  In the paper the step-by-step principles for making local model of electron
density are described. They are based on modulation principle - electron
density dependence on time is a product of a number of temporal variations
caused by solar radiation, magnetic activity, Earth orientation and unknown
additional periodical processes (not a sum, as they suppose sometimes when
making such models). A multiranges modulation principle is also suggested, that
allows automatically extend the set of parameters by using new ones, obtained
by filtration (or averaging) of basic set of parameters over the time. In the
paper we describe two approaches to the model creation - descriptional and
predictional ones.
  To test the approach three different models were created for daily electron
density logarithm using the described principles. We have used the data of
Irkutsk digisonde over the period 2003-2007 years for testing. It becomes clear
that a non-optimal choice of the number of model parameters could increase
prediction error, inspite the error over the set, used for analysis, will
decrease. It is shown that one year prediction has accuracy about 9-23%
depending on the height, and the highest error corresponds to the height about
200km. From the modelling we could also see that with increasing of the height
the number of parameters increases, and this could be caused by inaccuracy of
the model or by not taking additional physical mechanisms into consideration.
