Solar Wind Acceleration

How magnetic waves shape solar wind speeds

Alfven waves propagating through the solar wind continuously deposit energy into the plasma, accelerating it during its journey from the Sun to Earth. Speed distributions reveal that fast solar wind receives an additional 100+ km/s boost from wave forcing, explaining why observed speeds exceed predictions from thermal expansion alone.

Alfven wave forcing explains the fastest solar wind at Earth

A smooth black curve on a logarithmic vertical axis plots how frequently the solar wind arrives at Earth at each speed, from about 270 to 750 kilometers per second. The distribution rises sharply to a tall peak near 355 km/s (slow wind), then descends through an intermediate range before forming a lower, broader shoulder around 622 km/s (fast wind). Colored bands overlay the curve, each marking a characteristic speed range: cyan highlights the slow and fast wind peaks, magenta marks the transition zone where source regions shift, green spans the speeds predicted by a solar wind model with and without wave-driven acceleration, and orange marks speeds predicted from near-Sun energy measurements by Parker Solar Probe.

solar wind speedprobability distributionfast windslow windAlfvén wave forcingsource regionskinetic energy fluxisopoly modelParker Solar ProbeAlfvénicityAlfven wave accelerationsolar wind speed distributionbimodal distributionwave forcinginterplanetary acceleration

Related Figures

A two-dimensional heatmap plots kinetic energy flux (vertical axis, 0 to 1.4 milliwatts per square meter) against solar wind speed (horizontal axis, about 300 to 800 km/s).

Near-Sun energy predicts near-Earth fast wind speeds

Kinetic energy flux increases steadily with solar wind speed, and the horizontal orange band — derived from scaling near-Sun Parker Solar Probe energy measurements outward using empirically determined power laws — intersects the observed trend at speeds of 557-700 km/s.

Demonstrates how kinetic energy measured near the Sun by Parker Solar Probe predicts speeds at Earth, validating that wave-driven acceleration is complete by 0.2 AU

This chart shows the probability of observing the solar wind at different speeds, forming a tall peak around 355 km/s for slow wind and a smaller shoulder near 622 km/s for fast wind.

Derived speeds map onto the bimodal wind distribution

Characteristic speeds from compressibility analysis (this paper), wave activity analysis (companion paper), and independent energy budget studies all map onto distinct, physically meaningful portions of the bimodal speed distribution.

Places characteristic speeds from compressibility, wave activity, and energy budget analyses onto the bimodal speed distribution, showing how multiple independent approaches converge on a consistent picture of where wave-driven acceleration shapes the wind

Source

Characterizing the Impact of Alfven Wave Forcing in Interplanetary Space on the Distribution of Near-Earth Solar Wind Speeds

The Astrophysical Journal Letters (2025)

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Alfven wave forcing explains the fastest solar wind at Earth

Related Figures

Near-Sun energy predicts near-Earth fast wind speeds

Derived speeds map onto the bimodal wind distribution

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Source