Magnetic Field Curvature In A Filament Channel Derived From Oscillation Measurements And MHD Modeling

We have used measurements of repeated large amplitude longitudinal oscillations (LALOs) in an active region filament to diagnose the curvature of the magnetic field in the filament channel and compared the results with predictions of an MHD flux-rope model based on magnetograms of the region. In May and June of 2014 Active Region 12076 exhibited a complex of filaments undergoing repeated oscillations over the course of twelve days. The central filament channel exhibited emerging and then canceling magnetic flux that resulted in multiple activations, filament eruptions, and eight oscillation events, which we analyzed using GONG H-alpha data. Luna and Karpen (2012) model LALOs as oscillations of magnetized filament plasma moving along dipped magnetic field lines with gravity as a restoring force. Under this model the period of these oscillations can be used to estimate the curvature of the magnetic field in the location of the filament threads. Utilizing this, we find that the measured periods in the central filament ranging from 34-74 minutes should correspond to magnetic field curvatures of about 30-136 Mm. We also derive radii of curvature for the central filament channel using a flux-rope model that is based on an SDO/HMI magnetogram of the region. The rope is constructed using the analytic expressions by Titov et al. (2018) and then numerically relaxed towards a force-free state in the zero-beta MHD approximation, where gravity and thermal pressure are neglected. For comparison, we also employ a nonlinear force-free field (NLFFF) extrapolation of the active region. We compare the results of these different ways of attempting to determine the field in the filament channel.