Flux-tube geometry and solar wind speed during an activity cycle

Abstract

Context. The solar wind speed at 1 AU shows cyclic variations in latitude and in time which reflect the evolution of the global background magnetic field during the activity cycle. It is commonly accepted that the terminal (asymptotic) wind speed in a given magnetic flux-tube is generally anti-correlated with its total expansion ratio, which motivated the definition of widely used semiempirical scaling laws relating one to the other. In practice, such scaling laws require ad hoc corrections (especially for the slow wind in the vicinities of streamer/coronal hole boundaries) and empirical fits to in situ spacecraft data. A predictive law based solely on physical principles is still missing. Aims. We test whether the flux-tube expansion is the controlling factor of the wind speed at all phases of the cycle and at all latitudes (close to and far from streamer boundaries) using a very large sample of wind-carrying open magnetic flux-tubes. We furthermore search for additional physical parameters based on the geometry of the coronal magnetic field which have an influence on the terminal wind flow speed. Methods. We use numerical magneto-hydrodynamical simulations of the corona and wind coupled to a dynamo model to determine the properties of the coronal magnetic field and of the wind velocity (as a function of time and latitude) during a whole 11-yr activity cycle. These simulations provide a large statistical ensemble of open flux-tubes which we analyse conjointly in order to identify relations of dependence between the wind speed and geometrical parameters of the flux-tubes which are valid globally (for all latitudes and moments of the cycle). Results. Our study confirms that the terminal (asymptotic) speed of the solar wind depends very strongly on the geometry of the open magnetic flux-tubes through which it flows. The total flux-tube expansion is more clearly anti-correlated with the wind speed for fast rather than for slow wind flows, and effectively controls the locations of these flows during solar minima. Overall, the actual asymptotic wind speeds attained -especially those of the slow wind -are also strongly dependent on field-line inclination and magnetic field amplitude at the foot-points. We suggest ways of including these parameters in future predictive scaling laws for the solar wind speed.

The origin of slow Alfvénic solar wind at solar minimum

“…These observations agree with the idea that both the fast and slow Alfvénic solar wind were produced on open field lines rooted in coronal holes, but with varying magnetic field geometries in the lower corona. In particular, the amount the magnetic field expands in the low corona (up to 2.5r ), the footpoint magnetic field strength, and the magnetic field inclination at the solar surface are all thought to be important in shaping the properties of coronal hole wind (Suess et al 1984;Wang & Sheeley 1990;Bravo & Stewart 1997;Suzuki 2006;Pinto et al 2016;Réville & Brun 2017).…”

Magnetism, dynamo action and the solar-stellar connection

“…This value is actually found to change during the solar cycle and for various stellar parameters (Lee et al 2011;Réville et al 2015b). It is also shown that Potential field source surface extrapolation of the coronal field overestimates the expansion factor ( f = A1 A0 r 0 r 1 2 = B0 B1 r 0 r 1 2 ; with A0, A1 the flux tube surface area at the surface and far in the wind, B0, B1 the magnetic field at the same locations and r 0, r 1 the surface and distant radius) which is one of the key ingredients for determining the stellar wind terminal velocity, see, e.g., Wang and Sheeley (1990), NOAA's WSA-Enlil model (Arge and Pizzo 2000;Mays et al 2015) and Pinto et al (2016). More accurate solar wind models have also been computed over an 11-year cycle, showing that the Alfvén radius and torque exerted by the solar wind changes by a factor of 3 between the minimum and maximum phases of the activity cycle (Pinto et al 2011(Pinto et al , 2016Réville et al 2016).…”

Diagnosing solar wind origins usingin situmeasurements in the inner heliosphere

“…The reason slower speeds are not observable at the leading edge of high speed streams is because by 0.3 AU they have already been accelerated by the faster wind to form a co-rotating interaction region (Burlaga 1974;Pizzo 1991;McGregor et al 2011;Richardson 2018). Note that we have chosen to distinguish between the edges and the core of coronal holes; at the edge of coronal holes the magnetic field lines typically undergo large separations as a function of height in the corona, which has the effect of reducing both the wind speed (Levine et al 1977;Wang & Sheeley 1991;Cranmer et al 2007;Pinto et al 2016) and charge state ratios (Wang et al 2009). In the next two sections further evidence is used to predict which one of our three categories coronal hole edge wind is part of.…”