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As the solar photon fluxes directly control the production of photoelectrons, it is important to examine the influence of different solar irradiance models on the photoelectron fluxes. In this study, we present the implementation of the two recent solar irradiance models, the Flare Irradiance Spectral Model (FISM) and the Heliospheric Environment Solar Spectral Radiation (HESSR) model, to our SuperThermal Electron Transport (STET) model. In addition, we have proposed a new modification to the Hinteregger-81 model. The resultant photoelectron fluxes from the three solar irradiance models, the Hinteregger-81 model, FISM and the HESSR model, are compared and mostly vary within a factor of 2.

This article describes an update of the physical models that we use to reconstruct the FUV and EUV irradiance spectra and the radiance spectra of the features that at any given point in time may cover the solar disk depending on the state of solar activity. The present update introduces important modifications to the chromosphere–corona transition region of all models. Also, the update introduces improved and extended atomic data. By these changes, the agreement of the computed and observed spectra is largely improved in many EUV lines important for the modeling of the Earth’s upper atmosphere. This article describes the improvements and shows detailed comparisons with EUV/FUV radiance and irradiance measurements. The solar spectral irradiance from these models at wavelengths longer than ≈ 200 nm is discussed in a separate article.

The stratosphere is the region where the ozone chemistry is important for the balance of energy, and radiation in the near UV plays a fundamental role in the creation and destruction of ozone. However, the radiation in this range of wavelength has not been very well modeled. One of the most important elements, according to its abundance in the solar atmosphere, that contribute to the emission and absorption of radiation in the spectral range between 1900 and 3900 Å, is neutral nickel (Ni I). In this work we improve the atomic model of this element, taking into account 490 lines over the spectrum. We solve these lines in NLTE using the Solar Radiation Physical Modeling (SRPM) program and compare the results with observation of the quiet sun spectrum.

Intense heating in the quiet-Sun chromosphere raises the temperature from 4000 to 6500 K but, despite decades of study, the underlying mechanism remains a mystery. This study continues to explore the possibility that the Farley–Buneman instability contributes to chromospheric heating. This instability occurs in weakly ionized collisional plasmas in which electrons are magnetized, but ions are not. A mixture of metal ions generate the plasma density in the coolest parts of the chromosphere; while some ions are weakly magnetized, others are demagnetized by neutral collisions. This paper incorporates the effects of multiple, arbitrarily magnetized species of ions to the theory of the Farley–Buneman instability and examines the ramifications on instability in the chromosphere. The inclusion of magnetized ions introduces new restrictions on the regions in which the instability can occur in the chromosphere—in fact, it confines the instability to the regions in which heating is observed. For a magnetic field of 30 G, the minimum ambient electric field capable of driving the instability is 13.5 V/m at the temperature minimum.

Semiempirical atmospheric models of solar surface features as observed at moderate resolution are useful tools for understanding the observed solar spectral irradiance variations. Paper I described a set of models constructed to reproduce the observed radiance spectrum for solar surface features at ~2 arcsec resolution that constitute an average over small-scale features such as granulation. Paper II showed that a revision of previous models of low-chromospheric inter-network regions explains the observed infrared CO lines in addition to the UV and radio continuum from submillimeter to centimetric wavelengths. The present paper (1) shows that the Ca II H and K line wing observations are also explained by the new quiet-Sun-composite model, (2) introduces new low-chromospheric models of magnetic features that follow the ideas in Paper II, (3) introduces new upper chromospheric structures for all quiet-Sun and active-region models, and (4) shows how the new set of models explains EUV/FUV observations of spectral radiance and irradiance. This paper also discusses the chromospheric radiative-loss estimates in each of the magnetic features. The new set of models provides a basis for the spectral irradiance synthesis at EUV/FUV wavelengths based on the features observed on the solar surface.

Context. Chromospheric heating produces UV emissions that can only occur in an enhanced electron temperature medium. In the quiet Sun the radiative losses are orders of magnitude larger than those in the much hotter corona. Chromospheric heating mechanisms considered previously (e.g. shock waves and nanoflares) have failed to account for the observed persistency and uniformity of UV lines and continua. Also, resistive magnetic free-energy dissipation is not efficient enough in the highly electrically conductive solar atmosphere. Aims. In this paper we consider plasma effects in the low chromosphere and propose that the Farley-Buneman (hereafter FB) plasma-instability mechanism provides the mechanism for dissipating the energy of convectively driven motions of neutral atoms into chromospheric heating in the Sun and other cool stars that have a partially ionized chromosphere. Methods. Analysis of the ion acoustic sound speed and consideration of recent measurements of magnetic field in the quiet, internetwork, solar low chromosphere are carried out in the context of understanding the characteristics and onset of chromospheric heating. The FB instability is triggered by the cross-field motion of the partially ionized gas at velocities in excess of the ion acoustic velocity. The ions acquire their cross-field velocities through collisions with the much denser chromospheric neutral atoms. Estimates of cross-field velocities are obtained from consideration of both spectral line widths and convection numerical simulations that indicate values from a few to several km s -1 at the top of the practically radiative-equilibrium low chromosphere. Results. The FB instability is triggered by the cross-field motion of the neutral component of the partially ionized gas at velocities in excess of the ion acoustic velocity. This instability occurs in the solar chromosphere because electrons become strongly magnetized just above the photosphere, while heavy ions and protons remain unmagnetized, and only at the very top of the chromosphere do they become magnetized. Conclusions. We find that convective overshoot motions are drivers of the FB instability and provide enough energy to account for the upper chromospheric radiative losses in the quiet-Sun internetwork and network lanes.

We present a new, one-dimensional model of the solar atmosphere (called SRPM 305) at moderate angular resolution (~1''-2''). Key characteristics of the SRPM 305 model include (1) a minimum temperature of ~3800 K at a gas pressure of ~80 dyne cm-2 and (2) a rapid temperature rise above the temperature-minimum layer that results in substantial overionization of most elements when compared with LTE calculations. The model calculations reproduce the ~4300 K minimum brightness temperature of the UV continuum (between 1400-1500 A) observed by SUMER and the ~4400 K observed minimum radio-continuum brightness temperature (between wavelengths 0.01 and 100 mm). Neither the UV nor the radio continuum bear on the low-temperature minimum value because their broad intensity contribution functions cause the higher temperatures of the upper chromospheric layers to effectively hide the low minimum temperature region. The SRPM 305 model reproduces the observed intensities of CO lines at 4.466 μm, at both the disk center and near the limb, by using C and O abundances consistent with recent literature low values. The model also reproduces observed intensities of C I spectral lines at 5381 and 8337 A, CH lines at about 4306 A, the CN band head at 3883 A, and the O I lines at 7772, 7774, and 7776 A, respectively. Using the SRPM 305 model, we find no significant abundance variations between the photosphere and the low chromosphere. Consequently, the single-component model presented here matches several apparently contradictory observations and thereby resolves the controversy about the temperature minimum value.

In this paper we study and modify previous semiempirical models of the solar photosphere as observed at moderate spatial and temporal resolution (3 and 30 minutes, respectively) in the main quiet- and active Sun component features. Our present models are constructed to match the relevant available observations at this resolution for which a one-dimensional and time-independent stratification is reasonable. The models do not describe the fine structure and temporal variability observed in high-resolution images but correspond to a "radiation averaging" over the fine-structure and p-mode variations. We use the observed limb darkening in the range 0.3–2.4 m, as well as the absolute intensities and details of the spectral continua and lines in this range, to validate and adjust the models. Using the method described in a previous paper, we compute the emergent radiation from our models in full detail for the visible and IR continuum and the lines in the interval 0.3–5 m for which we have atomic data from NIST (13,000 lines used) and molecular data from HITRAN and Gray & Corbally (480,000 molecular lines used). The observations, abundances, and atomic/molecular data are improved over previous work and yield models that better fit the observations. In addition, we construct a new penumbra model. The visible and IR detailed spectra computed from these models provide insight for understanding the effects of magnetic fields on the solar irradiance and are useful tools for computing synthetic spectral irradiances in different solar activity configurations.

We study the plasma parameters in recent models of the observed magnetic features in the solar atmosphere and find that electrons are strongly magnetized in the chromosphere but protons are unmagnetized up to the transition region. Considering the magnetization and the classical Pedersen conductivity we find that magnetic diffusion is too small for effectively affecting propagating MHD waves of periods of a few minutes. However, the chromospheric-plasma parameters suggest a scenario in which upward-propagating fast-mode MHD waves of mHz frequencies would trigger the Farley-Buneman plasma instability at chromospheric layers where horizontal magnetic fields are present. We show that, because of the collisions between charged particles and neutral H atoms, the conditions in the chromosphere meet the instability criteria if the MHD wave velocity amplitude is lower but near the adiabatic sound speed. The instability growth is much faster than the wave frequency and the instability would quickly saturate. The electrostatic plasma waves resulting from the instability are expected to produce anomalous resistivity and wave energy dissipation that would heat the chromosphere as well as absorb the p-modes in magnetic regions.

The Spectral Irradiance Monitor (SIM) is a dual Fery prism spectrometer that employs 5 detectors per spectrometer channel to cover the wavelength range from 200 to 2700 nm. This instrument is used to monitor solar spectral variability throughout this wavelength region. Two identical, mirror-image, channels are used for redundancy and in-flight measurement of prism degradation. The primary detector for this instrument is an electrical substitution radiometer (ESR) designed to measure power levels ∼1000 times smaller than other radiometers used to measure TSI. The four complementary focal plane photodiodes are used in a fast-scan mode to acquire the solar spectrum, and the ESR calibrates their radiant sensitivity. Wavelength control is achieved by using a closed loop servo system that employs a linear charge coupled device (CCD) in the focal plane. This achieves 0.67 arcsec control of the prism rotation angle; this is equivalent to a wavelength positioning error of δλ/λ = 150 parts per million (ppm). This paper will describe the scientific measurement requirements used for instrument design and implementation, instrument performance, and the in-flight instrument operation modes.

The Spectral Irradiance Monitor (SIM) is a satellite-borne spectrometer aboard the Solar Radiation and Climate Experiment (SORCE) that measures solar irradiance between 200 and 2700 nm. This instrument employs a Fery prism as a dispersing element, an electrical substitution radiometer (ESR) as the primary detector, and four additional photodiode detectors for spectral scanning. Assembling unit level calibrations of critical components and expressing the sensitivity in terms of interrelated measurement equations supplies the instrument's radiant response. The calibration and analysis of the spectrometer's dispersive and transmissive properties, light aperture metrology, and detector characteristics provide the basis for these measurement equations. The values of critical calibration parameters, such as prism and detector response degradation, are re-measured throughout the mission to correct the ground-based calibration.

In this paper we describe the synthetic solar spectral irradiance (SSI) calculated from 2010 to 2015 using data from the Atmospheric Imaging Assembly (AIA) instrument, on board the Solar Dynamics Observatory spacecraft. We used the algorithms for solar disk image decomposition (SDID) and the spectral irradiance synthesis algorithm (SISA) that we had developed over several years. The SDID algorithm decomposes the images of the solar disk into areas occupied by nine types of chromospheric and 5 types of coronal physical structures. With this decomposition and a set of pre-computed angle-dependent spectra for each of the features, the SISA algorithm is used to calculate the SSI. We discuss the application of the basic SDID/SISA algorithm to a subset of the AIA images and the observed variation occurring in the 2010–2015 period of the relative areas of the solar disk covered by the various solar surface features. Our results consist of the SSI and total solar irradiance variations over the 2010–2015 period. The SSI results include soft X-ray, ultraviolet, visible, infrared, and far-infrared observations and can be used for studies of the solar radiative forcing of the Earth's atmosphere. These SSI estimates were used to drive a thermosphere–ionosphere physical simulation model. Predictions of neutral mass density at low Earth orbit altitudes in the thermosphere and peak plasma densities at mid-latitudes are in reasonable agreement with the observations. The correlation between the simulation results and the observations was consistently better when fluxes computed by SDID/SISA procedures were used.

Variations in the total radiative output of the Sun as well as the detailed spectral irradiance are of interest to terrestrial and solar-stellar atmosphere studies. Recent observations provide measurements of spectral irradiance variations at wavelengths in the range 1100-8650 ? with improved accuracy, and correlative studies give procedures for estimating the spectral irradiance changes from solar activity records using indicators such as those derived from Ca II K and Mg II indices. Here we describe our approach to physical modeling of irradiance variations using seven semiempirical models to represent sunspots, plage, network, and quiet atmosphere. This paper gives methods and details, and some preliminary results of our synthesis of the variations of the entire irradiance spectrum. Our calculation uses object-oriented programming techniques that are very efficient and flexible. We compute at high spectral resolution the intensity as a function of wavelength and position on the disk for each of the structure types corresponding to our models. These calculations include three different approximations for the line source function: one suited for the very strong resonance lines where partial redistribution (PRD) is important, another for the most important nonresonance lines, and another approximation for the many narrow lines that are provided in Kurucz's listings. The image analysis and calculations of the irradiance variation as a function of time will be described in a later paper. This work provides an understanding of the sources of variability arising from solar-activity surface structures. We compute the Ly? irradiance to within 3% of the observed values. The difference between our computations and the Neckel & Labs data is 3% or less in the near-IR wavelengths at 8650 ?, and less than 1% in the red at 6080 ?. Near 4100 ? we overestimate the irradiance by 9%-19% because of opacity sources missing in our calculations. We also compute a solar cycle variability of 49% in the Ly? irradiance, which is very close to observed values. At wavelengths between 4100 ? and 1.6 ?m, we obtain spectral irradiance variations ranging from -0.06% to 0.46% in the visible?the higher values correspond to the presence of strong lines. The variability in the IR between 1.3 and 2.2 ?m is ~-0.15%.

Empirical models of the solar chromosphere show intense electron heating immediately above its temperature minimum. Mechanisms such as resistive dissipation and shock waves appear insufficient to account for the persistence and uniformity of this heating as inferred from both UV lines and continuum measurements. This paper further develops the theory of the Farley-Buneman Instability (FBI) which could contribute substantially to this heating. It expands upon the single ion theory presented by Fontenla (2005) by developing a multiple ion species approach that better models the diverse, metal-dominated ion plasma of the solar chromosphere. This analysis generates a linear dispersion relationship that predicts the critical electron drift velocity needed to trigger the instability. Using careful estimates of collision frequencies and a one-dimensional, semi-empirical model of the chromosphere, this new theory predicts that the instability may be triggered by velocities as low as 4 km s^-1, well below the neutral acoustic speed. In the Earth's ionosphere, the FBI occurs frequently in situations where the instability trigger speed significantly exceeds the neutral acoustic speed. From this, we expect neutral flows rising from the photosphere to have enough energy to easily create electric fields and electron Hall drifts with sufficient amplitude to make the FBI common in the chromosphere. If so, this process will provide a mechanism to convert neutral flow and turbulence energy into electron thermal energy in the quiet Sun., 22 pages, 4 figures, updated and published in ApJ

Extreme ultraviolet (EUV) radiations (10--117 nm) from host stars play important roles in the ionization, heating, and mass loss from exoplanet atmospheres. Together with the host star's Lyman-alpha and far-UV (117--170 nm) radiation, EUV radiation photodissociates important molecules, thereby changing the chemistry in exoplanet atmospheres. Since stellar EUV fluxes cannot now be measured and interstellar neutral hydrogen completely obscures stellar radiation between 40 and 91.2 nm, even for the nearest stars, we must estimate the unobservable EUV flux by indirect methods. New non-LTE semiempirical models of the solar chromosphere and corona and solar irradiance measurements show that the ratio of EUV flux in a variety of wavelength bands to the Lyman-alpha flux varies slowly with the Lyman-alpha flux and thus with the magnetic heating rate. This suggests and we confirm that solar EUV/Lyman-alpha flux ratios based on the models and observations are similar to the available 10--40 nm flux ratios observed with the EUVE satellite and the 91.2--117 nm flux observed with the FUSE satellite for F5 V--M5 V stars. We provide formulae for predicting EUV flux ratios based on the EUVE and FUSE stellar data and on the solar models, which are essential input for modelling the atmospheres of exoplanets., To appear in the Astrophysical Journal, 37 pages, 9 figures, 6 tables

Neutral nickel (Ni I) is abundant in the solar atmosphere and is one of the important elements that contribute to the emission and absorption of radiation in the spectral range between 1900 and 3900 A. Previously, the Solar Radiation Physical Modeling (SRPM) models of the solar atmosphere considered only few levels of this species. Here we improve the Ni I atomic model by taking into account 61 levels and 490 spectral lines. We compute the populations of these levels in full NLTE using the SRPM code and compare the resulting emerging spectrum with observations. The present atomic model improves significantly the calculation of the solar spectral irradiance at near-UV wavelengths that are important for Earth atmo spheric studies, and particularly for ozone chemistry., Accepted for publication in The Astrophysical Journal

The far ultraviolet (FUV) continuum flux is recognized as a very sensitive diag- nostic of the temperature structure of the Sun's lower chromosphere. Until now analysis of the available stellar FUV data has shown that solar-type stars must also have chromospheres, but quantitative analyses of stellar FUV continua require far higher quality spectra and comparison with new non-LTE chromosphere models. We present accurate far ultraviolet (FUV, 1150-1500^{\circ}) continuum flux measurements for solar-mass stars, made feasible by the high throughput and very low detector background of the Cosmic Origins Spectrograph (COS) on the Hubbble Space Telescope. We show that the continuum flux can be measured above the detector background even for the faintest star in our sample. We find a clear trend of increasing continuum brightness temperature at all FUV wavelengths with decreasing rotational period, which provides an important measure of magnetic heating rates in stellar chromospheres. Comparison with semiempirical solar flux models shows that the most rapidly rotating solar-mass stars have FUV continuum brightness temperatures similar to the brightest faculae seen on the Sun. The thermal structure of the brightest solar faculae therefore provides a first-order estimate of the thermal structure and heating rate for the most rapidly rotating solar-mass stars in our sample., Comment: 21 pages, 5 figures, to appear in ApJ

Solar Ly-alpha irradiance measurements from the SOLar STellar Irradiance Comparison Experiment (SOLSTICE) on the Upper Atmosphere Research Satellite (UARS) have been made since 1991 October with a spectral resolution of 0.1 nm. The uniqueness of the small molecular oxygen cross section near Ly-alpha permits the Ly-alpha radiation to penetrate much deeper into the atmosphere than the other emissions near Ly-alpha. We have taken advantage of this phenomenon by performing solar occultation experiments near the Ly-alpha to evaluate precisely the instrument scattered light contribution. After correcting for scattered light, the broad wings of the solar Ly-alpha line can be extracted out to 5 nm from line center with a typical accuracy of +/-20%. The variability in the Ly-alpha wings near 2 nm from line center is about one-half that of the Ly-alpha core emission, defined within 0.1 nm from line center. These Ly-alpha profile measurements are found to be consistent with the Skylab radiance measurements and theoretical models of the Ly-alpha line profiles computed using partial redistribution of photons in the source function.

We present detailed calculations and results for a model case of two-dimensional small distortions of static, current-free equilibrium around a null-line magnetic configuration. This setup has been studied for fast energy release in flares, but has not been studied before in the context of the magnetic energy buildup that precedes the flare. Our analytical results are new because they incluce gas pressure and provide explicit formulae for the expressions of all quantities (and especially the electric current) and energy storage as functions of the values of boundary conditions

International audience; The Spectral Irradiance Monitor (SIM) on board the NASA SORCE satellite (Solar Radiation and Climate Experiment) was launched on 25 January 2003 and has been making twice-daily measurements of solar variability in the 220 to 1630 nm range and daily measurements in the 1600 to 2400 nm range. This study presents preflight and postlaunch calibration activities of the SIM instrument and its flight spare components as well as in-flight comparisons with the ATLAS 3 composite spectrum (Atmospheric Laboratory for Applications and Science) in the ultraviolet (UV), visible, and near infrared (NIR) as well as comparisons with the SOLSTICE (Solar Stellar Irradiance Comparison Experiment) in the UV. In the 258 to 1350 nm range, the SIM agrees with ATLAS 3 with a fractional difference of −0.021±0.021 (k=1, estimated standard deviation) and with the additional corrections discussed herein the agreement improves to −0.008±0.021 (k=1). In the ultraviolet (220–307 nm) the agreement between all the instruments in this study is better than 5%, but fractional differences reveal other instrument- and calibration-related differences. In the 1350 to 2400 nm range the agreement between SIM and ATLAS 3 is about 8%, so these SIM data are corrected to agree with ATLAS 3 in this range.