  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. 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. 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. 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.
