Properties of Magnetic Neutral Lines and Filament Formation

Chromospheric filaments form at a boundary (neutral lines) between opposite polarity magnetic fields. Both the total length of neutral lines and the number of filaments vary with solar cycle. However, our analysis indicates that the cycle variation in filaments is not due to the change in the total length of neutral lines. It is likely due to change in magnetic properties of neutral lines. In present study we compare properties of magnetic polarity inversion lines that have filaments above them and those without filaments, and investigate how these properties change during the solar cycle. Our results show that the gradient as well as convergence/divergence of magnetic field across neutral lines are not the major factors that determine the places of filament formation. We discuss the role of canceling magnetic features in filament formation and their cycle variation.