G. M. Lawrence

THE TOTAL IRRADIANCE MONITOR (TIM): SCIENCE RESULTS

The solar observations from the Total Irradiance Monitor (TIM) are discussed since the SOlar Radiation and Climate Experiment (SORCE) launch in January 2003. The TIM measurements clearly show the background disk-integrated solar oscillations of generally less than 50 parts per million (ppm) amplitude over the ∼2 ppm instrument noise level. The total solar irradiance (TSI) from the TIM is about 1361 W/m2, or 4–5 W/m2 lower than that measured by other current TSI instruments. This difference is not considered an instrument or calibration error. Comparisons with other instruments show excellent agreement of solar variability on a relative scale. The TIM observed the Sun during the extreme activity period extending from late October to early November 2003. During this period, the instrument recorded both the largest short-term decrease in the 25-year TSI record and also the first definitive detection of a solar flare in TSI, from which an integrated energy of roughly (6 ± 3) × 1032 ergs from the 28 October 2003 X17 flare is estimated. The TIM has also recorded two planets transiting the Sun, although only the Venus transit on 8 June 2004 was definitive.

Photofragment spectroscopy of ozone in the uv region 270–310 nm and at 600 nm

Both O(3P) and O(1D) atom fragments are observed in the photofragment spectroscopy of O3 in the Hartley band absorption region 270–300 nm. The quantum yield for O(3P) is 0.1 at 274 nm. The dissociation partner for O(3P) is O2(X3Σ−g) and that for O(1D) is O2(a 1Δg). The O2(X 3Σ−g) fragment is populated in a range of vibrational levels, v?0−10. For the O2(a 1Δg) fragment, all energetically accessible vibrational levels are populated at each energy of bombardment. In the Chappuis bands at 600 nm the O2(X 3Σ−g) photofragment is produced principally with v=0, 1. Photofragment angular distributions are measured in the uv for both O atom dissociation products at each wavelength of bombardment. A theoretical angular distribution for O(1D) is derived and the resultant prediction is in good agreement with the experimental results.

TIMED Solar EUV experiment

Abstract The Solar EUV Experiment (SEE) on the NASA Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) mission will measure the solar vacuum ultraviolet (VUV) spectral irradiance from 1 to 195 nm. To cover this wide spectral range two different types of instruments are used: a grating spectrograph for spectra above 25 nm and a set of silicon photodiodes with thin film filters for the soft x-ray (XUV) below 35 nm. Redundant channels of the spectrograph and XUV photodiodes provide in-flight calibration checks on the time scale of a week, and annual rocket underflight measurements provide absolute calibration checks traceable to radiometric standards. Both types of instruments have been developed and flight proven as part of a NASA solar EUV irradiance rocket experiment. The SEE instrument is currently on the NASA TIMED spacecraft at the Johns Hopkins University (JHU) Applied Physics Laboratory (APL). The TIMED launch is planned for October 2000 as a two year mission.

The yield of N/2D/ atoms in the dissociative recombination of NO/+/

The quantum yield or branching ratio of N(2D) atoms formed in the reaction e+NO+→N+O was measured to be 76±6%. Photoionization of buffered nitric oxide by a flash lamp was studied using time resolved atomic absorption. Atoms were produced both by direct photodissociation and by dissociative recombination, and these two effects were separated by means of SF6 as an electron scavenger.

TIMED solar EUV experiment

The solar EUV experiment (SEE) selected for the NASA Thermosphere, Ionosphere, and Mesosphere Energetics and Dynamics mission will measure the solar vacuum UV (VUV) spectral irradiance from 0.1 to 200 nm. To cover this wide spectral range two different types of instruments are used: grating spectrograph for spectra above 25 nm and a set of silicon soft x-ray (XUV) photodiodes with thin film filters for below 30 nm. Redundant channels of the spectrograph and XUV photodiodes provide in-flight calibration checks on the time scale of a week, and annual rocket underflight measurements provide absolute calibration checks traceable to radiometric standards. Both types of instrument have been developed and flight proven as part of a NASA solar EUV irradiance rocket experiment.

Mesospheric ozone depletion during the Solar Proton Event of July 13, 1982 Part I Measurement

The near infrared spectrometer and the ultraviolet spectrometer on the Solar Mesosphere Explorer (SME) observed the ozone density as a function of latitude and altitude during the solar proton event of July 13, 1982. Airglow at 1.27 µm was observed at the earths limb. The altitude profiles of the emission were inverted providing ozone densities. The ozone densities observed showed a clear decrease during the event. The maximum depletion seen was 70%. The decrease was observed in the northern high latitudes at mesospheric altitudes. The decrease was very short lived, lasting less than a day.

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.

Rocket-borne instrument with a high-resolution microchannel plate detector for planetary UV spectroscopy

A telescope-spectrograph employing a photon-counting microchannel plate (MCP)-CODACON detector has been built, tested, and flown on a sounding rocket. The detector uses a curved-channel MCP proximity focused onto a coded anode array of 1024 channels spaced 25.4-mm center to center. High quantum efficiency is obtained by depositing a cesium iodide photocathode on the front surface of the MCP. The instrument has obtained an ultraviolet (1500-1800-A) spectrum of Jupiter with a spectral resolution of 8 A, which is higher than that of any previously reported observation in this wavelength range.

Scientific objectives of the Solar Mesosphere Explorer mission

The 1981–82 Solar Mesosphere Explorer (SME) mission is described. The SME experiment will provide a comprehensive study of mesospheric ozone and the processes which form and destroy it. Five instruments will be carried on the spinning spacecraft to measure the ozone density and its altitude distribution from 30 to 80 km, monitor the incoming solar ultraviolet radiation, and measure other atmospheric constituent which affect ozone. The polar-orbiting spacecraft will be placed into a 3pm-3 am Sun-synchronous orbit. The atmospheric measurements will scan the Earths limb and measure: (1) the mesospheric and stratospheric ozone density distribution by inversion of Rayleigh-scattered ultraviolet limb radiance, and the thermal emission from ozone at 9.6 μm; (2) the water vapor density distribution by inversion of thermal emission at 6.3 μm; (3) the ozone photolysis rate by inversion of the O2(1Δg) 1.27 μm limb radiance; (4) the temperature profile by a combination of narrow-band and wide-band measurements of the 15 μm thermal emission by CO2; and, (5) theNO2 density distribution by inversion of Rayleighscattered limb radiance at 0.439 μm. The solar ultraviolet monitor will measure both the 0.2–0.31 μm spectral region and the Lyman-alpha (0.1216 μm) contribution to the solar irradiance. This combination of measurements will provide a rigorous test of the photochemical equilibrium theory of the mesospheric oxygen-hydrogen system, will determine what changes occur in the ozone distribution as a result of changes in the incoming solar radiation, and will detect changes that may occur as a result of meteorological disturbances.

The Total Irradiance Monitor (TIM): Instrument Calibration

The calibrations of the SORCE Total Irradiance Monitor (TIM) are detailed and compared against the designed uncertainty budget. Several primary calibrations were accomplished in the laboratory before launch, including the aperture area, applied radiometer power, and radiometer absorption efficiency. Other parameters are calibrated or tracked on orbit, including the electronic servo system gain, the radiometer sensitivity to background thermal emission, and the degradation of radiometer efficiency. The as-designed uncertainty budget is refined with knowledge from the on-orbit performance.