Center to limb variation of transition region Doppler shift in active regions

TL;DR

This study measures Doppler shifts in the transition region of active regions on the Sun, revealing consistent redshifts with center-to-limb variation, and challenges existing interpretations of these flows, suggesting alternative mechanisms.

Contribution

It provides the first comprehensive measurement of Doppler shifts in strong field regions of active regions, highlighting the variation and challenging previous models of flow origins.

Findings

Active regions show mean redshifts of 4-11 km/s in strong field areas.

Redshifts exhibit a mild center-to-limb variation with significant scatter.

Results suggest alternative explanations for lower transition region redshifts, beyond impulsive heating models.

Abstract

Studying Doppler shifts provides deeper insights into the flow of mass and energy in the solar atmosphere. We perform a comprehensive measurement of Doppler shifts in the transition region and its center-to-limb variation (CLV) in the strong field regions ($|\textbf{B}| \geq$ 50 G) of 50 active regions (ARs), using the \ion{Si}{4} 1394~{\AA} line recorded by the Interface Region Imaging Spectrometer(IRIS). To locate the ARs and identify strong field regions, we have used the magnetograms obtained by the Helioseismic and Magnetic Imager (HMI). We find that in strong field regions, on average, all the ARs show mean redshifts ranging between 4{--}11~ km/s, which varies with ARs. These flows show a mild CLV, with sizable magnitudes at the limb and substantial scatter at the mid-longitude range. Our observations do not support the idea that redshifts in the lower transition region (T $<\sim$ 0.1 MK) are produced by field-aligned downflows as a result of impulsive heating and warrant alternative interpretation, such as downflow of type-\rm{II} spicules in the presence of a chromospheric wall created by cooler type-\rm{I} spicules.