Solar Wind is the continuous outflow of charged particles from the upper atmosphere of the Sun into interplanetary space. It consists mainly of electrons and protons, with alpha particles and smaller quantities of heavier ions and other species. The Solar Wind is a plasma and carries mass, energy and magnetic field information away from the Sun. It extends throughout the heliosphere and interacts with planets, moons, comets and the interstellar medium.
Its properties vary considerably with solar activity and with the region of the Sun from which it originates. The existence of the Solar Wind is a fundamental reason why the Solar System is not simply an environment governed by gravity and electromagnetic radiation but also by a continuously expanding plasma flow.
The Solar Wind originates in the solar corona, where high temperatures allow particles to acquire sufficient energy to escape the Suns gravitational influence. The corona cannot be treated as a static atmosphere because the pressure and magnetic structure of the hot plasma support an expanding outflow. Solar Wind acceleration is described using magnetohydrodynamic and kinetic plasma theories, with different mechanisms contributing at different locations and scales. Open magnetic field structures provide pathways through which plasma can flow into interplanetary space. The resulting outflow eventually becomes approximately radial on large scales, although magnetic field geometry and solar rotation produce a spiral structure known as the Parker spiral.
The Solar Wind is commonly divided into slow and fast components based on typical flow speed and associated solar source regions. Fast Solar Wind is frequently associated with coronal holes and open magnetic field configurations, while slow Solar Wind has more complex origins involving boundaries between different magnetic structures and regions of the corona. The distinction is not simply a difference in speed because the two populations can also differ in composition, temperature, magnetic structure and variability. Streams of different speeds can interact as they propagate outward, producing compressed regions and shocks that influence the heliosphere and can affect planetary space environments.
The Solar Wind carries the Suns magnetic field outward, producing the interplanetary magnetic field. Because the Sun rotates while the plasma flows outward, magnetic field lines become wound into an approximate spiral geometry. The strength and direction of this field vary with distance, latitude, solar activity and transient disturbances. When Solar Wind structures encounter a planetary magnetic field, energy and momentum can be transferred into the planetary magnetosphere. The detailed interaction depends on magnetic field orientation, particle density, velocity and plasma properties. These interactions can drive magnetospheric currents and contribute to auroral phenomena.
Solar Wind interactions are particularly important at Earth. Earths magnetic field deflects much of the Solar Wind around the magnetosphere, forming a bow shock upstream and a magnetotail extending away from the Sun. Energy can enter the magnetosphere through processes including magnetic reconnection, particle acceleration and wave interactions. During periods of enhanced solar activity the Solar Wind can become faster, denser or more strongly magnetised. Such disturbances can produce geomagnetic storms and alter the radiation environment around Earth. The resulting space weather can affect satellites, communication systems, navigation, aviation operations and electrical power networks.
The Solar Wind also affects other planets and smaller bodies. Planets with strong intrinsic magnetic fields can form magnetospheres, while unmagnetised bodies interact with the Solar Wind through their atmospheres, ionospheres or surfaces. Mars has no global intrinsic magnetic field and therefore experiences substantial direct interaction between solar plasma and its upper atmosphere. Over geological timescales Solar Wind interaction can contribute to atmospheric escape, although the significance of this mechanism varies among planets and must be considered alongside thermal escape, photochemical processes and impacts. Comets develop plasma tails and complex interaction regions when solar radiation and Solar Wind interact with material released from their nuclei.
Solar Wind observations are obtained by spacecraft that directly measure particle distributions, plasma density, temperature, composition and magnetic fields. These measurements provide information about conditions in interplanetary space and help connect solar sources with effects observed near planets. Remote solar observations cannot by themselves measure all properties of the outflow because the three dimensional plasma state must be inferred from incomplete observations. Combining direct in situ measurements with imaging and magnetic field models is therefore essential for understanding propagation. Predicting the arrival of disturbances remains a significant scientific challenge because solar plasma structures evolve as they travel.
The Solar Wind is a major component of the heliosphere and acts as a medium through which solar activity propagates outward. It transports disturbances produced by Solar Flares and Coronal Mass Ejections, although electromagnetic radiation from a flare reaches Earth much faster than a material plasma disturbance. The Solar Wind also shapes the boundary between the solar environment and interstellar space. At large distances its pressure decreases as it expands, while the interstellar medium increasingly influences the overall structure of the heliosphere. Understanding the Solar Wind therefore requires linking solar atmospheric physics, plasma physics, planetary magnetospheres and the large scale interstellar environment.
