July 31, 2014

Atmospheric physics

Comparing the solar limb observed from the ground and from orbit reveals the role of the atmosphere. Seen from the ground, the limb may cover several arcseconds, which is why observing the diameter from orbit and from the ground simultaneously enables variations due to the Sun itself and variations due to Earth’s atmosphere to be separated.

But to extract information about the Sun from a ground-based measurement, we first need to understand the process by which the limb is altered by scattering and turbulence. Simultaneous ground-based and space-based measurements have been acquired to deduce the intrinsic effect of turbulence, using since two identical instruments. While atmospheric effects are easy to highlight, quantifying them is a different proposition. Turbulence conditions were measured by an independent instrument to gauge the degradation of the wavefront induced by atmospheric turbulence, and by locating the turbulent layers responsible for image distortion (Fried parameter r0, spatial coherence outer scale L0, isoplanatic angle 0, characteristic timescale(s) for wavefront evolution 0 and vertical turbulence profile Cn2(h)). These parameters were estimated by a turbulence model (Kolmogorov, von Karman, etc.) to be validated by observations, several models being good candidates (Voitsekhovich, 1995).

The role of the ultraviolet domain is taken into account in stratosphere-troposphere climate models. Variability in the UV spectrum must be precisely measured, more specifically that corresponding to ozone photochemistry, which conditions temperatures and dynamics in the stratosphere. The model devised by Rozanov et al., (2002) shows the spectral domains for which ozone temperature and ozone are the most sensitive. Using the variability of these domains measured by Picard is helping scientists to study the precise response of the stratosphere to UV variability and any coupling with the troposphere.