THE Lya LINES OF HI AND HeII: A DIFFERENTIAL HANLE EFFECT FOR EXPLORINGTHE MAGNETISM OF THE SOLAR TRANSITION REGION

摘要

The Lya line of HeII at 304 A is one of the spectral lines of choice for EUV channels of narrowband imagers on board space telescopes, which provide spectacular intensity images of the outer solar atmosphere. Since the magnetic field information is encoded in the polarization of the spectral line radiation, it is important to investigate whether the HeII line radiation from the solar disk can be polarized, along with its magnetic sensitivity. Here we report some theoretical predictions concerning the linear polarization signals produced by scattering processes in this strong emission line of the solar transition region, taking into account radiative transfer and the Hanle effect caused by the presence of organized and random magnetic fields. We find that the fractional polarization amplitudes are significant (～1%), even when considering the wavelength-integrated signals. Interestingly, the scattering polarization of the Lya line of He II starts to be sensitive to the Hanle effect for magnetic strengths B 100 G (i.e., for magnetic strengths of the order of and larger than the Hanle saturation field of the hydrogen Lya line at 1216 A). We therefore propose simultaneous observations of the scattering polarization in both Lya lines to facilitate magnetic field measurements in the upper solar chromosphere. Even the development of a narrowband imaging polarimeter for the He tt 304 A line alone would be already of great diagnostic value for probing the solar transition region.