Gérard Thuillier

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 DTM-2000 empirical thermosphere model with new data assimilation and constraints at lower boundary: accuracy and properties

Abstract The drag temperature model (DTM) is a semi-empirical model describing the temperature, density and composition of the Earths thermosphere. Its first version (Ann. Geophys. 34 (1978) 9) used direct measurements of exospheric temperature and atmospheric densities derived from satellite drag data. It has later been refined (J. Geodesy 72 (1998) 161). However, both models have their lower boundaries at 120 km , which are not constrained by observations. Consequently in the lower thermosphere, the modelled temperature and density structure is uncertain. For predicting satellite orbits in the lower thermosphere, more realistic density models are required. We present a new DTM model having the following improvements: (a) Temperature and its gradient at 120 km are represented in agreement with theory and observation, using incoherent scatter radar and satellite-borne interferometer data. (b) Atmosphere explorer (AE) data, which have not been assimilated in DTM-94, are used as they cover a complete solar activity cycle. (c) The Mg II index is used whenever possible to represent the solar UV and EUV emission intensity instead of the solar decimetre radio flux, since it is more representative of solar instantaneous chromospheric activity than F10.7 is. The basic DTM mathematical representation of temperature and composition is used, with, however, some additions and modifications to take into account the variations at 120 km altitude. The temperature modelling accuracy has improved by 5–8%, and there is no model bias as a function of solar activity. The oxygen and helium modelling has improved as well, and this is demonstrated by the estimated drag scale coefficients issued from precise orbit computation. The scaling coefficients estimated using DTM-2000 are systematically closer to unity than those resulting from employing DTM-94 and MSIS-86 in the orbit computation. The minor constituents (O2 and H) modelling is unchanged. The molecular nitrogen modelling is not improved, but this is, at least partly, caused by the poor data quality. Despite these improvements, semi-empirical thermosphere models still suffer of weaknesses. First, they assume a steady-state equilibrium which is not necessarily reached in any circumstances. Second, the basic process of atmospheric heating by EUV is assumed to be represented by some indices (Mg II, F10.7), while particle precipitation is represented by an index associated to a latitude without a longitude effect (Kp), although winds that transport energy suggest this effect. Third, the data accuracy and their incomplete geographical and temporal coverage are significant sources of uncertainty. Assimilation of a long time series of data, with a complete geographical coverage, and using the Mg II index will probably increase model accuracy from the present-day RMS of 19% to 10–15%. The complete CHAMP accelerometer data set may allow the achievement of that goal after 5 years of operations in 2005.

Validation of O(1S) wind measurements by WINDII: the WIND Imaging Interferometer on UARS

This paper describes the current state of the validation of wind measurements by the wind imaging interferometer (WINDII) in the O(1S) emission. Most data refer to the 90-to-110-km region. Measurements from orbit are compared with winds derived from ground-based observations using optical interferometers, MF radars and the European Incoherent-Scatter radar (EISCAT) during overpasses of the WINDII fields of view. Although the data from individual passes do not always agree well, the averages indicate good agreement for the zero reference between the winds measured on the ground and those obtained from orbit. A comparison with winds measured by the high resolution Doppler imager (HRDI) instrument on UARS has also been made, with excellent results. With one exception the WINDII zero wind reference agrees with all external measurement methods to within 10 m s−1 at the present time. The exception is the MF radar winds, which show large station-to-station differences. The subject of WINDII comparisons with MF radar winds requires further study. The thermospheric O(1S) emission region is less amenable to validation, but comparisons with EISCAT radar data give excellent agreement at 170 km. A zero wind calibration has been obtained for the O(1D) emission by comparing its averaged phase with that for O(1S) on several days when alternating 1D/1S measurements were made. Several other aspects of the WINDII performance have been studied using data from on-orbit measurements. These concern the instruments phase stability, its pointing, its responsivity, the phase distribution in the fields of view, and the behavior of two of the interference filters. In some cases, small adjustments have been made to the characterization database used to analyze the atmospheric data. In general, the WINDII characteristics have remained very stable during the mission to date. A discussion of measurement errors is included in the paper. Further study of the instrument performance may bring improvement, but the utimate limitation for wind validation appears to be atmospheric variability and this needs to be better understood.

Longitudinal structure in atomic oxygen concentrations observed with WINDII on UARS

WINDII, the Wind Imaging Interferometer on the Upper Atmosphere Research Satellite, began atmospheric observations on September 28, 1991 and since then has been collecting data on winds, temperatures and emissions rates from atomic, molecular and ionized oxygen species, as well as hydroxyl. The validation of winds and temperatures is not yet complete, and scientific interpretation has barely begun, but the dominant characteristic of these data so far is the remarkable structure in the emission rate from the excited species produced by the recombination of atomic oxygen. The latitudinal and temporal variability has been noted before by many others. In this preliminary report on WINDII results we draw attention to the dramatic longitudinal variations of planetary wave character in atomic oxygen concentration, as reflected in the OI 557.7 nm emission, and to similar variations seen in the Meinel hydroxyl band emission.

Solar Irradiance Reference Spectra

The solar spectrum is a key input for the study of the planetary atmospheres. It allows the understanding through theoretical modeling of the atmospheric properties (e.g., composition and variability). Furthermore, a reference model is useful for the preparation of instruments and platforms to be operated in space. New composite solar irradiance spectra are formed from 0.1 to 2400 nm using recent measurements for two distinct time periods during solar cycle 22. These two time periods correspond to the activity levels encountered during the ATmospheric Laboratory for Applications and Science (ATLAS) Space Shuttle missions which were moderately high (ATLAS 1, March 1992) and low (ATLAS 3, November 1994). The two reference times span approximately half of the total solar cycle amplitude in terms of the Mg II and F10.7 indices. The accuracy of the two presented spectra varies from 40% in the X-ray range to a mean of 3% in the UV, visible, and near IR ranges. After integration over all wavelengths, a comparison with the total solar irradiance measured at the same time shows an agreement of the order of 1%.

The Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite: A 20 year perspective

The Wind Imaging Interferometer (WINDII) was launched on the NASAs Upper Atmosphere Research Satellite on 12 September 1991 and operated until 2003. Its role in the mission was to measure vector winds in the Earths atmosphere from 80 to 110 km, but its measurements extended to nearly 300 km. The approach employed was to measure Doppler shifts from a suite of visible region airglow lines emitted over this altitude range. These included atomic oxygen O(1S) and O(1D) lines, as well as lines in the OH Meinel (8,3) and O2 Atmospheric (0,0) bands. The instrument employed was a Doppler Michelson Interferometer (DMI) that measured the Doppler shift as a phase shift of the cosinusoidal interferogram generated by single airglow lines. An extensive validation program was conducted after launch to confirm the accuracy of the measurements. The dominant wind field, the first one observed by WINDII, was that of the migrating diurnal tide at the equator. The overall most notable WINDII contribution followed from this; determining the influence of dynamics on the transport of atmospheric species. Currently, non-migrating tides are being studied in the thermosphere at both equatorial and high latitudes. Other aspects investigated included solar and geomagnetic influences, temperatures from atmospheric scale heights, nitric oxide concentrations and the occurrence of polar mesospheric clouds. The results of these observations are reviewed from a perspective of twenty years. A future perspective is then projected, involving more recently developed concepts. It is intended that this description will be helpful for those planning future missions.

The Visible Solar Spectral Irradiance from 350 to 850 nm as Measured by the Solspec Spectrometer During the Atlas I Mission

The SOLSPEC instrument has been built to carry out solar spectral irradiance measurements from 200 to 3000 nm. It consists of three spectrometers designed to measure the solar spectral irradiance in ultraviolet, visible, and infrared domains. It flew with the ATLAS I mission in March 1992. This paper is dedicated to the visible part of the solar spectrum. Comparisons with recent data are shown and differences below 450 nm are discussed.

Observations of the solar irradiance in the 200–350 nm interval during the ATLAS‐1 Mission: A comparison among three sets of measurements‐SSBUV, SOLSPEC, and SUSIM

The SOLSPEC, SSBUV, and SUSIM spectrometers simultaneously observed the solar spectral irradiance during the ATLAS-1 mission flown on board the Space Shuttle Atlantis in March 1992. The three instruments use different methods and means of absolute calibration and were each calibrated preflight and postflight. The three data sets are reported from 200 to 350 nm at 1.1 nm resolution. The method of comparing the three independent data sets is discussed. The importance of a common, precise wavelength scale is shown when comparing the data in wavelength regions of strong Fraunhofer lines. The agreement among the solar irradiance measurements is better than ±5%. The fact that the calibrations of the three instruments were based on three independent standards provides confidence that the absolute solar spectral irradiance in the range 200–350 nm is now known with an accuracy better than ±5%. The mean ATLAS-1 solar spectrum is compared with simultaneous solar observations from the UARS SOLSTICE and UARS SUSIM instruments. The two mean solar spectra agree to within ±3%.

The meridional wind component of the thermospheric tide observed by WINDII on UARS

Thermospheric meridional winds measured by the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS) are analyzed for migrating solar tides for March/April 1993. Tidal signatures are extracted by zonally averaging and binning the data in equally spaced intervals of local time, latitude and altitude. An examination of the binned data reveals vertically propagating tides in the lower thermosphere with the diurnal tide dominating at low latitudes. Above 120 km evidence of vertical propagation ceases and the vertical gradients are significantly reduced. In regions where both daytime and nighttime data are available Fourier decomposition of the binned winds is used to extract the diurnal component of the tide. This analysis reveals that the propagating diurnal tide attains a maximum amplitude of 65 m/s at 20°N and 20°S at 95 km and exhibits a vertical wavelength of about 25 km.

Ultraviolet solar irradiance measurement from 200 to 358 nm during spacelab 1 mission

The paper presents the results obtained from the UV-spectrometer of the ‘Solar Spectrum Experiment’ during the Spacelab 1 mission in December 1983. The irradiance data concern 492 passbands, which are located between 200 and 358 nm at almost equidistant wavelengths separated by about 0.3 nm. The passbands have a well-defined, bell-shaped profile with a full width at half maximum of about 1.3 nm. The data, which have an error budget between 4 and 5%, agree closely with the spectral distributions observed by Heath (1980) and Mentall et al. (1981) and confirm that the solar irradiance and the fluxes of Sun-like stars show about the same spectral distribution down to at least 240 nm.