Lawrence C. Puga

Detection and parameterization of variations in solar mid‐ and near‐ultraviolet radiation (200–400 nm)

Nimbus 7 and Solar Stellar Irradiance Comparison Experiment (SOLSTICE) spacecraft measurements of solar irradiance both exhibit variability at mid (200–300 nm) and near (300–400 nm) ultraviolet (UV) wavelengths that are attributable to the Suns 27-day solar rotation, even though instrument sensitivity drifts obscure longer-term, 11-year cycle variations. Competing influences of dark sunspots and bright faculae are the dominant causes of this rotational modulation. Parameterizations of these influences using a newly developed UV sunspot darkening index and the Mg index facular proxy replicate the rotational modulation detected in both the broadband Nimbus 7 filter data (275–360 nm and 300–410 nm) and in SOLSTICE 1-nm spectra from 200 to 400 nm. Assuming that these rotational modulation influences scale linearly over the solar cycle, long–term databases of sunspot and global facular proxies permit estimation of 11-year cycle amplitudes of the mid– and near–UV solar spectrum, unmeasured at wavelengths longward of 300 nm because of insufficient long-term repeatability (relative accuracy) of state-of-the-art solar radiometers at these wavelengths. Reconstructions of UV irradiances throughout the 11-year solar cycle indicate variabilities of 0.173 W/m2 (1.1%) in the integrated radiation from 200 to 300 nm and 0.24 W/m2 (0.25%) in radiation from 300 to 400 nm. These two UV bands thus contribute about 13% and 18%, respectively, to the 1.34 W/m2 (0.1%) total (spectrally integrated) radiative output solar cycle. The parameterizations allow customization of UV irradiance time series for specific wavelength bands required as inputs to general circulation model simulations of solar cycle forcing of global climate change, and have practical implications regarding the long-term repeatability required for future solar monitoring.

The Mg II index: A proxy for solar EUV

This paper shows that the Mg II core-to-wing ratio is a better proxy for Solar Extreme Ultraviolet (EUV) radiation, between 25 and 35 nm than is the F10.7 index. The He II 30.4 nm solar emission, by itself, is an important source of energy for the upper atmosphere. We will compare the NOAA Mg II Index and the F10.7 Index to the He II 30.4 data taken with the CELIAS/Solar EUV Monitor (SEM) on the Solar and Helospheric Observatory (SOHO).

The NOAA Mg II core-to-wing solar index: Construction of a 20-year time series of chromospheric variability from multiple satellites

The Mg II core-to-wing ratio is a measure of the amplitude of the chromospheric Mg II ion emission at 280 nm. It has been shown to have strong correlation with solar UV (150 – 400 nm) irradiance and other measures of solar activity and is relatively insensitive to errors introduced by variations in instrument sensitivity. For these reasons, the Mg II core-to-wing ratio has been used as a proxy for solar activity. Observations of the Mg II ratio started with the Solar Backscattered Ultraviolet spectrograph (SBUV) instrument on the Nimbus 7 spacecraft in November 1978. Since then, SBUV and SBUV2 instruments have flown regularly on the NOAA series of polar orbiting spacecraft, providing a nearly continuous set of daily solar observations. Measurements of the solar UV irradiance made from NOAA satellites ended in February 1998 when the NOAA9 satellite failed. Because it had been operating for nearly 12 years (at least 7 years beyond its expected lifetime), the NOAA 9 SBUV2 showed significant degradation near the end of its life. The most recent data required significantly more attention to extract the Mg II core-to-wing ratio. There have also been data gaps that require the augmentation of data from other sources. In particular, data from the UARS Solar Stellar Irradiance Comparison Experiment (SOLSTICE) instrument was used to bridge a 5 month gap in 1995. In this report we combine data from four different instruments to create the longest and most complete record of the Mg II core-to-wing ratio to date. We describe a modified analysis procedure that has allowed us to extend the Mg II index up to the beginning of 1998. Finally, we compare the time series of the Mg II ratio to similar measurements from the Solar Ultraviolet Spectral Irrandiance Monitor (SUSIM) instrument to determine the quality of the NOAA Mg II core-to-wing data set.

Change in the radiative output of the Sun in 1992 and its effect in the thermosphere

Ground and space measurements of the solar spectral irradiance at radio, visible, UV, and X ray wavelengths show a large decline in the first 6 months of 1992. This sustained drop in the solar output is important in understanding the connection between the emergent magnetic flux on the Sun and the radiative output as well as in understanding the effects of such change in the upper atmosphere of the earth. We present preliminary estimates of the observed changes as the means to spur inquiry into this solar event in the declining phase of solar cycle 22. Typical decreases are 15% in Lyman α and 40% in 10.7-cm radio flux. Mass spectrometer and incoherent scatter model calculations at 600 km in the thermosphere indicate a 30% decrease in the temperature and a 3X decrease in the density of the thermosphere near the altitude where both the Upper Atmosphere Research Satellite (UARS) and Hubble Space Telescope are flying. Decrease of the orbital period of the UARS shows the expected effect of decreasing density at flight altitude. Work in progress indicates that the output change results from the decline in solar magnetic flux to a lower level of activity in the southern hemisphere of the Sun.

Theoretical basis for operational ensemble forecasting of coronal mass ejections

We lay out the theoretical underpinnings for the application of the Wang-Sheeley-Arge-Enlil modeling system to ensemble forecasting of coronal mass ejections (CMEs) in an operational environment. In such models, there is no magnetic cloud component, so our results pertain only to CME front properties, such as transit time to Earth. Within this framework, we find no evidence that the propagation is chaotic, and therefore, CME forecasting calls for different tactics than employed for terrestrial weather or hurricane forecasting. We explore a broad range of CME cone inputs and ambient states to flesh out differing CME evolutionary behavior in the various dynamical domains (e.g., large, fast CMEs launched into a slow ambient, and the converse; plus numerous permutations in between). CME propagation in both uniform and highly structured ambient flows is considered to assess how much the solar wind background affects the CME front properties at 1 AU. Graphical and analytic tools pertinent to an ensemble approach are developed to enable uncertainties in forecasting CME impact at Earth to be realistically estimated. We discuss how uncertainties in CME pointing relative to the Sun-Earth line affects the reliability of a forecast and how glancing blows become an issue for CME off-points greater than about the half width of the estimated input CME. While the basic results appear consistent with established impressions of CME behavior, the next step is to use existing records of well-observed CMEs at both Sun and Earth to verify that real events appear to follow the systematic tendencies presented in this study.