Solar wind

History

In the 1950s, physicist Eugene Parker predicted that the hot solar corona could not be held by gravity and must expand as a supersonic flow (faster than the speed of sound). In the early 1960s, the first spacecraft confirmed his theory.

How it works

This can be compared to steam over a boiling pot: the outer atmosphere of the Sun (the corona) is heated to millions of degrees, causing particles to accelerate and fly off into space, dragging the magnetic field with them.

History

In the 1950s, physicist Eugene Parker predicted that the hot solar corona could not be held by gravity and must expand as a supersonic flow (faster than the speed of sound). In the early 1960s, the first spacecraft confirmed his theory.

How it works

This can be compared to steam over a boiling pot: the outer atmosphere of the Sun (the corona) is heated to millions of degrees, causing particles to accelerate and fly off into space, dragging the magnetic field with them.

History

The idea of particle outflow from the Sun was suggested as early as the early 20th century (Eddington assumed influence on comets), but the physical theory was developed by Eugene Parker in 1958. His work was met with skepticism until the Mariner 2 satellite registered a constant plasma flow in 1962. Further missions (Helios, Ulysses) revealed the three-dimensional structure of the wind.

How it works

The outflow is described by magnetohydrodynamic equations taking into account thermal conductivity and wave heating. In Parker's simplest model, the plasma pressure in the corona exceeds the pressure of the interplanetary medium, leading to continuous acceleration to supersonic speeds. The analytical solution of the equation of motion gives a transonic transition. The magnetic field frozen into the plasma creates the heliospheric current sheet and spiral structure. Modern models include kinetic effects and heating by coronal holes. Observations confirm predictions of the existence of slow and fast wind.

Open problems & fun facts

What is the physical mechanism that heats the solar corona to millions of degrees (coronal heating problem)?; What exactly accelerates the fast solar wind in coronal holes?; How does the solar wind interact with the local interstellar medium at the heliosphere boundary?; Can extreme solar wind from young Sun-like stars completely strip the atmospheres of exoplanets?

💡 During the solar superstorm of 1859 (the Carrington event), telegraph machines around the world sparked and worked while disconnected from batteries, and auroras were observed even in the tropics.