Martin Snow

Solar-Stellar Irradiance Comparison Experiment II (SOLSTICE II): Pre-Launch and On-Orbit Calibrations

The Solar–Stellar Irradiance Comparison Experiment {II (SOLSTICE {II), aboard the Solar Radiation and Climate Experiment (SORCE) spacecraft, consists of a pair of identical scanning grating monochromators, which have the capability to observe both solar spectral irradiance and stellar spectral irradiance using a single optical system. The SOLSTICE science objectives are to measure solar spectral irradiance from 115 to 320 nm with a spectral resolution of 1 nm, a cadence of 6 h, and an accuracy of 5%, to determine its variability with a long-term relative accuracy of 0.5% per year during a 5-year nominal mission, and to determine the ratio of solar irradiance to that of an ensemble of bright B and A stars to an accuracy of 2%. Those objectives are met by calibrating instrument radiometric sensitivity before launch using the Synchrotron Ultraviolet Radiation Facility at the National Institute for Standards and Technology in Gaithersburg, Maryland. During orbital operations irradiance measurements from an ensemble of bright, stable, main-sequence B and A stars are used to track instrument sensitivity. SORCE was launched on 25 January 2003. After spacecraft and instrument check out, SOLSTICE {II first observed a series of three stars to establish an on-orbit performance baseline. Since 6 March 2003, both instruments have been making daily measurements of both the Sun and stars. This paper describes the pre-flight and in-flight calibration and characterization measurements that are required to achieve the SOLSTICE science objectives and compares early SOLSTICE{II measurements of both solar and stellar irradiance with those obtained by SOLSTICE {I on the Upper Atmosphere Research Satellite.

A New Catalog of Ultraviolet Stellar Spectra for Calibration

The SOLar-STellar Irradiance Comparison Experiment (SOLSTICE) on the SOlar Radiation and Climate Experiment (SORCE) observes both the Sun and stars in the ultraviolet (115–300 nm). Prior to launch, it was calibrated at the SURF-III synchrotron. Spectra from the International Ultraviolet Explorer (IUE) corrected to the white dwarf flux scale are in good agreement with SOLSTICE observations, validating the two completely independent methods of calibration. Measurements of stars in the SOLSTICE catalog are then used to transfer this calibration to the SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) instrument. We describe the steps used to calculate the effective area for SPICAM to calibrate its stellar observations. Since only a handful of stars in the IUE archive have been converted to the white dwarf scale and many of them are relatively faint, the SOLSTICE catalog of bright stars can be an extremely useful resource for inflight calibration of ultraviolet spectrographs.

Midlatitude atmospheric OH response to the most recent 11-y solar cycle

The hydroxyl radical (OH) plays an important role in middle atmospheric photochemistry, particularly in ozone (O3) chemistry. Because it is mainly produced through photolysis and has a short chemical lifetime, OH is expected to show rapid responses to solar forcing [e.g., the 11-y solar cycle (SC)], resulting in variabilities in related middle atmospheric O3 chemistry. Here, we present an effort to investigate such OH variability using long-term observations (from space and the surface) and model simulations. Ground-based measurements and data from the Microwave Limb Sounder on the National Aeronautics and Space Administration’s Aura satellite suggest an ∼7–10% decrease in OH column abundance from solar maximum to solar minimum that is highly correlated with changes in total solar irradiance, solar Mg-II index, and Lyman-α index during SC 23. However, model simulations using a commonly accepted solar UV variability parameterization give much smaller OH variability (∼3%). Although this discrepancy could result partially from the limitations in our current understanding of middle atmospheric chemistry, recently published solar spectral irradiance data from the Solar Radiation and Climate Experiment suggest a solar UV variability that is much larger than previously believed. With a solar forcing derived from the Solar Radiation and Climate Experiment data, modeled OH variability (∼6–7%) agrees much better with observations. Model simulations reveal the detailed chemical mechanisms, suggesting that such OH variability and the corresponding catalytic chemistry may dominate the O3 SC signal in the upper stratosphere. Continuing measurements through SC 24 are required to understand this OH variability and its impacts on O3 further.

The Extreme Ultraviolet Sensor (EUVS) for GOES-R

Recognizing that the solar extreme ultraviolet (EUV) irradiance is an important driver of space weather, the National Oceanic and Atmospheric Administration (NOAA) has added an Extreme Ultraviolet Sensor (EUVS) to its Geostationary Operational Environmental Satellite (GOES) program, starting with the recently launched GOES-N, now designated GOES-13. For the GOES-R series (slated for launch starting in 2015) , the EUVS measurement concept has been redesigned. Instead of measuring broad bands spanning the EUV, the GOES-R EUVS will measure specific solar emissions representative of coronal, transition region, and chromospheric variability. From these measurements, the geo-effective EUV wavelength range from 5 to 127 nm can be reconstructed using models based on spectrally resolved measurements gathered over the full range of solar variability. An overview of the GOES-R EUVS design is presented. A description of the in-flight degradation tracking utilizing similar measurement and modeling techniques used to generate the EUV irradiance is also provided.

Cross-calibration of far UV spectra of solar system objects and the heliosphere

Distribution of interstellar H atoms in the heliosphere and backscattered solar Lyman-alpha.- Solar parameters for modeling interplanetary background.- 30 Years of Interplanetary Background Data: A Global View.- Lyman-alpha Models from Messenger and SOHO data.- New Horizons Cruise Observations of Lyalpha Emissions from the Interplanetary Medium.- A New Catalog of Ultraviolet Stellar Spectra.- Absolute Ultraviolet Irradiance of the Moon from SORCE SOLSTICE.- Lyman-alpha Observations of Comet Holmes from SORCE SOLSTICE and SOHO SWAN.

The disconnection events of 1986 April 13-18 and the cessation of plasma tail activity in comet Halley in 1986 May

The disconnection events (DEs) in the 1986 April 13-18 time period appear to be the last major ones of comet Halleys 1986 apparition. We determined the time of the first disconnection event in this sequence, April 13.9±0.1 days, from kinematic analysis of photographic images. We show here that at the time of this DE, comet Halley had just crossed a magnetic field sector boundary and was in a moderate-density solar wind region with a speed of approximately 350 km s -1 . A sequence of DEs began approximately 1 day after the April 13.9 DE and was easily visible on April 16 and 17. Based on IMP 8 data, we show that this sequence was associated with a complex magnetic structure of the solar wind, showing multiple reversals

Observations of 3C 273 with the Goddard High Resolution Spectrograph on the Hubble Space Telescope

The observations of the quasar 3C 273 taken with the Goddard High Resolution Spectrograph in 1991 February are presented here. We have included both the reduced raw data, and smoothed and deconvolved spectra. Also, a list of observed absorption lines is presented. The data comprise 11 spectra, including 1 low resolution observation and 10 medium resolution observations. The wavelength region covered ranged from about 1150 to 2820 A, but was not all inclusive. The procedures used to obtain and reduce the data, including corrections for fixed pattern noise, compensation for the effects of spherical aberration in the HST primary mirror, and objective detection of weak absorption lines, are described. We also have included a short discussion on the detection of galactic Ni II and Virgo cluster metal lines

Lyman-α Models for LRO LAMP from MESSENGER MASCS and SOHO SWAN Data

From models of the interplanetary Lyman-α glow derived from observations by the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) interplanetary Lyman-α data obtained in 2009–2011 on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft mission, daily all-sky Lyman-α maps were generated for use by the Lunar Reconnaissance Orbiter (LRO) LAMP Lyman-Alpha Mapping Project (LAMP) experiment. These models were then compared with Solar and Heliospheric Observatory (SOHO) Solar Wind ANistropy (SWAN) Lyman-α maps when available. Although the empirical agreement across the sky between the scaled model and the SWAN maps is adequate for LAMP mapping purposes, the model brightness values best agree with the SWAN values in 2008 and 2009. SWAN’s observations show a systematic decline in 2010 and 2011 relative to the model. It is not clear if the decline represents a failure of the model or a decline in sensitivity in SWAN in 2010 and 2011. MESSENGER MASCS and SOHO SWAN Lyman-α calibrations systematically differ in comparison with the model, with MASCS reporting Lyman-α values some 30 % lower than SWAN.

EUVS-C: the measurement of the magnesium II index for GOES-R EXIS

The EUV and X-ray Irradiance Sensors (EXIS) on the upcoming GOES-R mission will include a spectrograph to measure the Magnesium II doublet at 280 nm (channel C). The ratio of the core of this spectral feature to the line wings is the well-known Mg II index. This ratio is often used as a proxy for chromospheric activity, since changes in the index are highly correlated with changes in other chromospheric emission lines. As a ratio measurement, the Mg II index is relatively insensitive to instrumental effects. The A and B channels of the Extreme UltraViolet Sensor (EUVS) will make use of this fact and use the Mg II index measured by channel C in their degradation correction. EUVS C channel has sufficient spectral resolution and sampling to measure the Mg II index with high precision and will make this measurement at better than 30 s time cadence. This paper describes the design and measurement requirements of the C channel.

High Signal-to-Noise Ratio Observations of Weak Interstellar Absorption Lines Towards XI Ophiuchi With the Goddard High-Resolution Spectrograph Aboard the Hubble Space Telescope

We present an atlas and tabulation of weak interstellar absorption lines in the ultraviolet spectrum of £ Ophiuchi in four selected wavelength regions observed with the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope. The signal-to-noise ratio ranges from 150 to nearly 400, and the spectral resolving power exceeds 20 000, allowing 2cr detections of features as weak as Wx=0.8 mÂ. We report positive measurements of two lines of OH, and weak detections of P l, Tl n, and N v. Upper limits of WX<1 m are found for the molecules H20, HC1, SiO, NO , and CH2. Similar limits are found for heavy elements Te n, Co II, and Sb IL Three lines are present in our spectrum for which we have no identifications. They are found at wavelengths of \= 1229.84, 1313.98, and 1314.23 Â. However, none of the features reported by previous authors as unidentified absorption lines in the wavelength regions we have observed are present in our data.