Parker spiral

The solar wind is a continuing outflow of electrically charged particles from the Sun’s outer atmosphere. In the highly conducting plasma, magnetic field lines are carried along with the moving solar wind to a good approximation. Near the Sun, the open field is approximately radial. But the point on the Sun from which a field line emerges rotates as the wind parcel travels outward. The successive positions of that rotating source, combined with the outward flow, trace a spiral-shaped field pattern rather than a straight radial line. This geometry is often compared with water from a rotating garden sprinkler, although the magnetic field is embedded in a plasma and follows magnetohydrodynamic dynamics.

In the simplest steady, radial-flow picture, the field has both radial and azimuthal components. Close to the Sun the radial component is more prominent; farther out, the effect of solar rotation accumulates, and the field becomes more inclined relative to the Sun–spacecraft direction. The exact spiral angle depends on heliocentric distance, solar rotation, wind speed and latitude. Because solar-wind speed varies, the field is not a perfect smooth spiral everywhere: streams of different speeds, solar activity and evolving magnetic structures complicate the idealized pattern. The familiar Parker spiral is therefore a useful large-scale model, not a description of every local magnetic-field measurement.

The pattern follows from Eugene Parker’s theoretical work on the solar wind and interplanetary magnetic field. In 1958, Parker published a dynamical account of the solar wind and its associated magnetic field. NASA notes that the first measurement confirming the solar wind came in 1959, from a spacecraft operating beyond Earth’s orbit. Spacecraft observations have since mapped the solar wind and interplanetary field throughout the heliosphere, while later missions such as Parker Solar Probe have sampled the environment much closer to the Sun.

The spiral field guides the motion of charged particles, including energetic particles released in solar eruptions. The magnetic connection between a spacecraft and the Sun can influence when and how solar energetic particles reach it, and thus matters for interpreting observations and space-weather hazards. The field also forms part of the heliosphere’s global structure, the immense region in which the solar wind interacts with material from interstellar space. These applications depend on the broad magnetic geometry, while actual conditions can depart from the simple steady model.