An observing campaign ( SOHO JOP 139 ) , coordinated between ground based and SOHO instruments , has been planned to obtain simultaneous spectroheliograms of the same active region in several spectral lines .
The chromospheric lines Ca ii K , H \alpha and Na i D as well as He i 10830 , 5876 , 584 and He ii 304 Å lines have been observed .
The EUV radiation in the range \lambda < 500 Å and in the range 260 < \lambda < 340 Å has also been measured at the same time .
These simultaneous observations allow us to build semi-empirical models of the chromosphere and low transition region of an active region , taking into account the estimated total number of photoionizing photons impinging on the considered active region and their spectral distribution .
We obtained a model that matches very well all the observed line profiles , using a standard value for the He abundance ( [ He ] =0.1 ) and a modified distribution of microturbulence .
For this model we study the influence of the coronal radiation on the computed helium lines .
We find that , even in an active region , the incident coronal radiation has a limited effect on the UV He lines , while it results of fundamental importance for the D _ { 3 } and 10830 lines .
Finally we build two more models assuming values of He abundance [ He ] = 0.07 and 1.5 , only in the region where temperatures are larger than 1. \times 10 ^ { 4 } K. This region , between the chromosphere and transition region , has been indicated as a good candidate for processes that might be responsible for strong variations of [ He ] ( ) .
The set of our observables can still be well reproduced in both cases changing the atmospheric structure mainly in the low transition region .
This implies that , to choose between different values of [ He ] , it is necessary to constrain the transition region with different observables , independent on the He lines .