A New Relation between Central and Total Solar H I Lyman-α Irradiances, as measured by SOHO during Solar Cycle 23 (1996-2003)

Details A New Relation between Central and Total Solar H I Lyman-α Irradiances, as measured by SOHO during Solar Cycle 23 (1996-2003) A New Relation between Central and Total Solar H I Lyman-α Irradiances, as measured by SOHO during Solar Cycle 23 (1996-2003)

May 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004aas...204.9802e&link_type=abstract

American Astronomical Society Meeting 204, #98.02; Bulletin of the American Astronomical Society, Vol. 36, p.984

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Scientific paper

The spectral irradiance at the center of the solar H Ly-α line profile is the main excitation source responsible for the atomic hydrogen resonant scattering in cool material. It has therefore to be known with the best possible accuracy to model the H Ly-α emissions taking place in planetary, cometary and interplanetary environments. On the other hand, the only permanently monitored solar irradiance is the total one - either measured by near-Earth satellites, or deduced from its correlation with solar activity indexes. It is the reason why Vidal-Madjar (1975) using OSO-5 satellite H Ly-α data, established a semi-empirical formula allowing to deduce the central H Ly-α irradiance from the integrated one. This relation has been extensively used for three decades. In fact, at the low altitude of the OSO-5 orbit, the observed central part of the solar line profiles was deeply absorbed by a large column of exospheric atomic hydrogen. Consequently, the solar line center irradiances were not measured directly, but obtained by confronting the measurements with simulations of both the geocoronal absorption and the self-reversed shape of the central solar profile itself.
On the contrary, the SOHO/SUMER spectrometer orbiting well outside the H geocoronal envelope (at the L1 Sun-Earth Lagrange point), provided full Sun H Ly-α profiles, exempt from any central geocoronal absorption (Lemaire et al. (1998, 2002, 2004)). This has made it possible to directly measure the central H Ly-α solar irradiances as a function of the integrated ones, during eight years of Solar Cycle 23. The newly obtained relation confirms the general trend of the previous one, but it is characterized by significantly different coefficients. It will therefore provide new and more accurate inputs for the future modeling of the various Ly-α emissions occurring inside the Solar System.

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