Visible emission line spectroscopy of the solar corona during the 2019 total solar eclipse

Abstract

Spectroscopic measurements of the low solar corona are crucial to understanding the mechanisms that heat the corona and accelerate the solar wind, yet the lowest solar radii (R⊙) of the corona is difficult to observe. Our expedition collected narrow wavelengths of visible light at 530.3, 637.4, and 789.2 nm emitted by Fe XIV, X, and XI ions, respectively, from the total solar eclipse on 2019 July 2 at 20:40 UTC in Rodeo, Argentina with a bespoke 3-channel spectrometer. This paper describes the instrument, and data calibration method that enables diagnostics out to ≈1.0 R⊙ above the solar limb within a bright helmet streamer. We find that Fe X and XI lines are dominant through 0.3 R⊙, with Fe XIV maintaining a stronger signal at higher elevations. Thermal broadening is consistent with 4 MK for the cooler Fe X and XI lines, and 6 MK for the hottest Fe XIV line, which can be interpreted as differing density scale heights within isolated, isothermal flux tubes. The Doppler measurements correspond to bulk plasma motion ranging from -12 to +2.5 km s-1, with Fe XIV moving at nearly an assumed solid body rotation rate throughout 1.0 R⊙. After considering coronal rotation, these measurements are likely associated with plasma motion along the dominant longitudinal orientation of the magnetic field at the streamer base within 0.4R⊙. These results show that high resolution spectroscopy of visible light offers valuable diagnostics of the low corona, and lend insight into the interconnected loop complexity within helmet streamers.