High-speed solar wind

High-speed solar wind is the fast component of the solar wind characterized at 1 astronomical unit (AU) by bulk speeds commonly between about 600 and 800 km/s and by relatively low density and high Alfvénicity compared to slow wind. It originates primarily in coronal holes, regions of open magnetic field in the solar corona.

ESA visualization of magnetic waves channeling fast wind within coronal holes
Classification
Type	Solar wind regime
Also called	High-speed stream (HSS)[1]
Parent concept	Solar wind
Source regions	Coronal holes, open magnetic flux[2]
Typical near 1 AU
Speed	600–800 km/s, average ≈ 667 km/s[3]
Proton density	≈ 3 cm−3 (typical)[3]
Composition	Near-photospheric abundances with small FIP bias ≈ 1.7× and low O7+/O6+ < 0.1[4]
Turbulence	Strongly Alfvénic fluctuations typical of fast wind[5]
Heliospheric effects
Associated structures	Stream interaction regions and Corotating interaction regions[6]
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American solar physicist Steven R. Cranmer describes coronal holes as "the darkest and least active regions of the Sun" that are "associated with rapidly expanding open magnetic fields and the acceleration of the high-speed solar wind." Despite decades of study, the precise mechanisms feeding the solar wind from these regions remain unclear. As Cranmer notes, "it is still unknown to what extent the solar wind is fed by flux tubes that remain open (and are energized by footpoint-driven wave-like fluctuations), and to what extent much of the mass and energy is input intermittently from closed loops into the open-field regions."^[2]

When fast streams sweep past Earth, they are commonly referred to as high-speed streams. Their interaction with preceding slower solar wind creates stream interaction regions and Corotating interaction regions, which can trigger recurrent geomagnetic activity.^[6][7][8]

History

A cluster of double layers forming in an Alfvén wave, about a sixth of the distance from the left.

The theoretical foundation for high-speed solar wind was established by Eugene Parker, who predicted a supersonic outflow from the hot corona and demonstrated that magnetic fields would be carried outward with the expanding plasma.^[9] The discovery and application of Alfvén waves by Hannes Alfvén provided a crucial framework for understanding wave–plasma coupling, which remains central to modern fast-wind acceleration models.^[10]

An animation of the Parker Solar Probe's trajectory from August 7, 2018, to August 29, 2025:
  Parker Solar Probe ·   Sun ·   Mercury ·   Venus ·   Earth
For more detailed animation, see this video.

Key observational milestones followed these theoretical advances. Polar passes by the Ulysses spacecraft established that fast wind dominates at high latitudes during solar minimum, revealing the global structure of high-speed streams.^[11] More recently, Parker Solar Probe has provided unprecedented close-up observations of the source physics near the Sun, directly measuring magnetic switchbacks and microstreams that characterize young fast wind.^[12][13]

The characterization of high-speed wind has advanced through a comprehensive suite of space missions and observational techniques. In situ measurements from spacecraft including Helios, Ulysses, WIND, ACE, Solar Orbiter, and Parker Solar Probe have provided detailed plasma, magnetic field, and composition diagnostics spanning distances from 0.3 AU to high heliolatitudes. Complementing these direct measurements, remote sensing observations of coronal holes using extreme ultraviolet and X-ray imaging, Doppler dimming spectroscopy, and interplanetary scintillation techniques have helped constrain the source regions and acceleration profiles of fast wind.^{[2][6][14][7]}

Definition

CME erupts, generating high-speed solar wind

Space-physics studies commonly separates solar wind intervals by speed into fast, slow, or transitional. Many observational studies define fast wind as intervals with average proton bulk speed greater than about 600 km/s, slow wind as less than about 400 km/s, and intermediate values as mixed or transitional flow.^[15] In the inner heliosphere, the fast component tends to be Alfvénic and relatively homogeneous over hours to days, while the slow component is more variable in composition, temperature, and turbulence properties.^[16][14]

High-speed wind emerges along open magnetic flux that threads coronal holes. Italian space physicists Roberto Bruno and Vincenzo Carbone noted that fast wind is "less dense but hotter" than slow wind, reflecting the distinct thermodynamics of open-field regions.^[16] Results from NASA's Parker Solar Probe indicate that fast wind parcels originate deep within coronal holes, where interchange reconnection between open and closed fields creates microstream structure and associated magnetic switchbacks.^[13] These findings build on earlier coronal-hole studies that linked fast wind to open flux tubes and documented its connection to large polar holes during solar minimum.^[2]

At 1 AU, typical fast-wind speeds cluster near 700 km/s, with proton number densities lower than in slow wind and higher proton temperatures.^[14][16] The fluctuations are strongly Alfvénic, with velocity and magnetic-field variations closely correlated and with large directional rotations. Near the Sun, Parker Solar Probe has observed ubiquitous, short-duration magnetic-field reversals known as switchbacks and "Alfvénic velocity spikes," which are signatures of young, Alfvénic fast wind.^[12][17]

Composition and charge states also distinguish high-speed streams. Coronal-hole wind exhibits near-photospheric elemental abundances with a low degree of first ionization potential fractionation compared with slow wind, consistent with plasma release along open field lines.^[18] Charge-state ratios such as ${\displaystyle O^{7+}/O^{6+}}$ are typically lower than in slow wind, reflecting cooler coronal freeze-in temperatures in the source regions.^[18] Ion-kinetic features are prominent: Alpha particles often drift relative to protons along the magnetic field, and the magnitude of the alpha–proton drift is regulated by the wind's Alfvénicity, especially in fast streams.^[15]

Two broad classes of mechanisms are proposed for accelerating high-speed wind on open flux. Wave- or turbulence-driven models deposit energy from Alfvénic fluctuations into the outflow, while reconnection-driven scenarios feed mass and momentum onto open field lines through interchange reconnection at low coronal heights. Cranmer reviews evidence for wave–turbulence heating in coronal holes and connects it to fast wind emerging along "rapidly expanding open magnetic fields."^[2] American space physicist Stuart D. Bale and collaborators reported Parker Solar Probe observations that support nearly continuous interchange reconnection operating within coronal holes, arguing that the source is "deep within coronal holes."^[13] Parker Solar Probe has also crossed below the Alfvén critical surface, providing constraints on where the flow becomes super-Alfvénic and how waves couple to the wind.^[17]

Solar cycle

Main articles: Solar cycle and Solar wind

High-speed solar wind exhibits distinct patterns that vary with the solar magnetic cycle. During solar minimum, observations from the Ulysses spacecraft revealed that fast wind dominates the high-latitude polar regions, forming nearly uniform streams that fill the polar caps. In contrast, slow wind remains confined to a narrow equatorial belt. At solar maximum, this clear latitudinal separation breaks down, and fast streams can originate from coronal holes at mid-latitudes or near the equator.^[11][2] When equatorial coronal holes or their extensions point toward Earth, they generate high-speed streams that create recurrent 27-day patterns corresponding to the Sun's rotation period.^[1]

When a fast-moving stream encounters slower solar wind ahead of it, the interaction creates a compression region. At Earth's distance from the Sun (1 AU), this structure is called a stream interaction region. When this pattern persists over multiple solar rotations, it becomes a corotating interaction region (CIR). These compressed regions exhibit enhanced magnetic field strength and plasma density, and they can develop into shock waves as they propagate farther from the Sun.^[7][6] The high-speed stream that follows is characterized by elevated velocity and temperature but decreasing density—a well-documented sequence observed in spacecraft measurements and used in space weather forecasting.^[1]

High-speed streams and their associated CIRs are particularly effective at driving prolonged geomagnetic disturbances. According to NOAA space weather forecasters, while CIR-driven geomagnetic storms are typically less intense than those caused by coronal mass ejections, they can sustain energy input to Earth's magnetosphere "over a longer interval."^[8] These recurrent high-speed streams generate sequences of geomagnetic storms and prolonged auroral electrojet activity. They also frequently increase electron populations in the outer Van Allen radiation belts through wave-particle interactions that occur during extended periods of high solar wind speed.^[19][6]

See also

• Solar wind
• Coronal hole
• Parker Solar Probe
• Solar Orbiter
• Corotating interaction region
• Space weather