Elsevier

Icarus

Volume 178, Issue 2, 15 November 2005, Pages 429-433
Icarus

A new relation between the central spectral solar H I Lyman α irradiance and the line irradiance measured by SUMER/SOHO during the cycle 23

https://doi.org/10.1016/j.icarus.2005.05.002Get rights and content

Abstract

The spectral irradiance at the center of the solar H I Lyman α (λ0=121.5664nm, referred to as Lyα in this paper) line profile is the main excitation source responsible for the atomic hydrogen resonant scattering of cool material in our Solar System. It has therefore to be known with the best possible accuracy in order to model the various Lyα emissions taking place in planetary, cometary, and interplanetary environments. Since the only permanently monitored solar irradiance is the total one (i.e. integrated over the whole Lyα line profile), Vidal-Madjar [1975. Evolution of the solar Lyman alpha flux during four consecutive years. Solar Phys. 40, 69–86] using Orbiting Solar Observatory 5 (OSO-5) satellite Lyα data, established a semi-empirical formula allowing him to deduce the central spectral Lyα irradiance from the total one. This relation has been extensively used for three decades. But, at the low altitude of the OSO-5 orbit, the central part of the solar line profile was deeply absorbed by a large column of exospheric atomic hydrogen. Consequently, the spectral irradiance at the center of the line was obtained by a complex procedure confronting the observations with simulations of both the geocoronal absorption and the self-reversed shape of the solar Lyα profile. The SUMER spectrometer onboard SOHO positioned well outside the hydrogen geocorona, provided full-Sun Lyα profiles, not affected by such an absorption [Lemaire et al., 1998. Solar H I Lyman α full disk profile obtained with the SUMER/SOHO spectrometer. Astron. Astrophys. 334, 1095–1098; 2002. Variation of the full Sun Hydrogen Lyman α and β profiles with the activity cycle. Proc. SOHO 11 Symposium, ESA SP-508, 219–222; 2004. Variation of the full Sun Hydrogen Lyman profiles through solar cycle 23. COSPAR 2004 Meeting], making it—for the first time—possible to measure the spectral and total Lyα solar irradiances directly and simultaneously. A new relation between these two quantities is derived in an expression that is formally similar to the previous one, but with significantly different parameters. After having discussed the potential causes for such differences, it is suggested that the new relation should replace the old one for any future modeling of the numerous Lyα absorptions and emissions observed in the Solar System.

Introduction

The spectral irradiance at the center of the solar Lyα line profile is the main excitation source responsible for the atomic hydrogen resonant scattering in cool material. If one wishes to model precisely the Lyα emissions occurring in planetary, cometary or interplanetary environments, this central spectral irradiance has to be known with the best possible accuracy at any given time. On the other hand, the only permanently monitored solar irradiance is the one integrated over the whole solar Lyα profile—either measured by near-Earth satellites, or deduced from its correlation with solar activity indexes. This was the reason why Vidal-Madjar (1975) analyzing the OSO-5 satellite Lyα data, proposed a semi-empirical formula (referred to VM75 relation in the following), relating the central spectral irradiance to the line irradiance of Lyα. In the present work, owing to solar Lyα spectra measured by SUMER/SOHO well outside the hydrogen geocorona, we present a new and more direct determination of the relation between both quantities.

Section snippets

Previous determination

At the 500 km altitude of the OSO-5 orbit, the observed central part of the solar Lyα line profile was deeply absorbed by a large column of exospheric atomic hydrogen, as shown in Fig. 1: the measurement made by a hydrogen resonance cell provided the central irradiance of the solar Lyα profile convolved with the absorption function of the geocoronal hydrogen column present between the spacecraft and the Sun. Therefore, the actual spectral irradiance was not directly measured, but rather

Direct measurements of pure solar Lyα line profiles with SUMER/SOHO

Onboard the SOHO spacecraft, positioned well outside the hydrogen geocoronal envelope (at the L1 Sun–Earth Lagrange point), the SUMER spectrometer provided full-Sun Lyα profiles, exempt from any central geocoronal absorption Lemaire et al., 1998, Lemaire et al., 2002, Lemaire et al., 2004. The measurement method has been extensively described in these papers. In brief, it essentially consists in using the scattering properties of the SUMER instrument: with very long exposure times, the

The new directly measured relation between central and total solar irradiances

The line profile data collected during eight years of solar cycle 23 allowed us to measure directly the central Lyα solar irradiances as a function of the total ones. The results are shown in Fig. 3 in which the VM75 relation is also represented for comparison. Using the same units as in formula (1), the new analytical relation directly obtained from the SUMER/SOHO data is as follows:f1012s−1cm−2nm−1=0.64(F1011s−1cm−2)1.21±0.08.

The uncertainty corresponds to the standard deviation between the

Discussion

The newly measured relation confirms the general trend of the VM75 analytical relation. However, the new variation law is characterized by significantly different coefficients, providing smaller central irradiances than previously estimated for the same total irradiances, and also showing a central irradiance slope smaller than formerly estimated. Furthermore, the VM75 relation was shown to be valid only for the high and medium solar activities during cycle 20, but was clearly less accurate for

Conclusion

During the solar cycle 23, a relationship between the central and the total solar Lyα irradiances was directly derived from full-Sun H I Lyα solar profiles obtained by SUMER/SOHO. Compared to the previous VM75 relation deduced from the OSO-5 data obtained during the solar cycles 20–21, the new relation provides a lower central spectral irradiance (for a given line irradiance), gives a more gradual line center irradiance variation with activity (close to a linear relation), and applies to all

Acknowledgements

SOHO is a mission of international cooperation between ESA and NASA. The SUMER Project is financially supported by DLR, CNES, NASA and ESA PRODEX program (Swiss contribution). We thank the EIT team for the use of the He I 30.4 nm and Fe XI 17.1 nm images to determine the scattered radiation contribution. We acknowledge the use of SOLSTICE and TIMED/SEE data obtained from their web page. Finally, Charles Brossollet and Christophe Chevallier are acknowledged for their very useful contribution to

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