Interplanetary H Lyman-alpha 27-day modulations observed at Cassini and Voyager in 2003-2004

Details Interplanetary H Lyman-alpha 27-day modulations observed at Cassini and Voyager in 2003-2004 Interplanetary H Lyman-alpha 27-day modulations observed at Cassini and Voyager in 2003-2004

May 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusmsh23a..03p&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #SH23A-03

Other

2126 Heliosphere/Interstellar Medium Interactions, 2144 Interstellar Gas, 2151 Neutral Particles (7837)

Scientific paper

Interplanetary neutral hydrogen is a major component of the outer heliosphere. However, its density remains poorly constrained as published values differ widely. As part of an ISSI team effort to improve the interplanetary H parameters, we explore a calibration-independent method of estimating the H density. 27-day modulations in the solar Lyman-alpha intensity are observed by TIMED SEE and other spacecraft instruments near the Earth. An optically thin model for interplanetary Lyman-alpha brightness and its variation is applied to 2003- 2004 data from the Cassini Ultraviolet Imaging Spectrograph (UVIS) and Voyager Ultraviolet Spectrometer (UVS). It is found that this model adequately fits the Cassini data obtained from downwind inside Saturn's orbit, but overestimates the amplitude of the 27-day modulation at Voyager 1's upwind location from 88-92 A.U. This reduced amplitude in the outer heliosphere is interpreted as resulting from multiple scattering in optically thick hydrogen. The degree of the damping is an indicator of the hydrogen density. Monte Carlo calculations for this effect have been performed and the results will be described.

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