Radiation transport of heliospheric Lyman-α from combined Cassini and Voyager data sets

¹ Central Arizona College, 8470 N. Overfield Rd., Coolidge, AZ 85228, USA e-mail: wayne.pryor@centralaz.edu
² Space Environment Technologies, 1676 Palisades Dr., Pacific Palisades, CA 90272, USA
³ Department of Physics, Astronomy and Space Sciences Center, University of Southern California, Los Angeles, CA, 90089, USA
⁴ Lunar and Planetary Laboratory, University of Arizona, 1541 E University Blvd, Tucson, AZ 85721-0077, USA
⁵ Service d'Aéronomie, BP 3, 91371 Verrières-le-Buisson, France
⁶ Dept. of Physics, University of New Hampshire, Durham, NH 03824, USA
⁷ Laboratory for Atmospheric and Space Physics, University of Colorado, 1234 Innovation Dr., Boulder, CO 80303, USA
⁸ Dept. of Physics, University of Central Florida, 4000 Central Florida Boulevard, Building 12 Room 310, Orlando, FL 32816-2385, USA
⁹ Lomonosov Moscow State University, Department of Aeromechanics and Gas Dynamics, School of Mechanics and Mathematics, Moscow 119899, Russia
¹⁰ Space Research Institute (IKI) and Institute for Problems in Mechanics, Russian Academy of Sciences, Prospect Vernadskogo 101, Moscow 117526, Russia
¹¹ Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, CA 91109, USA
¹² Space Research Centre, Polish Academy of Sciences, Bartycka 18A, 00-716 Warsaw, Poland

Received: 17 October 2007
Accepted: 5 June 2008

Abstract

Aims. Heliospheric neutral hydrogen scatters solar Lyman-α radiation from the Sun with “27-day” intensity modulations observed near Earth due to the Sun's rotation combined with Earth's orbital motion. These modulations are increasingly damped in amplitude at larger distances from the Sun due to multiple scattering in the heliosphere, providing a diagnostic of the interplanetary neutral hydrogen density independent of instrument calibration.

Methods. This paper presents Cassini data from 2003-2004 obtained downwind near Saturn at ~10 AU that at times show undamped “27-day” waves in good agreement with the single-scattering models of Pryor et al. (1992, ApJ, 394, 363). Simultaneous Voyager 1 data from 2003-2004 obtained upwind at a distance of 88.8-92.6 AU from the Sun show waves damped by a factor of ~0.21. The observed degree of damping is interpreted in terms of Monte Carlo multiple-scattering calculations (e.g., Keller et al. 1981, A&A, 102, 415) applied to two heliospheric hydrogen two-shock density distributions (discussed in Gangopadhyay et al. 2006, ApJ, 637, 786) calculated in the frame of the Baranov-Malama model of the solar wind interaction with the two-component (neutral hydrogen and plasma) interstellar wind (Baranov & Malama 1993, J. Geophys. Res., 98, 15157; Izmodenov et al. 2001, J. Geophys. Res., 106, 10681; Baranov & Izmodenov 2006, Fluid Dyn., 41, 689).

Results. We conclude that multiple scattering is definitely occurring in the outer heliosphere. Both models compare favorably to the data, using heliospheric neutral H densities at the termination shock of 0.085 cm^-3 and 0.095 cm^-3. This work generally agrees with earlier discussions of Voyager data in Quemerais et al. (1996, ApJ, 463, 349) showing the importance of multiple scattering but is based on Voyager data obtained at larger distances from the Sun (with larger damping) simultaneously with Cassini data obtained closer to the Sun.