REAL AND VIRTUAL UNIPOLAR REGIONS

Difficulties in relating magnetograph measurements to the actual solar magnetic field are discussed. After a brief review both of problems i nherent in the nature of the measurements and of sources of instrument al error, we show that field measurements taken within the photosphere can map out large-scale regions of a single magnetic polarity even th ough these regions contain no footpoints of large-scale magnetic struc tures, but instead only aggregates of small, unresolved bipoles. This may occur wherever the density of unresolved bipoles has a preferred o rientation and a spatial variation along the direction of that orienta tion. We call these regions virtual unipolar regions, as they are not connected to regions of opposite polarity by field loops or lines pass ing through the corona. Investigation of these regions shows that they can arise at widely separated locations, and that they may evolve int o real unipolar magnetic regions which are connected to the chromosphe ric and coronal fields. These results can explain a number of puzzling aspects of magnetograph observations of the solar background magnetic field.