With the aim of studying active region fan loops using observations from the Hinode EUV Imaging Spectrometer ( EIS ) and Solar Dynamics Observatory ( SDO ) Atmospheric Imaging Assembly ( AIA ) , we investigate a number of inconsistencies in modeling the absolute intensities of Fe viii and Si vii lines , and address why spectroheliograms formed from these lines look very similar despite the fact that ionization equilibrium calculations suggest that they have significantly different formation temperatures : \log ( T _ { e } / K ) = 5.6 and 5.8 , respectively .
These issues are important to resolve because confidence has been undermined in their use for differential emission measure ( DEM ) analysis , and Fe viii is the main contributor to the AIA 131 Å channel at low temperatures .
Furthermore , the strong Fe viii 185.213 Å and Si vii 275.368 Å lines are the best EIS lines to use for velocity studies in the transition region , and for assigning the correct temperature to velocity measurements in the fans .
We find that the Fe viii 185.213 Å line is particularly sensitive to the slope of the DEM , leading to disproportionate changes in its effective formation temperature .
If the DEM has a steep gradient in the \log ( T _ { e } / K ) = 5.6 to 5.8 temperature range , or is strongly peaked , Fe viii 185.213 Å and Si vii 275.368 Å will be formed at the same temperature .
We show that this effect explains the similarity of these images in the fans .
Furthermore , we show that the most recent ionization balance compilations resolve the discrepancies in absolute intensities .
With these difficulties overcome , we combine EIS and AIA data to determine the temperature structure of a number of fan loops and find that they have peak temperatures of 0.8–1.2MK .
The EIS data indicate that the temperature distribution has a finite ( but narrow ) width < \log ( \sigma _ { T _ { e } } / K ) = 5.5 which , in one detailed case , is found to broaden substantially towards the loop base .
AIA and EIS yield similar results on the temperature , emission measure magnitude , and thermal distribution in the fans , though sometimes the AIA data suggest a relatively larger thermal width .
The result is that both the Fe viii 185.213 Å and Si vii 275.368 Å lines are formed at \log ( T _ { e } / K ) \sim 5.9 in the fans , and the AIA 131 Å response also shifts to this temperature .