Alfvén surface

The Sun does not have a solid surface. However, it does have a superheated atmosphere, made of solar material bound to the Sun by gravity and magnetic forces.[2] The Sun's corona extends far beyond the solar surface, or photosphere, and is considered the outer boundary of the Sun. It marks the transition to the solar wind which moves through the Solar 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 the Sun 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 the Sun is called the Alfvén surface.[3] 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 the Sun. This is the point where solar atmosphere ends and where solar wind begins.[2][4]

Adhikari, Zank, & Zhao (2019) define the Alfvén surface as:[5]

the location at which the large-scale bulk solar wind speed ${\displaystyle U}$ and the Alfvén speed ${\displaystyle V_{\text{A}}}$ are equal, and thus it separates sub-Aflvénic coronal flow |${\displaystyle U}$|≪|${\displaystyle V_{\text{A}}}$| from super-Alfvénic solar wind flow |${\displaystyle U}$|≫|${\displaystyle V_{\text{A}}}$|

DeForest, Howard, & McComas (2014) define the Alfvén surface as:[6]

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.

Researchers were unsure exactly where the Alfvén critical surface of the Sun lay. Based on remote images of the corona, estimates had put it somewhere between 10 and 20 solar radii from the surface of the Sun.[2] On April 28, 2021, during its eighth flyby of the Sun, NASA's Parker Solar Probe encountered the specific magnetic and particle conditions at 18.8 solar radii that indicated that it penetrated the Alfvén surface;[2][1] the probe measured the solar wind plasma environment with its FIELDS and SWEAP instruments.[3] This event was described by NASA as "touching the Sun".[2] During the flyby, Parker Solar Probe passed into and out of the corona several times. This proved the predictions that the Alfvén critical surface isn't shaped like a smooth ball, but has spikes and valleys that wrinkle its surface.[2]

• Kasper, Justin C.; Klein, Kristopher G. (1 June 2019). "Strong Preferential Ion Heating is Limited to within the Solar Alfvén Surface". The Astrophysical Journal Letters. 877 (2): L35. arXiv:1906.02763. Bibcode:2019ApJ...877L..35K. doi:10.3847/2041-8213/ab1de5. S2CID 174801124.
• Guilet, Jerome; Foglizzo, Thierry; Fromang, Sebastien (2010). "Dynamics of an Alfven surface in core collapse supernovae". The Astrophysical Journal. 729 (1): 71. arXiv:1006.4697. doi:10.1088/0004-637X/729/1/71. S2CID 118461285.

• "Solving the sun's super-heating mystery with Parker Solar Probe". University of Michigan News. 4 June 2019. Retrieved 30 July 2022.