Folding ion rays in comet C / 2004 Q 2 ( Machholz ) and the connection with the solar wind ?

Aims. The appearance of folding ion rays in cometary comae is still not very well understood, so our aim is to gain more insight into the role of the local solar wind in the formation of these structures. Methods. Comet C/2004 Q2 (Machholz) was intensively monitored during its closest approach to Earth (January 2005) with the CCD camera Merope mounted on the Flemish 1.2m Mercator telescope, in three different bands (Geneva U and B and Cousins I). Spectacular ion rays, thin ionic structures rapidly folding tailward, were recorded in the U band during one night, January 12th. Results. Data from the SOHO satellite that was extrapolated corotationally to the position of the comet showed that the ion rays were formed during a sudden change in the in-situ solar wind state. We were able to succesfully correlate a high-speed solar wind stream with the appearance of folding ion rays. Conclusions. To our knowledge, this is the first clear observational evidence that folding ion rays in cometary comae are produced by a sudden change in the local solar wind state.