ON THE SENSITIVITY OF STOKES PROFILES TO PHYSICAL QUANTITIES

A thorough analysis of the sensitivities of the four Stokes profiles t o the physical quantities involved in the LTE line formation is presen ted. We point out the ambiguities on the definition of a mean depth of formation of a given spectral line and on the use of Contribution Fun ctions for the ascription of a measure to a given depth in the atmosph ere. Response Functions behave like partial derivatives of the Stokes spectrum at a given depth of the atmosphere. They provide the sensitiv ities of the observed spectrum to the physical quantities characterizi ng the state of the atmosphere. After a theoretical generalization of any measured parameter over the spectrum, we extend the concept of Res ponse Functions for such parameters; in detail are discussed the prope rties and sensitivities of the equivalent width, the Stokes V peaks di stance, the line-ratio method, and the centre of gravity method. Of pa rticular interest are the following results: a) a constant and longitu dinal magnetic field can desaturate a spectral line with a Zeeman patt ern other than a pure Zeeman triplet; b) saturation is readily underst ood as a consequence of an enhancement of the photon supply; c) method s to measure magnetic field strength are sensitive to temperature vari ations, mostly if a field strength gradient is present through the pho tosphere.