrees, the retrieved scale height is about 4 km (i.e., 3.7 +/- 0.3 km between -40 degrees and 0 degrees and 4.5 +/- 0.8 km between -50 degrees and -40 degrees) and is constant with the height. %26lt;br%26gt;Between -60 degrees and -50 degrees, a scale height of 4 - 5 km is found at low temperatures (226-231 K), whereas at higher temperature, we are able to give a lower (2-3 km) and an upper (7-9 km) limit. %26lt;br%26gt;Between -70 degrees and -60 degrees no Limb Darkening is observed, suggesting a low scale height (i.e., %26lt; 1 km), as well as a low lapse rate.
- 出版日期2012-8
