Speaker
Description
First Name: Kyuhyoun
Last Name: Cho
Email Address: kcho@seti.org
Affiliation: LMSAL / SETI
All Authors: Bart De Pontieu, Paola Testa
Abstract: Nonthermal broadening in spectral lines formed in the solar transition region has been a subject of interest for several decades, as its underlying mechanisms remain unclear and it may provide key insights into the heating of the upper solar atmosphere. Previous studies have primarily focused on the relationship between broadening and magnetic field orientation, often reporting that regions where the magnetic field is perpendicular to the line of sight exhibit higher broadening. This suggests that transverse waves, such as Alfven waves, may play a role in the nonthermal broadening. However, in this study, we present an opposite finding: higher broadening is observed in regions where the magnetic field is aligned with the line of sight. By analyzing the Si IV 1403 Å line in three plage regions observed by the Interface Region Imaging Spectrograph, we derived nonthermal velocity maps. These were compared to magnetic field inclination maps calculated via nonlinear force-free field extrapolation, based on vector magnetic fields from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. Our results clearly demonstrate that higher broadening is preferentially associated with lower-inclination regions showing downflows, highlighting the magnetic field-aligned component of nonthermal broadening. We interpret this finding as the presence of velocity gradients within plasma flowing downward along the magnetic field in the transition region.