An Erupting Active Region Filament: Three-Dimensional Trajectory and Hydrogen Column Density

Abstract

From 15:33 through 16:02 UT on 13 June 1998, observations of an erupting filament as it crossed solar disk center were obtained with the NSO/KPVT and SOHO/CDS instruments as part of the SOHO Joint Observing Program 70. Context observations show that this event was the eruption of the north-east section of a small active region filament associated with NOAA 8237, that the photospheric magnetic field was changing in this active region between 12-14 June 1998, and that a coronal Moreton-wave disk event occurred, as well as a white-light CME off the south-west solar limb. The NSO/KPVT imaging spectroscopy data covered 512 × 512 arc sec of the disk center and were spectrally centered at the He i 1083 nm line and captured ± 1.0 nm of surrounding solar spectrum. The He i absorption line is seen blue-shifted to velocities of between 200 and 300 km s⁻¹. The true solar trajectory of the eruption is obtained by using the projected solar coordinates and by integrating the Doppler velocity. The filament travels with a total velocity of about 300 km s⁻¹ along a path inclined roughly 49 deg to the solar surface and rises to a height of just over 1.5 solar radii before it becomes too diffuse to follow. The filament also shows internal motions with multiple Doppler components shifted by ± 25 km s⁻¹. Finally, the KPVT data show no Stokes V profiles in the Doppler-shifted He i 1083.03 nm absorption to a limit of roughly 3×10⁻³ times the continuum intensity. The SOHO/CDS scanned the center of the KPVT FOV using seven EUV lines; Doppler-shifted filament emission is seen in lines from He i 58.4 nm, He ii 30.4 nm, O iv 55.5 nm, O v 63.0 nm, Ne vi 56.3 nm, and Mg x 61.0 nm representing temperatures from about 2×10⁴K through 1×10⁶K. Bound-free continuum absorption from H i, without confusion from foreground emission and line emission, is seen as the filament obscures underlying chromospheric emission. A fit to the wavelength dependence of the absorption from five lines between 55.5 to 63.0 nm yields a column density ξ_H I =4.8±2.5×10¹⁷ cm⁻². Spatial maps show that this filament absorption is more confined than the regions which show emission.