We present an investigation of a coronal cavity observed above the western limb in the coronal red line Fe X 6374 A using a telescope of Peking University and in the green line Fe XIV 5303 A 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 is 
constructed based on the magnetograms taken by the Helioseismic and Magnetic Imager on board 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 10747 A 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 10747 A, 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 10747 A and Fe X 6374 A.

https://iopscience.iop.org/article/10.3847/1538-4357/aaaf68/pdf

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

We report on early results from a suite of instruments for imaging and spectra we deployed to Salem, Oregon, for two minutes of totality at the August 21, 2017, total solar eclipse. Our instruments included refracting telescopes and telephoto lenses for use with CCD detectors and DSLR cameras, narrow-band filters at the wavelengths of coronal emission lines ([Fe XIV] 530.3 nm and [Fe X] 637.4 nm), and spectrographs. We also monitored the effect of the eclipse penumbra and umbra on the terrestrial atmosphere.

/pdf/images-and-spectra-of-the-2017-total-solar-eclipse-corona-16pl2bnnio.pdf

Images and Spectra of the 2017 Total Solar Eclipse Corona from our Oregon Site

Total solar eclipses, as the recent one seen across North America, are rare opportunities for optical spectroscopy of the corona. In view of the dearth of accurate rest-frame wavelength data, we measured 11 of the strongest optical coronal lines belonging to Fe X-XIV thereby proving the existence of the Fe XII line at 290.385(8) nm. Four lines, such as the green coronal line at 530.28113(13) nm, were measured with unprecedented precision, allowing in principle for absolute velocity determinations of plasmas with uncertainties of 0.08 km ${\mathrm{s}}^{\ensuremath{-}1}$. These results furthermore stringently benchmark the theory of complex open-$3p$-shell ions.

Laboratory precision measurements of optical emissions from coronal iron

Modulation using a rotating waveplate is the most popular way in astronomy to obtain radiation polarization states and thus the physical condition of celestial bodies. Modulation error analysis of the rotating quarter-waveplate polarimeter is presented in this paper. In terms of geometric dimensions, three modulation error sources are analyzed: the waveplate axial error, waveplate rotation axis tip-tilt error (zenithal error), and position error of the waveplate fast axis (azimuthal error). The dispersion deviation, as another dimension of modulator error, is also studied in this paper. In theory, two factors affect the accuracy of polarization measurement using waveplate polarimetry: retardance $ \delta $δand fast axis position $ \theta $θ. The temperature property of the waveplate, which represents the axial error, the waveplate mounting tip-tilt error, which represents the zenithal error, and the wavelength-based retardance characteristic, which represents the dispersion property of the waveplate, belong to the retardance error. The motorized rotary stage home position error and the random sample rotating position error, representing the azimuthal error, belong to the fast axis position error. These factors will be analyzed in detail here. Based on these analyses, the maximum allowable upper limits for each error source under $ 1 \times {10^{ - 4}} $1×10-4 polarization measurement accuracy are presented. Also, feasible solutions are proposed to address these errors.

Analysis of errors in polarimetry using a rotating waveplate.

We present an investigation of a coronal cavity observed above the western limb in the coronal red line Fe X 6374 {\AA} using a telescope of Peking University and in the green line Fe XIV 5303 {\AA} 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 3.1$\pi$, 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 10747 {\AA} 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 10747 {\AA}, 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 10747 {\AA} and Fe X 6374 {\AA}.

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

Hui Tian1*, ZhongQuan Qu2, YaJie Chen1, LinHua Deng2, ZhengHua Huang3, Hao Li2, Yue Zhong2, Yu Liang2, JingWen Zhang1, YiGong Zhang2, BaoLi Lun2, XiangMing Cheng2, XiaoLi Yan2, ZhiKe Xue2, YuXin Xin2, ZhiMing Song2, YingJie Zhu1, and Tanmoy Samanta1 1School of Earth and Space Sciences, Peking University, Beijing 100871,China; 2Yunnan Observatories, Chinese Academy of Sciences, Kunming 650011, China; 3Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai 264209, China

Observations of the solar corona during the total solar eclipse on 21 August 2017

Strict registration is critical for imaging polarimetry; a highly accurate registration approach to the coronal image and imaging polarimetry of the Fe 6374 angstrom red coronal line have been researched in this paper. In order to improve the registration accuracy, this paper proposes the idea of image enhancement based on blind deconvolution combined with noise-adaptive fuzzy equalization algorithms. After image enhancement, the cross-correlation registration algorithm achieves better results. To diagnose a low-temperature corona, the polarization brightness of the red coronal line data and the Mauna Loa Solar Observatory (MLSO)/KCor data are compared to study the structural features and polarization information of the inner corona. We found that the polarization brightness radiation of the red coronal line has overall features similar to that of the MLSO/KCor data, but it has more detailed features, such as the cavity, polar plume, and interplume characteristics. Active regions, polar plumes, and interplumes show similar performance in red coronal line intensity, polarization brightness, degree of linear polarization, K-corona, and electron density along with the radial height. An interesting finding is that the polarization brightness of the red coronal line changes drastically along with the radial height across the cavity above the solar west limb, while neither the K-corona nor the electron density changes obviously. This seems to be a piece of possible evidence indicating that the red coronal line is not unpolarized in the cavity region.

Registration and imaging polarimetry of the Fe 6374 Å red coronal line during the 2017 total solar eclipse

Finding out and identifying mechanisms responsible for coronal eruptions, coronal heating and solar wind acceleration form the cores in solar physics research, and such mechanisms are crucial for us to establish efficient models for predicting the catastrophic space weather and even for sun-like stars. As one important element of the Ground-based Advanced Solar Observatory (ASO-G), the proposed COronal Magnetism and Plasma ASsembled Scopes (COMPASS) consist of three coronagraphs of different sizes. They are respectively the coronal magnetism probing refractive Lyot coronagraph with an aperture of 1.26 m, called COMPT, improved from American COSMO-LC, a two-dimensional spectroscopic diagnosis coronagraph with an aperture of 0.30 m, named TSDC, and a small Lyot coronagraph with an aperture of 0.09 m attached to COMPT for monitoring. Observations of COMPT focus on the accurate measurement of vector magnetic fields and other physical parameters in a height range of 0.05–0.65 solar radius above solar limb by two observing modes, the integral field Stokes spectroscopy (IFS) realized by a pair of integral field units and corresponding spectrometers as well as two-dimensional imaging Stokes spectroscopy (TDIS) by dual tunable Lyot filters, and the reduced polarimetric optical switching is applied for high precision polarimetry, while observations from TSDC will provide information on the necessary physical parameters of coronal plasma in a field of view with height range from 0.10 to 1.5 solar radii, with help of Fabry-Perot interferometer to carry out the imaging spectroscopy as well as white light polarization imaging. These three coronagraphs form a complementary set. The scientific and observational goals are stated and then the preliminary optical, mechanical and electronic control designs of these coronagraphs are described. Finally the innovative ideas to serve the goals are outlined.

COronal Magnetism and Plasma ASsembled Scopes(COMPASS)

The utility model discloses a novel coke side diaphragm aims at providing a small novel coke side diaphragm. It includes that the middle part has the diaphragm body of iris opening, the incident surface of the diaphragm body is provided with the special -shaped recess around the iris opening, and the reflection mirror surface of this side end face of iris opening for having the constant slope, the reflection mirror surface stretches out one section distance of special -shaped recess, makes the circuit of its reverberation lie in outside the special -shaped recess. The utility model discloses can effectively satisfy the user demand of the telescope of adoption cassegrain system, both can provide field stop's effect for the terminal fitting, still can possess the supervision light path of predetermined visual field, can satisfy the required heat accuse function of solar telescope again.

Song Zhiming

Papers

Analysis of errors in polarimetry using a rotating waveplate.

Observations of the solar corona during the total solar eclipse on 21 August 2017

Registration and imaging polarimetry of the Fe 6374 Å red coronal line during the 2017 total solar eclipse

COronal Magnetism and Plasma ASsembled Scopes(COMPASS)

Novel coke side diaphragm