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Abstract Quiet-Sun regions cover most of the Sun's surface; their magnetic fields contribute significantly to solar chromospheric and coronal heating. However, characterizing the magnetic fields of the quiet Sun is challenging due to their weak polarization signal. The 4 m Daniel K. Inouye Solar Telescope (DKIST) is expected to improve our understanding of quiet-Sun magnetism. In this paper, we assess the diagnostic capability of the Diffraction Limited Near Infrared Spectropolarimeter (DL-NIRSP) instrument on DKIST for the energy transport processes in the quiet-Sun photosphere. To this end, we synthesize high-resolution, high-cadence Stokes profiles of the Fei630 nm lines using a realistic magnetohydrodynamic simulation, degrade them to emulate the DKIST/DL-NIRSP observations, and subsequently infer the vector magnetic and velocity fields. For the assessment, we first verify that a widely used flow tracking algorithm, the Differential Affine Velocity Estimator for Vector Magnetograms, works well for estimating the large-scale (>200 km) photospheric velocity fields with these high-resolution data. We then examine how the accuracy of the inferred velocity depends on the temporal resolution. Finally, we investigate the reliability of the Poynting flux estimate and its dependence on the model assumptions. The results suggest that the unsigned Poynting flux, estimated with existing schemes, can account for about 71.4% and 52.6% of the reference ground truth at $\log \tau =0.0$and $\log \tau =-1$. However, the net Poynting flux tends to be significantly underestimated. The error mainly arises from the underestimated contribution of the horizontal motion. We discuss the implications for DKIST observations. 

Abstract On 20 December 2015, three Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft detected a nightside magnetotail reconnection event in the early main phase of a major geomagnetic storm. The spacecraft (P5, P4, and P3) had their footprints located over North America near the Gillam ground magnetometer station in Canada. Multipoint observations, both in space and from the ground, allow for an examination of the spatiotemporal characteristics of the disturbance on the ground and the associated physical drivers in the magnetosphere and ionosphere. This study shows that the horizontal geomagnetic field d/dt localized (on the scale of 100–300 km) feature observed at Gillam ground magnetometer site was caused by an isolated substorm onset near that station driven by a nightside magnetotail reconnection event detected by three THEMIS spacecraft that were located near the central plasma sheet. A close inspection of equivalent ionospheric current and current amplitude maps derived from ground magnetometer measurements using the spherical elementary current system technique indicates that the location of the localization lies roughly between the upward and downward field aligned current system, which is consistent with other earlier studies. This event represents the first reported observation of ground d/dt localization that is directly linked to nightside magnetotail fast flow bursts and reconnection event during the onset phase of a major Geomagnetic disturbance (GMD). 

Abstract Delta (δ) sunspots sometimes host fast photospheric flows along the central magnetic polarity inversion line (PIL). Here we study the strong Doppler shift signature in the central penumbral light bridge of solar active region NOAA 12673. Observations from the Helioseismic and Magnetic Imager (HMI) indicate highly sheared and strong magnetic fields. Large Doppler shifts up to 3.2 km s⁻¹appeared during the formation of the light bridge and persisted for about 16 hr. A new velocity estimator, called DAVE4VMwDV, reveals fast converging and shearing motion along the PIL from HMI vector magnetograms, and recovers the observed Doppler signal much better than an old version of the algorithm. The inferred velocity vectors are largely (anti-)parallel to the inclined magnetic fields, suggesting that the observed Doppler shift contains a significant contribution from the projected field-aligned flows. High-resolution observations from the Hinode/Spectro-Polarimeter further exhibit a clear correlation between the Doppler velocity and the cosine of the magnetic inclination, which is in agreement with HMI results and consistent with a field-aligned flow of about 9.6 km s⁻¹. The complex Stokes profiles suggest significant gradients of physical variables along the line of sight. We discuss the implications on theδ-spot magnetic structure and the flow-driving mechanism.