Carrington Maps of the Upper Photosphere

Abstract

We have used images of the Sun's disk, obtained in the 6767 Å continuum with the Michelson Doppler Interferometer (MDI) on the Solar and Heliospheric Observatory (SOHO), to make Carrington maps of the upper photosphere during the years 1996-2005. Each map is constructed from observations near the limb where the continuum radiation originates relatively high in the photosphere and faculae have their greatest visibility. Consequently, the Carrington maps resemble spectroheliograms in temperature-sensitive photospheric lines and show the global distribution of faculae and all but the smallest sunspots (which are obscured by overlying faculae). A time-lapse sequence of the combined east-limb and west-limb maps shows the emergence of active regions and the evolution of large-scale patterns of faculae with an average temporal resolution of 14 days during the sunspot cycle. Also, a longitudinally averaged butterfly diagram of these maps shows that in each hemisphere there is a facula-free zone separating the old-cycle polar field from trailing-polarity flux that is migrating poleward from the sunspot belts. These facula-free zones coincide with the neutral zones of the axisymmetric component of photospheric magnetic field and their arrival at the poles in 2001 marks the reversal of the polar fields. We think that this mapmaking technique can be applied to white-light images obtained daily at the Mount Wilson Observatory since 1905 and that the resulting Carrington maps will provide details about the polar-field reversal process during past sunspot cycles when high-quality magnetograms were unavailable.