The solar magnetic field and the solar wind: Existence of preferred longitudes

Direct measurements of the solar wind speed and the radial component of the interplanetary magnetic field acquired over more than three solar cycles a re used to search for signatures of a persistent dependence of solar wind p roperties on solar longitude. Two methods of analysis are used, One finds t he rotation period that maximizes the amplitude of longitudinal variations of both interplanetary and near-Earth data mapped to the Sun. The other is based on power spectra of near-Earth and near-Venus data. The two methods g ive the same result. Preferred-longitude effects are found for a synodic so lar rotation period of 27.03 +/- 0.02 days. Such high precision is attained by using several hundred thousand hourly averages of the solar wind speed and magnetic field. The 27.03-day periodicity is dominant only over long pe riods of time; other periodicities are often more prominent for shorter int ervals such as a single solar cycle or less. The 27.03-day signal is strong er and more consistent in the magnetic field than in the solar wind speed a nd is stronger for intervals of high and declining solar activity than for intervals of low or rising activity. On average, solar magnetic field lines in the ecliptic plane point outward on one side of the Sun and inward on t he other, reversing direction approximately every 11 years while maintainin g the same phase. The data are consistent with a model in which the solar m agnetic dipole returns to the same longitude after each reversal.