Center-to-limb Variations in Solar Plage using IRIS Observations

Abstract: The center-to-limb variations (CLV) of transition region line Gaussian fit parameters in solar plage are reported for the first time. The Si iv 1402.77 A line observed by Interface Region Imaging Spectrograph (IRIS) is used. The spectral intensity increases linearly from the disk center to the solar limb. Similarly, the non-thermal velocity also increases linearly from 23.6 km/s (at the disk center) to 30.9 km/s (at the solar limb). On the other hand, the Doppler velocity decreases from 8.9$\pm$1.0 km/s at the disk center to 0.0 km/s at the limb. This CLV pattern in solar plages is consistent with the CLV pattern reported in the quiet-Sun (QS). However, the average values of the parameters in the solar plage are significantly higher than in the QS. The intensity and non-thermal velocity increase linearly with the magnetic field at the disk center while the Doppler velocity does not depend on the magnetic field. Due to the line-of-sight effect, the plasma column depth increases towards the solar limb which leads to a linear increase in the spectral intensity. Further, the increasing plasma column depth towards the solar limb adds more and more unresolved motions, and as a result, the non-thermal velocity increases from the disk center to the solar limb. In the solar plages, the higher plasma density due to the strong magnetic field leads to the higher intensity and non-thermal velocity compared to QS and coronal hole (CH)