arXiv subjects

Kosuke Namekata

Publications and source records attributed to Kosuke Namekata.

At least 37 records · Page 2Linked to original sources

arXiv2025

High-Time-Cadence Spectroscopy and Photometry of Stellar Flares on M-dwarf YZ Canis Minoris with Seimei Telescope and TESS. II. Statistical Properties of Blue/Red Asymmetries in the H$α$ Line

M-dwarfs frequently produce flares, and their associated coronal mass ejections (CMEs) may threaten the habitability of close-in exoplanets. M-dwarf flares sometimes show prominence eruption signatures, observed as blue/red asymmetries in the H$α$ line. In Paper I, we reported four candidates of prominence eruptions, which shows large diversity in their durations and velocities. In this study, we statistically investigate how blue/red asymmetries are related with their flare and starspot properties, using the dataset from 27 H$α$ flares in Paper I and previously reported 8 H$α$ flares on an M-dwarf YZ Canis Minoris. We found that these asymmetry events tend to show larger H$α$ flare energies compared to non-asymmetry events. In particular, 5 out of 6 blue asymmetry events are not associated with white-light flares, whereas all 7 red asymmetry events are associated with white-light flares. Furthermore, their starspot distributions estimated from the TESS light curve show that all prominence eruption candidates occurred when starspots were located on the stellar disk center as well as on the stellar limb. These results suggest that flares with lower heating rates may have a higher association rate with prominence eruptions and/or the possibility that prominence eruptions are more detectable on the limb than on the disk center on M-dwarfs. These results provide significant insights into CMEs that can affect the habitable world around M-dwarfs.

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arXiv2025

Temporal Variation of Flare Occurrence Rates via the Spot Evolution on the Sun and Solar-type Stars

The spot evolution on the Sun and solar-type stars is important for understanding the nature of consequential flaring activity. This study statistically investigates the variance of flare occurrence rate through the time evolution of spots on the Sun and solar-type stars. We have compiled the 28-year catalogs of solar flares and their source sunspots obtained from solar surface observations by NOAA and GOES for the Sun. Also, we combined the cataloged stellar flares with the time evolution of starspots estimated by light curves obtained by the 4-year Kepler mission for solar-type stars. For the obtained 24124 solar flares and 180 stellar flares, we calculate the flare occurrence distribution with respect to $t_\mathrm{flare}-t_\mathrm{max}$, which represents the timing of flare through the spot evolution, where $t_\mathrm{flare}$ is the flare occurrence time, and $t_\mathrm{max}$ is the time when the source spot takes its maximum area. When normalized by the spot lifetime, we found that the flare occurrence distribution for $t_\mathrm{flare}-t_\mathrm{max}$ shows a similar distribution regardless of spot size or flare energy, suggesting that the Sun and the solar-type star share the same physical process in the spot-to-flare activity. On this basis, we propose a formula for the time variation of the flare occurrence rate per spot. Also, the correlation between the temporal variation of flare occurrence rate and the time evolution of spot area and the lack of difference in flare occurrence rate between the emergence and decaying phases provide a milestone for the nature of flare-productive spots.

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arXiv2024

High-Time-Cadence Spectroscopy and Photometry of Stellar Flares on M-dwarf YZ Canis Minoris with Seimei Telescope and TESS. I. Discovery of Rapid and Short-Duration Prominence Eruptions

M-dwarfs show frequent flares and associated coronal mass ejections (CMEs) may significantly impact close-in habitable planets. M-dwarf flares sometimes show red/blue asymmetries in the H$α$ line profile, suggesting prominence eruptions as an early stage of CMEs. However, their high-time-cadence observations are limited. We conducted spectroscopic monitoring observations of the active M-dwarf YZ Canis Minoris with $\sim$1 minute time cadence using the Seimei telescope, simultaneously with the optical photometric observations by Transiting Exoplanet Survey Satellite. We detected 27 H$α$ flares with H$α$ energies ranging from 1.7 $\times$ 10$^{29}$ to 3.8 $\times$ 10$^{32}$ erg and durations from 8 to 319 minutes. Among them, we identified 3 blue asymmetry and 5 red asymmetry events based on criteria using the Bayesian Information Criterion. The maximum velocity of the blue- and red-shifted components ranges from 250 to 450 km s$^{-1}$ and 190 to 400 km s$^{-1}$, respectively. The duration and time evolution show variety, and in particular, we discovered rapid, short-duration blue/red asymmetry events with the duration of 6--8 minutes. Among the 8 blue/red asymmetry events, two blue and one red asymmetry events are interpreted as prominence eruptions because of their fast velocity and time evolution. Based on this interpretation, the lower limit of occurrence frequency of prominence eruptions can be estimated to be $\sim$1.1 events per day. Our discovery of short-duration events suggests that previous studies with low time cadence may have missed these events, potentially leading to an underestimation of the occurrence frequency of prominence eruptions/CMEs.

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arXiv2024

Rising Near-Ultraviolet Spectra in Stellar Megaflares

Flares from M-dwarf stars can attain energies up to $10^4$ times larger than solar flares but are generally thought to result from similar processes of magnetic energy release and particle acceleration. Larger heating rates in the low atmosphere are needed to reproduce the shape and strength of the observed continua in stellar flares, which are often simplified to a blackbody model from the optical to the far-ultraviolet (FUV). The near-ultraviolet (NUV) has been woefully undersampled in spectral observations despite this being where the blackbody radiation should peak. We present Hubble Space Telescope NUV spectra in the impulsive phase of a flare with $E_{\rm{TESS}} \approx 7.5 \times 10^{33}$ erg and a flare with $E_{\rm{TESS}} \approx 10^{35}$ erg and the largest NUV flare luminosity observed to date from an M star. The composite NUV spectra are not well represented by a single blackbody that is commonly assumed in the literature. Rather, continuum flux rises toward shorter wavelengths into the FUV, and we calculate that an optical $T=10^4$ K blackbody underestimates the short wavelength NUV flux by a factor of $\approx 6$. We show that rising NUV continuum spectra can be reproduced by collisionally heating the lower atmosphere with beams of $E \gtrsim 10$ MeV protons or $E \gtrsim 500$ keV electrons and flux densities of $10^{13}$ erg cm$^{-2}$ s$^{-1}$. These are much larger than canonical values describing accelerated particles in solar flares.

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arXiv2024

Multiwavelength Campaign Observations of a Young Solar-type Star, EK Draconis. II. Understanding Prominence Eruption through Data-Driven Modeling and Observed Magnetic Environment

EK Draconis, a nearby young solar-type star (G1.5V, 50-120 Myr), is known as one of the best proxies for inferring the environmental conditions of the young Sun. The star frequently produces superflares and Paper I presented the first evidence of an associated gigantic prominence eruption observed as a blueshifted H$α$ Balmer line emission. In this paper, we present the results of dynamical modeling of the stellar eruption and examine its relationship to the surface starspots and large-scale magnetic fields observed concurrently with the event. By performing a one-dimensional free-fall dynamical model and a one dimensional hydrodynamic simulation of the flow along the expanding magnetic loop, we found that the prominence eruption likely occurred near the stellar limb (12$^{+5}_{-5}$-16$^{+7}_{-7}$ degrees from the limb) and was ejected at an angle of 15$^{+6}_{-5}$-24$^{+6}_{-6}$ degrees relative to the line of sight, and the magnetic structures can expand into a coronal mass ejection (CME). The observed prominence displayed a terminal velocity of $\sim$0 km s$^{-1}$ prior to disappearance, complicating the interpretation of its dynamics in Paper I. The models in this paper suggest that prominence's H$α$ intensity diminishes at around or before its expected maximum height, explaining the puzzling time evolution in observations. The TESS light curve modeling and (Zeeman) Doppler Imaging revealed large mid-latitude spots with polarity inversion lines and one polar spot with dominant single polarity, all near the stellar limb during the eruption. This suggests that mid-latitude spots could be the source of the pre-existing gigantic prominence we reported in Paper I. These results provide valuable insights into the dynamic processes that likely influenced the environments of early Earth, Mars, Venus, and young exoplanets.

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arXiv2024

Assessing the capability of a model-based stellar XUV estimation

Stellar XUV (X-ray and extreme ultraviolet) emission drives the heating and chemical reactions in planetary atmospheres and protoplanetary disks, and therefore, a proper estimation of a stellar XUV spectrum is required for their studies. One proposed solution is to estimate stellar atmospheric heating using numerical models, although the validation was restricted to the Sun over a limited parameter range. In this study, we extend the validation of the model by testing it with the Sun and three young, nearby solar-type stars with available XUV observational data. We first test the model with the solar observations, examining its accuracy in activity minimum and maximum phases, its dependence on loop length, the effect of loop length superposition, and its sensitivity to elemental abundance. We confirm that the model spectrum is mostly accurate both in activity minimum and maximum, although the high-energy X-rays (< 1 nm) are underestimated in the activity maximum. Applying the model to young solar-type stars, we find that it can reproduce the observed XUV spectra within a factor of 3 in the range of 1-30 nm for stars with magnetic flux up to 100 times that of the Sun. For a star with 300 times the solar magnetic flux, although the raw numerical data show a systematically lower spectrum than observed, the spectra are in good agreement once corrected for the effect of insufficient resolution in the transition region. For all young solar-type stars, high-energy X-rays (< 1 nm) are significantly underestimated, with the deviation increasing with stellar magnetic activity. Our findings indicate that the stellar XUV spectrum can be reasonably estimated through a numerical model, given that the essential input parameters (surface magnetic flux and elemental abundance) are known.

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arXiv2024

Comparison of solar multifrequency microwave data with other solar indices for understanding solar and stellar microwave data

Thermal microwave emissions detected from stellar atmospheres contain information on stellar activity. However, even for the Sun, the relationship between multifrequency microwave data and other activity indices remains unclear. We investigated the relationships among the thermal microwave fluxes with 1, 2, 3.75 and 9.4 GHz, their circular polarizations, and several activity indices recorded during recent solar cycles and observed that these relationships can be categorized into two groups. In the first group, the relationship between the microwave fluxes and solar indices, which are strongly related to the active regions, can be well-fitted by using a linear function. In the second group, the fitting function is dependent on frequency. Specifically, the microwave fluxes at 1 and 2 GHz can be well-fitted to the total unsigned magnetic and extreme ultraviolet fluxes by employing a power-law function. The trend changes around 3.75 GHz, and of the trend for the 9.4 GHz fluxes can be fitted by using a linear function. For the first time, we present the relationship between circular polarization and solar indices. Moreover, we extrapolated these relationships of the solar microwave fluxes to higher values and compared them with the solar-type stars. We found that epsilon Eri, whose microwave emission originates from thermal plasma, follows the extrapolated relationship. However, to date, only one star's emission at 1--10 GHz has been confirmed as thermal emission. More solar-type stars should be observed with future radio interferometers to confirm that relationships based on solar data can be applied to stellar microwave data.

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arXiv2023

Multiwavelength observation of an active M-dwarf star EV Lac and its stellar flare accompanied by a delayed prominence eruption

We conducted 4-night multiwavelength observations of an active M-dwarf star EV Lac on 2022 October 24$-$27 with simultaneous coverage of soft X-rays (NICER; 0.2$-$12 $\mathrm{keV}$, Swift XRT; 0.2$-$10 $\mathrm{keV}$), near-ultraviolet (Swift UVOT/UVW2; 1600$-$3500 Å), optical photometry (TESS; 6000$-$10000 Å), and optical spectroscopy (Nayuta/MALLS; 6350$-$6800 Å). During the campaign, we detected a flare starting at 12:28 UTC on October 25 with its white-light bolometric energy of $3.4 \times 10^{32}$ erg. At about 1 hour after this flare peak, our $\mathrm{Hα}$ spectrum showed a blue-shifted excess component at its corresponding velocity of $\sim 100 \: \mathrm{km \: s^{-1}}$. This may indicate that the prominence erupted with a 1-hour delay of the flare peak. Furthermore, the simultaneous 20-second cadence near-ultraviolet and white-light curves show gradual and rapid brightening behaviors during the rising phase at this flare. The ratio of flux in NUV to white light at the gradual brightening was $\sim 0.49$, which may suggest that the temperature of the blackbody is low ($< 9000 \: \mathrm{K}$) or the maximum energy flux of a nonthermal electron beam is less than $5\times10^{11} \: \mathrm{erg \: cm^{-2} \: s^{-1}}$. Our simultaneous observations of NUV and white-light flare raise the issue of a simple estimation of UV flux from optical continuum data by using a blackbody model.

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arXiv2023

Multiwavelength Campaign Observations of a Young Solar-type Star, EK Draconis. I. Discovery of Prominence Eruptions Associated with Superflares

Young solar-type stars frequently produce superflares, serving as a unique window into the young Sun-Earth environments. Large solar flares are closely linked to coronal mass ejections (CMEs) associated with filament/prominence eruptions, but its observational evidence for stellar superflares remains scarce. Here, we present a 12-day multi-wavelength campaign observation of young solar-type star EK Draconis (G1.5V, 50-120 Myr age) utilizing TESS, NICER, and Seimei telescope. The star has previously exhibited blueshifted H$α$ absorptions as evidence for a filament eruption associated with a superflare. Our simultaneous optical and X-ray observations identified three superflares of $1.5\times10^{33}$ -- $1.2\times10^{34}$ erg. We report the first discovery of two prominence eruptions on a solar-type star, observed as blueshifted H$α$ emissions at speed of 690 and 430 km s$^{-1}$ and masses of $1.1\times10^{19}$ and $3.2\times10^{17}$ g, respectively. The faster, massive event shows a candidate of post-flare X-ray dimming with the amplitude of up to $\sim$10 \%. Several observational aspects consistently point to the occurrence of a fast CME associated with this event. The comparative analysis of the estimated length scales of flare loops, prominences, possible dimming region, and starspots provides the overall picture of the eruptive phenomena. Furthermore, the energy partition of the observed superflares in the optical and X-ray bands is consistent with flares from the Sun, M-dwarfs, and close binaries, yielding the unified empirical relations. These discoveries provide profound implications of impact of these eruptive events on the early Venus, Earth and Mars and young exoplanets.

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arXiv2023

APO & SMARTS flare star campaign observations I. Blue wing asymmetries in chromospheric lines during mid M dwarf flares from simultaneous spectroscopic and photometric observation data

We conducted the time-resolved simultaneous optical spectroscopic and photometric observations of mid M dwarf flare stars YZ CMi, EV Lac, and AD Leo. Spectroscopic observations were obtained using Apache Point Observatory 3.5m and Small \& Moderate Aperture Research Telescope System 1.5m telescopes during 31 nights. Among the 41 detected flares, seven flares showed clear blue wing asymmetries in the H$α$ line, with various correspondences in flare properties. The duration of the blue wing asymmetries range from 20 min to 2.5 hours, including a flare showing the shift from blue to red wing asymmetry. Blue wing asymmetries can be observed during both white-light and candidate non white-light flares. All of the seven flares showed blue wing asymmetries also in the H$β$ line, but there are large varieties on which other chromospheric lines showed blue wing asymmetries. One among the 7 flares was also observed with soft X-ray spectroscopy, which enabled us to estimate the flare magnetic field and length of the flare loop. The line-of-sight velocities of the blue-shifted components range from -73 to -122 km s$^{-1}$. Assuming that the blue-shifts were caused by prominence eruptions, the mass of upward moving plasma was estimated to be 10$^{15}$ -- 10$^{19}$ g, which are roughly on the relation between flare energy and erupting mass expected from solar coronal mass ejections (CMEs). Although further investigations are necessary for understanding the observed various properties, these possible prominence eruptions on M-dwarfs could evolve into CMEs, assuming the similar acceleration mechanism with solar eruptions.

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arXiv2023

Reconstructing the XUV Spectra of Active Sun-like Stars Using Solar Scaling Relations with Magnetic Flux

Kepler Space Telescope and Transiting Exoplanet Survey Satellite unveiled that Sun-like stars frequently host exoplanets. These exoplanets are subject to fluxes of ionizing radiation in the form of X-ray and extreme-ultraviolet (EUV) radiation that may cause changes in their atmospheric dynamics and chemistry. While X-ray fluxes can be observed directly, EUV fluxes cannot be observed because of severe interstellar medium absorption. Here, we present a new empirical method to estimate the whole stellar XUV (X-ray plus EUV) and FUV spectra as a function of total unsigned magnetic fluxes of stars. The response of the solar XUV and FUV spectrum (0.1-180 nm) to the solar total unsigned magnetic flux is investigated by using the long-term Sun-as-a-star dataset over 10 yrs, and the power-law relation is obtained for each wavelength with a spectral resolution of 0.1-1 nm. We applied the scaling relations to active young Sun-like stars (G-dwarfs), EK Dra (G1.5V), $π^1$ Uma (G1.5V) and $κ^1$ Ceti (G5V), and found that the observed spectra (except for the unobservable longward EUV wavelength) are roughly consistent with the extension of the derived power-law relations with errors of an order of magnitude. This suggests that our model is a valuable method to derive the XUV/FUV fluxes of Sun-like stars including the EUV band mostly absorbed at wavelengths longward of 36 nm. We also discuss differences between the solar extensions and stellar observations at the wavelength in the 2-30 nm band and concluded that simultaneous observations of magnetic and XUV/FUV fluxes are necessary for further validations.

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arXiv2023

Starspot mapping with adaptive parallel tempering. II. Application to TESS data for M-dwarf flare stars, AU Microscopii, YZ Canis Minoris, and EV Lacertae

Starspots and stellar flares are indicators of stellar magnetic activity.The magnetic energy stored around spots is thought to be the origin of flares, but the connection is not completely understood. To investigate the relation between spot locations deduced from the light curves and occurrence of flares therein, we perform starspot modeling for TESS light curves of three M-dwarf flare stars, AU Mic, YZ CMi, and EV Lac, using the code implemented in Paper I. The code enables to deduce multiple stellar/spot parameters by the adaptive parallel tempering algorithm efficiently. We found that flare occurrence frequency is not necessarily correlated with the rotation phases of the light curve for each star. The result of starspot modeling shows that either spot is always visible to the line of sight in all phases, and we suggest that this can be one of the reasons that there is no or less correlation between rotation phases and flare frequency. In addition, the amplitude and shape of the light curve for AU Mic and YZ CMi have varied in two years between different TESS Cycles. The result of starspot modeling suggests that this can be explained by the variations of spot size and latitude.

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arXiv2023

Detection of a high-velocity prominence eruption leading to a CME associated with a superflare on the RS CVn-type star V1355 Orionis

Stellar coronal mass ejections (CMEs) have recently received much attention for their impacts on exoplanets and stellar evolution. Detecting prominence eruptions, the initial phase of CMEs, as the blue-shifted excess component of Balmer lines is a technique to capture stellar CMEs. However, most of prominence eruptions identified thus far have been slow and less than the surface escape velocity. Therefore, whether these eruptions were developing into CMEs remained unknown. In this study, we conducted simultaneous optical photometric observations with Transiting Exoplanet Survey Satellite and optical spectroscopic observations with the 3.8m Seimei Telescope for the RS CVn-type star V1355 Orionis that frequently produces large-scale superflares. We detected a superflare releasing $7.0 \times 10^{35} \: \mathrm{erg}$. In the early stage of this flare, a blue-shifted excess component of $\mathrm{H α}$ extending its velocity up to $760-1690 \: \mathrm{km \: s^{-1}}$ was observed and thought to originate from prominence eruptions. The velocity greatly exceeds the escape velocity (i.e., $\sim 350 \: \mathrm{km \: s^{-1}}$), which provides important evidence that stellar prominence eruptions can develop into CMEs. Furthermore, we found that the prominence is very massive ($9.5 \times 10^{18} \: \mathrm{g} < M < 1.4 \times 10^{21} \: \mathrm{g}$). These data will clarify whether such events follow existing theories and scaling laws on solar flares and CMEs even when the energy scale far exceeds solar cases.

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arXiv2023

A Superflare on YZ Canis Minoris Observed by Seimei Telescope and TESS: Red Asymmetry of H$α$ Emission Associated with White-Light Emission

Active M-type stars are known to often produce superflares on the surface. Radiation from stellar (super-)flares is important for the exoplanet habitability, but the mechanisms are not well understood. In this paper, we report simultaneous optical spectroscopic and photometric observations of a stellar superflare on an active M dwarf YZ CMi with the 3.8-m Seimei telescope and the $Transiting\, Exoplanet\, Survey\, Satellite$. The flare bolometric energy was $1.3^{+1.6}_{-0.6} \times 10^{34} \,\rm{erg}$ and H$α$ energy was $3.0^{+0.1}_{-0.1} \times 10^{32} \,\rm{erg}$. The H$α$ emission line profile showed red asymmetry throughout the flare with a duration of $4.6-5.1 \,\rm{hrs}$. The velocity of the red asymmetry was $\sim 200-500 \,\rm{km\,s^{-1}}$ and line width of H$α$ was broadened up to $34 \pm 14$ $Å$. The redshifted velocity and line width of H$α$ line decayed more rapidly than the equivalent width, and their time evolutions are correlated with that of the white-light emission. This indicates a possibility that the white light, H$α$ red asymmetry, and H$α$ line broadening originate from nearly the same site, i.e., the dense chromospheric condensation region heated by non-thermal electrons. On the other hand, the flux ratio of the redshifted excess components to the central components is enhanced one hour after the flare onset. This may be due to the change of the main source of the red asymmetry to the post-flare loops in the later phase of the flare.

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arXiv2022

Hunting for stellar coronal mass ejections

Solar flares are often accompanied by filament/prominence eruptions, sometimes leading to coronal mass ejections (CMEs). By analogy, we expect that stellar flares are also associated with stellar CMEs whose properties are essential to know the impact on exoplanet habitability. Probable detections of stellar CMEs are still rare, but in this decade, there are several reports that (super-)flares on M/K-dwarfs and evolved stars sometimes show blue-shifted optical/UV/X-ray emissions lines, XUV/FUV dimming, and radio bursts. Some of them are interpreted as indirect evidence of stellar prominence eruptions/CMEs on cool stars. More recently, evidence of stellar filament eruption, probably leading to a CME, is reported even on a young solar-type star (G-dwarf) as a blue-shifted absorption of H$α$ line associated with a superflare. Notably, the erupted masses for superflares are larger than those of the largest solar CMEs, indicating severe influence on exoplanet environments. The ratio of the kinetic energy of stellar CMEs to flare energy is significantly smaller than expected from the solar scaling relation and this discrepancy is still in debate. We will review the recent updates of stellar CME studies and discuss the future direction in this paper.

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arXiv2022

Sun-as-a-star Analyses of Various Solar Active Events Using H$α$ Spectral Images Taken by SMART/SDDI

Sun-as-a-star analyses, in which observational data is spatially integrated, are useful for interpreting stellar data. For future applications to stellar observations, we performed Sun-as-a-star analyses of H$α$ spectra for various active events on the Sun, not only for flares and filament eruptions/surges on the solar disk, but also for eruptions of off limb prominences using H$α$ spectral images taken by the Solar Magnetic Activity Research Telescope / Solar Dynamics Doppler Imager (SMART/SDDI) at Hida Observatory, Kyoto University. All the analyzed events show emission relative to the pre-event state and the changes in their H$α$ equivalent widths are all on the orders of 10$^{-4}$ Å. Sun-as-a-star H$α$ spectra exhibit different features depending on the causes of the emission: (i) Flares show emission at the H$α$ line center, together with red asymmetry and line broadening, as reported in a previous study. (ii) Filament eruptions with and without flares show emission near the H$α$ line center, accompanied by blue-/red-shifted absorption. Notably, disappearance of dark filaments leads to the apparent enhancement of the H$α$ line center emission. (iii) Eruptions of off limb prominences show blue-/red-shifted emission. These spectral features enable us to identify the active phenomena on Sun-like stars. We have also found that even the filament eruptions showing red-shifted absorptions in Sun-as-a-star H$α$ spectra lead to coronal mass ejections (CMEs). This result suggests that even if the falling components of stellar filament eruptions are detected as red-shifted absorptions in H$α$ spectra, such stellar filament eruptions may also develop into CMEs.

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arXiv2022

Universal Scaling Laws for Solar and Stellar Atmospheric Heating: Catalog of Power-law Index between Solar Activity Proxies and Various Spectral Irradiances

The formation of extremely hot outer atmospheres is one of the most prominent manifestations of magnetic activity common to the late-type dwarf stars, including the Sun. It is widely believed that these atmospheric layers, the corona, transition region, and chromosphere, are heated by the dissipation of energy transported upwards from the stellar surface by the magnetic field. This is signified by the spectral line fluxes at various wavelengths, scaled with power-law relationships against the surface magnetic flux over a wide range of formation temperatures, which are universal to the Sun and Sun-like stars of different ages and activity levels. This study describes a catalog of power-law indices between solar activity proxies and various spectral line fluxes. Compared to previous studies, we expanded the number of proxies, which now includes the total magnetic flux, total sunspot number, total sunspot area, and the F10.7 cm radio flux, and further enhances the number of spectral lines by a factor of two. This provides the data to study in detail the flux-flux scaling laws from the regions specified by the temperatures of the corona (log(T/K)=6-7) to those of the chromosphere (log(T/K)~4), as well as the reconstruction of various spectral line fluxes of the Sun in the past, F-, G-, and K-type dwarfs, and the modeled stars.

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arXiv2022

Sun-as-a-star Analysis of H$α$ Spectra of a Solar Flare Observed by SMART/SDDI: Time Evolution of Red Asymmetry and Line Broadening

Stellar flares sometimes show red/blue asymmetries of H$α$ line, which can indicate chromospheric dynamics and prominence activations. However, the origin of asymmetries is not completely understood. For a deeper understanding of stellar data, we performed a Sun-as-a-star analysis of H$α$ line profiles of an M4.2-class solar flare showing dominant emissions from flare ribbons by using the data of the Solar Dynamics Doppler Imager onboard the Solar Magnetic Activity Research Telescope at Hida Observatory. The Sun-as-a-star H$α$ spectra of the flare show red asymmetry of up to $\sim$95 km s$^{-1}$ and line broadening of up to $\sim$7.5 Å. The Sun-as-a-star H$α$ profiles are consistent with spectra from flare regions with weak intensity, but they take smaller redshift velocities and line widths by a factor of $\sim$2 than those with strong intensity. The redshift velocities, as well as line widths, peak out and decay more rapidly than the H$α$ equivalent widths, which is consistent with chromospheric condensation model and spatially-resolved flare spectra. This suggests that as a result of superposition, the nature of chromospheric condensation is observable even from stellar flare spectra. The time evolution of redshift velocities is found to be similar to that of luminosities of near-ultraviolet rays (1600 Å), while the time evolution of line broadening is similar to that of optical white lights. These H$α$ spectral behaviors in Sun-as-a-star view could be helpful to distinguish whether the origin of H$α$ red asymmetry of stellar flares is a flare ribbon or other phenomena.

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