EDP Sciences
Free access
Volume 426, Number 3, November II 2004
Page(s) 867 - 874
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:200400028

A&A 426, 867-874 (2004)
DOI: 10.1051/0004-6361:200400028

Solar cycle dependence of the helium focusing cone from SOHO/UVCS observations

Electron impact rates and associated pickup ions
R. Lallement1, J. C. Raymond2, J.-L. Bertaux1, E. Quémerais1, Y.-K. Ko2, M. Uzzo2, D. McMullin3 and †D. Rucinski4

1  Service d'Aéronomie du CNRS, BP 3, 91371 Verrières-le-Buisson, France
    e-mail: rosine.lallement@aerov.jussieu.fr
2  Harvard Smithsonian Center for Astrophysics, 60 Garden Street, MS 50, Cambridge, MA 02138, USA
3  USC Space Sciences Center, University Park, SHS-274, Los Angeles, CA 90089-1341, USA
4  Space research Center, Bartycka 18A, 00716 Warsaw, Poland

(Received 12 March 2003 / Accepted 8 December 2003 )

The Ultraviolet Coronograph on board SOHO (UVCS) has observed the 58.4 nm glow of the interplanetary He focusing cone at regular intervals since 1996. The intensity decrease with time already observed during the first two years (Michels et al. 2002) has dramatically amplified during the solar activity increase. Intensities seem to reach a plateau in 2001. Using a model of the cone emission which takes into account both photoionization and electron impact ionization of neutral helium we show that the photoionization increase alone cannot explain the observed intensity drop. Data can be fitted if at minimum activity the electron impact ionization rate is the solar cycle average rate predicted by Rucinski & Fahr (1989), and if this rate is increased by a factor of about 3.5 between 1996 and 2001. Assuming the Rucinski and Fahr radial dependence, such high electron impact rates create averaged He + pickup ion (PUI) fluxes which may reach 50% of the fluxes of ions born after photoionization, or 35% of PUI total fluxes, as far as 1 AU from the Sun. In slow and dense solar wind enhancements, in particular in the presence of strong suprathermal tails, PUIs from electron impact could be the dominant species. This could explain a fraction of the observed correlation between He + and H + pickups and anticorrelation of He + fluxes with solar wind velocity.

Key words: interplanetary medium -- Sun: solar wind -- Sun: general -- plasmas

© ESO 2004