Alfvén surface

The Alfvén surface is the boundary separating a star's corona from the stellar wind defined as where the coronal plasma's Alfvén speed and the large-scale stellar wind speed are equal. It is named after Hannes Alfvén, and is also called Alfvén critical surface, Alfvén point, or Alfvén radius. Parker Solar Probe became the first spacecraft that crossed Alfvén surface of the Sun. Stars do not have a solid surface. However, they have a superheated atmosphere, made of solar material bound to the star by gravity and magnetic forces. The stellar corona extends far beyond the solar surface, or photosphere, and is considered the outer boundary of the star. It marks the transition to the solar wind which moves through the planetary system. This limit is defined by the distance at which disturbances in the solar wind cannot propagate back to the solar surface. Those disturbances cannot propagate back towards a star if the outbound solar wind speed exceeds Mach one, the speed of 'sound' as defined for the solar wind. This distance forms an irregular 'surface' around a star is called the Alfvén surface. 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. This is the point where solar atmosphere ends and where solar wind begins. Adhikari, Zank, & Zhao (2019) define the Alfvén surface as: the location at which the large-scale bulk solar wind speed and the Alfvén speed are equal, and thus it separates sub-Aflvénic coronal flow ||≪|| from super-Alfvénic solar wind flow ||≫|| DeForest, Howard, & McComas (2014) define the Alfvén surface as: a natural boundary that marks the causal disconnection of individual packets of plasma and magnetic flux from the Sun itself. The Alfvén surface is the locus where the radial motion of the accelerating solar wind passes the radial Alfvén speed, and therefore any displacement of material cannot carry information back down into the corona. It is thus the natural outer boundary of the solar corona, and the inner boundary of interplanetary space.