Parker solar wind is a theoretical model of the solar wind developed by American solar physicist Eugene Parker in 1958. The model describes how the Sun's hot outer atmosphere, the corona, expands continuously into interplanetary space as a supersonic plasma stream rather than remaining in static equilibrium. Parker demonstrated that the corona's high temperature creates a pressure gradient that overcomes the Sun's gravitational pull near the solar surface, causing the plasma to accelerate outward. This acceleration continues until the flow passes through a critical transition point where it becomes supersonic, typically occurring within a few solar radii from the Sun's surface. The theory successfully predicted solar wind velocities of several hundred kilometers per second at Earth's distance (1 astronomical unit or AU) and explained how the interplanetary magnetic field adopts a characteristic spiral pattern due to solar rotation.[1][2]
Parker coined the term "solar wind" to describe this continuous supersonic outflow, distinguishing it from the alternative subsonic "solar breeze" model. The Parker model encompasses three key components: the transonic wind solution, the spiral configuration of the interplanetary magnetic field, and the prediction of supersonic bulk velocities in interplanetary space. Parker's 1963 monograph Interplanetary Dynamical Processes provided a comprehensive treatment of the theory and its connection to early spacecraft observations, establishing it as a foundational text in heliophysics.[3][4]
History
Structure of a comet
Hints that the Sun emits a continuous outflow predate spaceflight. Observations of comet ion tails led Ludwig Biermann in the 1950s to posit a corpuscular solar emission, but a physical mechanism and global dynamics remained unclear. Parker's 1958 paper supplied both, framing the corona as a thermally driven, steady flow that must pass smoothly through a critical point and accelerate to supersonic speed.[1] British astronomer Joseph W. Chamberlain responded with an alternative exospheric model he termed the "solar breeze", a globally subsonic flow that declines with distance.[5] Italian plasma physicist Marco Velli later showed that "the breeze solution is unstable" to low frequency perturbations, strengthening the case that the physical solar outflow must be transonic.[6][7]
Mariner 2 engineering model
Confirmation arrived with Mariner 2 in 1962 when American space physicists Marcia Neugebauer and Conway W. Snyder reported a steady flow of ions from the direction of the Sun with velocities generally 400–700 km s−1, in line with Parker's prediction.[8] Subsequent missions, including Helios, Ulysses, Wind, and ACE, established that the supersonic wind is a persistent heliospheric feature and varies with solar latitude and the solar cycle.[9]
Since 2018 Parker Solar Probe has sampled the near-Sun wind inside 0.1 AU. Early results reported ubiquitous magnetic field reversals or "switchbacks" and Alfvénic patches, and later showed that the spacecraft entered regions magnetically connected to the corona. Reviews summarize switchbacks as "strong rotations of the magnetic-field vector", while new studies connect many of them to interchange reconnection in the lower atmosphere.[10][11][12][13] These measurements probe where Parker's thermal forcing couples to wave and reconnection physics that shape the nascent wind.
Parker's hydrodynamic wind model has become a fundamental framework for understanding stellar winds from other stars and atmospheric escape from strongly irradiated exoplanets. In exoplanet research, scientists commonly employ one-dimensional "Parker-type" hydrodynamic models within the planetary Hill sphere to simulate transonic atmospheric escape. These models help interpret observational signatures such as helium absorption at 1083 nm and Lyman-alpha absorption detected during planetary transits.[14][15]
When applied to stellar winds, the Parker framework is extended to incorporate additional physical processes including stellar rotation, magnetic field stresses, and non-isothermal energy transport mechanisms. Despite these complexities, the fundamental transonic flow structure and the existence of a critical point where the flow transitions from subsonic to supersonic speeds remain essential characteristics of the model.[16]
Physical model
Laboratory simulation of solar wind interaction with a magnetosphere using a terrella (magnetized anode globe) in an evacuated chamber, showing aurora-like Birkeland currents.
Parker treated the corona as a steady, spherically symmetric, single fluid outflow with pressure, density, and radial speed varying with distance. Combining mass conservation with radial momentum balance and an energy closure, typically isothermal or polytropic, yields a first order differential equation for the radial velocity. In the isothermal case the equation can be expressed so that its denominator vanishes at a critical radius, which requires the numerator to vanish there as well. The corresponding solution accelerates from subsonic to supersonic speed and is the unique transonic branch that satisfies regularity at the critical point and finite conditions at infinity.[1][16]
The critical radius depends on the gravitational potential and the effective coronal sound speed. For an isothermal sound speed of approximately 100 km s−1, the critical point occurs within a few solar radii of the Sun's surface, ensuring that the flow becomes supersonic well before reaching 1 AU. This prediction aligns with spacecraft observations of supersonic solar wind velocities in interplanetary space.[1][2]
The solar wind carries the solar magnetic field outward into interplanetary space. Due to solar rotation, the frozen-in magnetic field lines are stretched into an Archimedean spiral pattern known as the Parker spiral. In this configuration, the azimuthal angle of the magnetic field increases with heliocentric distance and decreases with wind speed. Parker predicted this spiral geometry in his 1958 analysis, and it was subsequently confirmed through statistical analysis of interplanetary magnetometer data from Mariner 2, Pioneer, and later space missions.[1][4][17]
In situ spacecraft measurements at 1 astronomical unit (AU) have revealed two distinct types of solar wind characterized by different velocities and origins. The slow solar wind typically flows at velocities around 400 km s−1, while the fast solar wind reaches speeds of 700 km s−1 or higher. These wind types exhibit fundamental differences in their solar sources and plasma properties. The fast solar wind originates primarily from coronal holes, regions of open magnetic field lines that appear as dark areas in solar X-ray images due to their reduced coronal density. The slow solar wind, by contrast, displays greater compositional variability and is associated with the heliospheric current sheet and the boundaries of helmet streamers, which are large-scale coronal magnetic structures that extend into interplanetary space. Spacecraft observations have demonstrated that velocity alone is insufficient for reliably distinguishing between these wind types. More accurate classification relies on plasma composition, ion charge states, and temporal variability patterns. This observational evidence has enhanced understanding of the original Parker model by revealing that the solar wind's structure is considerably more complex than a single, uniform outflow.[9][18][19]
Reception
Parker's transonic outflow model superseded earlier hydrostatic and subsonic theories by demonstrating that a hot corona must undergo continuous expansion rather than remain in static equilibrium. Modern space plasma physics recognizes this as an intrinsically multi-scale phenomenon, where the global expansion of the corona couples to kinetic processes across many orders of magnitude in both time and spatial scales.[2] Early critics of alternative exospheric models, such as Chamberlain's "solar breeze", argued that these subsonic solutions lacked the physically necessary transonic transition. This criticism was later validated when Italian plasma theorists demonstrated the mathematical instability of breeze solutions, establishing Parker's transonic model as the dynamically stable and physically correct description of solar wind acceleration.[6][7]
Beyond its foundational role in solar wind theory, Parker's model has proven essential for understanding turbulence in space plasmas. The solar wind serves as a natural laboratory for studying turbulent processes, characterized by its supersonic and super-Alfvénic flow properties. Research has shown how broadband fluctuations in this environment transfer energy and provide heating mechanisms for the plasma.[20] Modern understanding integrates Parker's original fluid acceleration framework with Alfvénic wave dynamics, turbulent cascades, and kinetic microinstabilities that govern particle distribution functions and energy partitioning between different plasma species.[2]
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