A force-free field (FFF) is determined solely by the normal components of magnetic field and current density on the entire boundary of the domain. Methods employing three components of magnetic field suffer from overspecification of boundary conditions and/or a nonzero divergence-B problem. A vector potential formulation eliminates the latter issue, but introduces difficulties in imposing the normal component of current density at the boundary. This paper proposes four different boundary treatment methods within the vector potential formulation. We conduct a comparative analysis of the vector potential FFF solvers that we have developed incorporating these methods against other FFF codes in different magnetic field representations. Although the vector potential solvers with the new boundary treatments do not outperform our poloidal-toroidal formulation code, they demonstrate comparable or superior performance compared to the optimization code in SolarSoftWare. The methods developed here are expected to be readily applied not only to force-free field computations but also to time-dependent data-driven simulations.

We investigate 20 post-coronal mass ejection (CME) blobs formed in the post-CME current sheet (CS) that were observed by K-Cor on 2017 September 10. By visual inspection of the trajectories and projected speed variations of each blob, we nd that all blobs except one show irregular \zigzag" trajectories resembling transverse oscillatory motions along the CS, and have at least one oscillatory pattern in their instantaneous radial speeds. Their oscillation periods are ranging from 30 to 91 s and their speed amplitudes from 128 to 902 kms􀀀1. Among 19 blobs, 10 blobs have experienced at least two cycles of radial speed oscillations with dierent speed amplitudes and periods, while 9 blobs undergo one oscillation cycle. To examine whether or not the apparent speed oscillations can be explained by vortex shedding, we estimate the quantitative parameter of vortex shedding, the Strouhal number, by using the observed lateral widths, linear speeds, and oscillation periods of the blobs. We then compare our estimates with theoretical and experimental results from MHD simulations and uid dynamic experiments. We nd that the observed Strouhal numbers range from 0.2 to 2.1, consistent with those (0.15{3.0) from uid dynamic experiments of blu spheres, while they are higher than those (0.15{0.25) from MHD simulations of cylindrical shapes. We thus nd that blobs formed in a post-CME CS undergo kinematic oscillations caused by uid dynamic vortex shedding. The vortex shedding is driven by the interaction of the outward-moving blob having a blu spherical shape with the background plasma in the post-CME CS.

The Korea Astronomy and Space Science Institute (KASI) has been developing a next-generation coronagraph (NGC) in cooperation with NASA to measure the coronal electron density, temperature, and speed simultaneously, using four different optical filters around 400 nm. KASI organized an expedition to demonstrate the coronagraph measurement scheme and the instrumental technology during the 2017 total solar eclipse (TSE) across the USA. The observation site was in Jackson Hole, Wyoming, USA. We built an eclipse observation system, the Diagnostic Coronal Experiment (DICE), composed of two identical telescopes to improve the signal-to-noise ratio. The observation was conducted at four wavelengths and three linear polarization directions in the limited total eclipse time of about 140 seconds. We successfully obtained polarization data for the corona but we were not able to obtain information on the coronal electron temperature and speed due to the low signal-to-noise ratio of the optical system and strong emission from prominences located at the western limb. In this study, we report the development of DICE and the observation results from the eclipse expedition. TSE observation and analysis with our self-developed instrument showed that a coronagraph needs to be designed carefully to achieve its scientific purpose. We gained valuable experience for future follow-up NASA-KASI joint missions: the Balloon-borne Investigation of the Temperature and Speed of Electrons in the Corona (BITSE) and the COronal Diagnostic EXperiment (CODEX).