The solar wind is a continuous stream of charged particles, primarily protons and electrons, ejected from the Sun's corona—the outermost layer of its atmosphere—into interplanetary space, traveling at speeds typically ranging from 300 to 800 kilometers per second (about 1 million miles per hour) and carrying the Sun's embedded magnetic field.[1][2] This plasma flow originates from the high temperatures in the corona, which exceed 1 million degrees Kelvin, causing the gas to expand supersonically outward in all directions from the Sun.[3] At Earth's orbit, approximately 1 astronomical unit from the Sun, the solar wind has a typical density of 3 to 10 particles per cubic centimeter, though this varies with solar activity.[4]The concept of the solar wind was first proposed in 1958 by physicist Eugene Parker, who theorized that the Sun's corona must expand into space as a dynamic flow to explain observed properties of the interplanetary medium and comet tails.[5] Parker's hypothesis faced initial skepticism but was definitively confirmed in 1962 by NASA's Mariner 2spacecraft, the first interplanetary probe, which measured a steady flux of solar particles during its journey to Venus.[6] Subsequent missions, such as the Wind spacecraft and the Parker Solar Probe launched in 2018, which has flown through the Sun's corona since 2021 including its closest approach to date in December 2024, have provided detailed in-situ observations, revealing the solar wind's structure and origins near the Sun.[7]Compositionally, the solar wind is dominated by fully ionized hydrogen (protons, about 95% of the ionic content) and helium (alpha particles, around 4%), with trace amounts of heavier elements like carbon, oxygen, and iron in various ionization states that reflect the corona's high temperatures.[1][8] It exhibits two primary streams: a slower, denser wind (300–500 km/s) from the Sun's equatorial regions and a faster, less dense wind (600–800 km/s) from polar coronal holes, with variations driven by the 11-year solar cycle.[9] The embedded magnetic field, spiraling outward due to the Sun's rotation, imprints a Parker spiral pattern on the flow, influencing its interactions across the solar system.[2]The solar wind profoundly shapes the heliosphere, the vast bubble of solar-influenced space enclosing the planets, by pushing against the interstellar medium and creating boundaries like the termination shock.[10] On Earth, it compresses the magnetosphere, triggering auroras through particle precipitation into the atmosphere and driving geomagnetic storms that can disrupt satellites, power grids, and communications during intense solar events.[11] Similar interactions occur at other planets, eroding atmospheres on Mars and Venus while protecting magnetized bodies like Jupiter from cosmic rays.[12] Ongoing research, including from the Parker Solar Probe's close solar encounters, continues to refine models of solar wind acceleration and its role in space weather forecasting.[7]
Fundamentals and Properties
Composition and Origin
The solar wind is a continuous stream of charged particles emanating from the Sun's corona, consisting primarily of protons, electrons, and alpha particles (helium nuclei) that flow outward at supersonic speeds exceeding the local sound speed in the plasma. This plasma originates in the outermost layer of the Sun's atmosphere, where high temperatures enable the particles to overcome gravitational binding and expand into the interplanetary medium.[1][2]The elemental composition of the solar wind is dominated by ionized hydrogen, accounting for approximately 95% of the ions as protons (H⁺), with helium making up about 4% primarily as doubly ionized alpha particles (He²⁺). Trace heavier elements constitute the remaining ~1%, including ions of oxygen, carbon, neon, magnesium, silicon, and iron, which are present in proportions reflecting fractionation processes related to their first ionization potentials in the solar atmosphere. Isotopic ratios in the solar wind, such as the ³He/⁴He ratio of roughly 4 × 10⁻⁴, provide insights into the primordial solar composition and are enhanced relative to meteoritic values due to processes in the corona.[13][14][15]The origin of the solar wind lies in the solar corona, a region where plasma temperatures routinely exceed 1 million Kelvin (1 MK), reaching up to 2–3 MK in some areas, which drives thermal expansion and allows particles to achieve escape velocities. This hot, tenuous plasma expands from coronal source regions, particularly open magnetic field structures like