Magnetic Structure of an Erupting Filament

The full 3D vector magnetic field of a solar filament prior to eruption is presented. The filament was observed with the Facility Infrared Spectropolarimeter at the Dunn Solar Telescope in the chromospheric He i line at 10830 on 2017 May 29 and 30. We inverted the spectropolarimetric observations with the Hanle and Zeeman Light code to obtain the chromospheric magnetic field. A bimodal distribution of field strength was found in or near the filament. The average field strength was 24 Gauss, but prior to the eruption we find the 90th percentile of field strength was 435 Gauss for the observations on May 29. The field inclination was about 67° from the solar vertical. The field azimuth made an angle of about 47°-65° to the spine axis. The results suggest an inverse configuration indicative of a flux rope topology. He i intensity threads were found to be coaligned with the magnetic field direction. The filament had a sinistral configuration as expected for the southern hemisphere. The filament was stable on 2017 May 29 and started to rise during two observations on May 30, before erupting and causing a minor coronal mass ejection. There was no obvious change of the magnetic topology during the eruption process. Such information on the magnetic topology of erupting filaments could improve the prediction of the geoeffectiveness of solar storms.