Authors: M. Casti (CUA, NASA/GSFC), C. N. Arge (NASA/GSFC), A. Bemporad (INAF – Turin Astrophysical Observatory), Rui F. Pinto (IRAP, Université Toulouse III – Paul Sabatier, CNRS, CNES), C. J. Henney (Air Force Research Laboratory)

The physics behind the release and acceleration of the solar wind is still a matter of study, and one of the aspects that needs further investigation is the identification of the region in the solar corona where the larger fraction of acceleration occurs. To address this matter, reliable measurements of the solar wind speed in a heliocentric range 1 to 10 solar radii are needed. Moreover, due to the continuous interactions between the coronal plasma and the magnetic field, it is important to correlate the measured outflow speed with the local configuration of the coronal magnetic field.

This study investigates a method to relate solar wind flow measurements to the extrapolated coronal magnetic field through 3D field line tracing. Outflow speed measurements are derived by applying the Doppler dimming technique to simultaneous coronagraphic observations of the solar corona in polarized visible light and in the UV Lyman-alpha line emission from the neutral hydrogen (i.e., 121.6 nm). The coronal magnetic field configuration is obtained using global maps of the photospheric magnetic field as input to the Wang-Sheeley-Arge (WSA) model, which exploits a combination of the magnetostatic Potential Field Source Surface (PFSS) and the Schatten Current Sheet (SCS) models. The two sets of data are then linked by associating the measured speed values (i.e., based on their longitude, polar angle, and heliocentric distance) with the magnetic field lines as determined by WSA.

To validate the method, we used observations acquired during the cycle 22/23 minimum, and we derived outflow speed as a function of distance from the center of the Sun for three classes of field lines based on magnetic field line foot-point distance to the nearest coronal hole boundary. The validation is contingent on verifying that the highest values of wind speed are associated with those field lines rooted mostly deep inside coronal holes (i.e., fast solar wind originates deep within the coronal holes, while slow wind is associated with source regions near coronal hole boundaries).