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The wind that inflates the heliosphere, the solar wind, blows continuously. It originates in the several-million-degree solar corona and is accelerated to supersonic speeds near the Sun. Like its coronal source, the solar wind is structured and variable. It varies in density, speed, and temperature, and in the strength and orientation of the magnetic field embedded in its flow (the interplanetary magnetic field, or IMF). At solar minimum, the heliosphere is dominated by a fast solar wind from high latitudes, while during the approach to and at solar maximum it is dominated by a slow and variable wind from all latitudes. This change in the structure of the solar wind reflects the dramatic reconfiguration of the corona that takes place as the polarity of the solar magnetic field reverses.
During this period, the solar wind is also increasingly disturbed in its flow by coronal mass ejections (CMEs), which occur over ten times more often at solar maximum than at solar minimum. CMEs are transient releases of huge quantities of coronal plasma and magnetic fields into the heliosphere, sometimes at initial speeds in excess of 1,000 kilometers per second. Fast CMEs drive powerful shock waves, which accelerate solar wind ions to energies high enough to penetrate a space suit or the hull of a spacecraft and can cause severe disturbances in the geospace environment when they encounter Earth’s magnetic field.
The eruption of CMEs and flares, the heating of the corona to temperatures several hundred times that of the Sun’s visible surface, the acceleration of the solar wind—all of these processes, whose detailed workings are still poorly understood, are driven or mediated by energy provided by magnetic fields generated within the upper third of the Sun’s interior, in the so-called convection zone. In this region, the rotational and turbulent convective motions of the electrically conducting plasma drive a magnetic dynamo that generates and maintains the Sun’s global magnetic field as well as smallerscale local fields. The magnetic fields thus generated emerge through the photosphere, forming sunspots and other active regions and creating the complex and dynamic coronal structures revealed in such stunning detail in recent images from the Transition Region and Coronal Explorer (TRACE) spacecraft.
As the solar wind flows away from the Sun and fills the heliosphere, it interacts in various and complex ways with the planets and other solar system bodies that it encounters. The nature of this interaction depends critically on whether the object has an internally generated magnetic field (Mercury, Earth, the giant outer planets) or not (Venus, Mars, comets, the Moon). For example, Mars has no strong global magnetic field, and the solar wind impinges directly on a significant fraction of its thin carbon dioxide atmosphere. The erosion of the atmosphere resulting from this interaction may have played an important role over the last 3 billion or so years in the evolution of Mars’s atmosphere and climate. In contrast, the terrestrial atmosphere is protected from direct exposure to the solar wind by Earth’s magnetic field, which forms a complex and dynamic structure—the magnetosphere—around which most of the solar wind is diverted.
The solar wind’s interaction with the magnetosphere, effected principally through a temporary merging of the interplanetary and terrestrial magnetic fields, stirs and energizes the magnetospheric plasmas and leads to periodic explosive releases of magnetic energy. In these events, known as magnetospheric substorms, powerful electrical currents flow between the magnetosphere and the ionosphere, injecting several billion watts of power into the upper atmosphere and producing often quite spectacular displays of the aurora borealis and australis—the Northern and Southern Lights. Unfortunately, at times of extreme magnetospheric disturbance, the beauty of the auroras can be accompanied by less benign phenomena—radiation belt enhancements, for example, and ionospheric disturbances— that can disrupt the communications, navigation, and power systems on which modern society so extensively depends.
- Solar Wind Turbulence (Matthaeus): Spacecraft venturing into the solar wind measure broadband fluctuations in all plasma variables, including the proton fluid velocity, the density, and the magnetic field. Such fluctuations are in many ways reminiscent of ordinary fluid turbulence; however, turbulence in the solar wind is also distinctly a plasma phenomenon, involving magnetic fields, kinetic effects, and interactions with charged particles. The observed turbulence is thought to enhance heating of the interplanetary medium, and it is responsible for scattering and transport of cosmic rays originating from both inside and outside the heliosphere. Solar wind turbulence also provides important clues about the nature of the lower solar corona. A substantial portion of the turbulence energy is distributed in a distinctive power-law spectral distribution over about three decades of spatial scale, and is highly reminiscent of ordinary fluid turbulence. Like its terrestrial counterparts, solar wind turbulence remains an incompletely understood topic but an important one, in that it mediates complex dynamical couplings between very large and very small scales, very slow fluid motions and very fast kinetic processes, and very-low-energy and very-high-energy plasma particles.