J. W. Harder

SEMIEMPIRICAL MODELS OF THE SOLAR ATMOSPHERE. III. SET OF NON-LTE MODELS FOR FAR-ULTRAVIOLET/EXTREME-ULTRAVIOLET IRRADIANCE COMPUTATION

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.

Semiempirical Models of the Solar Atmosphere. I. The Quiet- and Active Sun Photosphere at Moderate Resolution

Inthispaperwestudyandmodifyprevioussemiempiricalmodelsofthesolarphotosphereasobservedatmoderate spatial and temporal resolution (� 3 00 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 finestructure andp-modevariations. Weuse theobservedlimbdarkeningin therange0.3‐2.4 � m,as wellasthe absolute intensities and details of the spectral continua and lines in this range, to validate and adjust the models. Using the methoddescribedinapreviouspaper,wecomputetheemergentradiationfromourmodelsinfulldetailforthevisible and IR continuum and the lines in the interval 0.3‐5 � mf or 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.

The Signature of Solar Activity in the Infrared Spectral Irradiance

The effects of solar activity on the spectral irradiance have been studied using atmospheric semiempirical models developed from observations of the various surface features observed on the Sun. From these models, it has been the long-standing belief that the contributions of active regions to solar irradiance at wavelengths in the range of 1.2-3 μm is negative; that is, their net effect reduces the overall solar irradiance at these wavelengths by a small amount. For verifying the validity of the current modeling, we use the observed plage areas to compute the solar irradiance variations at two bands (centered at 0.516 and 1.553 μm wavelength). We compare in detail the predictions of the models by Fontenla et al. with measurements of the solar spectral irradiance variations obtained by the Spectral Irradiance Monitor instrument aboard the Solar Radiation and Climate Experiment spacecraft. The data comparison extends over a 6 month period in 2003 that covers several solar rotations. The comparison indicates that the variations in the short wavelength display good agreement between models and observations but also that the current models of IR spectral irradiance are inaccurate at the long wavelength. This disagreement in the IR may be due to the fact that, contrary to the current model assumptions, the presence of active regions on the disk increases the spectral irradiance at all wavelengths, even near 1.6 μm. Consequently, the modeling of solar spectral irradiance at wavelengths in the range around 1.6 μm has to be revised to match the new observations.

The influence of spectral solar irradiance data on stratospheric heating rates during the 11 year solar cycle

Heating rate calculations with the FUBRad shortwave (SW) radiation parameterization have been performed to examine the effect of prescribed spectral solar fluxes from the NRLSSI, MPS and IUP data sets on SW heating rates over the 11 year solar cycle 22. The corresponding temperature response is derived from perpetual January General Circulation Model (GCM) simulations with prescribed ozone concentrations. The different solar flux input data sets induce clear differences in SW heating rates at solar minimum, with the established NRLSSI data set showing the smallest solar heating rates. The stronger SW heating in the middle and upper stratosphere in the MPS data warms the summer upper stratosphere by 2 K. Over the solar cycle, SW heating rate differences vary up to 40% between the irradiance data sets, but do not result in a significant change of the solar temperature signal. Lower solar fluxes in the newer SIM data lead to a significantly cooler stratosphere and mesosphere when compared to NRLSSI data for 2007. Changes in SW heating from 2004 to 2007 are however up to six times stronger than for the NRLSSI data. Key Points: - Solar minimum and solar cycle differences in SW heating rates and temperature - Comparison of three spectral solar input data sets for solar cycle 22 - Comparison of the newly compiled SORCE-data with the commonly used NRLSSI-data

Solar Spectral Irradiance Monitor (SIM)

The Spectral Irradiance Monitor (SIM) will measure the solar spectral irradiance between 0.2 ?m to 2.0 ?m to a relative standard uncertainty of 3 ? 10?4 (1 s) and with a noise level of <1 ? 10?4. The instrument will be launched in 2002 on the SORCE mission of the Earth Observing System (EOS). Sunlight passes through a 0.3 mm ? 7 mm entrance slit and is modulated by a shutter at 0.1 Hz. Light then travels 400 mm to a Fery prism that disperses and focuses the light at the focal plane containing five separate exit slits, with an electrical-substitution radiometer (ESR) and four photodiode detectors. The ESR consists of two back-to-back 1 mm ? 10 mm diamond bolometers blackened with nickel phosphorus. The bolometer bridge, electrical power replacement, ESR temperature regulation, and prism rotation are under digital signal processor control. The measurement equation and the uncertainty budget for the SIM instrument are presented along with plans for the pre-flight characterization of the instrument.

Intercomparison of SCIAMACHY and SIM vis-IR irradiance over several solar rotational timescales

The two satellite spectrometers SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) aboard ENVISAT (Environmental Satellite), and SIM (Spectral Irradiance Monitor) aboard SORCE (Solar Radiation and Climate Experiment) observe since 2002 and 2003, respectively, daily solar spectral irradiance (SSI) not only in UV but extending to the visible and near- infrared (vis-NIR) regions. In this work, we intercompare (1) spectra and (2) timeseries of SSI measurements from SCIAMACHY and SIM. In (1) same-day (April 21, 2004) SSI measurements from these two instruments are compared to reference spectra from ground (new Kurucz), high-altitude (Hall and Anderson, Neckel and Labs, and Wehrli composite), and space (SOLSPEC/ATLAS 3, and SUSIM/UARS). In (2) timeseries of measurements (July 3 to August 21, 2004) covering several solar rotations in 2004 are compared to VIRGO sunphotometers (SPM) aboard SOHO. In general, SCIAMACHY and SIM are in agreement to within 4% over the common spectral domain and with respect to the other reference data. Apart from SSI and its variability, we integrate SSI over selected wavelength intervals and compare qualitatively to total solar irradiance (TSI) variability from PMOD/WRC and TIM/SORCE. Timeseries of integrated SSI in the vis (400–700 nm), NIR (700–1600 nm), and UV-vis-NIR (240–1600 nm) bands are compared. The overall rise and fall of integrated SCIAMACHY and SIM irradiances over several solar rotations are in good agreement and agree in most cases qualitatively with TSI variations in the visible and near IR. The application of White Light Source (WLS) corrections brings SCIAMACHY irradiances in closer agreement with SIM. Since WLS is also degrading with time, the WLS lamp ratios cannot be used for SSI degradation corrections after 2004.

Calibration of the total irradiance monitor

Launching on the SORCE (SOlar Radiation and Climate Experiment), the total irradiance monitor (TIM) will measure the total solar irradiance to a relative standard uncertainty of 10−4 and a noise level of 2×10−6 each 400 s. Because of the outstanding brightness, uniformity, stability and collimation of the Sun, this ambient temperature primary radiometer achieves an absolute accuracy comparable to that of a cryogenic radiometer. The major paradigm shift from previous solar radiometers is the use of phase-sensitive detection at the shutter fundamental. We describe the equivalence between replacement power and radiant power as a complex number, the ratio of the (complex) thermal impedances. Our aperture measurements are provided by NIST, with added corrections for diffraction and scattering. We measure the cavity reflection losses versus wavelength using laser scans. We have verified methods for measuring, in flight, the servo gain, cavity reflection changes and shutter-modulated infrared offset (dark signal).

Investigation of Solar Irradiance Variations and Their Impact on Middle Atmospheric Ozone

The satellite spectrometer SCIAMACHY aboard ENVISAT is a unique instrument that covers at a moderately high spectral resolution the entire optical range from the near UV (230 nm) to the near IR (2.4 μm) with some gaps above 1.7 μ. This broad spectral range allows not only the retrieval of several atmospheric trace gases (among them ozone), cloud and aerosol parameters, but also regular daily measurements of the spectral solar irradiance (SSI) with an unprecedented spectral coverage. The following studies were carried out with irradiance and ozone data from SCIAMACHY: a) SCIAMACHY SSI was compared to other solar data from space and ground as well as with SIM/SORCE (Solar Irradiance Monitor, the only other satellite instrument daily measuring the visible and near IR), in order to verify the quality of the SCIAMACHY measurements, b) an empirical solar proxy model, in short the SCIA proxy model, was developed that permits expressing the SCIAMACHY SSI variations by fitting solar proxies for faculae brightening and sunspot darkening, which then allows investigation of solar variability on time scales beyond the instrument life time, e.g. 11-year solar cycle, c) solar cycle SSI variations derived from empirical models (Lean2000, SATIRE, SCIA proxy) and different observations (SBUV composite, SUSIM) were compared for the three most recent solar cycles 21–23, and d) SCIAMACHY ozone limb profiles were analyzed to derive signatures of the 27-day solar rotation on stratospheric ozone. Our studies were complemented by investigations of daytime variations in mesospheric ozone (here data from SABER/TIMED), which were compared to results from the HAMMONIA chemistry climate model.

Hyperspectral Solar Spectral Measurements and Applications

Measurements of hyperspectral solar irradiance from aircraft and satellite are applied to a variety of cloud and aerosol remote sensing, and radiative energy budget applications.