Diagnosing the Magnetic Field Structure of a Coronal Cavity Observed during the 2017 Total Solar Eclipse

Abstract

We present an investigation of a coronal cavity observed above the western limb in the coronal red line Fe X 6374Å using a telescope of Peking University and in the green line Fe XIV 5303Å using a telescope of Yunnan Observatories, Chinese Academy of Sciences during the total solar eclipse on 2017 August 21. A series of magnetic field models are constructed based on the magnetograms taken by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory (SDO) one week before the eclipse. The model field lines are then compared with coronal structures seen in images taken by the Atmospheric Imaging Assembly on board SDO and in our coronal red line images. The best-fit model consists of a flux rope with a twist angle of 3.1π, which is consistent with the most probable value of the total twist angle of interplanetary flux ropes observed at 1 AU. Linear polarization of the Fe XIII 10747Å line calculated from this model shows a "lagomorphic" signature that is also observed by the Coronal Multichannel Polarimeter of the High Altitude Observatory. We also find a ring-shaped structure in the line-of-sight velocity of Fe XIII 10747Å, which implies hot plasma flows along a helical magnetic field structure, in the cavity. These results suggest that the magnetic structure of the cavity is a highly twisted flux rope, which may erupt eventually. The temperature structure of the cavity has also been investigated using the intensity ratio of Fe XIII 10747Å and Fe X 6374Å.

Images and Spectra of the 2017 Total Solar Eclipse Corona From Our Oregon Site

“…shows a stacked image of the white-light corona as processed by Roman Vanúr from the 800 mm telephoto lens, using 85 individual pictures of different exposures from 2 to 1/500 s. To show higher contrast that reveals the structure of the underlying magnetic field, we used an especially-high-frequency filter. Tian et al (2017) and Chen et al (2018) have described the coronal configuration and a coronal cavity over a prominence region, which also shows on our images. Hanaoka et al (2018) have described changes over an expanse of umbral motion across the United States.…”

Mapping the magnetic field in the solar corona through magnetoseismology

“…Direct measurements of the coronal magnetic field through spectropolarimetric observations have been attempted in the past two decades. The linear polarization of some coronal forbidden lines is sensitive to the magnetic field orientation in the plane of sky (POS) (e.g., [12][13][14]), and the degree of linear polarization could be used to diagnose some magnetic structures such as flux ropes [15][16][17]. Due to the weak field in the corona, circular polarization signals associated with the longitudinal Zeeman effect are usually very weak.…”

Microwave diagnostics of magnetic field strengths in solar flaring loops

“…We have constructed a series of magnetic field models by adjusting the axial and poloidal flux until a good match of the model field lines with the observed flare loops is achieved. For a more detailed description of this method, we refer to the recent publications of Su et al [53] and Chen et al [54]. Using the best-fit model we could have a reasonable estimation of the viewing angle for each flare loop.…”