  Current models of Titan ionosphere have difficulties in explaining the
observed electron density and/or temperature. In order to get new insights, we
re-analyzed the data taken in the ionosphere of Titan by the Cassini Langmuir
probe (LP), part of the Radio and Plasma Wave Science (RPWS) instrument. This
is the first of two papers that present the new analysis method (current paper)
and statistics on the whole dataset. We suggest that between 2 and 4 electron
populations are necessary to fit the data. Each population is defined by a
potential, an electron density and an electron temperature and is easily
visualized by a dinstinct peak in the second derivative of the electron
current, which is physically related to the electron energy distribution
function (Druyvesteyn method). The detected populations vary with solar
illumination and altitude. We suggest that the 4 electron populations are due
to photo-ionization, magnetospheric particles, dusty plasma and electron
emission from the probe boom, respectively.
