Hinode extreme-ultraviolet imaging spectrometer observations of a limb active region

Aims: We investigate the electron density and temperature structure of a limb active region.
Methods: We have carried out a study of an active region close to the solar limb using observations from the Extreme-ultraviolet Imaging Spectrometer (EIS) and the X-ray telescope (XRT) on board Hinode. The electron density and temperature distributions of the coronal emission have been determined using emission line intensity ratios. Differential emission measure (DEM) analysis and the emission measure (EM) loci technique were used to examine the thermal structure of the emitting plasma as a function of distance from the limb.
Results: The highest temperature and electron density values are found to be located in the core of the active region, with a peak electron number density value of 1.9 × 10¹⁰ cm^-3 measured using the Fe XII 186.887 Å to 192.394 Å line intensity ratio. The plasma along the line of sight in the active region was found to be multi-thermal at different distances from the limb. The EIS and XRT DEM analyses appear to be in agreement in the temperature interval from log T = 6.5-6.7.
Conclusions: Our results provide new constraints for models of coronal heating in active regions.