A Simulation Study on Terahertz Continuum-Wave Observations of Neptune’s Atmosphere Focusing on Future ALMA Observations

Abstract

The Atacama Large Millimeter-submillimeter Array (ALMA) is suitable for planetary atmospheric remote sensing because of its high spatial resolution that can resolve even the small planetary disk of Neptune. From the photometryof planetary atmospheric emissions continuum that are dominated by collision-induced absorption (CIA), the horizontal/verticaltemperature structurecan be estimated at different altitudes.To enable imaging photometry of Neptune using the ALMA, we developed radiative transfer code that includes the CIA opacity for application to the upper frequency ALMA Bands 7 to 10, in the 275 to 950GHz frequency range to investigate the structure ofNeptune’s atmosphere at altitudes that can be analyzed by continuum emissionobservations atspecific frequencies which are not affectedby the strong absorption of carbon monoxide. We used the CIA intensity parameter provided by the HIgh-resolution TRANsmission (HITRAN) database for the estimation of the absorption coefficient. The estimated disk-averaged brightness temperature agrees with previous continuum observations. The contribution function indicates that the entire range of ALMA Bands 7 to 10 enables us to investigate the vertical temperature structurefrom below the tropopause to the upper troposphere over the south pole, which is in the middle of previously detected stratospheric and tropospheric hot spots that exhibited ahigher (7-30K) brightness temperaturethan lower latitudinal regions.