Alfvén surface

However, they have a superheated atmosphere, made of solar material bound to the star by gravity and magnetic forces.

[5] The stellar corona extends far beyond the solar surface, or photosphere, and is considered the outer boundary of the star.

[6] It can also be described as a point where gravity and magnetic fields are too weak to contain heat and pressure that push the material away from a star.

[5] Adhikari, Zank, & Zhao (2019) define the Alfvén surface as:[7] the location at which the large-scale bulk solar wind speed

DeForest, Howard, & McComas (2014) define the Alfvén surface as:[8]a natural boundary that marks the causal disconnection of individual packets of plasma and magnetic flux from the Sun itself.

Alfvén surface separates the sub- and super-Alfvénic regimes of the stellar wind, which influence the structure of any magnetosphere/ionosphere around an orbiting planet in the system.

Based on remote images of the corona, estimates had put it somewhere between 10 and 20 solar radii from the surface of the Sun.

[5] At 09:33 UT on 28 April 2021 Parker Solar Probe entered the magnetized atmosphere of the Sun 13 million kilometres (8.1 million miles) above the photosphere, crossing below the Alfvén critical surface for five hours into plasma in causal contact with the Sun with an Alfvén Mach number of 0.79 and magnetic pressure dominating both ion and electron pressure.

NASA animation showing Parker Solar Probe passing through the Sun's outer atmosphere, its corona , in April 2021. The boundary at the edge of the corona is the Alfvén critical surface. Inside that surface (circle at left), plasma connects to the Sun by waves traveling back and forth to the surface. Beyond it (circle at right), the Sun's magnetic fields and gravity are too weak to contain the plasma and it becomes the solar wind , racing across the Solar System so fast that waves within the wind cannot make it back to the Sun. [ 1 ]
Simulated stellar wind environment for AU Microscopii . A translucent shade shows the resulting Alfvén surface of the stellar wind. [ 2 ]
Effect of magnetic field strength and geometry on the Alfvén surface. The AS is highly affected by the change in the surface geometry. [ 3 ]
Simulated stellar wind environment for the Proxima Centauri system. The purple isosurface corresponds to the Alfvén surface of the stellar wind. [ 4 ]