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Solar wind speed and flux-tube geometry


Pinto Rui




Auteur(s) supplémentaire(s)S. Brun, A. Rouillard
Institution(s) supplémentaire(s)Laboratoire AIM, SAp., CEA Saclay


The cyclic variations of the strength and geometry of the global background magnetic field strongly affect the solar wind flow and cause the segregation between the fast and slow wind flows. Fast wind flows develop exclusively within coronal holes, while the slow solar wind streams from the vicinities of the coronal hole boundaries (i.e, around streamers and pseudo-streamers) and/or active regions.
It is well known that the terminal (asymptotic) wind speed in a given magnetic flux-tube is generally anti-correlated with its total superradial expansion ratio, which motivated the definition of widely-used semi-empirical predictive laws (such as WSA). In practice, such scaling laws require ad-hoc corrections and empirical fits to \emph{in-situ} spacecraft data; a predictive law based solely on physical principles is still missing.
We investigate this problem by performing combined numerical simulations of the solar dynamo, corona and solar wind covering an 11 yr activity cycle. We analysed a large sample of open flux-tubes in order to investigate the dependence of the wind speed on geometrical parameters of the flux-tubes. We found that the total flux-tube expansion effectively control the locations of the slow and fast wind flows, but the actual asymptotic wind speeds attained -- specially those of the slow wind -- are also dependent on field-line inclination.
This is work is supported by the FP7 project #606692 (HELCATS).

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