Solar cycle changes in the polar solar wind

Abstract

Although the 11-yr cycle of solar activity was first recognized in 1843 (ref. 1) and was subsequently traced back to the seventeenth century^2–4, it is still only poorly understood. The most obvious indicators of the activity cycle are sunspots, flares, plages, and so on. The corona as seen during an eclipse also reflects the cycle, appearing flattened at the poles near solar minimum and almost spherical near the solar maximum. All such phenomena are intimately linked to the solar magnetic fields, which exhibit complex but systematic variations. Whereas there are satisfactory qualitative models (see refs 5, 6) for the magnetic variations, the underlying physics is still obscure. The solar cycle also influences the Earth causing, for example, changes in geomagnetic activity, the magnetosphere and aurorae, the ionosphere, the flux of galactic cosmic rays and possibly the weather. These modulations are not well understood but must be transmitted by one or more of the following: electromagnetic radiation; solar wind thermal plasma and its magnetic field; energetic particles. We here present new observations on the changing three-dimensional form of the solar wind, which helps to relate some of the modulations.