Chromospheres of F, G, K type stars. 7.Upper chromosphere and transition region

Basing of detailed energy balance between radiative losses and amount of mechanical energy with different flux spectra we made an attempt to reconstruct conditions for the formation of transition region chromosphere-corona. We confirmed that the shape of mechanical energy spectrum may be different for stars with different effective temperatures and we found that the gradient of microturbulence velocity in the chromosphere has a significant influence on formation region. Calculations show that transition regions are present in main sequence stars and absent for a Boo and α Tau, in agreement with observations. (author)