Data obtained on slant sensing paths Magadan-Irkutsk and Norilsk-Irkutsk indicate rapid increase of maximum applicable frequencies at morning and daylight hours in the period from September 24 to 26, 2005. At the same time, we observed increase of ionization according to the data of vertical sensing at the stations analyzed. The period considered is characterized by quiet geomagnetic conditions. We discuss possible mechanisms of observed variations of ionospheric parameters. Analysis included consideration of satellite data on vertical distribution of temperature in the stratosphere-mesosphere obtained by Microwave Limb Sounder (MLS) installed on the Earth Observing System (EOS) Aura satellite. Also we used data on the atomic oxygen to molecular nitrogen concentration ratios [O]/[N2] at altitudes of the lower thermosphere, collected by Global Ultraviolet Imager (GUVI), installed on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. Since the electron concentration at altitudes of the maximum of F2-layer of the ionosphere is proportional to the [O]/[N2] ratio, it is hypothesized that the observed effect in ionospheric parameters may be associated with the variations of temperature and composition of the neutral atmosphere (possibly, in the course of seasonal change of the atmosphere).