Venus limb temperatures from Akatsuki Longwave Infrared Radiometer (LIR) camera at all latitudes and local times

The  thermal infrared images from the LIR camera on Akatsuki  orbiter allow detrmining the altiutde and the temperature at the limb (unit slant optical depth).  With the near equatorial orbit of Akatsuki around Venus, each full disk image provide near pole to pole latitude coverage at the limb. T he limb temperatures provide a very useful to characterize the Venus thermal structure in an altitude reion (~ 80-90 km above the mean surface) where few direct measurements are available. By analyzing all available images acquired during the Akatsuki mission (7 December 2015 - February 2022), brightness temperatures are obtained at all local times and latitudes at the limb altitudes.  It has been possible to determine the limb location to a very high accuracy despite the low angular resolution of the LIR camera and the highly elliplital orbit which results in nominal pixel size ranging from ~50 km  forfull disk  images acquired at ~ 50,000 km that fill the 248 x 320 pixel sensor and as low as 300 km at apoapsis. Both sub-pixel sampling and Gauss fits to the gradient were used to determine the peak radial gradient of intensity to locate the slant unit optical depth.  The technique also yields an estimate of the vertical gradient of the brightness temperature which is a very good proxy for the atmospheric temperature lapse rate at the limb.