Chromospheric Models Research Articles

A 7 Day Multiwavelength Flare Campaign on AU Mic. I. High-time-resolution Light Curves and the Thermal Empirical Neupert Effect

We present light curves and flares from a 7 day, multiwavelength observational campaign of AU Mic, a young and active dM1e star with exoplanets and a debris disk. We report on 73 unique flares between the X-ray to optical data. We use high-time-resolution near-UV (NUV) photometry and soft X-ray (SXR) data from the X-ray Multi-Mirror Mission to study the empirical Neupert effect, which correlates the gradual and impulsive phase flaring emissions. We find that 65% (30 of 46) flares do not follow the Neupert effect, which is 3 times more excursions than seen in solar flares, and propose a four-part Neupert effect classification (Neupert, quasi-Neupert, non-Neupert types I and II) to explain the multiwavelength responses. While the SXR emission generally lags behind the NUV as expected from the chromospheric evaporation flare models, the Neupert effect is more prevalent in larger, more impulsive flares. Preliminary flaring rate analysis with X-ray and U-band data suggests that previously estimated energy ratios hold for a collection of flares observed over the same time period, but not necessarily for an individual, multiwavelength flare. These results imply that one model cannot explain all stellar flares and care should be taken when extrapolating between wavelength regimes. Future work will expand wavelength coverage using radio data to constrain the nonthermal empirical and theoretical Neupert effects to better refine models and bridge the gap between stellar and solar flare physics.

Monochromatic Two-Fluid Alfvén Waves in the Partially Ionised Solar Chromosphere

We present new results towards the explanation of the chromospheric-heating problem and the solar-wind origin, using a two-fluid model that takes into account the collisional interaction between ions (protons) and neutrals (hydrogen atoms). Our aim is to further reveal the mechanism behind chromospheric heating and plasma outflows. We simulate and analyse the propagation and evolution of Alfvén waves in the partially ionised solar chromosphere, consisting of ions + electrons and neutral fluids. The simplified model chromosphere is permeated by a vertical, uniform magnetic field. We perform numerical simulations in the framework of a quasi-1.5-dimensional (1.5D), two-fluid model in which Alfvén waves are excited by a harmonic driver in the transverse component of the ion and neutral velocities, operating in the chromosphere. In the case of a small-amplitude driver, Alfvén waves are weakly damped, and for the chosen wave periods of a few seconds, Alfvén waves manage to propagate through the chromosphere and enter the solar corona. Non-linear Alfvén waves excited by a large-amplitude driver cause significant chromospheric heating and plasma outflows. We thus conclude that two-fluid Alfvén waves with larger amplitudes can contribute to chromospheric heating and plasma outflows, which may result higher up in the solar-wind origin.

Dark Off-limb Gap: Manifestation of a Temperature Minimum and the Dynamic Nature of the Chromosphere

We study off-limb emission of the lower solar atmosphere using high-resolution imaging spectroscopy in the Hβ and Ca ii 8542 Å lines obtained with the CHROMospheric Imaging Spectrometer (CHROMIS) and the CRisp Imaging SpectroPolarimeter (CRISP) on the Swedish 1 m Solar Telescope. The Hβ line-wing images show the dark intensity gap between the photospheric limb and chromosphere, which is absent in the Ca ii images. We calculate synthetic spectra of the off-limb emissions with the RH code in one-dimensional spherical geometry and find good agreement with the observations. The analysis of synthetic line profiles shows that the gap in the Hβ line-wing images maps the temperature minimum region between the photosphere and chromosphere due to the well-known opacity and emissivity gap of Balmer lines in this layer. However, the observed gap is detected farther from the line core in the outer line-wing positions than in the synthetic profiles. We found that an increased microturbulence in the model chromosphere is needed to reproduce the dark gap in the outer line wing, suggesting that the observed Hβ gap is the manifestation of the temperature minimum and the dynamic nature of the solar chromosphere. The temperature minimum produces a small enhancement in synthetic Ca ii line-wing intensities. Observed off-limb Ca ii line-wing emissions show similar enhancement below the temperature minimum layer near the edge of the photospheric limb.

Fast optical flares from M dwarfs detected by a one-second-cadence survey with Tomo-e Gozen

Abstract We report on a one-second-cadence wide-field survey for M-dwarf flares using the Tomo-e Gozen camera mounted on the Kiso Schmidt telescope. We detect 22 flares from M3–M5 dwarfs with a rise time of 5 s ≲ trise ≲ 100 s and an amplitude of 0.5 ≲ ΔF/F⋆ ≲ 20. The flare light-curves mostly show steeper rises and shallower decays than those obtained from the Kepler one-minute cadence data and tend to have flat peak structures. Assuming a blackbody spectrum with a temperature of 9000–15000 K, the peak luminosities and energies are estimated to be 1029 erg s−1 ≲ Lpeak ≲ 1031 erg s−1 and 1031 erg ≲ Eflare ≲ 1034 erg, which constitutes the bright end of fast optical flares for M dwarfs. We confirm that more than $90\%$ of the host stars of the detected flares are magnetically active based on their Hα-emission-line intensities obtained by LAMOST. An estimated occurrence rate of detected flares is ∼0.7 per day per active star, indicating they are common in magnetically active M dwarfs. We argue that the flare light-curves can be explained by the chromospheric compression model: the rise time is broadly consistent with the Alfvén transit time of a magnetic loop with a length scale of lloop ∼ 104 km and a field strength of 1000 gauss, while the decay time is likely determined by the radiative cooling of the compressed chromosphere down near to the photosphere with a temperature of ≳ 10000 K. These flares from M dwarfs could be a major contamination source for a future search of fast optical transients of unknown types.

Modeling of chromospheric features and dynamics in solar plage

The chromosphere is a dynamic and complex layer where all the relevant physical processes happen on very small spatio-temporal scales. A few spectral lines that can be used as chromospheric diagnostics give us convoluted information that is hard to interpret without realistic theoretical models. What are the key ingredients that these models need to contain? The magnetic field has a paramount effect on chromospheric structuring. This is obvious from the ubiquitous presence of chromospheric dynamic fibrilar structures visible on the solar disk and at the limb. The numerical experiments presented in this manuscript illustrate the present state of modeling. They showcase to what extent our models reproduce various chromospheric features and their dynamics. The publication describes the effect different ingredients have on chromospheric models and provides a recipe for building one-to-one models. Combining these models with observations will provide insight into the physical processes that take place in the solar atmosphere.

Chromospheric extension of the MURaM code

Context. Detailed numerical models of the chromosphere and corona are required to understand the heating of the solar atmosphere. An accurate treatment of the solar chromosphere is complicated by the effects arising from non-local thermodynamic equilibrium (NLTE) radiative transfer. A small number of strong, highly scattering lines dominate the cooling and heating in the chromosphere. Additionally, the recombination times of ionised hydrogen are longer than the dynamical timescales, requiring a non-equilibrium (NE) treatment of hydrogen ionisation. Aims. We describe a set of necessary additions to the MURaM code that allow it to handle some of the important NLTE effects. We investigate the impact on solar chromosphere models caused by NLTE and NE effects in radiation magnetohydrodynamic simulations of the solar atmosphere. Methods. The MURaM code was extended to include the physical process required for an accurate simulation of the solar chromosphere, as implemented in the Bifrost code. This includes a time-dependent treatment of hydrogen ionisation, a scattering multi-group radiation transfer scheme, and approximations for NLTE radiative cooling. Results. The inclusion of NE and NLTE physics has a large impact on the structure of the chromosphere; the NE treatment of hydrogen ionisation leads to a higher ionisation fraction and enhanced populations in the first excited state throughout cold inter-shock regions of the chromosphere. Additionally, this prevents hydrogen ionisation from buffering energy fluctuations, leading to hotter shocks and cooler inter-shock regions. The hydrogen populations in the ground and first excited state are enhanced by 102–103 in the upper chromosphere and by up to 109 near the transition region. Conclusions. Including the necessary NLTE physics leads to significant differences in chromospheric structure and dynamics. The thermodynamics and hydrogen populations calculated using the extended version of the MURaM code are consistent with previous non-equilibrium simulations. The electron number and temperature calculated using the non-equilibrium treatment of the chromosphere are required to accurately synthesise chromospheric spectral lines.

Evolution of Solar-type Activity: A Brief Study of Ca ii Emission in Beta Hydri (G2 IV)

We present results about the decrease of stellar activity for stars like the Sun evolving toward the subgiant stage. Specifically, we examine two-component chromosphere models (acoustic and magnetic) for β Hydri (G2 IV) based on detailed time-dependent simulations while focusing on Ca ii emission. We convey models for two different photospheric magnetic filling factors.

Sun-as-a-star Analysis of Hα 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 the Hα 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 Hα 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 on board the Solar Magnetic Activity Research Telescope at the Hida Observatory. Sun-as-a-star Hα spectra of the flare show red asymmetry of up to ∼95 km s−1 and line broadening of up to ∼7.5 Å. The Sun-as-a-star Hα profiles are consistent with spectra from flare regions with weak intensity, but they take smaller redshift velocities and line widths by a factor of ∼2 than those with strong intensity. The redshift velocities, as well as line widths, peak out and decay more rapidly than the Hα equivalent widths, which is consistent with the 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 Hα spectral behaviors in Sun-as-a-star view could be helpful to distinguish whether the origin of Hα red asymmetry of stellar flares is a flare ribbon or other phenomena.

HST STIS Observations of ζ Aurigae A's Irradiated Atmosphere

The details of the processes responsible for heating the chromospheres of evolved cool stars remain uncertain. While most spectroscopic diagnostics measure spatially-integrated emission, here we examine diagnostics sensitive to localized atmospheric regions that are specific to cool evolved stars in binary systems with hot main-sequence companions: emission from Si i and C i resulting from the ultraviolet irradiation of the evolved star’s atmosphere. HST Space Telescope Imaging Spectrograph (STIS) high-spectral-resolution near-ultraviolet observations of ζ Aurigae A+B (K4 Ib + B5 V) were obtained at three orbital phases, including total eclipse, to search for Si i and the corresponding C i line emission. Si i 2987.645 Å emission was detected at phases ϕ = .101 and .448 (from periastron) in-line with predictions from a previous study of optical Si i 3905 Å and Si i 4102 Å emission lines. No other Si i line emission is apparent, and the analogous C i lines at 2478.561 Å and 2582.901 Å also are not detected. High-spectral-resolution HST STIS and Goddard High Resolution Spectrograph spectra confirm the results of a previous study that showed that the intrinsic chromospheric fluxes on the visible hemisphere of ζ Aur A, observed during total eclipse, are representative of the single K supergiant λ Vel (K4 Ib). Furthermore, the HST spectra show that the chromospheric turbulent velocities are very close to those in this spectral-type proxy. These combined results highlight the importance of detailed spatially-resolved chromospheric models of ζ Aur systems, based on sequences of atmospheric eclipse spectra, to help constrain the poorly understood mechanisms that heat the atmospheres and drive stellar winds in cool evolved stars.

The Atmospheric Response to High Nonthermal Electron-beam Fluxes in Solar Flares. II. Hydrogen-broadening Predictions for Solar Flare Observations with the Daniel K. Inouye Solar Telescope

Abstract Redshifted components of chromospheric emission lines in the hard X-ray impulsive phase of solar flares have recently been studied through their 30 s evolution with the high resolution of the Interface Region Imaging Spectrograph. Radiative-hydrodynamic flare models show that these redshifts are generally reproduced by electron-beam-generated chromospheric condensations. The models produce large ambient electron densities, and the pressure broadening of the hydrogen Balmer series should be readily detected in observations. To accurately interpret the upcoming spectral data of flares with the DKIST, we incorporate nonideal, nonadiabatic line-broadening profiles of hydrogen into the RADYN code. These improvements allow time-dependent predictions for the extreme Balmer line wing enhancements in solar flares. We study two chromospheric condensation models, which cover a range of electron-beam fluxes (1 − 5 × 1011 erg s−1 cm−2) and ambient electron densities (1 − 60 × 1013 cm−3) in the flare chromosphere. Both models produce broadening and redshift variations within 10 s of the onset of beam heating. In the chromospheric condensations, there is enhanced spectral broadening due to large optical depths at Hα, Hβ, and Hγ, while the much lower optical depth of the Balmer series H12−H16 provides a translucent window into the smaller electron densities in the beam-heated layers below the condensation. The wavelength ranges of typical DKIST/ViSP spectra of solar flares will be sufficient to test the predictions of extreme hydrogen wing broadening and accurately constrain large densities in chromospheric condensations.

Chromospheric activity in 55 Cancri: II. Theoretical wave studies versus observations

ABSTRACT In this study, we consider chromospheric heating models for 55 Cancri in conjunction with observations. The theoretical models, previously discussed in Paper I, are self-consistent, non-linear, and time-dependent ab-initio computations encompassing the generation, propagation, and dissipation of waves. Our focus is the consideration of both acoustic waves and longitudinal flux tube waves amounting to two-component chromosphere models. 55 Cancri, a K-type orange dwarf, is a star of low activity, as expected by its age, which also implies a relatively small magnetic filling factor. The Ca ii K fluxes are computed (multi-ray treatment) assuming partial redistribution and time-dependent ionization. The theoretical Ca ii H+K fluxes are subsequently compared with observations. It is found that for stages of lowest chromospheric activity the observed Ca ii fluxes are akin, though not identical, to those obtained by acoustic heating, but agreement can be obtained if low levels of magnetic heating – consistent with the expected photospheric magnetic filling factor – are considered as an additional component; this idea is in alignment with previous proposals conveyed in the literature.

Accretion-powered Outflows in AGB Stars

AbstractOne of the big challenges for 21st century stellar astrophysics is the impact of binary interactions on stellar evolution. Such interactions are believed to play a key role in the death throes of 1-8 M⊙ stars, as they evolve from the AGB stars into Planetary Nebulae. X-ray surveys of UV-emitting AGB stars show that ∼40% of objects with FUV emission and GALEX FUV/NUV flux ratios ≳0.2 have variable X-ray emission characterized by very high temperatures (Tx∼35-160 MK) and luminosities (Lx∼0.002-0.2L⊙). We hypothesize that such AGB stars have accretion and (accretion-powered) outflows associated with a close binary companion. UV spectroscopy with HST/STIS of our brightest object (Y Gem) shows the presence of infalling and outflowing gas, providing direct kinematic confirmation of this hypothesis. However, the UV-emitting AGB star population is dominated by objects with little or no FUV emission, and we do not know whether the UV emission from these is intrinsic to the AGB star or extrinsic (i.e., due to binarity). Here we present the first results from a large grid of simple chromospheric models to help discriminate between the intrinsic and extrinsic mechanisms of UV emission for AGB stars.

The CARMENES search for exoplanets around M dwarfs

The He I infrared (IR) line at a vacuum wavelength of 10 833 Å is a diagnostic for the investigation of atmospheres of stars and planets orbiting them. For the first time, we study the behavior of the He I IR line in a set of chromospheric models for M-dwarf stars, whose much denser chromospheres may favor collisions for the level population over photoionization and recombination, which are believed to be dominant in solar-type stars. For this purpose, we use published PHOENIX models for stars of spectral types M2 V and M3 V and also compute new series of models with different levels of activity following an ansatz developed for the case of the Sun. We perform a detailed analysis of the behavior of the He I IR line within these models. We evaluate the line in relation to other chromospheric lines and also the influence of the extreme ultraviolet (EUV) radiation field. The analysis of the He I IR line strengths as a function of the respective EUV radiation field strengths suggests that the mechanism of photoionization and recombination is necessary to form the line for inactive models, while collisions start to play a role in our most active models. Moreover, the published model set, which is optimized in the ranges of the Na I D2, Hα, and the bluest Ca II IR triplet line, gives an adequate prediction of the He I IR line for most stars of the stellar sample. Because especially the most inactive stars with weak He I IR lines are fit worst by our models, it seems that our assumption of a 100% filling factor of a single inactive component no longer holds for these stars.

Nonlinear Convergence of Solar-like Stars Chromospheres Using Millimeter, Submillimeter, and Infrared Observations

Abstract In this work, we present a new methodology to fit the observed and synthetic spectrum of solar-like stars at millimeter, submillimeter, and infrared wavelengths through semiempirical models of the solar chromosphere. We use the Levenberg–Marquardt algorithm as a Nonlinear method, PakalMPI as the semiempirical model of the solar chromosphere, and recent observations from the Atacama Large Millimeter/submillimeter Array of Alpha Centauri A as a test case. Our results show that we can use solar chromospheric semiempirical models as an input model to reproduce the observed spectrum of solar-like stars. The new profiles show similarities to the solar chromosphere as a minimum of temperature (without the restriction from CO emission) and a plateau in the high chromosphere. Our method provides a new fast numerical tool to estimate the physical conditions of solar-like stars.

Generation of nonlinear magnetic flux tube wave energies in Procyon A

Abstract The aim of the current study is the computation of the magnetic flux tube wave energies and fluxes generated in the convection zone of Procyon A. This is a subgiant of spectral type F5 IV-V showing chromospheric and coronal activities. The mechanisms responsible for the generation of different wave modes include the interaction of the thin and vertically oriented magnetic flux tube embedded in magnetic-free regions with turbulence in the convection zone of Procyon A. We are considering longitudinal, transverse and acoustic wave modes. Turbulence in the convection zone is modeled by the extended Kolmogorov turbulent energy spectrum and the modified Gaussian frequency factor. Different magnetic flux tube models with different degrees of magnetic activities were considered. The current approach takes the nonlinear effects into consideration. The results show that there is enough wave energy in the three different forms to heat the outer layers of the star. The obtained acoustic wave energies are larger than those of the longitudinal tube wave energies compared to the solar case. This can be explained by the relatively low magnetic field strength. On the other side, our computations show the importance of the transverse wave energies compared to the energies carried by the longitudinal waves. The former waves carry energy several (between 2 and 14) times higher than the latter. The obtained wave energies are essential for constructing time-dependent model chromospheres and for the predictions of atmospheric oscillations to be compared e.g. with the data collected by the CoRoT and Kepler missions.

Linear Acoustic Waves in a Nonisothermal Atmosphere. II. Photospheric Resonator Model of Three-minute Umbral Oscillations

Abstract The velocity oscillations observed in the chromosphere of sunspot umbrae resemble a resonance in that their power spectra are sharply peaked around a period of about three minutes. In order to describe the resonance that leads to the observed 3-minute oscillations, we propose the photospheric resonator model of acoustic waves in the solar atmosphere. The acoustic waves are driven by the motion of a piston at the lower boundary, and propagate in a nonisothermal atmosphere that consists of the lower layer (photosphere), where temperature rapidly decreases with height, and the upper layer (chromosphere), where temperature slowly increases with height. We have obtained the following results: (1) The lower layer (photosphere) acts as a leaky resonator of acoustic waves. The bottom end is established by the piston, and the top end by the reflection at the interface between the two layers. (2) The temperature minimum region partially reflects and partially transmits acoustic waves of frequencies around the acoustic cutoff frequency at the temperature minimum. (3) The resonance occurs in the photospheric layer at one frequency around this cutoff frequency. (4) The waves escaping the photospheric layer appear as upward-propagating waves in the chromosphere. The power spectrum of the velocity oscillation observed in the chromosphere can be fairly well reproduced by this model. The photospheric resonator model was compared with the chromospheric resonator model and the propagating wave model.

Origin of the chromospheric three-minute oscillations in sunspot umbrae

Context. Sunspot umbrae show a change in the dominant period of their oscillations from five minutes (3.3 mHz) in the photosphere to three minutes (5.5 mHz) in the chromosphere. Aims. In this paper, we explore the two most popular models proposed to explain the three-minute oscillations: the chromospheric acoustic resonator and the propagation of waves with frequency above the cutoff value directly from lower layers. Methods. We employ numerical simulations of wave propagation from the solar interior to the corona. Waves are driven by a piston at the bottom boundary. We have performed a parametric study of the measured chromospheric power spectra in a large number of numerical simulations with differences in the driving method, the height of the transition region (or absence of transition region), the strength of the vertical magnetic field, and the value of the radiative cooling time. Results. We find that both mechanisms require the presence of waves with periods in the three-minute band at the photosphere. These waves propagate upward and their amplitude increases due to the drop of the density. Their amplification is stronger than that of evanescent low-frequency waves. This effect is enough to explain the dominant period observed in chromospheric spectral lines. However, waves are partially trapped between the photosphere and the transition region, forming an acoustic resonator. This chromospheric resonant cavity strongly enhances the power in the three-minute band. Conclusions. The chromospheric acoustic resonator model and the propagation of waves in the three-minute band directly from the photosphere can explain the observed chromospheric three-minute oscillations. They are both important in different scenarios. Resonances are produced by waves trapped between the temperature minimum and the transition region. Strong magnetic fields and radiative losses remove energy from the waves inside the cavity, resulting in resonances with weaker amplitude.

Resolving the extended stellar atmospheres of asymptotic giant branch stars at (sub)millimetre wavelengths

Context. The initial conditions for mass loss during the asymptotic giant branch (AGB) phase are set in their extended atmospheres, where, among others, convection and pulsation driven shocks determine the physical conditions. Aims. High resolution observations of AGB stars at (sub)millimetre wavelengths can now directly determine the morphology, activity, density, and temperature close to the stellar photosphere. Methods. We used Atacama Large Millimeter/submillimeter Array (ALMA) high angular resolution observations to resolve the extended atmospheres of four of the nearest AGB stars: W Hya, Mira A, R Dor, and R Leo. We interpreted the observations using a parameterised atmosphere model. Results. We resolve all four AGB stars and determine the brightness temperature structure between 1 and 2 stellar radii. For W Hya and R Dor we confirm the existence of hotspots with brightness temperatures > 3000 to 10 000 K. All four stars show deviations from spherical symmetry. We find variations on a timescale of days to weeks, and for R Leo we directly measure an outward motion of the millimetre wavelength surface with a velocity of at least 10.6 ± 1.4 km s−1. For all objects but W Hya we find that the temperature-radius and size-frequency relations require the existence of a (likely inhomogeneous) layer of enhanced opacity. Conclusions. The ALMA observations provide a unique probe of the structure of the extended AGB atmosphere. We find highly variable structures of hotspots and likely convective cells. In the future, these observations can be directly compared to multi-dimensional chromosphere and atmosphere models that determine the temperature, density, velocity, and ionisation structure between the stellar photosphere and the dust formation region. However, our results show that for the best interpretation, both very accurate flux calibration and near-simultaneous observations are essential.

The CARMENES search for exoplanets around M dwarfs

Chromospheric modeling of observed differences in stellar activity lines is imperative to fully understand the upper atmospheres of late-type stars. We present one-dimensional parametrized chromosphere models computed with the atmosphere code PHOENIX using an underlying photosphere of 3500 K. The aim of this work is to model chromospheric lines of a sample of 50 M2–3 dwarfs observed in the framework of the CARMENES, the Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs, exoplanet survey. The spectral comparison between observed data and models is performed in the chromospheric lines of Na I D2, Hα, and the bluest Ca II infrared triplet line to obtain best-fit models for each star in the sample. We find that for inactive stars a single model with a VAL C-like temperature structure is sufficient to describe simultaneously all three lines adequately. Active stars are rather modeled by a combination of an inactive and an active model, also giving the filling factors of inactive and active regions. Moreover, the fitting of linear combinations on variable stars yields relationships between filling factors and activity states, indicating that more active phases are coupled to a larger portion of active regions on the surface of the star.

Possibility of chromospheric back-radiation influencing the lithium line formation in Spite plateau stars

Context.Spectroscopically determined lithium abundances of metal-poor turn-off dwarfs are known to be nearly constant (Spite plateau), but manifestly lower than the primordial value expected from the standard cosmological model. However, abundance determination by using conventional model atmospheres may not necessarily be correct since the existence of high-temperature chromosphere even in very old stars has been confirmed.Aims.The aim of this study is to examine how the extra UV flux possibly irradiated from the chromosphere could affect the formation of the Li I6708 line, and whether or not its influence might lead to a solution of the Li abundance discrepancy.Methods.A simple model chromosphere of a uniform thin gray slab emitting only thermal radiation is assumed, characterized by optical thickness and temperature. By taking into account this incident radiation in the surface boundary condition, non-local thermodynamical equilibrium calculations for neutral Li atoms are carried out in order to see how the equivalent widths and the resulting abundances are affected by these parameters.Results.If the parameters are appropriately chosen, the strength of the Li I6708 line can be reduced by a factor of ~2–3 due to overionization caused by enhanced UV radiation, leading to an apparent lowering of the abundance by ~0.3–0.5 dex, which is consistent with the discrepancy in question. Moreover, the observed slight metallicity-dependent trend of the plateau can also be reproduced as a result of the change in atmospheric transparency.Conclusions.Superficial underestimation of Li abundances due to considerable overionization caused by chromospheric radiation may be regarded as a ponderable interpretation for the cosmological Li problem. The touchstone to verify this model would be to check the existence of significantly enhanced UV radiation in these Spite plateau stars, which should be detected if this scenario is valid, although very few such UV spectrophotometric observations have been done to date.