arXiv subjects

Kyuhyoun Cho

Publications and source records attributed to Kyuhyoun Cho.

At least 19 recordsLinked to original sources

arXiv2026

Evidence for anisotropic non-Maxwellian velocity distributions in the solar transition region

Spectroscopic observations of the solar transition region have long shown excess line broadening and non-Gaussian profiles, but their spatial distribution and prevalence are not well established, and their physical origin remains the subject of debate. Here we analyze SiIV line profiles in full-disk mosaics of observations with Interface Region Imaging Spectrograph (IRIS; De Pontieu et al. 2014), and find that non-Gaussian k-like profiles, indicative of suprathermal velocity distributions, are pervasive (comprising ~60% of the observed profiles). In addition, we find that their occurrence depends strongly on the angle between the line of sight and the magnetic field as inferred from NLFFF extrapolations. k-like profiles become increasingly prevalent when the magnetic field is oriented transverse to the observer, whereas Gaussian profiles become more prevalent when the field is aligned with the line of sight. This geometric dependence provides evidence that the observed velocity distributions in the transition region are non-Maxwellian and anisotropic. Our findings provide evidence for the importance, in the solar transition region, of kinetic processes, and the key role the magnetic field plays in the superposition of signals, and/or the generation or relaxation of non-Maxwellian particle distributions, likely caused by magnetic energy release through reconnection. These effects are not captured by the magnetohydrodynamic approaches that are typically invoked to study the solar atmosphere. Our results highlight the need and provide constraints for more advanced multi-fluid and/or kinetic modeling of the solar transition region.

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arXiv2026

Predicting the thermodynamics in the chromosphere from the translation of SDO data into the IRIS$^{2}$ inversion results using a visual transformer model

We present SDO2IRIS$^2$: a visual transformer model that translates a combination of images of the chromosphere and transition region (TR), observed by AIA, and a line-of-sight magnetogram, provided by HMI, into temperature, line-of-sight velocity (v$_{los}$), velocity of the turbulent motions (v$_{turb}$), and electron density (n$_{e}$) in the chromosphere. Using the thermodynamic variables obtained from the inversion of the chromospheric lines Mg II h&k, observed by IRIS, as the target of the model, and the intensity images in the chromosphere and TR, and the photospheric magnetogram as the input, the predicted T and n$_{e}$ show a strong correlation ($\approx 0.80$) for $\approx$80% of the test inverted data, a moderate-to-strong correlation ($\approx0.63$) for 70% of the v$_{turb}$ of the target test inverted data, while for the $v_{los}$, the correlation is weak. Therefore, the predicted values by SDO2IRIS$^2$ may be used as an estimation of the thermodynamics in the chromosphere, either as a stand-alone result or as complementary information to other chromospheric data observed simultaneously. The execution time employed by SDO2IRIS$^2$ to obtain the thermodynamic values in the chromosphere is of the order of a few minutes, being $\le10$ minutes when using a CPU, and $\le5$ minutes when using a GPU. SDO2IRIS$^2$ opens a new avenue for the use of SDO data thanks to the inversions provided by IRIS observables.

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arXiv2025

Coronal Abundance Fractionation Linked to Chromospheric Transverse MHD Waves in a Solar Active Region Observed with FISS/GST and EIS/Hinode

Elemental abundances in the solar corona differ from those in the photosphere, with low first ionization potential (FIP) elements being enhanced, a phenomenon known as the FIP effect. This enhancement is attributed to ponderomotive forces linked to magnetohydrodynamic (MHD) waves, particularly incompressible transverse waves. Our study investigates the relationship between coronal abundance fractionation and chromospheric transverse MHD waves by examining the spatial correlation between FIP fractionation and these waves and by analyzing their properties to test the ponderomotive force model. We used H alpha data from the Fast Imaging Solar Spectrograph at the Goode Solar Telescope to detect chromospheric transverse MHD waves and \ion{Si}{X} (low FIP) and \ion{S}{X} (high FIP) spectra from Hinode EUV Imaging Spectrometer to determine relative abundances in an active region. Extrapolated linear force free magnetic fields from Solar Dynamics Observatory/Helioseismic and Magnetic Imager magnetograms further linked the observed chromospheric waves with coronal composition. Approximately 400 wave packets were identified and characterized by their period, velocity amplitude, propagation speed, and direction. These incompressible or weakly compressible waves were mainly observed near loop footpoints in the sunspot penumbra and superpenumbral fibrils. Regions of high FIP fractionation coincided with closed magnetic fields where these waves were present, and low-frequency, downward-propagating waves comprised about 43/% of the total. Our results demonstrate a strong correlation between coronal abundance fractionation and chromospheric transverse MHD waves, supporting the view that the FIP effect is driven by the ponderomotive force from these waves.

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arXiv2024

On the Nature of Nonthermal Broadening of Spectral Lines Observed by IRIS

The origin of nonthermal broadening in solar spectra is one of the long-standing questions in solar physics. Various processes have been invoked - including unresolved flows, waves, and turbulent processes - but definitive answers are lacking. To investigate the physical processes responsible for nonthermal broadening, we examine its relation with the angle between the magnetic field and the line of sight in three different closed-field regions above plage regions at different locations on the solar disk. We obtained the nonthermal width of transition-region Si IV 1403 Å spectra observed in active regions by the Interface Region Imaging Spectrograph, after subtraction of the thermal and instrumental line broadening. To investigate the dependence of the measured broadening on the viewing angle between the line of sight and magnetic field direction, we determined the magnetic field direction at transition-region heights using nonlinear force-free extrapolations based on the observed photospheric vector magnetic field taken by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We found that the nonthermal broadening shows a correlation with downward motion (redshifts)and alignment between the magnetic field and the observer's line-of-sight direction. Based on the observed correlations, we suggest that velocity gradients within plasma flowing down along the magnetic field may lead to a significant portion of the observed nonthermal broadening of transition-region spectral lines in closed fields above plage regions.

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arXiv2024

A model of umbral oscillations inherited from subphotospheric fast-body modes

Recently, complex horizontal patterns of umbral oscillations have been reported, but their physical nature and origin are still not fully understood. Here we show that the two-dimensional patterns of umbral oscillations of slow waves are inherited from the subphotospheric fast-body modes. Using a simple analytic model, we successfully reproduced the temporal evolution of oscillation patterns with a finite number of fast-body modes. In this model, the radial apparent propagation of the pattern is associated with the appropriate combination of the amplitudes in radial modes. We also find that the oscillation patterns are dependent on the oscillation period. This result indicates that there is a cutoff radial mode, which is a unique characteristic of the model of fast-body modes. In principle, both internal and external sources can excite these fast-body modes and produce horizontal patterns of umbral oscillations.

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arXiv2024

Large-amplitude transverse MHD waves prevailing in the H$α$ chromosphere of a solar quiet region revealed by MiHI integrated field spectral observations

The investigation of plasma motions in the solar chromosphere is crucial for understanding the transport of mechanical energy from the interior of the Sun to the outer atmosphere and into interplanetary space. We report the finding of large-amplitude oscillatory transverse motions prevailing in the non-spicular Halpha chromosphere of a small quiet region near the solar disk center. The observation was carried out on 2018 August 25 with the Microlensed Hyperspectral Imager (MiHI) installed as an extension to the spectrograph at the Swedish Solar Telescope (SST). MiHi produced high-resolution Stokes spectra of the Halpha line over a two-dimensional array of points (sampled every 0.066 arcsec on the image plane) every 1.33 s for about 17 min. We extracted the Dopple-shift-insensitive intensity data of the line core by applying a bisector fit to Stoke I line profiles. From our time-distance analysis of the intensity data, we find a variety of transverse motions with velocity amplitudes of up to 40 km/s in fan fibrils and tiny filaments. In particular, in the fan fibrils, large-amplitude transverse MHD waves were seen to occur with a mean velocity amplitude of 25 km/s and a mean period of 5.8 min, propagating at a speed of 40 km/s. These waves are nonlinear and display group behavior. We estimate the wave energy flux in the upper chromosphere at 3 x 10^6 erg cm^-2 s^-1. Our results contribute to the advancement of our understanding of the properties of transverse MHD waves in the solar chromosphere.

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arXiv2023

A Statistical Study of IRIS Observational Signatures of Nanoflares and Non-thermal Particles

Nanoflares are regarded as one of the major mechanisms of magnetic energy release and coronal heating in the solar outer atmosphere. We conduct a statistical study on the response of the chromosphere and transition region to nanoflares, as observed by the Interface Region Imaging Spectrograph (IRIS), by using an algorithm for the automatic detection of these events. The initial atmospheric response to these small heating events is observed, with IRIS, as transient brightening at the footpoints of coronal loops heated to high temperatures (>4 MK). For four active regions, observed over 143 hours, we detected 1082 footpoint brightenings under the IRIS slit, and for those we extracted physical parameters from the IRIS Mg II and Si IV spectra that are formed in the chromosphere and transition region, respectively. We investigate the distribution of the spectral parameters, and the relationship between the parameters, also comparing them with predictions from RADYN numerical simulations of nanoflare-heated loops. We find that these events, and the presence of non-thermal particles, tend to be more frequent in flare productive active regions, and where the hot Atmospheric Imaging Assembly 94 Å emission is higher. We find evidence for highly dynamic motions characterized by strong Si IV non-thermal velocity (not dependent on the heliocentric x coordinate, i.e., on the angle between the magnetic field and the line-of-sight) and asymmetric Mg II spectra. These findings provide tight new constraints on the properties of nanoflares, and non-thermal particles, in active regions, and their effects on the lower atmosphere.

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arXiv2021

Investigation of the subsurface structure of a sunspot based on the spatial distribution of oscillation centers inferred from umbral flashes

The subsurface structure of a solar sunspot is important in the stability of the sunspots and the energy transport therein. Two subsurface structure models have been proposed, the monolithic and cluster models, but no clear observational evidence supporting a particular model has been found so far. To obtain clues about the subsurface structure of sunspots, we analyzed umbral flashes in merging sunspots registered by IRIS Mg II 2796 Angstrom slit-jaw images (SJIs). Umbral flashes are regarded as an observational manifestation of magnetohydrodynamic (MHD) shock waves originating from convection cells below the photosphere. By tracking the motion of individual umbral flashes, we determined the position of the convection cells that are the oscillation centers located below the umbra. We found that the oscillation centers are preferentially located at dark nuclei in the umbral cores rather than in bright regions such as light bridges or umbral dots. Moreover, the oscillation centers tend to deviate from the convergent interface of the merging sunspots where vigorous convection is expected to occur. We also found that the inferred depths of the convection cells have no noticeable regional dependence. These results suggest that the subsurface of the umbra is an environment where convection can occur more easily than the convergent interface, and hence support the cluster model. For more concrete results, further studies based on umbral velocity oscillations in the lower atmosphere are required.

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arXiv2020

The chromospheric component of coronal bright points. Coronal and chromospheric responses to magnetic-flux emergence

We investigate the chromospheric counterpart of small-scale coronal loops constituting a coronal bright point (CBP) and its response to a photospheric magnetic-flux increase accompanied by co-temporal CBP heating. We used co-observations from the AIA and HMI/SDO, together with data from the Fast Imaging Solar Spectrograph taken in the Halpha and Ca II 8542 lines. We used a new multi-layer spectral inversion technique to derive the temporal variations of the temperature of the Halpha loops (HLs). We find that the counterpart of the CBP, as seen at chromospheric temperatures, is composed of a bundle of dark elongated features named in this work Halpha loops, which constitute an integral part of the CBP loop magnetic structure. An increase in the photospheric magnetic flux due to flux emergence is accompanied by a rise of the coronal emission of the CBP loops, that is a heating episode. We also observe enhanced chromospheric activity associated with the occurrence of new HLs and mottles. While the coronal emission and magnetic flux increases appear to be co-temporal, the response of the Halpha counterpart of the CBP occurs with a small delay of less than 3 min. A sharp temperature increase is found in one of the HLs and in one of the CBP footpoints estimated at 46% and 55% with respect to the pre-event values, also starting with a delay of less than 3~min following the coronal heating episode. The low-lying CBP loop structure remains non-potential for the entire observing period. The magnetic topological analysis of the overlying corona reveals the presence of a coronal null point at the beginning and towards the end of the heating episode. The delay in the response of the chromospheric counterpart of the CBP suggests that the heating may have occurred at coronal heights.

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arXiv2020

Impulsive wave excitation by rapidly changing granules

It is not yet fully understood how magnetohydrodynamic waves in the interior and atmosphere of the Sun are excited. Traditionally, turbulent convection in the interior is considered to be the source of wave excitation in the quiet Sun. Over the last few decades, acoustic events observed in the intergranular lanes in the photosphere have emerged as a strong candidate for a wave excitation source. Here we report our observations of wave excitation by a new type of event: rapidly changing granules. Our observations were carried out with the Fast Imaging Solar Spectrograph in the H$α$ and Ca II 8542 $\unicode{xC5}$ lines and the TiO 7057 $\unicode{xC5}$ broadband filter imager of the 1.6 m Goode Solar Telescope at the Big Bear Solar Observatory. We identify granules in the internetwork region that undergo rapid dynamic changes such as collapse (event 1), fragmentation (event 2), or submergence (event 3). In the photospheric images, these granules become significantly darker than neighboring granules. Following the granules' rapid changes, transient oscillations are detected in the photospheric and chromospheric layers. In the case of event 1, the dominant period of the oscillations is close to 4.2 min in the photosphere and 3.8 min in the chromosphere. Moreover, in the Ca II$-$0.5 $\unicode{xC5}$ raster image, we observe repetitive brightenings in the location of the rapidly changing granules that are considered the manifestation of shock waves. Based on our results, we suggest that dynamic changes of granules can generate upward-propagating acoustic waves in the quiet Sun that ultimately develop into shocks.

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arXiv2020

Inference of chromospheric plasma parameters on the Sun

The solar chromosphere can be observed well through strong absorption lines. We infer the physical parameters of chromospheric plasmas from these lines using a multilayer spectral inversion. This is a new technique of spectral inversion. We assume that the atmosphere consists of a finite number of layers. In each layer the absorption profile is constant and the source function varies with optical depth with a constant gradient. Specifically, we consider a three-layer model of radiative transfer where the lowest layer is identified with the photosphere and the two upper layers are identified with the chromosphere. The absorption profile in the photosphere is described by a Voigt function, and the profile in the chromosphere by a Gaussian function. This three-layer model is fully specified by 13 parameters. Four parameters can be fixed to prescribed values, and one parameter can be determined from the analysis of a satellite photospheric line. The remaining 8 parameters are determined from a constrained least-squares fitting. We applied the multilayer spectral inversion to the spectral data of the H$α$ and the Ca II 854.21 nm lines taken in a quiet region by the Fast Imaging Solar Spectrograph (FISS) of the Goode Solar Telescope (GST). We find that our model successfully fits most of the observed profiles and produces regular maps of the model parameters. The combination of the inferred Doppler widths of the two lines yields reasonable estimates of temperature and nonthermal speed in the chromosphere. We conclude that our multilayer inversion is useful to infer chromospheric plasma parameters on the Sun.

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arXiv2020

The Application of the Filtered Backprojection Algorithm to Solar Rotational Tomography

Solar rotational tomography (SRT) is an important method to reconstruct the physical parameters of the three-dimensional solar corona. Here we propose an approach to apply the filtered backprojection (FBP) algorithm to the SRT. The FBP algorithm is generally not suitable for SRT due to the several issues with solar extreme ultraviolet (EUV) observations, in particular a problem caused by missing data because of the unobserved back side of corona hidden behind the Sun. We developed a method to generate a modified sinogram which resolves the blocking problem. The modified sinogram is generated by combining the EUV data at two opposite sites observed by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). We generated the modified sinogram for about one month in 2019 February and reconstructed the three-dimensional corona under the static state assumption. In order to obtain the physical parameters of the corona, we employed a DEM inversion method. We tested the performance of the FBP algorithm with the modified sinogram by comparing the reconstructed data with the observed EUV image, electron density models, previous studies of electron temperature, and an observed coronagraph image. The results illustrate that the FBP algorithm reasonably reconstructs the bright regions and the coronal holes, and can reproduce their physical parameters. The main advantage of the FBP algorithm is that it is easy to understand and computationally efficient. Thus, it enables us to easily probe the inhomogeneous coronal electron density and temperature distribution of the solar corona.

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arXiv2020

Strong Blue Asymmetry in Hα line as a Preflare Activity

Chromospheric activities prior to solar flares provide important clues to solar flare initiation, but are as yet poorly understood. We report a significant and rapid H$α$ line broadening before the solar flare SOL2011-09-29T18:08, that was detected using the unprecedented high-resolution H$α$ imaging spectroscopy with the Fast Imaging Solar Spectrograph (FISS) installed on the 1.6 m New Solar Telescope (NST) at Big Bear Solar Observatory.The strong H$α$ broadening extends as a blue excursion up to -4.5 A and as a red excursion up to 2.0 A, which implies a mixture of velocities in the range of -130 km/s to 38 km/s by applying the cloud model, comparable to the highest chromospheric motions reported before. The H$α$ blueshifted broadening lasts for about 4 minutes, and is temporally and spatially correlated with the start of a rising filament, which is later associated with the main phase of the flare as detected by the Atmosphere Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO). The potential importance of this H$α$ blueshifted broadening as a preflare chromospheric activity is briefly discussed within the context of the two-step eruption model.

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arXiv2020

Source Depth of Three-minute Umbral Oscillations

We infer the depth of the internal sources giving rise to three-minute umbral oscillations. Recent observations of ripple-like velocity patterns of umbral oscillations supported the notion that there exist internal sources exciting the umbral oscillations. We adopt the hypothesis that the fast magnetohydrodynamic (MHD) waves generated at a source below the photospheric layer propagate along different paths, reach the surface at different times, and excited slow MHD waves by mode conversion. These slow MHD waves are observed as the ripples that apparently propagate horizontally. The propagation distance of the ripple given as a function of time is strongly related to the depth of the source. Using the spectral data of the Fe I 5435A line taken by the Fast Imaging Solar Spectrograph of the Goode Solar Telescope at Big Bear Solar Observatory, we identified five ripples and determined the propagation distance as a function of time in each ripple. From the model fitting to these data, we obtained the depth between 1000 km and 2000 km. Our result will serve as an observational constraint to understanding the detailed processes of magnetoconvection and wave generation in sunspots.

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arXiv2019

Positive and negative feedback of AGN outflows in NGC 5728

We present a spatially-resolved analysis of ionized and molecular gas in a nearby Seyfert 2 galaxy NGC 5728, using the VLT/MUSE and ALMA data. We find ionized gas outflows out to ~kpc scales, which encounter the star formation ring at 1 kpc radius. The star formation rate of the encountering region is significantly high (~1.8 M$_{\rm sol}/yr/kpc^2$) compared to other regions in the ring. In contrast, the CO (2-1) emission is significantly weaker by a factor of ~3.5, indicating very high star formation efficiency. These results support the positive feedback scenario that the AGN-driven outflows compress the ISM in the ring, enhancing the star formation activity. In addition, we detect outflow regions outside of spiral arms, in which gas is likely to be removed from the spiral arms and no clear sign of star formation is detected. The overall impact of AGN outflows on the global star formation in NGC 5728 is limited, suggesting the feedback of the low-luminosity AGN is insignificant.

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arXiv2019

The Observational Evidence for the Internal Excitation of Sunspot Oscillations Inferred from the Fe I 5435 A Line

The umbral oscillations of velocity are commonly observed in the chromosphere of a sunspot. Their sources are considered to be either the external p-mode driving or the internal excitation by magnetoconvection. Even though the possibility of the p-mode driving has been often considered, the internal excitation has been rarely investigated. We report the identification of the oscillation patterns that may be closely related to the events of internal excitation from the observations of velocity oscillations in the temperature minimum region of two sunspots. The velocities were determined from the spectral data of the Fe I 5435 A line, a magnetically insensitive line, taken with the Fast Imaging Solar Spectrograph of the 1.6 m Goode Solar Telescope at the Big Bear Solar Observatory. As a result, we identified 4 oscillation patterns of 2.0 * 10^3 km coherent size that were clearly identified for about 7.9 minutes with oscillation amplitude of 9.3 * 10^-2 km/s. The power of the oscillations in these centers was concentrated in the 3 minute band. All the oscillation centers were located above the umbral dots undergoing noticeable morphological and dynamical changes that may be regarded as an observable signature of small-scale magnetoconvection inside the umbrae. Our results support the notion that magnetoconvection associated with umbral dots inside sunspots can drive the 3 minute umbral oscillations.

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arXiv2019

The Physical Nature of Spiral Wave Patterns in Sunspots

Recently, spiral wave patterns (SWPs) have been detected in 3 minute oscillations of sunspot umbrae, but the nature of this phenomenon has remained elusive. We present a theoretical model that interprets the observed SWPs as the superposition of two different azimuthal modes of slow magnetoacoustic waves driven below the surface in an untwisted and non-rotating magnetic cylinder. We apply this model to SWPs of the line-of-sight (LOS) velocity in a pore observed by the Fast Imaging Solar Spectrograph installed at the 1.6 m Goode Solar Telescope. One- and two-armed SWPs were identified in instantaneous amplitudes of LOS Doppler velocity maps of 3 minute oscillations. The associated oscillation periods are about 160 s, and the durations are about 5 minutes. In our theoretical model, the observed spiral structures are explained by the superposition of non-zero azimuthal modes driven 1600 km below the photosphere in the pore. The one-armed SWP is produced by the slow-body sausage (m = 0) and kink (m = 1) modes, and the two-armed SWP is formed by the slow-body sausage (m = 0) and fluting (m = 2) modes of the magnetic flux tube forming the pore.

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arXiv2017

Observation of a Large-scale Quasi-circular Secondary Ribbon associated with Successive Flares and a Halo CME

Solar flare ribbons provide an important clue to the magnetic reconnection process and associated magnetic field topology in the solar corona. We detected a large-scale secondary flare ribbon of a circular shape that developed in association with two successive M-class flares and one CME. The ribbon revealed interesting properties such as 1) a quasi-circular shape and enclosing the central active region; 2) the size as large as 500\arcsec\, by 650\arcsec\,, 3) successive brightenings in the clockwise direction at a speed of \kms{160} starting from the nearest position to the flaring sunspots, 4) radial contraction and expansion in the northern and the southern part, respectively at speeds of $\leq$ \kms{10}. Using multi-wavelength data from \textit{SDO}, \textit{RHESSI}, XRT, and Nobeyama, along with magnetic field extrapolations, we found that: 1) the secondary ribbon location is consistent with the field line footpoints of a fan-shaped magnetic structure that connects the flaring region and the ambient decaying field; 2) the second M2.6 flare occurred when the expanding coronal loops driven by the first M2.0 flare encountered the background decayed field. 3) Immediately after the second flare, the secondary ribbon developed along with dimming regions. Based on our findings, we suggest that interaction between the expanding sigmoid field and the overlying fan-shaped field triggered the secondary reconnection that resulted in the field opening and formation of the quasi-circular secondary ribbon. We thus conclude that interaction between the active region and the ambient large-scale fields should be taken into account to fully understand the entire eruption process.

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