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We present an investigation of a coronal cavity observed above the westernlimb in the coronal red line Fe X 6374 {\AA} using a telescope of PekingUniversity and in the green line Fe XIV 5303 {\AA} using a telescope of YunnanObservatories, Chinese Academy of Sciences during the total solar eclipse on2017 August 21. A series of magnetic field models are constructed based on themagnetograms taken by the Helioseismic and Magnetic Imager onboard the SolarDynamics Observatory (SDO) one week before the eclipse. The model field linesare then compared with coronal structures seen in images taken by theAtmospheric 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 3.1$\pi$,which is consistent with the most probable value of the total twist angle ofinterplanetary flux ropes observed at 1 AU. Linear polarization of the Fe XIII10747 {\AA} line calculated from this model shows a "lagomorphic" signaturethat is also observed by the Coronal Multichannel Polarimeter of the HighAltitude Observatory. We also find a ring-shaped structure in the line-of-sightvelocity of Fe XIII 10747 {\AA}, which implies hot plasma flows along a helicalmagnetic field structure, in the cavity. These results suggest that themagnetic structure of the cavity is a highly twisted flux rope, which may erupteventually. The temperature structure of the cavity has also been investigatedusing the intensity ratio of Fe XIII 10747 {\AA} and Fe X 6374 {\AA}.

Diagnosing the magnetic field structure of a coronal cavity observed during the 2017 total solar eclipse