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Abstract We report on a flare-driven coronal rain event observed along postflare loops during the decay phase of an X1.6-class solar flare. Although high-resolution studies of flare-driven coronal rain have been conducted, imaging spectroscopic studies are rare due to observational difficulties. Our observation taken with the Fast Imaging Solar Spectrograph, installed at the 1.6 m Goode Solar Telescope of the Big Bear Solar Observatory, provided unprecedented high-resolution spectroscopic imaging data of coronal rain in the Hαand Caii854.2 nm lines. We identify two locations along postflare loops with rain displaying distinctly different thermal properties, different Doppler velocities, and different patterns of acceleration and deceleration. We also observed intense brightening at one footpoint of coronal rain, where the spectroscopic analysis reveals an energy conversion process resulting in significant localized chromospheric heating. We thoroughly investigate the footpoint brightening Doppler velocities and compare their spectral line profiles to typical flare-ribbon line profiles. We estimate the spatial scale of the fine structure of the coronal rain and the footpoint brightening. Our results provide important insights into the dynamic and thermal properties of flare-driven coronal rain and the related chromospheric response, which will help validate the flare-driven modeling of coronal rain. 

Context.The elemental abundance in the solar corona differs from that in the photosphere, with low first ionization potential (FIP) elements showing enhanced abundances, a phenomenon known as the FIP effect. This effect is considered to be driven by ponderomotive forces associated with magnetohydrodynamic (MHD) waves, particularly incompressible transverse waves. Aims.We aim to investigate the relationship between coronal abundance fractionation and transverse MHD waves in the chromosphere. We focus on analyzing the spatial correlation between the FIP fractionation and these waves, while exploring wave properties to validate the ponderomotive-force-driven fractionation model. Methods.We analyzed the Hαdata from the Fast Imaging Solar Spectrograph of the Goode Solar Telescope to detect chromospheric transverse MHD waves, and Si X(low FIP) and S X(high FIP) spectra from the EUV Imaging Spectrometer on board Hinode to determine the relative abundance in an active region. By extrapolating linear-force-free magnetic fields with Solar Dynamics Observatory/Helioseismic and Magnetic Imager magnetograms, we examine the connection between chromospheric waves and coronal composition. Around 400 wave packets were identified, and their properties, including the period, velocity amplitude, propagation speed, and propagation direction, were studied. Results.These chromospheric transverse MHD waves, mostly incompressible or weakly compressible, are found near loop footpoints, particularly in the sunspot penumbra and superpenumbral fibrils. The highly fractionated coronal region is associated with areas where these waves were detected within closed magnetic fields. Our examination of the statistics of wave properties revealed that downward-propagating low-frequency waves are particularly prominent, comprising about 43% of the detected waves. Conclusions.The correlation between abundance fractionation and transverse MHD waves, along with wave properties, supports the hypothesis that FIP fractionation occurs due to the ponderomotive force from transverse MHD waves in the chromosphere. Additionally, the observed characteristics of these chromospheric waves provide valuable observational constraints for understanding the FIP fractionation process. 

Abstract Small-scale jets, such as chromospheric and transition region (TR) network jets, are of great interest regarding coronal heating and solar wind acceleration. Spectroscopic analysis based on multiple spectral lines with different formation temperatures is essential for understanding the physical properties and driving mechanisms of jets. Here, we conduct an investigation of the physical properties of a small-scale chromospheric jet in a quiet-Sun network region and its TR counterpart. This jet is recorded from formation to extinction using the Fast Imaging Solar Spectrograph at the Goode Solar Telescope and the Interface Region Imaging Spectrograph. The chromospheric component of the jet exhibits a high line-of-sight speed of up to 45 km s⁻¹during its ascending phase, accompanied by spectral profiles akin to rapid blueshifted excursion and downflowing rapid redshifted excursion during the descending phase. Using a cloud model combined with a Multi-Layer Spectral Inversion, we quantify the jet’s temperature during its ascending phase, which starts at approximately 11,000 K and increases by only 1000 K over 1 minute, much smaller than a few 10⁴K, the excess temperature expected in an ideal gas reconnection jet at an outflow speed of 45 km s⁻¹. The TR counterpart exhibits a Siiv1394 Å line profile with a non-Gaussian shape, including a blueshifted component and a large nonthermal width. Our results suggest that if the jet is driven by magnetic reconnection in the chromosphere, the heat released by the reconnection may be mostly used to ionize the hydrogen rather than to increase the temperature so that the gas may appear almost isothermal. 

Abstract Minifilament eruptions producing small jets and microflares have mostly been studied based on coronal observations at extreme-ultraviolet and X-ray wavelengths. This study presents chromospheric plasma diagnostics of a quiet-Sun minifilament of size ∼ 2″ × 5″ with a sigmoidal shape and an associated microflare observed on 2021 August 7 17:00 UT using high temporal and spatial resolution spectroscopy from the Fast Imaging Solar Spectrograph (FISS) and high-resolution magnetograms from the Near InfraRed Imaging Spectropolarimeter (NIRIS) installed on the 1.6 m Goode Solar Telescope at Big Bear Solar Observatory. Using FISS Hαand Caii8542 Å line spectra at the time of the minifilament activation we determined a temperature of 8600 K and a nonthermal speed of 7.9 km s⁻¹. During the eruption, the minifilament was no longer visible in the Caii8542 Å line, and only the Hαline spectra were used to find the temperature of the minifilament, which reached 1.2 × 10⁴K and decreased afterward. We estimated thermal energy of 3.6 × 10²⁴erg from the maximum temperature and kinetic energy of 2.6 × 10²⁴erg from the rising speed (18 km s⁻¹) of the minifilament. From the NIRIS magnetograms we found small-scale flux emergence and cancellation coincident with the minifilament eruption, and the magnetic energy change across the conjugate footpoints reaches 7.2 × 10²⁵erg. Such spectroscopic diagnostics of the chromospheric minifilament complement earlier studies of minifilament eruptions made using coronal images. 

Abstract We report a magnetic relaxation process inside a sunspot associated with the evolution of a transient light bridge (LB). From high-resolution imaging and spectro-polarimetric data taken by the 1.6 m Goode Solar Telescope installed at Big Bear Solar Observatory, we observe the evolutionary process of a rapidly evolving LB. The LB is formed as a result of the strong intrusion of filamentary structures with relatively horizontal fields into the vertical umbral field region. A strong current density is detected along a localized region where the magnetic field topology changes rapidly in the sunspot, especially in the boundary region between the LB and the umbra, and bright jets are observed intermittently and repeatedly in the chromosphere along this region through magnetic reconnection. In the second half of our observation, the horizontal component of the magnetic field diminishes within the LB, and the typical convection structure within the sunspot, which manifests itself as umbral dots, is restored. Our findings provide a comprehensive perspective not only on the evolution of an LB itself but also on its impacts in the neighboring regions, including the chromospheric activity and the change of magnetic energy of a sunspot. 

Abstract We report the detection of transverse magnetohydrodynamic waves, also known as Alfvénic waves, in the chromospheric fibrils of a solar-quiet region. Unlike previous studies that measured transversal displacements of fibrils in imaging data, we investigate the line-of-sight (LOS) velocity oscillations of the fibrils in spectral data. The observations were carried out with the Fast Imaging Solar Spectrograph of the 1.6 m Goode Solar Telescope at the Big Bear Solar Observatory. By applying spectral inversion to the Hαand Caii8542 Å line profiles, we determine various physical parameters, including the LOS velocity in the chromosphere of the quiet Sun. In the Hαdata, we select two adjacent points along the fibrils and analyze the LOS velocities at those points. For the time series of the velocities that show high cross-correlation between the two points and do not exhibit any correlation with intensity, we interpret them as propagating Alfvénic wave packets. We identify a total of 385 Alfvénic wave packets in the quiet-Sun fibrils. The mean values of the period, velocity amplitude, and propagation speed are 7.5 minutes, 1.33 km s⁻¹, and 123 km s⁻¹, respectively. We find that the detected waves are classified into three groups based on their periods, namely, 3, 5, and 10 minute bands. Each group of waves exhibits distinct wave properties, indicating a possible connection to their generation mechanism. Based on our results, we expect that the identification of Alfvénic waves in various regions will provide clues to their origin and the underlying physical processes in the solar atmosphere. 

Abstract A multilayer spectral inversion (MLSI) model has recently been proposed for inferring the physical parameters of plasmas in the solar chromosphere from strong absorption lines taken by the Fast Imaging Solar Spectrograph (FISS). We apply a deep neural network (DNN) technique in order to produce the MLSI outputs with reduced computational costs. We train the model using two absorption lines, H α and Ca ii 8542 Å, taken by FISS, and 13 physical parameters obtained from the application of MLSI to 49 raster scans (∼2,000,000 spectra). We use a fully connected network with skip connections and multi-branch architecture to avoid the problem of vanishing gradients and to improve the model’s performance. Our test shows that the DNN successfully reproduces the physical parameters for each line with high accuracy and a computing time of about 0.3–0.4 ms per line, which is about 250 times faster than the direct application of MLSI. We also confirm that the DNN reliably reproduces the temporal variations of the physical parameters generated by the MLSI inversion. By taking advantage of the high performance of the DNN, we plan to provide physical parameter maps for all the FISS observations, in order to understand the chromospheric plasma conditions in various solar features. 

Abstract Recent observations provided evidence that the solar chromosphere of sunspot regions is pervaded by Alfvénic waves—transverse magnetohydrodynamic (MHD) waves (Alfvén waves or kink waves). In order to systematically investigate the physical characteristics of Alfvénic waves over a wide range of periods, we analyzed the time series of line-of-sight velocity maps constructed from the H α spectral data of a small sunspot region taken by the Fast Imaging Solar Spectrograph of the Goode Solar Telescope at Big Bear. We identified each Alfvénic wave packet by examining the cross-correlation of band-filtered velocity between two points that are located a little apart presumably on the same magnetic field line. As result, we detected a total of 279 wave packets in the superpenumbral region around the sunspot and obtained their statistics of period, velocity amplitude, and propagation speed. An important finding of ours is that the detected Alfvénic waves are clearly separated into two groups: 3-minute period (<7 minutes) waves and 10-minute period (>7 minutes) waves. We propose two tales on the origin of Alfvénic waves in the chromosphere; the 3-minute Alfvénic waves are excited by the upward-propagating slow waves in the chromosphere through the slow-to-Alfvénic mode conversion, and the 10-minute Alfvénic waves represent the chromospheric manifestation of the kink waves driven by convective motions in the photosphere.