  We study the generation of parallel electric fields by virtue of propagation
of ion cyclotron waves (with frequency 0.3 w_{ci}) in the plasma with a
transverse density inhomogeneity. Using two-fluid, cold plasma linearised
equations, we show for the first time that E_|| generation can be understood by
an analytic equation that couples E_|| to the transverse electric field of the
driving ion cyclotron wave. In this simplified model, the generated E_||
amplitude e.g. for plausible solar coronal parameters attains values of 90 V/m
for the mass ratio m_i/m_e=262, within a time corresponding to 3 periods of the
driving ion cyclotron wave. By considering the numerical solutions we also show
that the cause of E_|| generation is electron and ion flow separation (which is
not the same as electrostatic charge separation) induced by the transverse
density inhomogeneity. The model also correctly reproduces the previous kinetic
results in that only electrons are accelerated (along the background magnetic
field), while ions do not accelerate substantially. We also investigate how
E_|| generation is affected by the mass ratio and found that amplitude attained
by E_|| decreases linearly as inverse of the mass ratio m_i/m_e, i.e. E_||
\propto 1/m_i. This result contradicts to the earlier suggestion by Genot et al
(1999, 2004) that the cause of E_|| generation is the polarisation drift of the
driving wave, which scales as \propto m_i. Also, for realistic mass ratio of
m_i/m_e=1836 our empirical scaling law is producing E_||=14 V/m (for solar
coronal parameters).
