Solar wind

The solar wind is a continuous stream of charged particles. It originates from the corona, which is the Sun's outermost atmospheric layer. This stream is composed of plasma, a state of matter filled with charged particles. The solar wind is vital to understanding how the Sun influences the entire solar system. It carries the interplanetary magnetic field along with it as it travels through space.

The mechanism of the solar wind begins with the extreme heat of the corona. Scientists determined that the corona reaches temperatures of about one million degrees Celsius. This intense heat provides particles with high kinetic energy. This energy allows the particles to overcome the Sun's gravitational pull. The flow moves from the corona through a boundary called the Alfvén surface. As the plasma moves further from the Sun, it undergoes a transition. It moves from subsonic speeds to supersonic speeds, meaning it travels faster than fast magnetosonic waves.

The composition of this plasma is quite complex. It consists mostly of electrons, protons, and alpha particles. However, it also contains trace amounts of heavier ions and atomic nuclei. These include elements such as carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron. There are even rarer traces of isotopes like phosphorus and nickel isotopes, such as 58Ni, 60Ni, and 62Ni. The density, temperature, and speed of the wind vary based on time and solar latitude.

History shows that many scientists contributed to our understanding of this phenomenon. In 1859, Richard C. Carrington and Richard Hodgson observed a solar flare. A solar flare is a sudden, localized increase in brightness on the solar disc. Carrington suspected a connection between this flare and a powerful geomagnetic storm that followed. Later, Kristian Birkeland suggested the Sun emits both positive and negative rays. He proposed that the Earth is continually bombarded by these electric corpuscles. In the 1950s, Ludwig Biermann noted that comet tails always point away from the Sun. He postulated that the solar wind pushes these tails away.

In 1958, astrophysicist Eugene Parker provided a mathematical model for the solar wind. He argued that a million-degree corona cannot remain static. Instead, pressure forces drive a radially expanding flow. Parker's theory predicted that the wind would transition to supersonic flow at about four solar radii from the photosphere. Although one reviewer called his work "utter nonsense," the editor Subrahmanyan Chandrasekhar chose to publish it.

Parker's model also predicted the Parker spiral, which is the shape of the magnetic field lines in the ecliptic plane.

Spacecraft observations eventually confirmed these theoretical predictions. In 1959, the Soviet Luna 1 probe became the first to directly observe and measure the solar wind. Later, the American Mariner 2 mission identified two distinct components: a low-speed and a high-speed solar wind. In 1999, the ACE and WIND spacecraft observed a massive 98% decrease in solar wind density. This event caused a "polar rain" where a visible aurora appeared over the North Pole. This happened because energetic electrons flowed toward Earth in narrow beams called "strahl."

The solar wind connects many different astronomical concepts. It interacts with Earth's magnetosphere to create phenomena like the aurora, or northern and southern lights. It also drives geomagnetic storms that can change magnetic field lines. On a larger scale, the solar wind defines the heliosphere. In 2010, the Voyager 1 probe determined that the solar wind velocity slowed to zero at its location. At this point, the wind no longer moves outward but moves sideways into the tail of the heliosphere.

Modern missions continue to explore these mysteries. The STEREO mission uses stereoscopy to study coronal mass ejections and the corona. Its cameras can image the solar wind itself through Thomson scattering of sunlight. Today, the Parker Solar Probe is studying the Sun up close. It helps scientists investigate why the wind accelerates faster than thermodynamic expansion alone can explain. This research helps us understand the complex relationship between the Sun and the planets it supports.